Soil Organic Carbon Distribution Patterns Across Tillage and Nitrogen Treatments in a Five-Year Straw-Return Maize Field

Under full straw return, changes in cropland management may be reflected first in the vertical distribution of soil organic carbon (SOC) rather than in total SOC stock. However, how tillage practices and nitrogen application are related to SOC profile distribution and associated biological processes remains unclear. Based on a five-year field experiment in the brown soil region of Northeast China, this study compared no-tillage/deep tillage rotation (NPT), continuous deep tillage (PT), and continuous rotary tillage (RT) under two nitrogen rates: 150 kg N ha−1 (LN) and 240 kg N ha−1 (HN). SOC, total nitrogen (TN), labile C and N fractions, microbial biomass, and enzyme activities were measured in the 0–5, 5–15, and 15–30 cm soil layers. After five years of treatment application, total SOC stock in the 0–30 cm profile did not differ significantly among treatments (p > 0.05), whereas SOC and TN stocks showed distinct vertical distribution patterns. PT and RT were associated with higher SOC, TN, and labile C and N levels mainly in the 0–15 cm layer, whereas NPT showed an advantage for SOC and TN retention in the 15–30 cm layer. The highest SOC stock in the 15–30 cm layer occurred under NPT-LN, reaching 25.91 Mg ha−1 and exceeding other treatments by 12.5–44.1%. High nitrogen application increased dissolved inorganic nitrogen (DIN) and TN stock, but did not further increase the total SOC stock. Biological indicators showed clear depth-dependent responses, and RDA combined with exploratory SEM suggested that SOC-associated pathways shifted from management and inorganic N-related associations in the surface layer to stronger coupling among input position, labile C–N status, microbial processing, and retention conditions in the 15–30 cm layer. Overall, under five years of full straw return, tillage and nitrogen treatments were associated with SOC distribution patterns within the plough layer rather than a significant increase in total SOC stock. NPT combined with low nitrogen application may represent a favorable management option for enhancing SOC retention in the 15–30 cm lower plough layer.