Effects of no-tillage and stover mulching on the transformation and utilization of chemical fertilizer N in Northeast China

Microbial immobilization-remineralization of nitrogen (N) and mineral fixation-release of ammonium play essential roles in the retention of fertilizer N in the soil and the subsequent crop N uptakes. No-till farming with stover mulching leaves a significant amount of crop residues on the soil that provide carbon (C) substrates for microbes and regulate soil N availability to crops. However, crop residue removal, especially maize stover, for bioenergy feedstocks may threaten ecological benefits of no-till farming with stover mulching on N retention in the soil. Thus, it is of great importance to understand whether no-till farming should come with different levels of stover mulching to regulate N transformation to three main N pools as well as crop yields. With the help of a 15N-labeled tracer approach, we conducted a long-term maize cropping field study in the black soil of Northwest China with the following five treatments: (1) ridge tillage (RT); (2) no-tillage with 0 % mulching (NT-0); (3) no-tillage with 33 % mulching (NT-33); (4) no-tillage with 67 % mulching (NT-67); (5) no-tillage with 100 % mulching (NT-100). Our research showed that stover mulching (NT-33, NT-67, and NT-100) significantly increased maize yields from averaging 12038–13904 kg ha−1 and crop N uptakes by averaging 16 % than RT. In the seedling stage, compared to RT, residual 15N-labeled N decreased by 13 % in the topsoil of NT-0, NT-67 and NT-100, but increased by 15 % in all no-till treatments at 20−40 cm soil layer. Further assessment of three soil N pools revealed that at top 20 cm soil layer, maize stover mulching significantly reduced 15N-labeled mineral N by 33 % and 52 % compared to RT and NT-0 in the seedling and tasseling stages, respectively, but increased 15N-labeled fixed NH4+ and organic N in the seedling stage, potentially enhancing the fertilizer N transformation from mineral N to fixed NH4+ and organic N reserves. At 20–40 cm soil layer, 15N-labeled fixed NH4+ increased in NT-67 and NT-100 than NT-0 in the seedling stage. Our results suggest that no-till farming should accompany crop residue mulching to enhance fertilizer N processes to more stable N reserves of fixed NH4+ and organic N rather than labile mineral N pool. Thus, indiscriminate harvesting of crop residues for biofuel feedstocks may deteriorate soil nutrient cycling to reduce N use efficiency in the soil-crop system, leading to a decline of crop N uptakes and yields.