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Because of the effect of nitrogen (N) fertilization on soil-derived N uptake, N difference (FNRDiff) and 15 N tracer (FNR15N) methods may give significantly different estimates of the fertilizer N recovery (FNR) in crops. To better understand quantitative differences in FNR (∆FNR) between the two methods, we evaluated the effect of fertilizer N input rates on ∆FNR and soil-derived N uptake by considering data from field 15 N tracer experiments of cereal crops across China, including N-unfertilized treatments. The FNRDiff was significantly higher than the corresponding FNR15N in wheat (38% vs. 36%), maize (38% vs. 31%), and rice (40% vs. 31%) under fertilizer applications of 40–500 kg N ha−1. Except in wheat at 250–500 kg N ha−1, FNRDiff was higher than FNR15N at various N input rates in the three crops, and ∆FNR decreased with increasing rates of applied N. The ∆FNR apparently occurred because added-N interaction (ANI) increased soil-derived N uptake (by 5% for wheat, 11% for maize, and 24% for rice). For wheat in particular, ANI values decreased significantly as N input rates increased, with a switch from positive to negative above 252 kg N ha−1. This result suggests that the decrease in soil-derived N uptake is mainly caused by the negative rhizosphere N priming and the law of diminishing returns (current-season non-fertilizer N input or previous-season residual fertilizer N) under high N input, but not by the N pool substitution. Finally, we propose a new method for accurately distinguishing direct and indirect pathway contributions to ∆FNR. This study helps to improve understanding of the underlying mechanisms of discrepancies between the N difference method and the 15 N tracer method for estimating the fertilizer N recovery in crops. [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate plant-available N pools and the role of N management index (NMI) in the surface (0-20 cm) of a fluvo-aquic soil after 18 years of fertilization treatments under a wheat-maize cropping system in the North China Plain. The experiment included seven treatments: (1) NPK, balanced application of chemical fertilizer NPK; (2) OM, application of organic manure; (3) 1/2OMN, application of half organic manure plus chemical fertilizer NPK; (4) NP, application of chemical fertilizer NP; (5) PK, application of chemical fertilizer PK; (6) NK, application of chemical fertilizer NK; and (7) CK, unfertilized control. Total organic N (TON), microbial biomass N (MBN), labile N (LN), inorganic N (ION, including ammonium (NH)-N and nitrate (NO)-N) contents, net ammonification rate (NAR), net nitrification rate (NNR), net N mineralization rate (NNMR), and NMI in the fertilized treatments were higher than in the unfertilized treatment. Application of chemical fertilizer N (NPK, NP, and NK) increased ION in soils, compared with application of organic N or control. Nitrate N prevailed over exchangeable NH-N in all treatments. Nitrogen storage of the OM- and 1/2OMN-treated soils increased by 50.0% and 24.3%, respectively, over the NPK-treated soil, which had 5.4-22.5% more N than NP-, PK-, and NK-treated soils. The MBN, LN, and ION accounted for 1.7-2.4%, 25.7-34.2%, and 1.4-2.9% of TON, respectively, in different fertilization treatments. The surface soils (0-20-cm layer) in all treatments mineralized 43.6-152.9 kg N ha year for crop growth. Microbial biomass N was probably the better predictor of N mineralization, as it was correlated significantly ( P < 0.01) with NNMR. The OM and 1/2OMN treatments were not an optimal option for farmers when the crop yield and labor cost were taken into consideration but an optimal option for increasing soil N supply capacity and N sequestration in soil. The NPK treatment showed the highest crop yields and increased soil N fractions through crop residues and exudates input, and thus, it may be considered as a sustainable system in the North China Plain. [ABSTRACT FROM AUTHOR]

Using an alkaline calcareous soil, experiments were conducted to elucidate the effects of nitrification inhibitor dicyandiamide (DCD) on the fate of N-labelled urea applied to cotton, maize, and wheat under greenhouse conditions. Combined effects of DCD and two levels of wheat straw (applied to cotton) and of fertilizer application method (conventional broadcast vs. point injection in maize and wheat) on the recovery of the fertilizer N were also studied. High soil temperatures prevailed under cotton and maize, whereas the soil temperature was relatively moderate during the wheat growing season. The fertilizer N loss under cotton was lowest (44% of the applied) when urea was applied alone; the loss increased due to DCD (54%) or wheat straw (50-54%) and was highest (63-64%) when DCD and wheat straw were applied together. Under maize also, DCD increased the loss of the fertilizer N applied by the conventional method (51% without DCD vs. 66% with DCD) or by point injection (26% without DCD vs. 42% with DCD). With the conventional method under wheat, DCD had no effect on the fertilizer N loss (34-37% of the applied). The fertilizer N loss under wheat was least (16%) when urea solution was point-injected but increased (24-26%) due to DCD or/and when pH of the urea solution was reduced to 2. Besides, DCD significantly reduced the fertilizer N uptake and increased the fertilizer N immobilization in soil under cotton and maize. However, DCD applied in combination with a higher level of wheat straw significantly increased the cotton dry matter and N yields due to increased N availability from sources other than the fertilizer. The results suggested that the use of DCD may not be beneficial in alkaline calcareous soils and that point injection of urea solution without any amendment is more effective in conserving the fertilizer N as compared to the conventional broadcast method. [ABSTRACT FROM AUTHOR]