Returning residue to soils is not only an effective nutrient management method, but also can reduce the air pollution caused by residue burning, which has become an important factor in global warming. However, it is not clear whether returning residue to the soil can affect the nitrogen mineralization and the nitrogen cycle process, and the environmental impact caused by the nitrogen loss in gaseous forms. Therefore, a pot experiment was conducted to study the effects of residue placement on the nitrogen turnover process, including microbial biomass N (MBN) and C (MBC), inorganic N, crop N uptake, and the contribution of residue-derived N to maize at different maize growth stages. Three treatments were assessed: no residue addition (T0), residue addition to the soil surface (T1), and residue incorporation into the 0–10 cm soil layer (T2). Soil samples were taken at the 0–5 and 5–10 cm layers for all residue treatments. Residue retention (T1 and T2) significantly affected the MBC and MBN contents and decreased MBC/MBN ratio at different maize growth stages. MBC/MBN markedly increased at the R1 stage compared to other growth stages. The differences in total inorganic nitrogen (TIN) were attributed to the balance in net N immobilization and net mineralization in the different maize growth stages. In addition, T2 significantly increased the residue-derived N source for maize by 11.3% compared to T0 in the R3 growth stage. Overall, relative to T1, T2 is a better agriculture management measure to promote N transformation and supply, and enhance residue-derived N release and uptake in maize.

To better understand the mechanism of nitrogen (N) distribution, absorption, utilization and loss in fertilizer under different tillage practices, a study was conducted to quantitatively explore the fate of fertilizer N in the soil–plant–atmosphere using the ¹⁵N labelling technique under the long-term conservation tillage experiment in Northeast China. The test crop used was corn. This study compared the residual amount of ¹⁵N fertilizer in soil, the content of ¹⁵N fertilizer N in particle organic nitrogen (PON), light fraction organic matter nitrogen (LFOMN) and heavy fraction organic matter N (HFOMN) under different tillage practices. In addition, N uptake, utilization and distribution by corn, the emission of N₂O and the gas loss of fertilizer N, and the fertilizer N utilization rate were also taken into account. The results showed that no tillage (NT) had a significantly lower amount of residual ¹⁵N fertilizer than a moldboard plow (MP) (p < 0.05). In general, the content under NT at the 0–30 cm soil layer was 7.85% lower than that of MP. NT led to significantly greater PON and LFOMN of soil organic N compared to MP (p < 0.05). ¹⁵N from N uptake, fertilizer absorption and utilization under NT were significantly higher than that under MP (p < 0.05), the soil N absorbed by plants under NT or MP was greater than 70%. The distribution of ¹⁵N from N fertilizer in each corn part increases in this order: seed > leave > sheath > stem > bract > ear; about 57.91–64.92% of ¹⁵N is distributed in the grain. NT resulted in significantly lower average and cumulative N₂O emissions than those from MP based on the static closed chamber approach (p < 0.05). The average and cumulative emissions of soil fertilizer ¹⁵N-N₂O under MP were also significantly greater than that of NT. Among the N₂O emissions, 15.3% and 22.98% came from fertilizer N under NT and MP, respectively. On average, 0.1–0.16% of fertilizer N was lost in the form of N₂O. There was a significant difference in fertilizer utilization between NT and MP, and NT was 4.23% larger than MP (p < 0.05). These one year findings suggest that NT plays a positive role in improving the N absorption and utilization of fertilizer in a Chinese mollisol and long-term effects need to be further studied.