The purpose of this study is to clarify the differential effects of the application rate of slow-release nitrogen fertilizer (SRF_N) on the nitrogen (N) accumulation dynamics, nutrient organ N distribution and transportation, yield, and N utilization efficiency of maize harvested using grain-type machines. This has significant implications for the scientific application of SRF_N, as well as for reducing its application rate and improving its efficiency, in the agro-pastoral transitional zone of northern China. In a long-term positioning experiment that began in 2018, five treatments consisting of different SRF_N application rates were set up, namely, N120 (120 kg ha⁻¹), N180 (180 kg ha⁻¹), N240 (240 kg ha⁻¹), N300 (300 kg ha⁻¹), and N360 (360 kg ha⁻¹), with no fertilization during the growth period used as control (CK) treatment. To explore the characteristics of nitrogen accumulation dynamics in maize populations and the main factors affecting maize yield formation under the different SRF_N application rate treatments, this study adopted a combination of quantitative analyses and model fitting, including logistic models, principal component analysis, and structural equation modeling. The research results show that SRF_N application increased the aboveground N accumulation of the maize population, and the fitting effect of the logistic models was significant. The maximum rate of N accumulation in both years showed a trend of first increasing and then decreasing with the increase in the SRF_N application rate. Compared with CK, SRF_N application reduced the proportion of N distribution in the nutrient organs during the R6 stage, and it increased the N transport from the nutrient organs to the grains after the VT-R1 stage. With the increase in the SRF_N application rate, both the economic yield and biological yield showed a single peak curve change and were maximized in the N240 treatment. The economic yield reached 15,342.07 kg ha⁻¹ in 2020 and 16,323.51 kg ha⁻¹ in 2021, increasing by 36.2% and 61.7% compared with CK, respectively. The apparent N fertilizer recovery rate, N uptake efficiency, N agronomic efficiency, and N fertilizer partial productivity all gradually decreased with the increase in the SRF_N application rate. In maize populations, an appropriate SRF_N application rate can adjust the characteristic parameters during the aboveground N accumulation rapid growth period, increase the N accumulation amount in aboveground parts, promote the transport of N from nutrient organs to grains, and improve yield. An application of 180–240 kg ha⁻¹ SRF_N is recommended for maize cultivation in the agro-pastoral transitional zone of northern China, as it is beneficial for stabilizing and increasing maize yield, as well as reducing the rate and improving the efficiency of N fertilizer.

The degradation of soil structure and organic carbon in the black soil region of Northeast China threatens sustainable crop production, and straw return combined with appropriate tillage may offer an effective strategy for soil restoration. However, the depth-dependent mechanisms by which straw management–tillage systems regulate aggregate stability, labile carbon distribution, and soil quality under long-term straw return remain unclear. Based on a seven-year field experiment established in 2018 in eastern Inner Mongolia, this study compared conventional tillage with straw removal (CT) and four straw-return practices: deep tillage with straw return (DTS), deep harrowing with straw return (DHS), rotary tillage with straw return (RTS), and no-tillage with straw mulching and direct seeding (NTS). Soil aggregate stability, soil organic carbon (SOC), SOC stock, labile organic carbon fractions, microbial biomass carbon, carbon-acquiring enzyme activity, available nutrients, and soil quality index (SQI) were evaluated across the 0–60 cm soil profile. Straw return combined with tillage improved soil structure, carbon accumulation, nutrient availability, and SQI, with stronger responses observed under NTS and DTS. NTS mainly increased macroaggregate formation and the accumulation of SOC, light fraction organic carbon (LFOC), and particulate organic carbon (POC) in the 0–20 cm layer and was associated with the highest SOC stock in the whole profile. In contrast, DTS showed greater increases in SOC stock, dissolved organic carbon (DOC), readily oxidizable organic carbon (ROC), available nutrients, and SQI in the 20–40 cm layer, likely related to deeper straw incorporation. Partial least squares (PLS) path modeling indicated that labile carbon pools were closely associated with the relationship between aggregate stability and soil quality improvement. These results suggest that NTS appears promising for surface soil conservation, whereas DTS may be beneficial for improving subsoil fertility and carbon sequestration in black soil farmland.