Effects of N fertilization and maize straw on the transformation and fate of labeled (15NH4)2SO4 among three continuous crop cultivations

The transformation and fate of fertilizer N applied to soil is particularly important for increasing N retention in soil-crop systems and minimizing N loss from agroecosystems. However, it is obscure how the transformation and fate of fertilizer N will be affected by N input rates and crop residue amendment. In present study, an outdoor experiment was conducted in Mollisols of Northeast China throughout three crop cultivations using 15N-labeled tracer techniques to evaluate the effects of N fertilization levels and maize straw application on the conversion of applied fertilizer N to newly synthesized organic N, recently fixed NH4+, and mineral N in soil-crop systems. Generally, six treatments were set up: no N fertilization (CK), application of only maize straw (M, 16.49 mg N kg−1 soil), low N input levels (N1, 44.64 mg N kg−1 soil), N1 plus maize straw (N1+M, 61.13 mg N kg−1 soil), high N input levels (N2, 89.28 mg N kg−1 soil), and N2 plus maize straw (N2+M, 105.77 mg N kg−1 soil). The results showed that the majority of applied fertilizer N was rapidly transformed into newly synthesized organic N, followed by recently fixed NH4+. Compared with the N1 and/or N2 treatments, maize straw amendment in the N1+M and N2+M treatments significantly increased the residual amount of applied fertilizer N in soil by accelerating the conversion of fertilizer N to newly synthesized organic N or recently fixed NH4+. Among three crop cultivations, the percentages of recovery of applied fertilizer N in crops (PR-Ncrop) and in soil (PR-Nsoil) were averagely 61.1% and 15.8%, respectively, while the percentage of N loss (PL-N) was 23.1%. Applying maize straw significantly elevated the PR-Ncrop and PR-Nsoil but decreased the PL-N compared to those without maize straw. These results suggested that the application of maize straw with high C/N ratio is important to retain applied fertilizer N in soil-crop systems. Balancing carbon and nitrogen inputs in soil through proper combination of fertilizer N and maize straw will be a potentially practical management to increase fertilizer N recovery in crop and soil, and therefore reduce fertilizer N loss from crop-soil systems.