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Increasing amounts of crop residues are being returned to croplands. Understanding nitrogen (N) availability in crop residues under various N fertilization regimes is important in optimizing N management. Pot experiments were conducted to investigate the contribution, fate and residual effects of urea and maize residue-N using a 15N isotope technique. Four N regimes were tested: three basal–topdressing ratios of N applied as urea (100–0, 75–25, 50–50) and one basal application of 75% N as urea and 25% N as manure (75U+25M). An average of 31.4% wheat N uptake was derived from urea, 9.2% from maize residue, and 59.5% from the soil in the first season. During the growing stages of wheat, maize residue contributed 0.3–4.8% and 3.1–13.2% to soil mineral and microbial biomass N, respectively, and those originated from urea was 1.0–4.2% and 4.6–16.8%, respectively. Regarding the fate of urea and residue-N after the first season, 35.9% and 16.9% of urea-15N and residue-15N were recovered by wheat, 28.1% and 46.9% remained in the soil, and 36.0% and 36.2% were lost. The contribution of urea to crop N uptake and N recovery efficiency increased, while that of residue-N decreased with increasing proportion of topdressing N. Substituting 25% mineral N with manure decreased the urea-15N loss without negative effects on crop dry matter and N uptake. Residual urea-15N and maize residue-15N from the previous season contributed 3.9% and 3.0% to summer maize N uptake. Additionally, 29.3% of residue-15N remained in the soil after the second season, while only 18.6% of urea-15N remained. Our study suggests that fertilizer and crop residue are actively involved in soil N transformation and plant N nutrition, emphasizing the capacities of organic residues to sustainably supply nutrients. Considering the utilization of both N fertilizer and maize residue, we may suggest a 75–25 split in N fertilizer application, but more appropriate options need to be further assessed under different cropping systems. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, water and nitrogen interaction on soil profile water extraction and evapo-transpiration (ET) was investigated taking a field experiment on a clay loam soil (Typic Haplustept) at the Indian Agricultural Research Institute, New Delhi with four consecutive crops (maize–wheat–maize–wheat) taken from July 2002 to April 2004. Three levels of water regime, namely W1, W2 and W3 referring to limited, medium and maximum irrigation were applied to each crop depending on the seasonal rainfall and the critical crop growth stage. The three water regimes were used with five nitrogen levels from T1 to T5, (T1, 0% N; T2, 75% N; T3, 100% N; T4, 150% N; T5, 100% N from organic source) in a split plot design for the four crops grown in sequence. Significant water and nitrogen interaction was observed for ET and soil profile water extraction pattern. Averaged across nitrogen treatments, ET in W2 and W3 were higher by 17 and 26%, respectively than W1 in maize 2002 and by 12 and 19% in maize 2003. In case of wheat, ET in W2 and W3 were higher by 27 and 58% than W1 in 1st crop and by 37 and 70% in 2nd crop. The effect of nitrogen regime, however, was prominent in both crops of maize and wheat, with significantly higher profile soil moisture depletion in T4 of each water regime. In all cases, lowest water depletion was observed in control plots receiving 0% N. In both crops, water extraction from surface 60cm was highest in W3 followed by W2 and W1. In maize, the % extraction from 0 to 60cm layer varied from 71 to 76% (W1), 70–79% (W2) and 75–82% (W3), whereas the values for wheat were 70–77, 72–79 and 75–83% for W1, W2 and W3, respectively. The 90–120cm layer contributed only 3–14% to total water extraction in both the crops. From 90 to 120cm layer, higher extraction was observed in W1 as compared to W3. The extraction values in W1, W2 and W3 in maize were 9–13, 7–14 and 3–9, respectively, whereas the corresponding values in wheat were 8–14, 5–12 and 3–7% for the three water regimes. Effect of nitrogen treatments on water extraction from deeper layer was observed with higher extraction in highest fertilized treatment (T4) as compared to other treatments. [Copyright &y& Elsevier]