In Northeast China, the establishment of irrigated paddy fields manifests on soil characterized as upland soils. However, the implications of soil conversion from upland soil to paddy soil for soil aggregates, soil organic carbon (SOC), and enzyme activity within soil aggregates remain poorly understood. Exploring the repercussions of soil conversion on SOC is paramount in delineating enhanced strategies for ameliorating soil structure and bolstering organic carbon sequestration within terrestrial ecosystems. Therefore, this study aimed to quantify the impact of land use modifications on SOC content and enzyme activity within soil aggregates. In this study, paddy (rice field) and upland (maize field) plots were selected from Mollisols in Northeast China, which is characterized by akin soil type, level topography, and climatic conditions. The results indicated that microaggregates represented the predominant fraction in both land use types, ranging from 36.96% to 48.99%, with a notably higher proportion in paddy soil compared to upland soil. After 40 years of rice cultivation, a significant decrease of 9.90% and 2.97% was observed in mean weight diameter and geometric mean diameter, respectively. In paddy soils, the SOC content in aggregates of varying sizes had the following order: macroaggregates (26.41 g kg⁻¹) < microaggregates (21.91 g kg⁻¹) < silt + clay (15.55 g kg⁻¹) fractions. Similarly, in upland soil, the highest SOC content was found in macroaggregates, with the following sequence: macroaggregates (21.67 g kg⁻¹) < microaggregates (17.44 g kg⁻¹) < silt + clay (15.03 g kg⁻¹) fractions. β-glucosidase (BG) displayed the highest enzyme activities, with average values of 95.99 nmol h⁻¹ g⁻¹ in paddy soil and 85.34 nmol h⁻¹ g⁻¹ in upland soil. Macroaggregate fractions exhibited the highest BG activity in both soil types (paddy: 112.49 nmol h⁻¹ g⁻¹, upland: 96.71 nmol h⁻¹ g⁻¹). In conclusion, the conversion from upland fields to paddy fields changes the occurrence mechanism of SOC in the aggregate, which is an important way of sustainable C sequestration in cropland ecosystems.

The process of straw decomposition is highly complex and is regulated by a multitude of interacting factors. However, how the straw return rate influences the metabolic byproducts, extracellular enzymes, and microbial communities during straw decomposition remains an unresolved question. To address this, we integrated straw and soil chemistry, amplicon sequencing, untargeted metabolomics, and enzyme assays across four straw return rates (no return, 1/3, 1/2, and the Full treatment) at 30 and 90 d. Straw mass loss was greatest under the 1/2 return treatment at 90 d, reaching 53.7%, whereas soil organic carbon (SOC) and total nitrogen (N) were highest under the 1/3 return, exceeding the control by 17.6% and 17.2%, respectively. This indicates that decomposition and short-term soil C and N accumulation were decoupled. Microbial communities underwent clear temporal turnover, and hydrolytic enzymes increased from 30 to 90 d and were positively associated with mass loss. In contrast, oxidative enzymes showed no positive association with decomposition. Metabolomic profiles shifted from early-stage labile compounds to later-stage aromatic and phenolic compounds, and straw and soil metabolomes were closely coupled in a stage-specific manner (Procrustes M² = 0.15, p = 0.001; Mantel r = 0.69, p = 0.001). Mantel tests further indicated that the metabolome was significantly associated with return rate and SOC. Partial least squares path modeling revealed that the direct and indirect pathways linking return rate, metabolome, enzymes, and decomposition were reorganized between 30 and 90 d. Overall, straw return rate influenced decomposition through stage-dependent biochemical and microbial changes, and straw mass loss and short-term soil C and N accumulation were decoupled, responding nonlinearly and peaking under different return rates.

Soil organic carbon (SOC) in mountain forest ecosystems exerts critical controls over regional carbon balance and climate feedback loops. This study collected 209 stratified topsoil (0–30 cm) samples across temperate mountain forests of Northeast China, conducted a boosted regression tree (BRT) modeling framework integrated with space-for-time substitution, and established 15 combined thermopluviometric sensitivity scenarios to simulate SOC shifts under diversified climate disturbances. Tenfold cross-validation yielded a model mean R² of 0.62, revealing mean annual temperature (MAT, RI = 35.31%) as the most influential predictor of SOC spatial variation, followed by elevation (ELE, RI = 19.11%), while single-season NDVI and soil particle fractions showed weak predictive capacity. Multi-scenario spatial simulation outputs demonstrated that simultaneous warming and aridification drastically reduce the coverage of high SOC zones, whereas increased precipitation can partially offset temperature-induced carbon mineralization losses. Terrain-mediated SOC spatial stratification remained stable across all climate backgrounds. This study quantifies the layered environmental association hierarchy of mountain SOC and generates spatially explicit modeled carbon sink projections under climate change. The terrain-dependent SOC response patterns provide operable differentiated carbon regulation guidance: humid low-lying convergence zones require long-term soil moisture conservation, while arid steep ridges need native mixed forest restoration to lift baseline carbon storage capacity, supporting precise watershed climate adaptation and targeted forest carbon sink management for temperate mountain regions.

CONTEXT: Sustainable agriculture depends on the simultaneous delivery of multiple services, namely crop production, soil organic carbon (SOC) sequestration and greenhouse gas (GHG) mitigation, rather than any single outcome, an integrated capacity termed agroecosystem multiservices (AMS). Direct straw incorporation is applied to more than 60% of Chinese cropland but frequently generates trade-offs among these functions. Indirect straw incorporation (ISI), in which straw is modified before or during soil application, has been proposed to relax such trade-offs, yet its integrated effects remain poorly quantified because most studies rely on single indicators and on no-straw controls that cannot isolate the effect of straw processing.OBJECTIVE: We aimed to quantify whether, where and for how long ISI enhances AMS relative to direct straw incorporation, and to identify the cropping-system, climatic, edaphic and management conditions that govern its performance.METHODS: We synthesized 81 peer-reviewed field studies (457 paired observations) conducted across China, using direct straw incorporation at equal straw input as a unified control. Seven ISI practices were evaluated within a single AMS framework by combining response-ratio meta-analysis (random-effects models) with partial-correlation network analysis, random forest, segmented regression and piecewise structural equation modelling.RESULTS AND CONCLUSIONS: Relative to direct incorporation, ISI significantly increased crop yield (+4.19%) and SOC (+7.25%) and reduced global warming potential (GWP; -22.7%), numerically raising composite AMS by 5.52%. However, this overall AMS effect was statistically non-significant, reflecting strong cross-site heterogeneity rather than an absence of benefit: AMS improved significantly in upland and paddy-upland rotation systems (notably under biochar and ammoniated incorporation) but not in continuously flooded paddies, where CH4-driven trade-offs offset yield and SOC gains. Individual services were governed by distinct drivers: yield gains were largest under cool climates (MAT < 14 ℃), upland systems and loam soils; SOC accrual under neutral-to-alkaline soils (pH > 7.5), high nitrogen input and crop rotation; and GWP mitigation in low-sand soils. Critically, the three services were temporally decoupled: yield benefits were transient (~4 years), SOC accumulated persistently, whereas GWP mitigation showed no temporal trend. ISI is therefore a cropping-system- and time-dependent strategy rather than a universal solution.SIGNIFICANCE: By benchmarking ISI against an equal-input direct-return control and by resolving the spatial and temporal boundaries of its benefits, this study provides a systems-level framework for evaluating the multiple services of agricultural systems and offers cropping-system- and region-specific guidance for sustainable straw management and climate-smart agriculture.