Influence of 15N-labeled ammonium sulfate and straw on nitrogen retention and supply in different fertility soils

Microbial immobilization/mineralization and mineral fixation/release of ammonium are important for N retention and supply. However, the rates of such processes vary among different fertility soils and fertilization management practices. Three long-term different fertilized soils were used to simulate a range in soil fertility level and incubated with different N amendments for 144 days. The dynamics of ¹⁵N derived from ammonium sulfate (AS) or straw in different soil N pools and the ammonium sulfate-N or straw-N retention and supply were studied. In the absence of straw, the amount of ammonium sulfate-N present as fixed ammonium was 1.1–3.5-fold higher than that present as soil microbial biomass N (SMBN), although ammonium sulfate-derived SMBN and its mineralization increased by increasing soil fertility level. Straw addition significantly (P < 0.05) enhanced the relative importance of the SMBN pool on ammonium sulfate-N retention and supply compared with the fixed ammonium-N pool, and the former exceeded the latter in higher fertility soils. Regardless of soil fertility levels, straw addition significantly blocked the release of ammonium sulfate-N from the fixed ammonium-N pool. The SMBN pool was more important in straw-N retention and supply than the fixed ammonium-N pool, confirming that straw-N cycling depended more on biotic processes. The percentage of mineralized ammonium sulfate-N or straw-N from SMBN was higher than that released from fixed ammonium, indicating the higher availability of SMBN. Generally, the mineral fixation/release of ammonium was the main process for mineral fertilizer N retention and supply in the low fertility soil with or without straw addition, whereas microbial immobilization/mineralization became the main process in the high fertility soil with straw addition. Our results gave insights on the ammonium sulfate-N or straw-N retention and supply in different fertility soils, providing suggestions for optimizing straw management and synchronizing N supply with crop demand.