Predicting Long-Term Maize Straw Decomposition from Incorporation Amount and Depth in the Black Soil Region of Northeast China

Straw incorporation, as a widely recommended agronomic practice, has been continuously enhancing global crop production and soil–water conservation. However, the absence of a direct predictive capability for the long-term residual biomass of incorporated straw, based on management practices, constrains an accurate assessment of its effectiveness for soil conservation. To address these knowledge gaps, this study conducted systematic 4-year in situ monitoring of decomposition pits with varying incorporation amounts (A6 with 6 kg ha−1, A8 with 8 kg ha−1, A10 with 10 kg ha−1, A12 with 12 kg ha−1, and A14 with 14 kg ha−1) and burial depths (D1 with 0–10 cm, D2 with 10–20 cm, D3 with 20–30 cm, D4 with 30–40 cm, D5 with 40–50 cm) to analyze long-term decomposition dynamics. Furthermore, time-dependent equations for post-incorporation residual biomass were developed based on management variables (incorporation amount and burial depth) to enhance the accuracy of soil loss prediction. The results showed that the higher incorporation amounts accelerated decomposition, with the residual straw ratios (RSRs) reduced by 27.4–62.2% compared to lower amounts at equivalent burial depths. Decomposition slowed with depth, and the RSR increased significantly with greater burial depth, rising at rates of 0.2–1.2% cm−1 (p < 0.05). The RSR decreased significantly with longer incorporation duration at rates of 6.9–18.6% a−1 (p < 0.05), with deeper soil layers exhibiting greater decline rates than shallower depths. The relationship between RSR and landfill amount (m), burial depth (d), and landfill years (a) is represented as follows: RSR = 101.62 a−1 m−0.54 d0.45 (R2 = 0.76). Based on this equation, the soil loss ratios (SLRs) under continuous straw incorporation for 4 years were estimated, and the results suggest that constant straw incorporation exerts cumulative effects, progressively reducing the SLR. This study provides the theoretical foundation for promoting and managing straw incorporation practices.