Exploring the spatiotemporal pattern of water quality and identifying pollution sources is crucial for achieving precise management of reservoir watersheds. This study is based on monthly water quality data from 9 monitoring stations in the upstream watershed of Guanting Reservoir in 2024, combined with an improved water quality index method (WQI) and multivariate statistical analysis (clustering, discrimination, principal component and factor analysis), to reveal the spatiotemporal variation characteristics of water quality and pollution sources. The results show (1) significant spatiotemporal heterogeneity. In terms of time, the water quality is worst during the summer rainy season (June August), indicating that the pollution load input from surface runoff exceeds the dilution effect of rainfall. In terms of space, the water quality deteriorates significantly downstream along the river network, with the most prominent pollution occurring in the entrance area. (2) The results also show clear identification of key indicators and dominant pollution sources. Discriminant analysis shows that BOD₅ and DO are key indicators for distinguishing rainy and dry seasons, while TN, TP, COD, COD_Mn, and F can effectively distinguish spatial clusters. Factor analysis further revealed that organic pollution (originating from domestic and industrial wastewater) and nutrient pollution (mainly from agricultural non-point sources) are the dominant factors. This study confirms that pollution input during rainfall is the primary driving factor for water quality deterioration, and human activities have led to the cumulative effect of pollutants along the river network. Based on this suggestion, differentiated and precise governance strategies should be implemented according to the spatiotemporal differentiation characteristics to improve the water environment quality of the upstream watershed of Guanting Reservoir.

To clarify the role and mechanism of sod-seeding patterns in improving soil fertility and mitigating nitrous oxide (N₂O) emissions in peach orchards, we conducted a study since 2023. Taking clean tillage (CK) as the control, three sod-seeding patterns—Trifolium repens–Lolium perenne mixed sowing (TPr), T. repens single sowing (Tr), and L. perenne single sowing (Pr)—were tested to analyze soil physicochemical properties, nitrogen cycle functional genes, and N₂O emission-related genes, and to explore the driving mechanism of N₂O mitigation. Results showed that all three sod-seeding patterns significantly reduced soil pH and bulk density, increased soil electrical conductivity and mean aggregate size, and improved soil nutrient status compared with CK; TPr performed best, significantly enhancing soil enzyme activities related to carbon and nitrogen cycles. Sod-seeding patterns had no significant effect on genes involved in assimilatory nitrate reduction, denitrification, or nitrification, but significantly increased dissimilatory nitrate reduction (DNRA) and nitrogen degradation gene abundances, and reduced N₂O-producing gene (amoA + amoB, nirS + nirK) abundances. Field monitoring indicated TPr reduced N₂O emissions by 34.0%, 35.7%, and 41.0%, relative to CK, Pr, and Tr, respectively. Structural equation modeling revealed that sod-seeding reduced N₂O emissions mainly by decreasing soil NH₄⁺-N content and nirS + nirK abundance. In conclusion, sod-seeding patterns improve soil fertility and mitigate N₂O emissions in peach orchards, with TPr showing the best comprehensive benefits.

Soil degradation in the agro-pastoral ecotone of North China severely constrains the productivity of vital forage crops like oat (Avena sativa L.). While biochar and humic acid are known soil amendments, their combined synergistic effects and optimal application ratios remain poorly understood. This study aimed to investigate the integrated effects of biochar and humic acid on soil properties and oat productivity to identify an optimal co-application strategy for this fragile ecosystem. A three-year field experiment (2023–2025) was conducted using a randomized complete block design. Treatments included a control (CK), sole biochar (B1H0), sole humic acid (B0H1), and three combined biochar-humic acid ratios (7:3, 5:5, and 3:7 by mass; B7H3, B5H5, B3H7). We measured key soil physicochemical and biological properties, as well as oat growth, forage yield, and quality parameters. Partial least squares structural equation modeling (PLS-SEM) was employed to elucidate the causal pathways linking amendments, soil quality, and crop performance. The combined application of biochar and humic acid demonstrated significant synergistic effects, outperforming sole applications in improving soil health and oat productivity. The B5H5 (1:1 ratio) treatment was the most effective, significantly reducing soil bulk density by up to 22.4% and increasing the mean weight diameter of water-stable aggregates by 40.7% compared to the CK. This treatment also led to the greatest increases in soil organic matter (68.9%), microbial biomass carbon (142.2%), and the activities of sucrase (139.8%) and urease. Consequently, the B5H5 treatment enhanced oat forage yield by 56.1% and improved its nutritional quality by reducing acid detergent fiber and neutral detergent fiber content by 22.9% and 12.7%, respectively. The PLS-SEM analysis confirmed that the positive effects of the amendments on crop performance were primarily mediated through the enhancement of soil quality (path coefficient β = 0.75, p < 0.001). The integrated application of biochar and humic acid, particularly at a 1:1 mass ratio, is a highly effective strategy for restoring degraded soils in the agro-pastoral ecotone. These findings provide a strong scientific basis for promoting this co-amendment approach as a sustainable solution to enhance agricultural resilience and productivity in fragile environments.

Soil degradation in the agro-pastoral ecotone of North China severely constrains the productivity of vital forage crops like oat (Avena sativa L.). While biochar and humic acid are known soil amendments, their combined synergistic effects and optimal application ratios remain poorly understood. This study aimed to investigate the integrated effects of biochar and humic acid on soil properties and oat productivity to identify an optimal co-application strategy for this fragile ecosystem. A two-year field experiment was conducted using a randomized complete block design. Treatments included a control (CK), sole biochar (B1H0), sole humic acid (B0H1), and three combined biochar-humic acid ratios (7:3, 5:5, and 3:7 by mass; B7H3, B5H5, B3H7). We measured key soil physicochemical and biological properties, as well as oat growth, forage yield, and quality parameters. Partial least squares structural equation modeling (PLS-SEM) was employed to elucidate the causal pathways linking amendments, soil quality, and crop performance. The combined application of biochar and humic acid demonstrated significant synergistic effects, outperforming sole applications in improving soil health and oat productivity. The B5H5 (1:1 ratio) treatment was the most effective, significantly reducing soil bulk density by up to 22.4% and increasing the mean weight diameter of water-stable aggregates by 40.7% compared to the CK. This treatment also led to the greatest increases in soil organic matter (68.9%), microbial biomass carbon (142.2%), and the activities of sucrase (139.8%) and urease. Consequently, the B5H5 treatment enhanced oat forage yield by 56.1% and improved its nutritional quality by reducing acid detergent fiber and neutral detergent fiber content by 22.9% and 12.7%, respectively. The PLS-SEM analysis confirmed that the positive effects of the amendments on crop performance were primarily mediated through the enhancement of soil quality (path coefficient β = 0.75, p < 0.001). The integrated application of biochar and humic acid, particularly at a 1:1 mass ratio, is a highly effective strategy for restoring degraded soils in the agro-pastoral ecotone. These findings provide a strong scientific basis for promoting this co-amendment approach as a sustainable solution to enhance agricultural resilience and productivity in fragile environments.