Soil erosion results in dissolved organic matter (DOM) loss and is one of the main paths of soil carbon loss. Bacteria affect the generation and transformation of DOM. However, the effect of bacteria on the composition and slope distribution of DOM has rarely been investigated under field conditions. Based on a long-term experiment of three gradients (3°, 5°, 8°) in a black soil erosion area of Northeast China, the content, composition, and source of DOM were studied. The results showed that the DOM of the 3° and 5° slope was enriched midslope, and the DOM of the 8° slope was enriched downslope. Parallel factor (PARAFAC) analysis indicated that the main substances in DOM were fulvic-like acid, humic-like acid, tryptophan-like protein, and soluble microbial metabolites. The upslope and downslope soils of 3° and 5° slopes showed high DOM bioavailability, while the downslope soil of the 8° slope showed high DOM bioavailability. The content of new DOM in downslope soil increased with the gradient. Bacteria played an important role in the synthesis and transformation of DOM and affected its composition and slope distribution. Verrucomicrobiota, Firmicutes, Planctomycetota, and Gemmatimonadota were the main factors affecting soil DOM. The results could be helpful in understanding the loss mechanism of DOM in eroded black soil and provide support for soil carbon sequestration.

Inversion tillage and organic fertilizer application can break the plow pan and improve soil quality. However, the effects of combining these practices on the soil microbial resource limitation and maize yield in Haplic Chernozem are unclear. In this research, a field experiment was established in 2018, and soil samples were collected in 2021 in Longjiang County in Northeast China, which is a Haplic Chernozem region. Four treatments comprising conventional tillage (T15), conventional tillage with organic fertilizer (T15+M), inversion tillage (T35), and inversion tillage with organic fertilizer (T35+M) were randomly arranged with four replications. Compared with T15 and T15+M treatments, soil bulk density significantly decreased by 11.1–16.3% in the 15–35 cm layer under T35 and T35+M treatments, accompanied by the improvement in soil pore structure (e.g., soil porosity, circularity, and Euler number). T15+M treatment significantly increased soil organic carbon and soil nutrient contents by 11.1–16.3% and 3.9–24.5% in the 0–15 cm layer compared with other treatments. However, soil organic carbon, total nitrogen, available phosphorus content, microbial biomass, and enzymatic activities reached the maximum values in the 0–35 cm layer under T35+M treatment. In addition, T35+M treatment had the highest maize yield and sustainable yield index. Extracellular enzymatic stoichiometry suggested that soil microorganisms are generally co-limited by carbon and phosphorus in Haplic Chernozem. However, T35+M treatment significantly reduced soil microbial resource limitation, which was one important factor impacting maize yield and sustainability. Random-forest and partial least-squares path modeling showed that T35+M treatment could reduce soil microbial resource limitation and increase the stability and sustainability of the maize yield by improving soil available nutrients, microbial biomass, and pore structure. Therefore, the incorporation of inversion tillage and organic fertilizer is a suitable soil management practice in view of increasing soil quality and crop yields in a Haplic Chernozem region.

Semi-arid Chernozem farmland in Northeast China faces the dual challenges of water scarcity and excessive nitrogen (N) fertilisation, yet quantitative understanding of coupled water–N use efficiency and environmental trade-offs under climatic variability remains limited. Using the WHCNS (Water, Heat, Carbon and Nitrogen Simulator) model calibrated and validated against field data (2022–2023) from Qiqihar, we quantified crop yield, water use efficiency (WUE), System-Level Nitrogen Conversion Efficiency (SLNCE) and N losses under five management regimes encompassing conventional fertilisation, straw mulching and organic manure substitution. Long-term climate adaptability was evaluated using precipitation scenarios from 2001 to 2023. Straw mulching reduced evapotranspiration by 12.2% and increased WUE to 2.09 kg·m⁻³. Combined chemical and organic fertiliser application (T223) achieved the highest yield (9745.9 kg·ha⁻¹) and WUE (2.14 kg·m⁻³), whereas a 30% reduction in chemical fertiliser with straw return (T323) reduced yield and SLNCE, indicating insufficient N supply. Organic substitution achieved the highest SLNCE (35.9 kg·kg⁻¹), exceeding the straw-only treatment by 29.1%. Nitrate leaching below 1 m was negligible, whereas gaseous N losses were the dominant loss pathway and increased with rainfall. SLNCE responded nonlinearly to precipitation, peaking at 420–451 mm; extreme drought years constrained yield through nitrogen limitation, whereas wet years increased gaseous N losses while maintaining relatively high yields. These results show that organic substitution can simultaneously enhance WUE and SLNCE, providing a climate-adaptive strategy for sustainable water–N management in semi-arid Chernozem soils.