Analysis of the Available Straw Nutrient Resources and Substitution of Chemical Fertilizers with Straw Returned Directly to the Field in China

: Returning straw to the ﬁeld is the most signiﬁcant straw utilization technique for China’s green development. It can provide nutrients for crop growth and improve soil organic matter content. However, there are no standard parameters for measuring the nutrient content of straw directly returned to the ﬁeld. In addition, the nutrient content of straw in the ﬁeld is disregarded and the least researched. Therefore, to address these issues, the current research examines the main nutrient composition of straw returned directly to ﬁelds and the potential substitution of straw for chemical fertilizers. This study used the latest data on the crop straw-to-grain ratio and straw’s direct return to the ﬁeld from the Database of Agricultural Crops Straws Resources in China (DACSRC) as the basis for a detailed estimation of the amount of crop straw nutrient resources and straw’s direct return to the ﬁeld. The straw nutrient resources were measured based on straw yield and the nitrogen (N), phosphorous (P 2 O 5 ), and potassium (K 2 O) contents. The results reveal that the maximum amount of straw returned directly to the ﬁelds in China was 517 teragram (Tg), while 128 Tg (25%) of the total was not collected but left on the ﬁeld. The North China region had an enormous amount of direct straw return at 176 Tg (34%), while South China had the least amount at 26 Tg (5%). The amounts of seasonally available N, P 2 O 5 , and K 2 O from straw directly returned to ﬁelds are 2 Tg, 1 Tg, and 7 Tg, respectively. In addition, the amount of available straw nutrient resources returned to the ﬁeld per hectare (ha) was 78 kg. The quantity of straw-originating seasonally available nutrients corresponds to 23% of the chemical fertilizers applied, which can substitute 10% N, 9% P 2 O 5 , and 58% K 2 O. The study provides critical insights on effective ways to improve straw fertilization through an increased rate of straw returned directly to ﬁelds and releasing seasonal nutrients.


Introduction
Crop straw is rich in nutrients such as nitrogen, phosphorus, and potassium. The decomposition of straw in agricultural soil can release mineral nutrients such as nitrogen, phosphorus, and potassium [1][2][3]. Returning straw to the soil can efficiently raise its organic matter content, improve its physical and chemical characteristics, increase soil fertility, and influence the activity of soil microorganisms, all of which are beneficial for the longterm sustainability of agricultural output [4][5][6][7][8]. Integration of straw return and fertilizer application can also enhance crop plants' uptake and transport of nitrogen, phosphorus, and potassium nutrients and improve the efficiency of nutrient recycling and fertilizerutilization rate in farmland [7,[9][10][11].
As an important initiative in sustainable agricultural practices, directly returning straw to the field is considered the most economical and ecological method of straw utilization [12]. Directly returning straw to the field can be scientific or nonscientific.
Using the appropriate straw quantity based on the cultivated land's chemical and physical composition constitutes the scientific method for returning straw directly to the field. This approach combines good agronomic practices (GAP) with a standard amount of straw that has been scientifically validated. On the other hand, using unconventional or excessive straw application techniques without properly managing the straw resources on the field is referred to as returning straw to the field in an unscientific manner. The scientific approach of directly returning straw improves the quality of cultivated land [13][14][15][16] and has become an important means to effectively control straw's open burning and reduce carbon sequestration [17]. However, unscientific and direct straw-returning practices may have negative effects such as reducing the crop germination rate and increasing crop diseases and insect pests [18][19][20]. Therefore, it is of great significance to properly evaluate the technology of directly returning straw for the effective management of cultivated lands and the sustainable development of agriculture.
At present, due to the lack of detailed and authoritative sources of parameters, many studies on the nutrient resources of straw returning and the potential of chemical fertilizer substitution can only roughly obtain data such as the straw-to-grain ratio and strawreturning ratio from the relevant published literature as the basis for calculation. In addition, in terms of straw-return nutrient resource estimation, most of the existing studies have analyzed the theoretical straw-return nutrient resource under the scenario of full straw return (i.e., 100% straw-return ratio) [17,21,22]. However, by combining information from various studies, only a small amount of research could determine the actual straw-return ratio for the nutrient composition of the returned straw [23][24][25]. Most of this research failed to consider important factors such as variations in straw-return rates across regions and crops. Moreover, the data of the straw-to-grain ratio are generally outdated, which may lead to large errors between the estimation results and reality, thus failing to accurately reflect the nutrient resource status of straw returned to the field under realistic agricultural production conditions.
To address these issues, the current research used the latest data on the crop straw-tograin ratio and directly returning straw to the field from the Database of Agricultural Crops Straws Resources in China (DACSRC). The analysis of the amount of crop straw nutrient resources and straw directly returned to the field was performed in detail. Compared with previous studies, the straw-to-grain ratio and the amount of straw returned directly to the field in this study were obtained from the latest measurements and survey results of different crops and regions in China. This can accurately reflect the nutrient resources of straw under the realistic level of straw directly returned to the field. Secondly, unlike previous research, the current study considers not just the conventional method of "directly returning straw to the field" but also the stubble portion of the crop left upright in the field after harvest that could not be gathered off the field [26,27]. The general statistics of the direct straw return to the field only refer to the collectable and usable volume of straw, excluding the root stubble fraction. The collectable and usable volume of straw resources refers to the maximum amount that could be collected from the field and utilized under actual cultivation management, particularly for crop-management practices [28]. At present, the level of agricultural mechanization is increasing in many countries including China, Bangladesh [29], and African countries [30], whiles Myanmar is a new frontier for mechanization research [31]. A dynamic assessment of the amount of nutrient resources returned to the field, including the amount of straw residue returned to the field and its potential for fertilizer substitution, will help to clarify the potential of the remaining uncollectable, upright stubble in the field in terms of nutrient availability. In addition, it will also help to dynamically adjust fertilizer-application rates in conjunction with optimal fertilizer-application rates and to scientifically develop fertilizer-management strategies [26,[32][33][34]. Given this, the nutrient resource contribution of the stubble portion of the crops left in the field cannot be ignored. Finally, this study also assessed the effective nutrient resources and fertilizer-substitution potential of the major crops of straw at the national scale based on the seasonal nutrient release rates of different crops, which is more comprehensive than previous studies.
The amount of crop straw and its nutrient resources in China is huge. Based on the geographical characteristics, the full and reasonable utilization of straw's nutrient resources among different regions in China and the scientific and accurate assessment of the potential of replacing chemical fertilizers with effective nutrient resources in the season of straw's direct return to the field are important ways to realize the reduction of chemical fertilizer application and the increase of efficiency in China. This paper aims to provide a theoretical basis and data for the efficient utilization of straw resources and green agricultural production in different regions in China.

Study Area
The study covers 31 provinces, cities, and autonomous regions of China, excluding Hong Kong, Macau, Taiwan, and the South China Sea Islands. The paper considers the differences in China's farming systems' zoning and economic development levels and the differences in straw-return levels between different regions and crops. This paper divides China into six major agricultural zones [32] based on maintaining the integrity of provincial boundaries (Figure 1), and the agro-ecosystems and cultivation conditions in each region are as follows. The main crops cultivated in these regions are rice, wheat, corn, soybeans, cotton, peanuts, and rapeseed. Similarly, the total land under cultivation for these selected crops was 124 million hectares, corresponding to 91% of the total cultivated land for crops in China. the crops left in the field cannot be ignored. Finally, this study also assessed the effective nutrient resources and fertilizer-substitution potential of the major crops of straw at the national scale based on the seasonal nutrient release rates of different crops, which is more comprehensive than previous studies. The amount of crop straw and its nutrient resources in China is huge. Based on the geographical characteristics, the full and reasonable utilization of straw's nutrient resources among different regions in China and the scientific and accurate assessment of the potential of replacing chemical fertilizers with effective nutrient resources in the season of straw's direct return to the field are important ways to realize the reduction of chemical fertilizer application and the increase of efficiency in China. This paper aims to provide a theoretical basis and data for the efficient utilization of straw resources and green agricultural production in different regions in China.

Study Area
The study covers 31 provinces, cities, and autonomous regions of China, excluding Hong Kong, Macau, Taiwan, and the South China Sea Islands. The paper considers the differences in China's farming systems' zoning and economic development levels and the differences in straw-return levels between different regions and crops. This paper divides China into six major agricultural zones [32] based on maintaining the integrity of provincial boundaries (Figure 1), and the agro-ecosystems and cultivation conditions in each region are as follows. The main crops cultivated in these regions are rice, wheat, corn, soybeans, cotton, peanuts, and rapeseed. Similarly, the total land under cultivation for these selected crops was 124 million hectares, corresponding to 91% of the total cultivated land for crops in China.  The northeast zone is composed of four provinces (districts), which include Liaoning, Jilin, Heilongjiang, and Inner Mongolia. The arable land in this region is fertile and concentrated, suitable for mechanized farming. Rainfall is abundant, with 500-700 mm of annual precipitation, 80-180 days of frost-free period, and the first frost in early-and mid-September. The northeast zone records 1300-3700 • C of cumulative temperature ≥10 • C, The data on crop yield, crop-sown area, straw-to-grain ratio, collection coefficient, and the amount of collectable straw returned to the field were obtained from the 2020 DACSRC [32]. The cultivated land area and fertilizer-application rate data were retrieved from the China Statistical Yearbook 2021 [35]. The nutrient content of crop straw (air-dried basis) is shown in Table 1, and the nutrient-release rate (%) of straw in the current season data was derived from the published literature (Table 2) [23].

Evaluation Method
The amount of straw returned directly to the field in the DACSRC is the amount returned directly to the field in the traditional way, which is the amount of collectable and usable straw. Moreover, the uncollectable and usable straw returned directly to the field refers to the amount of stubble left in the field. In this study, the contributions of the amount of collectable and usable straw returned directly to the field and the amount of stubble left in the field were considered together when measuring the nutrient resources of straw returned directly to the field ( Figure 2). The amount of stubble left in the field can be calculated from the difference between the amount of straw output and the amount of collectable and usable straw. Straw output is the product of crop yield and the straw-to-grain ratio, and the amount of collectable and usable straw is the product of the straw output and collection coefficient. Therefore, Equation (1) was used for the estimation:  The amount of stubble left in the field can be calculated from the difference between the amount of straw output and the amount of collectable and usable straw. Straw output is the product of crop yield and the straw-to-grain ratio, and the amount of collectable and usable straw is the product of the straw output and collection coefficient. Therefore, Equation (1) was used for the estimation: where M T is the amount of straw directly returned to the field (kg ha −1 ), W Pij is the crop yield of the j-th crop in the i-th province (city or autonomous region) (kg ha −1 ), S Gij is the straw-to-grain ratio of the j-th crop in the i-th province (city or autonomous region), S Cij is the i-th province and is the collectable coefficient of the j-th crop in the i-th province (city or autonomous region), and M Sij is the amount of collectable straw that can be directly returned to the field for the j-th crop in the i-th province (city or autonomous region) (kg ha −1 ). The quantity of nitrogen (N) nutrients from straw return was calculated as shown in Equation (2): where M n is the nitrogen (N) nutrient resources from the straw return (kg ha −1 ), W j is the amount of j-th crop straw directly returned to the field (kg/ha), and N j is the unit content of nitrogen nutrient in the j-th crop straw (%). The quantity of phosphorus (P 2 O 5 ) nutrients from straw return was calculated based on Equation (3): where M p is the phosphorus (P 2 O 5 ) nutrient resources from the straw return (kg ha −1 ), P j is the unit content of phosphorus nutrients in the j-th crop straw (%), and the constant 2.29 is the conversion from elemental phosphorus to P 2 O 5 [21]. The quantity of potassium (K 2 O) nutrients from straw return was calculated based on Equation (4): where M k is the amount of potassium (K 2 O) nutrient resources from the straw return (kg ha −1 ), K j is the unit content of potassium in the j-th crop straw (%), and the constant 1.2 is the conversion coefficient from elemental potassium to K 2 O [21]. The amount of in-season effective nutrient resources derived from direct straw return refers to the amount of inorganic nutrient resources that are effectively absorbed and utilized by crops in the current season during direct straw return [22,24]. Due to the slow decomposition of straw, it is difficult for straw to release nutrients quickly once it is returned to the field. Therefore, the seasonal effectiveness after directly returning straw to the field should be considered when studying the potential of nutrient resources derived from straw return replacing chemical fertilizers [21]. The in-season effective nutrient resources of direct straw return can be calculated as follows (Equation (5)): where W E is the amount of in-season effective nutrient resources from direct straw return (kg ha −1 ), M ij is the amount of straw directly returned for the j-th crop in the i-th province (city or autonomous region) (kg ha −1 )), and R j is the nutrient release rate of the j-th crop in the current season (%).

Data Processing
The data were calculated using Microsoft Excel 2016, which was produced by Microsoft Corporation, based in Redmond, WA, USA. Distribution maps were plotted using ArcGIS 10.6, which was produced by Environmental Systems Research Institute Corporation, based in Redlands, CA, USA, and Origin 2022, which was produced by Origin Lab Corporation, based in Redlands, MA, USA.

Directly Returning Main Crop Straws to the Field
The results showed that the total amount of major crop straw that can be directly returned to the field is 517 Tg in China, accounting for 64% of the total straw production ( Figure 3). Among these, the amount of stubble left in the field is 128 Tg, 25% of the total amount of straw directly returned. In addition, the amount of collectable and usable straw returned directly to the field is 389 Tg, corresponding to 75% of the total straw amount. the Southeast China region had the least straw return at 26 Tg, corresponding to only 5% of the total straw return. The main reason for the difference in the amount of straw directly returned to the field in the northeastern and southern regions is that the northeastern region is dominated by corn, rice, and soybean, with a large straw production accounting for 24% of the national straw production. Among them, the amount of straw directly returned to the field accounted for 43% of the total collectable and usable straw, while the southern region is dominated by rapeseed and rice straw, accounting for a low proportion of the national straw production. The straw directly returned to arable land is 4045 kg per hectare at the national level. The straw directly returned to arable land in the North China region is 7252 kg per hectare, which is the highest among the six regions. This is followed by the middle and lower reaches of the Yangtze River region, with the amount of straw directly returned to arable land is 6695 kg per hectare. However, the straw-return level in the northwestern and southwestern regions is relatively low, with an amount of straw directly returned to arable land of 2052 kg per hectare and 2043 kg per hectare.
The findings revealed variations in the proportion of straw returned to the field in the different regions. Given this, North China, South China, and the middle and lower reaches of the Yangtze River region have higher direct straw-return to straw-production ratios of 77%, 76%, and 71%, respectively. Meanwhile, the proportions of the direct strawreturn ratio in the Northeast, Northwest, and Southwest China regions are almost 50% each, which is relatively lower than in the other regions. Figure 4 shows the amount of major crop straw returned to the field. The three major grain of crops, rice, wheat, and maize contribute the largest shares of 168 Tg, 142 Tg, and 151 Tg, which account for 32%, 27%s and 29% of the direct return of the main crop straw. The direct return of soybean, cotton, rapeseed, and peanut straw contributes relatively lower proportions of 3%, 4%, 3%, and 1%, respectively.  The findings reveal a significant variation in China's regional distribution of straw. The amount of straw directly returned to the field has a higher potential in the North China region, the middle and lower reaches of the Yangtze River region, and the Northeast China region, while the Northwest, Southwest, and Southeast China regions had the least share. The North China region had the largest amount of direct straw return, reaching 176 Tg and accounting for 34% of China's total crop straw return. The middle and lower reaches of the Yangtze River had the second-highest amount of straw return of 149 Tg (29%). In addition, the straw return in Northeast China region is 98 Tg (19%), whereas the Southeast China region had the least straw return at 26 Tg, corresponding to only 5% of the total straw return. The main reason for the difference in the amount of straw directly returned to the field in the northeastern and southern regions is that the northeastern region is dominated by corn, rice, and soybean, with a large straw production accounting for 24% of the national straw production. Among them, the amount of straw directly returned to the field accounted for 43% of the total collectable and usable straw, while the southern region is dominated by rapeseed and rice straw, accounting for a low proportion of the national straw production. The straw directly returned to arable land is 4045 kg per hectare at the national level. The straw directly returned to arable land in the North China region is 7252 kg per hectare, which is the highest among the six regions. This is followed by the middle and lower reaches of the Yangtze River region, with the amount of straw directly returned to arable land is 6695 kg per hectare. However, the straw-return level in the northwestern and southwestern regions is relatively low, with an amount of straw directly returned to arable land of 2052 kg per hectare and 2043 kg per hectare.
The findings revealed variations in the proportion of straw returned to the field in the different regions. Given this, North China, South China, and the middle and lower reaches of the Yangtze River region have higher direct straw-return to straw-production ratios of 77%, 76%, and 71%, respectively. Meanwhile, the proportions of the direct straw-return ratio in the Northeast, Northwest, and Southwest China regions are almost 50% each, which is relatively lower than in the other regions. Figure 4 shows the amount of major crop straw returned to the field. The three major grain of crops, rice, wheat, and maize contribute the largest shares of 168 Tg, 142 Tg, and 151 Tg, which account for 32%, 27%s and 29% of the direct return of the main crop straw. The direct return of soybean, cotton, rapeseed, and peanut straw contributes relatively lower proportions of 3%, 4%, 3%, and 1%, respectively.  Regarding the proportion of major crop straws returned to the field, wheat straw has the highest return level of 80% of the straw production. The contribution of wheat straw is followed by rice (75%) and cotton (74%). For soybean, rapeseed, and maize straw, the amount of direct straw return accounts for 64%, 60%, and 50%, respectively. Further, with 25%, peanut straw has the lowest percentage.

Nutrient Resources of Major Crop Straws Returned to the Field
The results highlight that the major crop straw's direct return to the field is 14 Tg. Among these, 3 Tg of nutrient resources are from the stubble left in the field, accounting for 25%. The nutrient resources from the amount of collectable and usable straw returned directly to the field are 10 Tg, corresponding to 75% of the total nutrient resources. Likewise, N, P2O5, and K2O nutrient resources from the straw return are 4 Tg, 1 Tg, and 8 Tg, accounting for 31%, 9%, and 59% of the total nutrient resources of the major crop straws returned to the field. Figure 5 shows the regional distribution of nutrient resources from straw return. The middle and lower reaches of the Yangtze River region have the largest amount of strawreturn nutrient resources at 4 Tg, of which N, P2O5, and K2O are 1 Tg, 0.4 Tg, and 3 Tg, respectively. This is followed by the North China and Northeast China regions, where the  Regarding the proportion of major crop straws returned to the field, wheat straw has the highest return level of 80% of the straw production. The contribution of wheat straw is followed by rice (75%) and cotton (74%). For soybean, rapeseed, and maize straw, the amount of direct straw return accounts for 64%, 60%, and 50%, respectively. Further, with 25%, peanut straw has the lowest percentage.

Nutrient Resources of Major Crop Straws Returned to the Field
The results highlight that the major crop straw's direct return to the field is 14 Tg. Among these, 3 Tg of nutrient resources are from the stubble left in the field, accounting for 25%. The nutrient resources from the amount of collectable and usable straw returned directly to the field are 10 Tg, corresponding to 75% of the total nutrient resources. Likewise, N, P2O5, and K2O nutrient resources from the straw return are 4 Tg, 1 Tg, and 8 Tg, accounting for 31%, 9%, and 59% of the total nutrient resources of the major crop straws returned to the field. The nutrient resources from the straw return of the three major grain crops of rice, wheat, and maize are 5 Tg, 3 Tg, and 4 Tg, accounting for 39%, 21%, and 28%. Additionally, the largest amount of N and K2O is from rice straw return, which is 1 Tg (33%) and 4 Tg (43%), respectively. The largest share of P2O5 nutrient resources is from maize straw at 0.5 Tg, accounting for 35%.
The evaluation of the amount of straw nutrient resources per unit area of cultivated land shows that at the national level, the amount of nutrient resources per unit area for the major crop straw is 106 kg ha −1 . Based on this, the N and P2O5 per unit area of arable land are 33 kg ha −1 and 10 kg ha −1 . The K2O nutrient resource per unit of arable land is the highest compared with other selected nutrients, reaching 63 kg ha −1 .

The Amount of in-Season Effective Nutrient Resources from Direct Straw Return
The total in-season effective nutrient resources of the main crop straw directly returned to the field is 10 Tg. Among these, the amount of in-season effective nutrients from the amount of stubble left in the field is 3 Tg, which accounts for 25% of the total value. The amount of collectable and usable straw returned directly to the field contributes to 8 Tg of in-season effective nutrients and represents 75% of the total in-season effective nutrient resources. The in-season effective N, P2O5, and K2O nutrients from straw return are 2 Tg, 1 Tg, and 7 Tg, respectively, contributing 22%, 9%, and 70% to the total in-season effective nutrient resources. Figure 6 illustrates the regional distribution of the in-season effective nutrient resources from straw returned to the field. The middle and lower reaches of the Yangtze River have the largest amount of in-season effective nutrient resources at 3 Tg. In this regard, the in-season effective N, P2O5, and K2O are 0.6 Tg, 0.3 Tg, and 2 Tg, with the highest percentage of in-season effective K2O estimated at 33%. The North China region has the second-highest amount of in-season effective nutrient resources at 3 Tg. This region also has the highest amount of in-season effective N compared with other regions, reaching 0.7 Tg (32%) of the total national value. In contrast, the Northwest China region has the least amount of in-season effective nutrient resources from the straw return, which is 0.6 Tg (6%). The nutrient resources from the straw return of the three major grain crops of rice, wheat, and maize are 5 Tg, 3 Tg, and 4 Tg, accounting for 39%, 21%, and 28%. Additionally, the largest amount of N and K 2 O is from rice straw return, which is 1 Tg (33%) and 4 Tg (43%), respectively. The largest share of P 2 O 5 nutrient resources is from maize straw at 0.5 Tg, accounting for 35%.
The evaluation of the amount of straw nutrient resources per unit area of cultivated land shows that at the national level, the amount of nutrient resources per unit area for the major crop straw is 106 kg ha −1 . Based on this, the N and P 2 O 5 per unit area of arable land are 33 kg ha −1 and 10 kg ha −1 . The K 2 O nutrient resource per unit of arable land is the highest compared with other selected nutrients, reaching 63 kg ha −1 .

The Amount of in-Season Effective Nutrient Resources from Direct Straw Return
The total in-season effective nutrient resources of the main crop straw directly returned to the field is 10 Tg. Among these, the amount of in-season effective nutrients from the amount of stubble left in the field is 3 Tg, which accounts for 25% of the total value. The amount of collectable and usable straw returned directly to the field contributes to 8 Tg of in-season effective nutrients and represents 75% of the total in-season effective nutrient resources. The in-season effective N, P 2 O 5 , and K 2 O nutrients from straw return are 2 Tg, 1 Tg, and 7 Tg, respectively, contributing 22%, 9%, and 70% to the total in-season effective nutrient resources. Figure 6 illustrates the regional distribution of the in-season effective nutrient resources from straw returned to the field. The middle and lower reaches of the Yangtze River have the largest amount of in-season effective nutrient resources at 3 Tg. In this regard, the inseason effective N, P 2 O 5 , and K 2 O are 0.6 Tg, 0.3 Tg, and 2 Tg, with the highest percentage of in-season effective K 2 O estimated at 33%. The North China region has the second-highest amount of in-season effective nutrient resources at 3 Tg. This region also has the highest amount of in-season effective N compared with other regions, reaching 0.7 Tg (32%) of the total national value. In contrast, the Northwest China region has the least amount of in-season effective nutrient resources from the straw return, which is 0.6 Tg (6%). For the major crop straws, the amount of in-season effective nutrient resources for rice, wheat, and maize straw is 4 Tg, 2 Tg, and 3 Tg, accounting for 40%, 21%, and 28%, respectively. Likewise, the in-season effective K2O from rice straw had the highest share at 3 Tg, representing 42% of the total value. Maize straw provides the most in-season effective nutrient of N and P2O5, which are 0.7 Tg and 0.3 Tg, accounting for 33% and 39% of the total nutrient resources from straw return.
From the results, the amount of in-season effective nutrient resources at the national level for arable land is 78 kg ha −1 . Among them, the effective N, P2O5, and K2O per unit of the arable land area is 17 kg ha −1 , 7 kg ha −1 , and 54 kg ha −1 .

Substitute Potential of in-Season Effective Nutrients for Chemical Fertilizers
In 2020, the output of major crops in China was 726 Tg, and the total sown area was 124 million hectares (Table 3). In terms of the optimal rate of fertilization, the average application rates of N, P2O5, and K2O application are 168 kg ha −1 , 83 kg ha −1 , and 97 kg ha −1 , with an application ratio of 1:0.49:0.58 (N:P2O5:K2O). Note: The optimal fertilizer-application rates for different crops were taken from the average of the optimal fertilizer-application rates in different regions of China, and the data were mainly obtained from the recommended fertilizer-application rates of soil-testing formulations in different regions For the major crop straws, the amount of in-season effective nutrient resources for rice, wheat, and maize straw is 4 Tg, 2 Tg, and 3 Tg, accounting for 40%, 21%, and 28%, respectively. Likewise, the in-season effective K 2 O from rice straw had the highest share at 3 Tg, representing 42% of the total value. Maize straw provides the most in-season effective nutrient of N and P 2 O 5 , which are 0.7 Tg and 0.3 Tg, accounting for 33% and 39% of the total nutrient resources from straw return.
From the results, the amount of in-season effective nutrient resources at the national level for arable land is 78 kg ha −1 . Among them, the effective N, P 2 O 5 , and K 2 O per unit of the arable land area is 17 kg ha −1 , 7 kg ha −1 , and 54 kg ha −1 .

Substitute Potential of in-Season Effective Nutrients for Chemical Fertilizers
In 2020, the output of major crops in China was 726 Tg, and the total sown area was 124 million hectares (Table 3). In terms of the optimal rate of fertilization, the average application rates of N, P 2 O 5 , and K 2 O application are 168 kg ha −1 , 83 kg ha −1 , and 97 kg ha −1 , with an application ratio of 1:0.49:0.58 (N:P 2 O 5 :K 2 O). Note: The optimal fertilizer-application rates for different crops were taken from the average of the optimal fertilizer-application rates in different regions of China, and the data were mainly obtained from the recommended fertilizer-application rates of soil-testing formulations in different regions of China, the recommended fertilizerapplication rates of nutrient expert systems, and published academic papers and master's and doctoral theses from 2010-2017 [21]. Different crops' optimal chemical fertilizer-application rates can be calculated based on the optimal fertilizer (nutrition) amount. The in-season effective nutrient resources from the main crop straw return account for 23% of the optimal chemical fertilizer-application rate, indicating that it can replace 10% of the N chemical fertilizer application, 9% of P 2 O 5 fertilizer application, and 58% of K 2 O fertilizer application (Figure 7). The amount of inseason effective nutrient resources from rice straw return accounts for the largest proportion of the optimal fertilizer-application amount for rice, which can replace 12% N chemical fertilizer application, 15% P 2 O 5 fertilizer application, and 88% K 2 O fertilizer application. This is followed by the in-season effective nutrients from wheat straw, which can substitute 12% of the N fertilizer application, 8% of the P 2 O 5 , and 77% of K 2 O fertilizer application. In the case of maize straw return, the in-season nutrient resources are equivalent to 8% N, 10% P 2 O 5 , and 40% K 2 O of the fertilizer-application rate. The amount of in-season effective N nutrient resources from rapeseed straw return makes up 6% of the recommended optimum fertilizer application, which is the least proportion of the optimum fertilizer application. The amount of in-season effective P 2 O 5 and K 2 O from peanut straw return accounts for the least proportion of the optimum fertilizer application for peanuts, which is only 3% and 11%, respectively. of China, the recommended fertilizer-application rates of nutrient expert systems, and published academic papers and master's and doctoral theses from 2010-2017 [21].
Different crops' optimal chemical fertilizer-application rates can be calculated based on the optimal fertilizer (nutrition) amount. The in-season effective nutrient resources from the main crop straw return account for 23% of the optimal chemical fertilizer-application rate, indicating that it can replace 10% of the N chemical fertilizer application, 9% of P2O5 fertilizer application, and 58% of K2O fertilizer application (Figure 7). The amount of in-season effective nutrient resources from rice straw return accounts for the largest proportion of the optimal fertilizer-application amount for rice, which can replace 12% N chemical fertilizer application, 15% P2O5 fertilizer application, and 88% K2O fertilizer application. This is followed by the in-season effective nutrients from wheat straw, which can substitute 12% of the N fertilizer application, 8% of the P2O5, and 77% of K2O fertilizer application. In the case of maize straw return, the in-season nutrient resources are equivalent to 8% N, 10% P2O5, and 40% K2O of the fertilizer-application rate. The amount of inseason effective N nutrient resources from rapeseed straw return makes up 6% of the recommended optimum fertilizer application, which is the least proportion of the optimum fertilizer application. The amount of in-season effective P2O5 and K2O from peanut straw return accounts for the least proportion of the optimum fertilizer application for peanuts, which is only 3% and 11%, respectively. According to the data on the chemical fertilizer-application amount of N, P 2 O 5 , K 2 O, and compound fertilizer in China Statistical Yearbook 2021 [35], the application amount of N, P 2 O 5 , and K 2 O fertilizer was calculated. The N: P 2 O 5 : K 2 O content of compound fertilizer was calculated according to the research results of Liu et al. [23] from 2010 to 2016, and the application of N, P 2 O 5 , and K 2 O fertilizer in each province was included ( Figure 8). Overall, in 2020, the amount of chemical fertilizer application and the amount of available nutrient resources in the current season of straw in China was 62 Tg, of which the amount of chemical fertilizer application was 53 Tg, and the amount of available nutrient resources in the current season of straw was 10 Tg. The effective nutrient resources of straw in the current season accounted for 19% of the chemical fertilizer application amount, among which the effective nutrient resources of N, P 2 O 5 , and K 2 O straw in the current season accounted for 9%, 5%, and 71% of the national chemical fertilizer-application amount. The recommended amount of fertilizer for major crops in China is 43 Tg. From the perspective of regional distribution, except for the northeastern region, the fertilizer application rate in North China, the middle and lower reaches of the Yangtze River, the southwestern region, the northwestern region, and the southeastern region exceeded the recommended fertilizer-application rate. In general, the available nutrient resources of straw in North China, the middle and lower reaches of the Yangtze River, and Northeast China have great potential to replace chemical fertilizers.
Agriculture 2023, 13, x FOR PEER REVIEW 13 of 22 straw in the current season accounted for 19% of the chemical fertilizer application amount, among which the effective nutrient resources of N, P2O5, and K2O straw in the current season accounted for 9%, 5%, and 71% of the national chemical fertilizer-application amount. The recommended amount of fertilizer for major crops in China is 43 Tg. From the perspective of regional distribution, except for the northeastern region, the fertilizer application rate in North China, the middle and lower reaches of the Yangtze River, the southwestern region, the northwestern region, and the southeastern region exceeded the recommended fertilizer-application rate. In general, the available nutrient resources of straw in North China, the middle and lower reaches of the Yangtze River, and Northeast China have great potential to replace chemical fertilizers.

The Role of Straw in Nutrient Cycling and Soil Improvement
Crop straw is rich in nutrients such as N, P 2 O 5 , and K 2 O, and is a common organic fertilizer for farmland. The results of this study showed that the nutrient content of K 2 O in rice straw was as high as 2.06%, and the effective nutrient resource of K 2 O in season was the highest at 3 Tg, accounting for 42% of the total nutrient resource of K 2 O in season; the nutrient content of maize straw was 0.87% and 0.31%. Maize straw provides the greatest amount of in-season effective nutrients N and P 2 O 5 at 0.7 Tg and 0.3 Tg, accounting for 33% and 39% of the total nutrient resources from straw return. When straw, an agricultural nutrient resource, is returned to the soil as an organic fertilizer, a large amount of nutrients are released into the soil and further absorbed by the crop, and with the involvement of microorganisms, this allows more nutrients to be used to maximum benefit in the ecological cycle. Numerous studies have shown that straw recycling is an environmentally friendly form of biomass treatment that improves the soil's physical structure, increases the organic matter and nutrient content, and improves soil fertility. Returning straw to the field reduced the K 2 O deficit caused by earlier straw removal [40]. Currently, the return of crop straw to the field has become an important method of balancing and adjusting soil nutrients and improving soil conditions. It is a key measure in developing productive farmlands. It has a significant effect on the enhancement of resource utilization efficiency, cost reduction, and sustainable agricultural development.
As a soil conditioner, straw increases soil carbon sequestration, improves soil's physicochemical properties, and enhances soil's microbial activity, which improves soil fertility and promotes crop growth and development [41,42]. Returning straw to the field is closely related to soil's organic matter and N effectiveness and can improve soil structure and fertilizer utilization. The direct return of straw to the field improves the soil quality, increasing the energy microorganisms derived from the soil, which positively affects the growth and activity of microorganisms. The direct return of straw to the field can promote the microbial sequestration of fertilizer N, which helps to reduce the risk of active N export to the environment and improves soil N supply, storage capacity, and soil quality [43]. Based on crop-specific recycling targets, increasing the quantity and quality of straw's return to the field should help achieve the dual goals of increasing crop yields and promoting green development [40].

Standardization of Direct Straw-Return Technology
Based on the direct straw-return level in different regions, the regions with a higher proportion of straw directly returned to the field as a percentage of straw production are North China, South China, and the middle and lower reaches of the Yangtze River, reaching 77%, 76%, and 71%, respectively. The northeastern, northwestern, and southwestern regions have a relatively low proportion of straw directly returned to the field as a percentage of straw production, which is almost 50% per region. From the direct return level of the straws of different crops, wheat straw directly returned to the field accounted for 80% of straw production, the highest level of direct return of straw, followed by rice and cotton, for which the direct return level of straw was 75% and 74%. Soybean and rape straw directly returned to the field accounted for 64% and 60% of straw production, respectively, whereas corn straw directly returned to the field accounted for 50%. However, the lowest level of straw directly returned to the field was peanut, which accounted for only 25% of peanut straw production.
Direct straw-return technology is an important technique in modern agricultural practice. Studies have shown that about "two-thirds" of straw return can effectively improve soil quality, alleviate soil nutrient loss, improve soil fertilizer supply level and soil microbial activity [44], and increase yield by 5%~30% compared with lack of straw return [45,46]. Crop residue return provides energy and a favorable environment for microbial growth and subsequent amassing of soil enzymes [47]. Many previous studies have revealed the roles of N and P sources in the soil's microbial community structure and their enzyme activities [48][49][50]. In addition, maize straw return increased the number and diversity of soil microorganisms, which could affect the living environment of pathogens and pests in soil [51]. However, when the straw is returned to the field excessively or in an unstandardized way, some dangers inhibit crop emergence and reduce crop yield due to increased pests and disease infestation [52]. Rotary tillers are mostly used to plough topsoil in the northeast plain area, and the tillage depth is generally less than 15 cm. Uneven mixing of straw and soil often occurs, and it is easy to cause straw agglomeration in the soil, which affects the emergence rate and quality of maize to a certain extent [18]. The eggs and larvae carried by crop straw can be directly brought into the soil through straw return, which causes the accumulation of pathogenic bacteria in the soil and aggravates the occurrence of seedling diseases and soil-borne diseases [19]. Straw mulching is the main reason for increasing crop stem borers and underground pests, and straw crushing ploughing and burying and rotary-tillage mixed burying often have obvious inhibitory effects on these two types of pests. Therefore, standardizing direct straw-return technology is of great significance to the sustainable development of agriculture and the construction of cultivated land fertility.
Due to the impact of climatic conditions, soil types, cropping techniques, and pertinent legislation in various places, the degree of straw directly returned to the fields in China varies significantly from region to region. In this regard, each region should take different measures according to local conditions to appropriately increase the level of straw directly returned to the fields and standardize the technology needed for direct straw return to the fields to improve the capacity of cultivated land. For northern China, the promotion of mechanized straw-return technology can be updated to achieve a full straw return in areas where it is available. In northern China's wheat-maize multi-cropping regions and southern China's rice-wheat multi-cropping regions, wheat straw mulching is a widespread practice. Therefore, it is crucial to adjust to the local conditions in addition to expanding the area of mixed and wheat straw mulching in accordance with the needs of the next crop. Additionally, to prevent a buildup of wheat straw in the field, the straw should be completely pulverized and distributed uniformly. The black soil area in northeast China can further focus on straw directly returned to the fields. High-powered machinery, straw-return machinery, and no-till seeders can be configured as well as straw crushing and deep plowing, and returning straw to the fields can be carried out according to local conditions [53]. Due to the vast differences in landforms and mechanized straw-return levels in southwest China's hilly and mountainous areas, the promotion of compost and carbon-based fertilizer return techniques is encouraged. The total amount of straw returned directly to the field in the northwest is relatively low, but its level of straw-feed utilization is relatively high [54]. In this regard, straw over-belly return technology can be increased in the northwest. It also plays an important role in improving the strength of arable land.

Enhancing the Amount of Nutrient Resources from Straw Directly Returned to the Field
In this paper, we estimated the nutrient resources derived from the direct return of major crop straws to the field in China to be 14 Tg in 2020, which is relatively low compared with the results of other studies [21,[23][24][25]55]. The reasons for this are attributed to three aspects. One is that the latest crop straw-grain ratios used in this study are generally lower than those used in previous studies. For example, the maize straw-to-grain ratio in northeastern China in this study was determined to be 0.91, while the maize strawto-grain ratios in northeastern China used by Chai et al. [24,56] and Liu et al. [23] were 1.87 and 1.1, respectively. In some other studies, the national maize straw-to-grain ratios were uniformly taken in the range of 1.2 to 1.4 [21,25,55], which is higher than the values used in this study. With the targeted selection of new crop varieties and continuous improvement of cultivation practices, the crop harvest index has increased, and the overall straw-to-grain ratio is gradually decreasing [27]. Studies have shown that the maize strawto-grain ratio has gradually decreased from 1.2 in 2009 to 0.84 in 2018 [57]. In contrast, the straw-to-grain ratios used in previous studies were generally outdated. The straw-to-grain ratio cited by Chai et al. [56] was derived from the "Notice on the Final Evaluation of the Comprehensive Utilization Plan of Crop Straw" issued by the General Office of the National Development and Reform Commission (NDRC) and the General Office of the former Ministry of Agriculture in 2015. With the second national pollution source census carried out in 2017, the straw-to-grain ratios of major crops such as corn and wheat were re-monitored by sub-region, and the monitoring results were all lower than before [58]. Second, the estimated range of crop straw directly returned to the field varies. For instance, earlier research [23,24] included straw in situ burning in the scope of field return. Other authors [21,25,55] performed the nutrient resource estimation of straw returned to the field under the assumption of a full straw return. Third, some studies used more crop species, while this study focused on major crops. Li et al. [25] conducted a study on maize, rice, wheat, other cereal crops, beans, cotton, peanuts, rape, and potatoes, while Liu et al. [24] included sugar (sugarcane and sugar beet), tobacco, vegetables, and melons (watermelon, melon, and other melons) in addition to the abovementioned crops.
Studies have shown that the amount of nutrients left in the stubble of the country's major crops to be returned to the field accounts for about one-quarter of the total nutrient resources returned directly from straw. Mechanical harvesting has a higher stubble height and leaves more stubble in the field than manual harvesting methods. With the increasing area of machine-harvested crops in China, the amount of stubble left in the field has gradually increased [26], leading to the increased nutrient resources of stubble being left in the field. In addition to this, the root biomass below the ground is also a source of soil organic matter and mineral nutrients. Although fresh organic matter in agricultural soils is mainly derived from crop roots and returned crop straw [59], the present study did not consider root biomass as a straw category. Therefore, the nutrient contribution of root biomass was also not analyzed in this study. Currently, some regions are actively developing off-field straw-utilization models such as using straw to feed livestock and for energy to reduce carbon emissions in China [60]. Implementing these off-field utilization models on arable land-quality conservation and greenhouse gas emission reduction will gradually become a concern for researchers and government departments. The contribution of straw stubble and subsurface root biomass in the field for provision of nutrient resources to cropland will also be an important research component.
This research calculated the nutrient content of nitrogen, phosphorus, and potassium in straw left as stubble. It is expected to provide some research foundations for future studies on crop root nutrient resources. In future studies, and including carbon nutrient calculations, the role of crop residues in solid carbon reduction will be investigated.

Increasing the Effective Nutrient Resources of Straw in Season
The effective nutrient resources of the in-season straws in 2020 from major crops were estimated to be 10 Tg. Among them, the seasonal effective nutrient resources of N, P 2 O 5 , and K 2 O were 2 Tg, 1 Tg, and 7 Tg, accounting for 22%, 9%, and 70% of the total seasonal effective nutrient resources of straw directly returned to the field. This result is slightly lower than that of Li et al. [55], which may be explained by the fact that this paper measured the nutrient resources of straw based on the actual level of straw directly returned to the field instead of the nutrient resources of straw under the condition of the level of full straw return to the field. In this study, we also analyzed the potential of chemical fertilizer substitution by calculating the ratio of effective nutrient resources in the season of directly returning straw to the field to the optimal amount of chemical fertilizer application by releasing nutrients in the season of directly returning straw to the field. The results showed that the effective nutrient resources of the main crop straw returning to the field in the current season accounted for 23% of the optimum chemical fertilizer application, which is equivalent to replacing 10% N, 9% P 2 O 5 , and 58% K 2 O of fertilizer application.
The results showed that straw return could partially replace nitrogen fertilizer and help to reduce the amount of nitrogen fertilizer applied to farmland. The replacement rate of nitrogen fertilizer by returning straw to the fields was 10-30% [10,61,62]. When straw was returned to the fields, it was applied to the soil through decomposition and mineralization. This provides nitrogen nutrients and energy for crops and soil microorganisms after mineralization [63] and changes the aggregate structure of the soil, favoring the establishment and growth of microorganisms. At the same time, straw addition significantly increased the abundance and diversity of soil microorganisms in the rice-wheat rotation, but the effect of fertilizer application alone was not significant. Geisseler and Scow [64] found that the long-term application of chemical fertilizers reduced the stability of farmland ecosystems and soil's bacterial diversity. However, the combination of returning straw to the field and using fertilizer can significantly increase the soil's nutrient content, which improves the diversity and stability of the soil's microbial communities and enhances the soil's enzyme activity, thereby increasing crop yield [65,66]. In practice, it is important to note that straw decomposition is generally slow but faster in the early stage (about the first 2 months), and the release of nitrogen is also rapid. The cumulative release rate of nitrogen can reach 40-50% [67,68]. In the early stage of straw decomposition, there is a tendency for microorganisms to compete with the crop for nutrients, which could lead to a decrease in soil nitrogen availability [21,69]. The C/N ratio of straw controls the release of nitrogen from the soil and influences the level of microbial activities [70]. In the case of high crop yields and large amounts of straw return, there may be a shortage of soil nitrogen supply in the early stages of subsequent crop growth. Further, its suitable C/N inputs can reduce the risk of disease epidemics due to crop residues and achieve a more stable soil-crop health system by influencing the soil's microbial activity mechanisms in response to different levels of crop residues and mineral nutrient inputs [71,72]. Therefore, it is necessary to include a certain amount of nitrogen fertilizer in the straw return. Studies demonstrated that reducing soil nitrogen availability due to straw return can be addressed by adjusting the C/N ratio of straw and nitrogen fertilizer. In the rice-wheat rotation system along the Huai River in Anhui Province, the most significant increase in soil inorganic nitrogen content and the best nitrogen uptake and actual yield of rice were achieved when the total C/N ratio of straw to chemical nitrogen fertilizer was adjusted to 18:1 under straw-return conditions [68,73]. Proper nitrogen fertilizer management is also an important measure to achieve rapid straw decomposition and high crop yields. Nitrogen fertilizer-application trials with straw return in Hubei Province also showed that stable and high crop yields could be achieved after fertilizing, moving forward under the condition of paddy-upland rotation [74].
Studies revealed that crop straw has a high potassium content. Under the condition of straw return, the potassium-release rate from straw of major grain crops can reach about 85% [24]. Crop straw is therefore an important source of potassium in agricultural production, and straw return can bring a large amount of potassium to the soil [56]. If the potassium nutrient resources of straw are fully utilized, the amount of potassium fertilizer input can be greatly reduced, especially in soils with a high potassium-supply capacity. The total amount of straw returned to the fields can basically meet the potassium requirements of the current season's crop and ensure high yields [75]. In the early stage of straw decomposition, potassium nutrients are released rapidly, and the cumulative release rate is high. During fertilizer application, it may be appropriate to consider delaying the application of potassium fertilizer to allow for full potassium replacement in the early stages of straw return [68]. Among the different crops, rice straw is the richest in potassium resources. When rice straw is returned to the fields, the nutrient release is fast in the early stage and slow in the later stage, with a high release rate in the current season, so potassium supplementation through rice straw return is encouraged [76]. Compared to potassium and nitrogen nutrients, the phosphorus content of straw is low, and its contribution to soil phosphorus is small in the short term. However, straw return will have an important impact on the transformation of soil phosphorus and its effects [77]. Field-positioning trials in the double-season rice areas of Jiangxi Province showed that three consecutive years of straw return could significantly increase the available phosphorus content of the soil [78]. Under the rice-rape rotation system in Hubei Province, treatment with straw return combined with chemical fertilizer for 3 years resulted in significantly higher levels of available phosphorus in the soil than with chemical fertilizer alone [79]. In rice rotation and the double-season cropping rice areas in northeastern China, the middle and lower reaches of the Yangtze River, and southern China, rice straw return can be combined with the appropriate amount of straw-decomposing agents. This can further increase the phosphorus nutrient-release rate in the current season to fully utilize the effective nutrient potential of straw as a replacement for chemical fertilizers.
For China, due to various factors such as climatic conditions, tillage methods, and soil types, there may be some differences in the ratio of straw to grain, the effective nutrientrelease rate, and the optimal fertilization coefficient between different regions and different crops. Therefore, it is not comprehensive enough to calculate the potential of returning straw to the field in the current season based on the optimal fertilizer-application rate provided in the existing literature. In order to systematically evaluate the potential of returning straw to the field to replace chemical fertilizers in the current season, the internal relationship between the seasonal nutrient-release rate of straw return and the replaceable amount of chemical fertilizers still needs further study to promote green and efficient agricultural production in China. For global application, the current study has some limitations since it is only applied to China's conditions and does not cover different regions of the world. However, variations in crop straw returned to the soil in different regions of the world depend on many factors such as climatic conditions, soil-management practices, and soil and crop type. Given this, future research could explore the straw returned to the field in other geographical areas to achieve a comprehensive global significance.

Conclusions
Returning straw to the field is important for replenishing soil nutrients, reducing the amount of chemical fertilizers, and improving soil fertility. Making full use of the effective nutrient resources in the season for straw's direct return to the field in China is an important effort for promoting the realization of chemical fertilizer-application reduction and efficiency in China. This study shows that the effective nutrient resource amount derived from straw directly returned to the field in 2020 for the season was 10 Tg, and the N, P 2 O 5 , and K 2 O were 2 Tg, 1 Tg, and 7 Tg, respectively. This is equivalent to replacing 10% N, 9% P 2 O 5 , and 58% K 2 O of the fertilizer application. The study further shows that the portion of straw that fails to be collected and remains in the field has a significant nutrient resource and the potential to replace chemical fertilizers, accounting for one-quarter of the total nutrient resources directly returned to the field by straw. China's straw and nutrient resources are dominated by the three major food crops of rice, wheat, and maize, which are mainly concentrated in the middle and lower reaches of the Yangtze River, North China, and Northeast China. The available nutrient resources of straw in North China, the middle and lower reaches of the Yangtze River, and Northeast China have great potential to replace chemical fertilizers. The study results can provide some reference for reducing chemical fertilizer application in different regions of China by combining straw's return to the field with the application of chemical fertilizer. In future research, we will further study the nutrient contribution of returning straw directly to the field and the potential of alternative chemical fertilizers in combination with the actual application of chemical fertilizers and organic fertilizers in different regions so as to provide more comprehensive and systematic guidance for the study of organic fertilizers in different regions of China.
Author Contributions: J.S., conceptualization, data curation, methodology, formal analysis, and writing-original draft; C.G., validation and writing-review and editing; P.A.S., supervision and funding acquisition; J.X., validation and formal analysis; L.Z., formal analysis and writing-review and editing; Y.B., conceptualization, supervision, and funding acquisition; Y.W., supervision, validation, and writing-review and editing. All authors have read and agreed to the published version of the manuscript.