Reduction in Nitrogen Fertilization Rate for Spring Wheat Due to Carbon Mineralization-Induced Nitrogen Mineralization

: Using predicted potential N mineralization (PNM) from its relationship with CO 2 flush at 1 d incubation (CF) of soil samples in recommended N rates can reduce N fertilization rates for crops. This study used predicted PNM at the 0–15 cm depth to reduce N fertilization rates and examined spring wheat ( Triticum aestivum L.) yields at two sites (Froid and Sidney) in Montana, USA. Cropping sequences at Froid were fall and spring till continuous spring wheat (FSTCW), no-till continuous spring wheat (NTCW1), no-till spring wheat–pea ( Pisum sativum L.) (NTWP1), and spring till spring wheat–fallow (STWF). At Sidney, cropping sequences were conventional till spring wheat–fallow (CTWF), no-till spring wheat–fallow (NTWF), no-till continuous spring wheat (NTCW2), and no-till spring wheat–pea (NTWP2). Soil samples collected to a depth of 15 cm in September 2021 at both sites were analyzed for CF, PNM, and NO 3 -N contents, from which the reduction in N fertilization rate (RNFA) and the amount of N fertilizer applied (ANFA) to 2022 spring wheat were determined. In April 2022, spring wheat was grown with or without predicted PNM and annualized crop yields were compared. The CF and PNM were 114–137% greater for NTWP1 than STWF at Froid and 26–80% greater for NTCW2 than CTWF and NTWF at Sidney. The reduction in N fertilization rate was 26–102% greater for NTWP1 at Froid and 8–10% greater for NTCW2 and NTWF than other cropping sequences at Sidney. Annualized crop yield was 26–60% lower for crop–fallow than continuous cropping, but was not significantly different between with or without PNM at both sites. Using PNM can significantly reduce N fertilization rates for crops while sustaining dryland yields.


Introduction
Nitrogen, required in large amounts by crops, is usually applied from synthetic N fertilizers to sustain crop yields and quality [1][2][3].Because of high energetic cost of production, N fertilizer is one of the most expensive inputs applied to crops [4].As the demand for food grows to feed the growing population, the global consumption of N fertilizer will continue to increase to enhance crop production [5].Because crops are able to use only 40-60% of the applied N fertilizer, the remaining N accumulates in the soil as residual N after crop harvest [6].This residual N, if not removed by crops, can degrade soil quality by enhancing soil acidity and reduce environmental quality by increasing N leaching to the groundwater and emissions of N 2 O, a potent greenhouse gas that contributes to global warming and climate change [4,[7][8][9].Therefore, improved techniques are needed to reduce N fertilization rates for crops while enhancing yields and quality and sustaining soil and environmental quality.
One of the techniques to reduce N fertilization rates is to use potential N mineralization (PNM).The PNM refers to N mineralized from soil organic matter during a crop growing season which can amount to as much as <20-200 kg N ha −1 [10,11].Because of the long time required to determine PNM, it is usually ignored when N fertilization rates are recommended for crops [10].This results in the application of N fertilizers that exceeds crop requirements which increases the accumulation of soil residual N [12,13].Although soil NO 3 -N content to a depth of 60 cm is often used to adjust N fertilization rates to crops, this constitutes a small portion of the applied N fertilizer [14][15][16].Therefore, it is imperative that PNM in addition to soil NO 3 -N content be used when N fertilization rates are recommended for crops.
The CO 2 flush at 1 d incubation (CF) of prewetted dry soil is an important soil health indicator that measures microbial activity [17,18].During this incubation, N is also mineralized from soil organic matter due to enhanced microbial activity [17,18].Numerous researchers [17,19,20] have reported that CO 2 evolved during the incubation of prewetted air-dried soil is related to PNM, from which PNM can be estimated and used for adjusting N fertilization rates for crops.However, the incubation period for CO 2 flush determination in their methods is 7-10 d, which is impractical to use when there is a short window period (between snow melting and crop planting) of applying N fertilizers for crops in the spring.Sainju et al. [21] demonstrated that CF was strongly correlated to PNM determined during anaerobic incubation of soil for 10 d.The predicted PNM determined from such a relationship can be used to reduce N fertilization rates for crops, thereby reducing the negative impact on the soil and the environment [17,19,20].
This study further extrapolated the correlation between PNM and CF as observed by Sainju et al. [21] by determining their linear relationship to predict PNM after determining CF for soil samples collected from various cropping sequences at two dryland farming sites in Montana, USA.The predicted PNM was used in addition to soil NO 3 -N content to a depth of 15 cm to reduce N fertilization rates and examined their effects on crop yields.It was hypothesized that predicted PNM, reduction in N fertilization rate (RNFR), and annualized crop yields would be greater for continuous cropping than crop-fallow and that crop yields would be similar between no-PNM with soil NO 3 -N content and high N fertilization rate and PNM with soil NO 3 -N content and low N fertilization rate for all cropping sequences.The objectives of this study were to: (1) determine the effect of cropping sequence on CF, predicted PNM, and soil NO 3 -N content at 0-15 cm, and (2) find how much N fertilizer is reduced by comparing spring wheat yields with or without using predicted PNM in N fertilization rates for spring wheat in all cropping sequences at two dryland field sites in Montana, USA.
Cropping sequences at Froid were fall and spring till continuous spring wheat (FSTCW), no-till continuous spring wheat (NTCW1), no-till spring wheat-pea (NTWP1), and spring till spring wheat-fallow (STWF, traditional system), with each crop phase occurring in every year.In FSTCW, a tandem disc tilled plots to 8 cm depth in the fall and a field cultivator in the spring.In STWF, a field cultivator tilled plots to 8 cm depth in the spring and during fallow periods as needed.Cropping sequences were arranged in a randomized block design with four replications.The plot size was 30 m × 12 m.In Sidney, cropping sequences were conventional till spring wheat-fallow (CTWF, traditional system), no-till spring wheat-fallow (NTWF), no-till continuous spring wheat (NTCW2), and no-till spring wheat-pea (NTWP2) which were arranged in a randomized block design with three replications.All phases of crops in the rotations were present in each year.A field cultivator plowed plots to 8 cm depth in the spring and during fallow periods as needed in CTWF.Other cropping sequences did not receive tillage.The plot size was 12.0 m × 6.0 m.
Spring wheat and pea were planted at 80 and 110 kg ha −1 , respectively, at a row spacing of 20 cm in 28 and 29 April 2022 at Froid and Sidney, respectively.Crops were fertilized at planting with a banded application of P and K fertilizers at 11 kg P ha −1 and 27 kg K ha −1 from monoammonium phosphate (MAP) and muriate of potash, respectively, 5 cm to the side and 5 cm below seeds.At the same time, spring wheat received broadcast N fertilization of 95 kg N ha −1 from urea and banded N fertilization of 5 kg N ha −1 from MAP for a total of 100 kg N ha −1 while pea received banded application of N fertilizer at 5 kg N ha −1 from MAP.For cropping sequences with PNM, N fertilization rate for spring wheat included fertilizer N, PNM, and soil NO 3 -N content to a depth of 15 cm.For cropping sequences without PNM, N fertilization rate included fertilizer N and soil NO 3 -N content.All crops also received herbicides and pesticides as needed.In 2022, pea in 30 and 31 July and spring wheat in 14 and 15 August at Froid and Sidney, respectively, were harvested using a combine harvester from an area of 11.0 m × 1.5 m and grain yields determined in an oven-dried basis (65 • C for 7 d).After grain harvest, crop residues were returned to the soil.

Soil Sampling and Analysis
After previous crop harvest in September 2021, soil samples at the 0-15 cm depth were collected from five locations within a plot using a hand probe (2.5 cm inside diameter), composited, air dried, and ground to 2 mm.An additional undisturbed soil core was collected at the same depth and oven dried at 105 • C for 24 h, from which the bulk density was determined by dividing the weight of the oven-dried soil by the volume of the core.The CF in each soil sample was determined by measuring the CO 2 evolution using the infrared gas analyzer from a 40 g soil prewetted with water at 50% water-filled pore space in a 250 mL jar closed with lids containing ports where two solenoids were connected to the analyzer [24].The analyzer measured CO 2 flush every hour for 24 h at 400 mL min −1 for 3 min.The PNM was predicted from the linear relationship between CF and PNM for the same soils reported by Sainju et al. [21].In that study, CF was determined as above and PNM was determined by measuring NH 4 -N concentration using an autoanalyzer where 10 g air-dried soil was mixed with 30 mL of distilled water and anaerobically incubated for 10 d at 40 • C [25].The NO 3 -N concentration in the air-dried soil was determined by using an autoanalyzer after extracting 10 g soil with 20 mL of 2 M KCl for 1 h.The concentrations of CF, PNM, and NO 3 -N were converted into contents by multiplying their concentrations by the bulk density and the thickness of the soil layer.The RNFR without predicted PNM was the same as soil NO 3 -N content.With predicted PNM, RNFR was calculated by summing PNM and NO 3 -N content.The amount of N fertilizer applied for spring wheat (ANFA) in each cropping sequence without predicted PNM was calculated by deducting soil NO 3 -N content from the recommended N fertilization rate (100 kg N ha −1 ).With predicted PNM, ANFA was calculated by deducting the sum of soil NO 3 -N content and predicted PNM from the recommended N fertilization rate.

Data Analysis
For calculating annualized crop yield, yield during the fallow phase was considered zero for STWF at Froid and for CTWF and NTWF at Sidney.For NTWP1 at Froid and NTWP2 at Sidney, annualized yield was calculated by averaging the yields of spring wheat and pea.To determine the effect of predicted PNM on RNFR, ANFA, and annualized crop yield, the main plot for all cropping sequences at both sites were separated into two split-plot treatments of predicted PNM (with and without predicted PNM), resulting in split plot sizes of 15 m × 12 m at Froid and 6 m × 6 m at Sidney.While CF and predicted PNM were measured for cropping sequences with predicted PNM, same data for soil NO 3 -N content was used to calculate RNFR and ANFA for split-plot treatment of predicted PNM (with and without predicted PNM) in each cropping sequence.
Data for CF, predicted PNM, and soil NO 3 -N content were analyzed using the MIXED procedure of SAS [26] where the fixed effect was cropping sequence and the random effect was replication.Data for RNFR, ANFA, and annualized crop yield were analyzed using the MIXED procedure of SAS for a split-plot experiment [26] where the main plot treatment was cropping sequence and the split-plot treatment was predicted PNM (with and without predicted PNM).Fixed effects were cropping sequence, predicted PNM, and their interaction; and random effects were replication and replication × cropping sequence interaction.A least square means test [26] was used to separate means and interactions when significant at p ≤ 0.05 after adjusting for threshold values, unless otherwise mentioned.

Soil Parameters
The CF at 0-15 cm was 114% greater for NTWP1 than STWF at Froid (Table 1).At Sidney, CF was 53-80% greater for NTCW2 than CTWF and NTWF.Averaged across cropping sequences, CF was greater at Froid than at Sidney.Soil NO 3 -N content at 0-15 cm was 48% greater for NTWP1 than NTCW1 at Froid and 130-191% greater for CTWF and NTWF than NTCW2 and NTWP2 at Sidney.Averaged across treatments, NO 3 -N content was not significantly different between Froid and Sidney (14.4-15.5 kg N ha −1 ).Sainju et al. [21] reported very strong correlation between CF and PNM at 0-15 cm for the same soil samples collected in 2019 at Froid and Sidney.This was further extrapolated by using linear regression analysis to determine the relationship between CF and PNM (Figure 1) which showed that an increase in CF concentration by 1 g CO 2 -C kg −1 increased PNM concentration by 0.52 g N kg −1 at Froid and Sidney.Based on these relationships, the predicted PNM converted into mass area −1 basis at 0-15 cm was 137% greater for NTWP1 than STWF at Froid (Table 1).Similarly, the predicted PNM was 26-27% greater for NTCW2 than CTWF and NTWF.Averaged across cropping sequences, predicted PNM was greater at Sidney than at Froid.

Reduction in Nitrogen Fertilization Rate and the Amount of Nitrogen Fertilizer Applied for Crops
The RNFR stemming from soil NO3-N content without predicted PNM was similar to soil NO3-N content, which ranged from 7 kg N ha −1 for NTCW2 to 22 kg N ha −1 for NTWF at Sidney (Table 2).The values for RNFR for cropping sequences at Froid were in between these ranges.With predicted PNM, RNFR ranged from 39 kg N ha −1 for STWF at Froid to 82 kg N ha −1 for NTCW2 at Sidney.The RNFR was 16-40 kg N ha −1 greater for NTWP1 than other cropping sequences and 21-24 kg N ha −1 greater for FSTCW and NTCW1 than STWF at Froid.Similarly, RNFR was 6-8 kg N ha −1 greater for NTCW2 and NTWF than CTWF and NTWP2 at Sidney.Averaged across cropping sequences, RNFR was similar between Froid and Sidney without predicted PNM but was greater at Sidney than at Froid with predicted PNM.

Reduction in Nitrogen Fertilization Rate and the Amount of Nitrogen Fertilizer Applied for Crops
The RNFR stemming from soil NO 3 -N content without predicted PNM was similar to soil NO 3 -N content, which ranged from 7 kg N ha −1 for NTCW2 to 22 kg N ha −1 for NTWF at Sidney (Table 2).The values for RNFR for cropping sequences at Froid were in between these ranges.With predicted PNM, RNFR ranged from 39 kg N ha −1 for STWF at Froid to 82 kg N ha −1 for NTCW2 at Sidney.The RNFR was 16-40 kg N ha −1 greater for NTWP1 than other cropping sequences and 21-24 kg N ha −1 greater for FSTCW and NTCW1 than STWF at Froid.Similarly, RNFR was 6-8 kg N ha −1 greater for NTCW2 and NTWF than CTWF and NTWP2 at Sidney.Averaged across cropping sequences, RNFR was similar between Froid and Sidney without predicted PNM but was greater at Sidney than at Froid with predicted PNM.As a result of RNFR, ANFA without predicted PNM ranged from 79 kg N ha −1 for NTWF to 93 kg N ha −1 for NTCW2 at Sidney, with ANFA values in between these ranges for cropping sequences at Froid (Table 2).The ANFA was 6 kg N ha −1 greater for NTCW1 than NTWP1 at Froid and 11-14 kg N ha −1 greater for NTCW2 and NTWP2 than other cropping sequences at Sidney.With predicted PNM, ANFA ranged from 19 kg N ha −1 for NTCW2 at Sidney to 61 kg N ha −1 for STWF at Froid.The ANFA was 21-40 kg N ha −1 greater for STWF than other cropping sequences and 16-19 kg N ha −1 greater for FSTCW and NTCW1 than NTWP1 at Froid.Similarly, ANFA was 6-8 kg N ha −1 greater for CTWF and NTWP2 than NTCW2 and NTWF at Sidney.Averaged across cropping sequences, ANFA was similar between Froid and Sidney without predicted PNM, but was greater at Froid than at Sidney with predicted PNM.

Annualized Crop Yield
Annualized crop yield without predicted PNM was 18-60% greater for NTWP1 than other cropping sequences and 26-35% greater for FSTCW and NTCW1 than STWF at Froid (Table 3).At Sidney, annualized crop yield was 47-49% greater for NTCW2 and NTWP2 than CTWF and NTWF.With predicted PNM, annualized crop yield was 31-59% greater for FSTCW, NTCW1, and NTWP1 than STWF at Froid and 42-51% greater for NTCW2 and NTWP2 than CTWF and NTWF at Sidney.There was no significant difference in annualized crop yield with or without predicted PNM for any cropping sequence both at Froid and Sidney.Annualized crop yield with or without predicted PNM, averaged across-cropping sequences, was greater at Sidney than at Froid.

Soil Carbon Dioxide Flush
The greater CF for NTWP1 than STWF at Froid and greater for NTCW2 than CTWF and NTWF at Sidney (Table 1) were likely due to increased crop residue returned to the soil from continuous cropping as opposed to crop-fallow systems.As crops were grown every year in continuous cropping systems, increased crop residue returned to the soil through stems, leaves, and roots may have increased C mineralization and therefore CF.In crop-fallow systems, absence of crops during the fallow period probably reduced crop residue, thereby decreasing CF.The quality and quantity of crop residue returned to the soil due to differences in cropping systems can influence CF [27,28].Sainju et al. [21,29] also observed greater CF in continuous cropping than in crop-fallow systems.
Non-significant difference in CF between FSTCW and NTCW1 at Froid and between CTWF and NTWF at Sidney suggests that tillage did not affect CF.Minimum tillage conducted with a tandem disc and a field cultivator to a depth of 8 cm probably had minimum impact on CF under dryland cropping systems of the US northern Great Plains.This was similar to those reported by Sainju et al. [21,29].The greater CF at Froid than at Sidney was probably related to soil texture, as climatic conditions were similar between the two sites.It is likely that crop residue and soil organic matter mineralize more rapidly in coarse texture (sandy loam) soil, thereby increasing CF at Froid than in fine texture (loam) soil at Sidney.

Predicted Potential Nitrogen Mineralization
The significant linear relationship between PNM and CF (Figure 1) indicates that C and N mineralization from soil organic matter occur concurrently because of increased microbial activity.This is because C and N behave similarly during their storage and mineralization imposed by management practices and soil and climatic conditions [10,11].Various researchers [17,19,20] have reported a significant relationship between PNM and CF.The lower R 2 value obtained for the relationship between PNM and CF at Sidney than at Froid was possibly related to variation in soil texture.It is likely that the stronger relationship between PNM and CF occurred for coarse soil texture (sandy loam) at Froid compared to the weaker relationship for fine soil texture (loam) at Sidney.
The trends in predicted PNM due to variations in cropping sequences were similar to those observed for CF, as CF and PNM were strongly related (Figure 1).The greater crop residue input increased predicted PNM for NTWP1 compared to STWF at Froid and increased for NTCW2 compared to CTWF and NTWF at Sidney (Table 1).Tillage also did not affect predicted PNM, as there was no significant difference in PNM between FSTCW and NTCW1 at Froid and between CTWF and NTWF at Sidney.In contrast to CF, however, greater predicted PNM at Sidney than at Froid was likely due to higher soil organic matter content (27.9 g kg −1 at Sidney vs. 25.7 g kg −1 at Froid at 0-15 cm) and increased crop residue returned to the soil from greater crop yield (2.80 Mg ha −1 at Sidney vs. 2.02 Mg ha −1 at Froid) that enhanced PNM.

Soil Nitrate-Nitrogen Content
Increased N contribution by pea residue due to its higher tissue N concentration stemming from N fixation from the atmosphere than spring wheat residue may have increased soil NO 3 -N content for NTWP1 compared to NTCW1 at Froid (Table 1).This was similar to that reported by Sainju et al. [30].In contrast, increased mineralization of soil organic matter due to higher soil temperature and water content during the fallow period that stimulate microbial activity likely increased NO 3 -N content for CTWF and NTWF compared to NTCW2 and NTWP2 at Sidney.Several researchers [31,32] reported that soil NO 3 -N content was greater for crop-fallow than continuous cropping systems due to increased mineralization of organic matter during the fallow period when soil temperature and water content were higher.Average soil NO 3 -N content, however, did not differ between Froid and Sidney, suggesting that soil texture may have little effect on NO 3 -N content.

Reduction in Nitrogen Fertilization Rate and the Amount of Nitrogen Fertilizer Applied
Accounting for soil NO 3 -N content with or without predicted PNM reduced N fertilization rates from the recommended N fertilization rate of 100 kg N ha −1 for spring wheat in all cropping sequences at Froid and Sidney (Table 2).As much as 7-22 kg N ha −1 of N fertilization rates were reduced for spring wheat when soil NO 3 -N content was accounted for and 39-82 kg N ha −1 of N fertilization rates were reduced when both predicted PNM and NO 3 -N content were accounted for, depending on cropping sequences, in dryland cropping systems.Without predicted PNM, increased N contribution from pea residue increased RNFR for NTWP1 at Froid, and enhanced N mineralization from soil organic matter during the fallow period increased RNFR for CTWF and NTWF at Sidney.With predicted PNM, increased predicted PNM and NO 3 -N content enhanced RNFR for NTWP1 compared to other cropping sequences at Froid.Similarly, greater predicted PNM and NO 3 -N content increased RNFR for NTCW2 and NTWF, respectively, compared to CTWF and NTWP2 at Sidney.Increased predicted PNM similarly increased RNFR for all cropping sequences at Sidney compared to Froid with predicted PNM, but a non-significant difference in NO 3 -N content resulted in similar RNFR between the two sites without predicted PNM.Nitrogen fertilizer is an expensive chemical input for crop production, and reduced N fertilization rates to crops can lessen negative impacts on soil and environmental quality [4,[7][8][9].
Because of RNFR, ANFA also reduced the recommended N rate from 100 kg N ha −1 to 79-92 kg N ha −1 without predicted PNM and to 19-61 kg N ha −1 with predicted PNM, depending on cropping sequences (Table 2).Increased RNFR reduced ANFA for NTWP1 compared to NTCW1 at Froid and for CTWF and NTWF compared to other cropping sequences at Sidney without predicted PNM.With predicted PNM, increased RNFR reduced ANFA for NTWP1 compared to other cropping sequences at Froid and reduced ANFA for NTCW2 and NTWF compared to CTWF and NTWP2 at Sidney.A non-significant difference in RNFR resulted in similar ANFA between Froid and Sidney without predicted PNM, but increased RNFR decreased ANFA at Sidney compared to Froid with predicted PNM.

Annualized Crop Yield
Increased soil N availability due to greater soil NO 3 -N content (Table 1) increased annualized crop yield for NTWP1 compared to other cropping sequences without predicted PNM at Froid (Table 3).With predicted PNM, crop yield was still greater for NTWP1 due to increased predicted PNM and NO 3 -N content, although not significantly different from FSTCW and NTCW1.Lack of crops during the fallow period reduced annualized crop yield with or without predicted PNM for STWF.Crop yields were slightly but not significantly decreased for non-predicted PNM compared to predicted PNM for all cropping sequences.
At Sidney, greater N availability due to increased ANFA (Table 2) increased annualized crop yield for NTCW2 and NTWP2 compared to other cropping sequences without predicted PNM.With predicted PNM, greater N availability from predicted PNM (Table 1) increased crop yield for NTCW2 and NTWP2 compared to other cropping sequences.Absence of crops during the fallow period decreased annualized crop yields for CTWF and NTWF with or without predicted PNM.As at Froid, crop yields slightly decreased but were not significantly different for non-predicted and predicted PNM for all cropping sequences.
The greater crop yield for NTWP1 than for NTCW1 at Froid without predicted PNM was probably due to the rotational benefit of pea on spring wheat.Several researchers [33,34] have reported that pea enhances succeeding crop yields in the rotation by increasing soil water conservation and N supply and reducing weed and disease pressure.Such benefits to enhance crop yield, however, disappeared when predicted PNM was included at Froid.At Sidney, no such benefit to enhance crop yield occurred with or without predicted PNM.It is likely that coarse-textured soil is probably more responsive to such benefits to enhance crop yield at Froid compared to fine-textured soil at Sidney.A non-significant difference in crop yields between FSTCW and NTCW1 at Froid and between CTWF and NTWF at Sidney with or without predicted PNM shows that tillage had no effect on crop yield.The greater soil organic matter and predicted PNM likely increased crop yields at Sidney compared to those at Froid.The non-significant difference in crop yields with or without PNM for all cropping sequences at both Froid and Sidney suggests that predicted PNM can be used to reduce N fertilization rates to spring wheat while sustaining yields.

Conclusions
The results of this study suggest that N fertilization rates for spring wheat can be reduced by including soil NO 3 -N content during the adjustment of recommended N fertilization rates.Such reduction in N fertilization rates can be further lowered by as much as 39-82 kg N ha −1 , depending on cropping sequences, when predicted PNM from its relationship with CF is included along with soil NO 3 -N content in the recommended N fertilization rate.Although CF, predicted PNM, RNFR, and annualized crop yields were greater for continuous cropping than crop-fallow systems, crop yields did not vary with or without PNM for all cropping sequences at two dryland sites.Nitrogen fertilization rate for spring wheat can be significantly reduced by including predicted PNM along with soil NO 3 -N content without altering crop yields in dryland cropping systems of the northern Great Plains, USA.More studies, however, may be needed by repeating the experiments across years in various locations with different soil and climatic conditions, as predicted PNM and soil NO 3 -N content can vary among cropping systems from one site to other.
Funding: This research received no external funding.

Table 1 .a
Soil CO2 flush at 1 d incubation (CF), predicted potential N mineralization (PNM), and NO3-N content as affected by cropping sequence at Froid and Sidney, Montana, USA.Cropping sequences at Froid are FSTCW, fall and spring till continuous spring wheat; NTCW1, no-till continuous spring wheat; NTWP1, no-till spring wheat-pea; and STWF, spring till spring wheat-fallow.Cropping sequences at Sidney are CTWF, conventional till spring wheat-fallow; NTCW2, no-till continuous spring wheat; NTWF, no-till spring wheat-fallow; and NTWP2, no-till spring wheat-pea.b Numbers followed by different letters within a column at a site are significantly different at p ≤ 0.05 by the least square means test.

Table 1 .
Soil CO 2 flush at 1 d incubation (CF), predicted potential N mineralization (PNM), and NO 3 -N content as affected by cropping sequence at Froid and Sidney, Montana, USA.
a Cropping sequences at Froid are FSTCW, fall and spring till continuous spring wheat; NTCW1, no-till continuous spring wheat; NTWP1, no-till spring wheat-pea; and STWF, spring till spring wheat-fallow.Cropping sequences at Sidney are CTWF, conventional till spring wheat-fallow; NTCW2, no-till continuous spring wheat; NTWF, no-till spring wheat-fallow; and NTWP2, no-till spring wheat-pea.b Numbers followed by different letters within a column at a site are significantly different at p ≤ 0.05 by the least square means test.

Table 2 .
Reduction in N fertilization rate (RNFR) from the recommended N fertilization rate for spring wheat (100 kg N ha −1 ) and the amount of N fertilizer applied (ANFA) with or without predicted potential N mineralization (PNM) as affected by cropping sequence at Froid and Sidney, Montana, USA.
a Cropping sequences at Froid are FSTCW, fall and spring till continuous spring wheat; NTCW1, no-till continuous spring wheat; NTWP1, no-till spring wheat-pea; and STWF, spring till spring wheat-fallow.Cropping sequences at Sidney are CTWF, conventional till spring wheat-fallow; NTCW2, no-till continuous spring wheat; NTWF, no-till spring wheat-fallow; and NTWP2, no-till spring wheat-pea.b Numbers followed by different letters within a column at a site are significantly different at p ≤ 0.05 by the least square means test.

Table 3 .
Annualized crop yield with or without predicted potential N mineralization (PNM) as affected by cropping sequence at Froid and Sidney, Montana, USA.NTWP1, no-till spring wheat-pea; and STWF, spring till spring wheat-fallow.Cropping sequences at Sidney are CTWF, conventional till spring wheat-fallow; NTCW2, no-till continuous spring wheat; NTWF, no-till spring wheat-fallow; and NTWP2, no-till spring wheat-pea.b Numbers followed by different letters within a column at a site are significantly different at p ≤ 0.05 by the least square means test.
a Cropping sequences at Froid are FSTCW, fall and spring till continuous spring wheat; NTCW1, no-till continuous spring wheat;