Biopolymeric superabsorbent hydrogels: impact on soil moisture release pattern, crop and water productivity of soybean–wheat under different irrigation regimes in Indo-Gangetic plains of India

Environmental crises, declining factor productivity, and shrinking natural resource threatened global agricultural sustainability. The task is much more daunting in the Indo-Gangetic northern plains of India, where depletion of the underground water table and erratic rains due to the changing climate pose a major challenge to agriculture. To address these challenges a eld investigation was carried out during 2016–18 to test the ecacy of biopolymeric superabsorbent hydrogels namely Pusa Hydrogel (P-hydrogel: a semi-synthetic cellulose derivative-based product) and kaolin derivative of Pusa Hydrogel (K-hydrogel: semi-synthetic cellulose derivative) to assess their effect on crop and water productivity, soil moisture, root dynamics, and economics of soybean (Glycine max L.)–wheat (Triticum aestivum L.) system under three irrigation regimes namely full irrigation, limited irrigation and rainfed. The results revealed that the full irrigation along with P-hydrogel led to enhanced grain yield, biomass yield, and water productivity (WP) of soybean (1.61–10.5%, 2.2–9.5%, and 2.15–21.8%, respectively) and wheat (11.1–18.3%, 12– 54% and 11.1–13.1%, respectively) over control plots. The best performance of P-hydrogel was observed under full irrigation compared to K-hydrogel (both at 2.5 and 5.0 kg − 1 ) and control. Plots treated with P-hydrogel retained 3.0– 5.0% higher soil moisture compared to no-hydrogel plots, while K-hydrogel treated plots held the lower moisture (4.0– 6.0%) than the control. In terms of protability, full irrigation along with P-hydrogel plots registered 12.97% higher economic returns over control. The results suggested that full irrigation along with P hydrogels (2.5 kg ha − 1 ) is a viable option for sustainable production of soybean-wheat systems in the Indo-Gangetic plains of India and other similar eco-regions of the world. P-and K-hydrogels. These results demonstrate that during sucient rainfall years, addition P-hydrogel K-hydrogel in absence of irrigation superior soybean yields. In rabi season under less or negligible rainfall conditions, external application of irrigation enhances the water holding capacity of hydrogels, hydrogels release the crop needs which yields the respective plots. Equally, limited irrigations and rainfed


Introduction
The growing paucity of water has emerged as the most limiting factor for crop production, particularly in arid-and semi-arid agro-ecologies. In India, the per capita water availability has declined from 5,177 m 3 in 1951 to 1,441 m 3 in 2015 and is expected to decline to 1,174 m 3 by 2051 1 . Serious concerns are being raised over the sustainability of farming techniques involving massive water consumption [2][3][4] . In such a scenario, precise technologies aiming at reducing consumptive-use (CU) of available water without compromising productivity need to be invented and introduced in crop production. The use of specialty polymers termed superabsorbent or hydrogels have been reported very effective in enhancing retention of the applied water in the soil around the root zone by minimizing percolation and evaporative losses, thus ensuring a better and prolonged supply of moisture to the crop [5][6] . The uses of such materials become more relevant under the conditions of limited water availability such as in arid and semi-arid regions. These materials in granular form hold water and make it available for longer periods through its sustained release to the soil in their zone of application [7][8][9][10][11] . Hydrogels applications improved soil water holding capacity (WHC), resulting in delayed onset of a permanent wilting point under intense evaporation 6,12 . Hydrogels absorb and retain water 171-402%, mass per mass (m/m) 13 , 80-180 times, m/m 8 , and 67-376 times, m/m 14 under laboratory conditions. Therefore, the water runoff losses were reduced whereas in ltration rates got enhanced 15 , thus improved soil moisture retention enhanced sorghum biomass yield under rainfed conditions 16 . Improved hydro-physical and chemical conditions of soil through an increase in waterstable soil aggregates and retention pores, decrease in transmission pores and a lowering of soil penetration resistance leads to hydrogel effects 10,17 . Besides the sustained release of water, hydrogels have also been reported to in uence nutrient-use e ciency (NUE) by trapping the nutrients in the swollen mass and reducing their losses 9,10 .
The performance of hydrogels depends on the soil and crop types. The addition of polymer in saline soil had positive effects on plant growth, yield, and available moisture content in corn 18 . Likewise, better performance in sandy loam soils over the clay and clay loam soils has been reported 5 . The addition of hydrogels in sandy soil enhanced water availability to plants by reducing drainage loss, increasing retention pores, and reducing soil hydraulic conductivity 19,20 . The light-textured soils characterized by low fertility and moisture de cit resulted in abysmally low crop yields (< 1 to 2 Mg ha − 1 ) in drought-prone areas 16 . Crop production in drought-prone areas is constrained largely by variable rainfall conditions. Thus, rainfall variability coupled with drought waves causes 6-14% lower WP in wheat due to higher growth e ciency under the increasing CO 2 concentration 21 . The WP of cereal crops decreased with climate change due to higher growth period temperature and increased evapotranspiration 22 . Rainfall variability reduces the WP of soybean 22,23 and wheat 24 .
Various studies reported the bene cial effect of hydrogels in terms of higher soil moisture content and enhanced yields by 12-31% in rice 2,25 , 5-11% in wheat 6 , 31-36% in maize 12 , and 16.4-24.7% in mustard [26][27][28] . Similarly, water productivity (WP) with hydrogels enhanced by 22.5% in Indian mustard 27 and 97.1% in maize 29 under de cit irrigated conditions over no-hydrogel applied plots. Interestingly, hydrogels have also been reported to improve the quality of agricultural produce in terms of fruit and ower size and color 6 . Despite offering several advantages, the use of hydrogels in agriculture remained very limited mainly because of high application rates (50-225 kg ha − 1 ) 30,31 which incurred higher production cost 17 . Therefore, indigenous biopolymeric polyacrylate hydrogel, P-hydrogel (minimum water-absorbing capacity 350 times, m/m), and its kaolin based derivative (water-absorbing capacity 1000 times, m/m) was developed for effective moisture conservation at a lower rate of application (2.5-5.0 kg ha − 1 ) 32 .
Depletion of the underground water table is much faster in Indo-Gangetic plains (IGP), mainly due to intense cultivation of high water demanding crops (e.g. rice), changes in cropping pattern (towards more economical crops) coupled with surface water quality reduction 33 . Rice-wheat system is the dominant cropping system in the IGP of India, which requires a lot of water, labor, and energy. Under this situation, shifting towards soybean-wheat cropping systems may be a more economical and water-saving practices in rapidly water declining regions like IGP.
Soybean-wheat cropping, besides being more pro table, is a resource-and energy-use e cient production system 34 .
E cient water use is a major factor in achieving productivity goals in sustainable production systems. Hence, the development of water-saving technology/practices should be a prime focus to the researchers and policy planners designing sustainable agricultural planning. The effect of hydrogels in soybean-wheat systems has not been studied so far. Hence, it was hypothesized that the application of hydrogels may increase soil moisture retention capacity and changes the crop phenology which may improve crop productivity and pro tability of soybean-wheat system under various irrigation management practices. Therefore, the present study was conducted with the following objectives, 1) to nd out the effect of biopolymeric superabsorbent hydrogels on soil moisture release pattern, rooting behavior, and crop phenology of soybean-wheat system and, 2) to assess the effect of hydrogels on crop and water productivity and pro tability of the soybean-wheat system.

Results
Soybean seed and biomass yield. A signi cant (P ≤ 0.05) effect of irrigation regimes on seed and biomass yields of the soybean was observed during 2017 and 2018 (Table 3). Yields in the full irrigation applied plots were signi cantly higher as compared to rainfed or limited irrigation regimes. The magnitude of increase being 4.6-9.8% in 2017 and 5.2-12.5% in 2018. The yields differed signi cantly in hydrogel applied treatments also. Maximum seed (1.22-1.37 Mg ha − 1 ) and biomass yields (4.9-5.4 Mg ha − 1 ) were recorded under P-hydrogel over control and K-hydrogel treatments. Irrigation regimes and hydrogel interaction effects were also signi cant ( Table 3). The marginally higher yield (1.37 and 1.26 Mg ha − 1 ) and biomass yields (5.85 and 5.02 Mg ha − 1 ) were observed under full irrigation with Phydrogel during the 1st and 2nd years, respectively. Contrastingly, limited irrigation plots recorded signi cantly lower seed yields as compared to full irrigation and rainfed plots. However, control plots performed better than K-hydrogel plots with 2-13% and 19-25% higher seed and biomass yields. Among the two tested levels of K-hydrogel, application of K-hydrogel at 2.5 kg ha − 1 recorded higher seed yield but 5.0 kg ha − 1 applied plots recorded higher above-ground biomass. Under rainfed regimes also, P-hydrogel and K-hydrogel treatments were superior in terms of seed (15-27%) and biomass yields (36-54%) over control during both the study years. Mg ha − 1 ) and rainfed regime (grain yield, 2.85-4.09 Mg ha − 1 ; biomass yield, 9.70-13.20 Mg ha − 1 ). The grain and biomass yields increased signi cantly (P ≤ 0.05) with P-hydrogel application, the magnitude of increase being 3.0-15.0 and 2.0-6.0% over control, respectively. However, control plots exhibited slightly higher (~ 2%) seed and biomass yields over K-hydrogel (both at 5.0 and 2.5 kg ha − 1 ) but differences were non-signi cant. The interaction effect of hydrogels and irrigation regimes on wheat yields was signi cant in both the study years. Full and limited irrigations with P-hydrogel treatments led to an increase in grain yield (11-18%) and biomass yield (1.2-9.8%) as compared to control. Limited irrigations and rainfed plots with no-hydrogel registered signi cantly higher grain (9-11% and 3-7%) and biomass yields (4-9% and 2-6%) as compared to K-hydrogel treatments, respectively. Soybean-wheat system yields. Irrigation regimes expressed a signi cant in uence on wheat equivalent yield (Table 3). During both, study years, the wheat equivalent yield (WEY) was signi cantly higher under full irrigation (10-32%) as compared to the limited irrigation and rainfed regimes. Water productivity (WP) and irrigation water productivity (IWP). In soybean, WP (8.1-21.0%) and IWP (126-817%) were signi cantly higher in rainfed regimes compared to those in full and limited irrigations, respectively (Table 4).
Among hydrogels, the application of P-hydrogel resulted in a signi cant increase in WP (7-41%) and IWP (2-22%) over control. Interaction effects of irrigation regimes and hydrogels for the two years found signi cant, full, and limited irrigations with P-hydrogel exhibited the highest WP (3-22%) and IWP (1.2 to 5.0%) over control. However, lower WP (9-15%) and IWP (5-17%) were recorded under K-hydrogel applied plots over control plots. The WP and IWP of wheat were higher by 41-213% and 49-311% in rainfed plots as compared to the full and limited irrigations, respectively during 2017-18 and 2018-19. Unlike irrigation regimes, the hydrogel application did not affect the WP and IWP of wheat signi cantly during 2017-18 (Table 4), however in 2018-19, WP and IWP were maximum in P-hydrogel applied plots (252 and 457 kg ha-cm − 1 , respectively) which were signi cantly higher by 4.9% over the other K-hydrogel and control plots. Interestingly, K-hydrogel applied at both levels (5.0 and 2.5 kg ha − 1 ) recorded 5-12 and 6-14% lower WP and IWP as compared to control plots, respectively.
Wheat root length and volume. The total root length and volume of wheat were recorded for different irrigation regimes during 2018-19 ( Water retention-release pattern in hydrogels. Moisture retention by the two hydrogels, each at two application rates, was studied at 0.33 and 1.0 bars. At both the pressure points, P-hydrogel retained a signi cantly higher amount of water compared to K-hydrogel and control (no-hydrogel). P-hydrogel held more water at suction 0.33 bar ( eld capacity) as compared to control and K-hydrogel. K-hydrogel releases more water (~ 5%, v/v) compared to Pusa hydrogel (2-3%, v/v), and also the Pusa hydrogel has higher retention at 1 bar indicating its effectiveness is limited in dry soil water condition. While P-hydrogel still has the potential to hold signi cantly more water and exhibit greater residual soil moisture (available for plants) over K-hydrogel and control (Fig. 7). Whereas, K-hydrogel had a nonsigni cant effect on soil moisture retention and release pattern as compared to control.
Pro tability analysis of soybean-wheat system. ANOVA revealed a signi cant (P ≤ 0.05) effect of irrigation regimes and hydrogels on soybean and wheat pro tability during 2018-19 (Table 5). The Gross and net pro tability of soybean was signi cantly higher by 4.1-5.9% and 5.8-12.0% in full irrigation applied plots over the rainfed regime and limited irrigation, respectively. In wheat, there was a 20-49% enhancement in gross pro tability and 37-109% in net pro tability due to full irrigation over limited irrigation and rainfed regimes. Also, signi cant increases in wheat gross pro tability (5-15%) and net pro tability (8-18%) were found with the application of P-hydrogel compared to no-hydrogel and K-hydrogel. While in both soybean and wheat, K-hydrogel at 5.0 kg ha − 1 resulted in signi cantly lower net pro tability by 3-16% over control. Bene t: cost ratio (B:C) in soybean did not differ signi cantly with irrigation regimes, however, the effect of hydrogel application was signi cant. A higher B:C ratio of 2.38 was recorded under full irrigations than the other irrigation regimes. Among hydrogels, a signi cantly higher B:C ratio was recorded in control plots (1.72 and 2.38) for soybean and wheat over K-hydrogel but it was at-par with P-hydrogel plots.

Discussion
Irrigation effects on productivity, phenology, root attributes and soil moisture dynamics. More stable and signi cantly higher soybean and wheat yields were recorded under adequate irrigation during both the study years (2017-18 and 2018-19); grain and biomass yields of soybean and wheat were increased signi cantly by 4.6-9.8% and 5.2-12.5%, and 24-49% and 12-54%, respectively under adequate irrigation compared to rainfed plot due to better growth and development of soybean and wheat crops observed owing to favorable soil moisture content under full irrigation regimes. Infrequent and adequate irrigation applied plots the surface layers remained wet for a longer duration, maintaining favorable conditions during owering to maturity time resulting in higher water and nutrient uptake 28,35,36 and nally enhanced yield parameters and yield 37,38 compared to limited-irrigated or rainfed plots. Such enhancements in soybean and wheat yields caused a 10-32% improvement in system productivity. Concurrently, wheat crop took 2-3 days more to attain anthesis, milking, and maturity under full and limited irrigations over rainfed plots due to optimum moisture regime favoring continued photosynthesis, plant growth, and delaying its life cycle 35,36 .
The rate of carbon assimilation over transpired water is denoted as WUE, which acts as a bridge between the carbon and water cycle in agro-ecosystems 39 . In the current investigation, a higher WP of soybean and wheat was observed under rainfed conditions compared to ample or even limited irrigation, involving a much higher amount of water-use without showing proportionate yield increments. Parihar et al. 40 have also earlier reported that soil moisture conserved in the seed-zone not only provided better crop establishment and growth but also increased WP.
Incidentally, relatively more frequent irrigations scheduled at 40% depletion of available soil moisture in full irrigated plots resulted in higher soil moisture content at all the crop growth stages. Positive impact on water balance (consumptive use-cumulative pan evaporation, CPE) of soybean was much higher during 2018 over 2017 due to higher rainfall and lower irrigation water application. However, the impact on water balance (consumptive usecumulative pan evaporation, CPE) of wheat was much higher during 2017 over 2018 (Fig. 8).
Appreciably higher root length and volume under full and limited irrigation plots were observed as compared to rainfed plots (Fig. 2). In irrigated plots, the roots were concentrated in the upper layer and had greater horizontal development, which might be due to better moisture availability 28,36 . In rainfed plots, root length and volume were negatively affected due to relatively de cient moisture conditions, where roots did not proliferate as much as under full irrigation and hence were not able to extract water from deeper layers. Higher root mass and density in deeper soil layers enhance the water extraction capacity for increased wheat yield under terminal drought stress 41 . While, large root mass may aggravate water stress in the topsoil layer thus reducing stomatal conductance and photosynthesis 42 .
Hydrogel effects on productivity, phenology, root attributes and soil moisture dynamics. Hydrogels have been used as water-retaining polymers in agriculture 2,6,16,25−27 since they can retain a great amount of water when incorporated in soil and release it slowly more or less matching with plant requirement leading to improved crop growth and yield under water-stressed conditions 43 . In the current study, the applied hydrogels exhibited a signi cant effect on soybean and wheat growth, especially under limited irrigation and rainfed conditions. A signi cant (P ≤ 0.05) enhancement in soybean seed yield (6-25%) and wheat grain yield (3-15%) in P-hydrogel applied plots compared to no-hydrogel (control) and K-hydrogel applied plots were observed (Table 3). All the other parameters being constant, the increase in yield may be attributed to the extended availability of water to plants in the polymer treatments during periods of water stress 10,11,43 . Similarly, Jarvis and Davies 44 reported that the increased photosynthesis rate and leaf relative water content in plants under superabsorbent polymers would enhance growth under drought-stress conditions. Our results are consistent with earlier studies which showed that higher crop yields were attained under hydrogel application in rice 2,25 , wheat 6 , maize 12 and mustard 26,27 .
Islam et al. and Shekari et al. 29,37 studied water and yield interaction and concluded that the polymers enhanced the WHC of the soils, which is more bene cial for enhancing water and nutrient uptake by wheat and thus leads to higher above-ground biomass. They found that there was a signi cant effect of irrigation regimes and superabsorbent polymers on total above-ground biomass and water productivity. Our results also suggested that the application of Phydrogel @ 2.5 kg ha − 1 resulted in a signi cant increase in WP and IWP over no-hydrogel plots (Table 4). Thus, the application of P-hydrogel to the soil surface helped in conserving soil moisture in the root zone, and successive slow release of moisture led to crop yield enhancement with lesser water consumption. In contrast, K-hydrogel at 5.0 and 2.5 kg ha − 1 recorded 5-12 and 6-14% lower WP and IWP compared to no-hydrogel plots. This could be due to proportionately more loss of water in evapotranspiration or slow or no release of water in K-hydrogel applied plots.
Concurrently, K-hydrogel, kaolin-based super absorbent hydrogel apparently because of the presence of kaolin tends to hold water in its matrix more e ciently in a moisture-rich environment. The difference in the relative retention release ratios observed under rainfed and full irrigated conditions can be understood in terms of macromolecular expansion in presence of plenty of water (full irrigation and high rains), which led to higher water absorption and retention. Contrastingly, K-hydrogel releases more water (~ 5%, v/v) compared to Pusa hydrogel (2-3%, v/v), thus the released water was prone to evaporation losses. The presence of limited water with less rainfall or limited irrigated conditions led to suboptimal release absorption.
Phenology is an important criterion to decide the yield potential of any crop 36 . Underwater stress conditions any reduction in the number of days taken to attain different phenophases of the wheat will greatly affect the yield potential. The same was re ected in this study, where wheat took a signi cantly greater number of days to attain anthesis (93.0 days) in P-hydrogel applied plots over no-hydrogel and K-hydrogel (Fig. 1). Thus, P-hydrogel favored wheat plant growth owing to better soil moisture regimes and enhanced root-attributes. A non-signi cant effect of hydrogels was, however, observed for days to milking and maturity, which could be due to moisture stress during these stages of crop development.
Hydrogels also showed a signi cant (P ≤ 0.05) effect on root length and volume of wheat during 2019 (Fig. 2). Nohydrogel plots exhibited signi cantly higher root length (781.6 cm) and volume (16.1 cm 3 ) of wheat as compared to K-hydrogel and P-hydrogels. Due to water stress, roots explored lower horizons and had a vertical distribution in the lower moisture regimes and in control plots. However, P-hydrogel applied plots showed signi cantly lower root length and higher root volume (Fig. 3) because of better soil moisture availability to wheat plants, thus enhancing their water stress tolerance capacity and nally leading to better crop yields. However, Rezashateri et al. 45 reported that hydrogels seemed to increase root growth and decrease irrigation frequency initially for crop plants.
It is evident from Figs. 5 & 6 that application of hydrogels improved the soil water content under all the irrigation regimes, and eventually, there was an improvement in growth, yield, and higher WP (Tables 3 and 4). Further, Phydrogel applied plots had 3-5% higher soil moisture content than no-hydrogel applied plots. Hydrogel applied plots had similar moisture release patterns with no-hydrogel plots, but the amount of WHC varied among the hydrogels and it was slightly higher under P-hydrogel. Thus, P-hydrogel controls water movement and releases water in synchrony with crop needs. The enhanced water content in the soil pro le in hydrogel applied plots might have improved soil physical conditions 17 . In concurrence with Fig. 7, P-hydrogel retained a higher amount of water compared to Khydrogel and control (no-hydrogel) at both pressure points due to empty drainage pores and a larger proportion of soil capillary pores being lled with water, improving water availability to the crops in the long-run. Marginal but consistently higher soil moisture in P-hydrogel indicates a better soil-water regime, which could be of great signi cance when the soil moisture becomes limiting (limited and rainfed conditions), and therefore, facilitated higher root growth and yields.
The cost incurred and pro tability was the two attributes that de ne the adoption of any new technology on a large scale. However, hydrogel applied plots (both P-hydrogel and K-hydrogel) recorded a higher cost of production (Table 5) due to the higher cost incurred on the hydrogels. Whereas, a signi cant increase in wheat pro tability was found with the application of P-hydrogel due to higher grain yields compared to-hydrogel and K-hydrogel.
Contrastingly, K-hydrogel at 5.0 kg ha − 1 resulted in 3-16% lower net pro tability over no-hydrogel. It could be associated with the higher cost of hydrogel but comparatively lower seed and grain yields in both test crops.
Irrigation and hydrogel effects on productivity, root attributes and water productivity. Higher soybean yield in nohydrogel plots with full and limited irrigations than hydrogel plots during 2017-18 and 2018-19, could be because the application of extra irrigations in the rainy season which coupled with su cient rainfall could have caused excess water stress in the hydrogel applied plots, resulting in comparatively lower yields. While in rainfed plots, there was a signi cant increase in soybean yields in P-hydrogel and K-hydrogel over control (no-hydrogel). In wheat, full and limited irrigations coupled with P-hydrogel applied plots exhibited the highest yields over control and rainfed with Pand K-hydrogels. These results demonstrate that during su cient rainfall years, the addition of P-hydrogel and Khydrogel in absence of irrigation could produce superior soybean yields. In rabi season under less or negligible rainfall conditions, external application of irrigation enhances the water holding capacity of hydrogels, thus hydrogels release water as per the crop needs which resulted in higher yields in the respective plots. Equally, limited irrigations and rainfed plots with no-hydrogel registered higher wheat yield than K-hydrogel, could be due to lower soil moisture retention and higher release by K-hydrogel with an increase in soil matric suction resulted in loss of moisture through evaporation (Fig. 7).
Implications of hydrogels application under zero tillage/conservation agriculture (CA). Over the few decades due to excess tillage, input factor productivity of major crops and cropping systems have been declined. Thus, zero tillage (ZT) or no-tillage gained momentum in India and at the global level to reclaim depleted soil conditions and also to enhance input factor productivity. As a sustainable approach, the CA concept has been steadily increased globally with 124.8 M ha area 46 . In India, ZT/CA area expanded to 1.5 million hectares 47 and expansion is less due to variable climatic conditions (rainfall dependent area is ~ 60%), soil type, and small landholdings. Hence, under CA/ZT situations, the application of hydrogels is presumed to be bene cial in enhancing crop productivity through enhanced soil moisture content. The major constraint under long-term residue retention ZT/CA elds is soil application of hydrogels and subsequent increase in the cost of hydrogel application. But hydrogel application methodologies like seed coating and slurry application enhance crop yield and water productivity signi cantly than soil application 48 . It is evident from the ndings that crop yields enhanced at ~ 5.6% in Kharif and ~ 14.7% in with hydrogel application; it is presumed to produce additional ~ 7.34 million tons of food grains production (46% contribution from rainfed areas towards total grain production of 294 MT in India during 2020). Thus, hydrogel application is not only a viable sustainable crop production option under conventional tillage systems but it can be a good approach for yield enhancement under the ZT system in the Indo-Gangetic plains of India.

Conclusions
The study proved the hypothesis that soil application of P-hydrogel with full irrigations led to signi cant improvement in soybean and wheat productivity, WP, rooting behavior and pro tability of soybean-wheat cropping system. water ratio) 49 . The initial experimental soils had 1.28 g cm − 3 (0-15cm) bulk density, 4.8 mm h − 1 in ltration rate, 37.9, 8.6, and 29.3 % (volume/volume, v/v) moisture content at eld capacity, permanent wilting point, and available moisture content, respectively.
The rainfall received during the soybean crop seasons (July-October 2017- 18 and 2018-19) was 803 and 919 mm, respectively. The rst three months (July-September) witnessed > 80% of the total yearly rainfall during both years. The rainfall received during the winter season (October-April 2017-18 and 2018-18) was 32.4 and 135.1 mm, respectively. The average rainfall, weekly temperatures (T max and T min ), and daily pan evaporation during study periods are depicted in Fig. 1 (a-b).
Test materials and levels of application. P-hydrogel (Trade name: Pusa Jal Nidhi) was procured from M/S KCH India (P) Ltd, Chennai, India, and used as such. While, K-hydrogel was synthesized in the laboratory using derivative cellulose, kaolin, and acrylamide, by free radical polymerization technique 50 with minor modi cations. A mixture of bio-degradable cellulolytic derivatives and clay along with venyl monomers was added to warm water for the synthesis of K-hydrogel. A free radical initiator was added to the homogenized mixture with constant stirring. After a speci c interval of time (6-12 hours), the gel mass obtained was subjected to post reaction washing and drying to yield bio-polymeric grafted and cross-linked polyacrylate superabsorbent hydrogels. The salient characteristics of the two test hydrogels are given in Table 1. The two test hydrogels were applied as P-hydrogel @ 2.5 kg ha − 1 and Khydrogel @ 2.5 and 5.0 kg ha − 1 . These were applied manually after mixing with < 0.02mm sieved soil to make bulk. The application was done at the time of sowing at 5-6 cm soil depth in the seed zone as per the treatments. The entire dose of hydrogels in a single split has been applied in each crop season (i.e. 2 times each in soybean and wheat) Experimental design and management practices. The eld experiment was conducted in triplicate employing a splitplot design. The treatments included three irrigation regimes namely full irrigation, limited irrigation, and rainfed plots assigned randomly to main plots. In rainfed plots, the necessary base irrigation (5.0 cm) was given in wheat to obtain a uniform crop stand. Sub-plots in each main plot comprised four treatments namely control (no hydrogel), Khydrogel at 2.5 kg ha − 1 , K-hydrogel at 5.0 kg ha − 1 , and P-hydrogel at 2.5 kg ha − 1 assigned randomly. The agronomic practices used in the study are listed in Table 2. All the sub-plots were of a uniform size of 16.2 m 2 (4.5 × 3.6 m). To prevent peripheral water movement from irrigated plots to the rainfed plots, a 2-m wide buffer area was maintained. To reduce the weed problem, the post-emergence application of sulphosulfuron + metsulphuron @ 0.075 ml L − 1 was done in wheat at 25 DAS. Based on the rainfall observed, 3 (2017) and 2 (2018), and 2 (2017) and 1 (2018) irrigations in soybean were given under full and limited irrigation treatments, respectively. The corresponding irrigations given in full and limited irrigation plots of wheat were 5 and 3; 4 and 2, respectively. In both the crops, each irrigation provided a 5.0 cm equivalent of water using area×volume basis (Q = A×V), and scheduling was done at 40 and 70% depletion of available moisture. The conventional ood irrigation system was adopted to impose the irrigation treatments in both crops.
Data collection and analyses. Roots were sampled at the anthesis stage by taking soil core at 0-30 cm soil depth, washed by placing them in nylon nets to remove soil debris and other extraneous materials. These were then scanned through an image scanner (Epson V700, Indonesia) and the length and volume were retrieved by using Win RHIZO version 5.0 (Regent Instruments Inc., Quebec City, Canada). The number of days when 50% of the wheat plant population reached a particular phenological stage was recorded as a period of attainment of that stage 51 . Plants were harvested from the net-plot area of 9.0 m 2 (excluding 1.8 m and 0.3 m border area at both ends) in each plot to determine grain-and biomass-yields. After sun-drying for seven days, above-ground biomass from each plot was weighed, grains (~ 14% grain moisture) separated by a mechanical thresher, cleaned, and weighed.
Soil water content measurements. Retention and release of water by the hydrogel amended soil samples were measured using a pressure plate apparatus (Soil moisture Equipment Corp., CA). Soils from the experimental site were mixed with hydrogels similarly as in the case of eld applications. The soil was packed in small volumes (cylindrical; 5 cm diameter, 5 cm height) to the bulk densities as observed in the eld, capillary-saturated, and subjected to 0.33 and 1 bar suction in pressure plate apparatus. For each suction, 5 replicates were used, and water retention was determined from the difference between weights of soil samples after equilibrium with each suction, and weights of the oven-dried soils thereafter. The volumetric water content (volume/volume, v/v) was determined by Water productivity (WP) and irrigation water productivity (IWP) estimations. WP of the crops under different treatments was computed by dividing the grain yield (kg ha − 1 ) with the amount of irrigation water (cm) and effective rainfall (Rainfall more than 6.25 mm on any day is considered as ineffective and it should be multiplied with 0.65 to get effective rainfall, www.fao.org/3/x5560e/x5560e03.htm) from the respective plots as per the Eq. 1 52 : WP (kg ha cm − 1 ) = Ye ÷ {Iw + Er} (1) where, WP = water productivity (kg ha cm − 1 ), Ye = Economic yield (seed/grain, kg ha − 1 ), Iw = irrigation water applied (cm), Er = effective rainfall (cm).

Yield measurements
The productive capacity of the soybean-wheat systems under different irrigation regimes and hydrogels was measured in terms of wheat grain equivalent yield (WGEY) at a price scale. The WGEY was calculated by using the Farm pro tability analysis. The relative economics of treatments was calculated using the minimum support price of soybean and wheat declared by the Government of India during 2017-18 and 2018-19, and costs incurred in operations involved in raising crops from eld preparation to harvesting and storage. Since no cost is incurred on the amount of irrigation water in India, only cost incurred on irrigation water application at a rate of US$ 5.33 per irrigation has been used to calculate the cost of irrigation. The cost of K-hydrogel and P-hydrogel was US$11.97 kg − 1 and US$12.68 kg − 1 , respectively. Pro tability (US$ ha − 1 ) was calculated by adding the yearly net pro tability of soybean and wheat during both the study years. The minimum support price (MSP) for soybean was US$ 429. Statistical analyses. Soybean and wheat data recorded during the two years were analyzed through the analysis of variance (ANOVA) test for a split-plot design using SAS 9.3 software (SAS Institute, Cary, NC). The signi cance of the difference between the main plots (soil moisture levels), sub-plots (hydrogels), and their interactions was tested using F-test. Means were compared by Duncan's Multiple Range Test (DMRT).
Authors have con rmed that, all plant investigations were carried out in accordance with appropriate national, international, or institutional guidelines. Figure 1 Weekly average temperature, evapotranspiration (ETo) and rainfall during soybean (a) and wheat (b) during 2017-18