Objectives Studying the effects of straw and livestock manure application on the gross N transformation characteristics in cropland soil can provide a deep understanding of the mechanisms of N supply and retention in soil, which is of great significance for the rational application of organic materials to improve soil fertility.

Methods Indoor incubation experiment was used for the research. The test soil was black soil, and the test organic materials were pig manure (PM), chicken manure (CM), and maize straw (MS), the treatment without organic materials addition was used as the control (CK). The soil was paired labelled with ¹⁵N isotope and incubated for one week at 25°C and 60%WHC (water holding capacity). On the 0, 1, 2, 4, and 7 day of incubation, soil samples were collected for measurement of inorganic and organic nitrogen contents, and their ¹⁵N abundances. The rates of gross N mineralization, gross nitrification, NH₄⁺ immobilization and NO₃⁻ immobilization were calculated using FLUAZ numerical optimization model.

Results The gross N mineralization rate of PM, CM, and MS treatment was N 7.86, N 6.23, and N 6.79 mg/(kg∙d), which was 2.71, 1.94, and 2.20 folds higher than that of CK N 2.12 mg/(kg∙d). The gross N immobilization rate of PM, CM, and MS treatment was N 2.85, N 4.11, N 10.65 mg/(kg∙d), which was 1.32, 2.34, and 7.66 folds of that of CK N 1.23 mg/(kg∙d). The gross N mineralization rates of PM and CM treatments were greater than the gross N immobilization rates, leading to net N mineralization in soil, while the reverse was true for MS, leading to the net N immobilization in soil. The immobilization rate of NH₄⁺ by soil microorganisms was significantly higher than that of NO₃⁻, contributing 72%–99% of total inorganic N immobilization rate. Livestock manure and maize straw showed different immobilization effect on soil inorganic N. Compared with CK, PM and CM were recorded 2.36 and 3.40 times higher NH₄⁺ immobilization rates, but similar NO₃⁻ immobilization rates; MS was recorded 6.35 and 101 times higher NH₄⁺ and NO₃⁻ immobilization rates. The gross nitrification rate of CK was N 5.78 mg/(kg∙d), PM and CM treatments increased that to 2.13 and 1.35 times higher, whereas MS treatment decreased that to N 0.68 mg/(kg∙d). The ratios of gross nitrification to NH₄⁺ immobilization rate (GN/ I_\mathrmNH_4^+ ratio) in PM and CM treatments were larger than 1, being 4.36 and 1.92, respectively, while the ratio in MS treatment was 0.09, less than 1.

Conclusions Although straw can promote the mineralization of soil organic nitrogen, it also enhances the immobilization of inorganic nitrogen by microorganisms, resulting in a net immobilization of nitrogen in the soil. This is because the gross mineralization rate of organic nitrogen is lower than the immobilization rate of inorganic nitrogen, which favors the retention of inorganic nitrogen. When pig manure and chicken manure are applied, the gross mineralization rate of soil nitrogen exceeds the rate of inorganic nitrogen immobilization, particularly the biological immobilization rate of ammonium nitrogen, leading to net nitrogen mineralization. Moreover, due to the GN/ I_\mathrmNH_4^+ ratio (ratio of gross nitrification rate to ammonium nitrogen immobilization rate) being greater than 1, potentially causing nitrate nitrogen accumulation in the soil and nitrogen diffusion from the soil into the environment.