Soil structure controls the trade-off between soil health and erodibility along cascade-reservoir slopes in a dry-hot valley
Description
Table summarises the descriptive statistics and indicates clear spatial contrasts in soil nutrient status, biological activity, and potential stressors across reservoir segments. Across all samples, SOC ranged from 6.3 to 35.5 g kg⁻¹ with a mean of 17.6 g kg⁻¹, TN ranged from 0.77 to 3.65 g kg⁻¹ with a mean of 1.39 g kg⁻¹, and available phosphorus ranged from 4.5 to 48.7 mg kg⁻¹ with a mean of 15.1 mg kg⁻¹. Soil pH was narrowly buffered between 6.8 and 7.8. Trace metals also exhibited strong spatial gradients: Mn and Zn were dominant, ranging from 579 to 1136 mg kg⁻¹ and from 65 to 371 mg kg⁻¹, respectively, while Pb declined sharply from 211 mg kg⁻¹ at RH to 20 mg kg⁻¹ at RT. Chromium ranged from approximately 88 to 142 mg kg⁻¹. Copper showed pronounced local enrichment, peaking at BHT RT at about 177 mg kg⁻¹ rather than at RH, suggesting strong site-specific controls. Spatial differences were also evident in microbial and biochemical properties. Microbial biomass varied widely, with SMBC ranging from about 118 to 684 mg kg⁻¹ and SMBN from about 2.7 to 70.8 mg kg⁻¹. Enzyme activities differed among segments, and aggregate-size distributions were broadly stable, with silt plus clay remaining close to 6 % and only modest variation in macroaggregate and microaggregate proportions. Seasonal shifts further modified these spatial patterns, but the direction and magnitude of change differed among reservoirs and segments. In BHT, urease activity increased from an average of 0.55 mg g⁻¹ in the dry season to 0.70 mg g⁻¹ in the wet season, whereas NAG declined markedly in the wet season and SAP showed mixed seasonal responses across segments. BG varied strongly among locations, reaching its minimum at WDD RT in the wet season at approximately 0.25 μmol g⁻¹ dw h⁻¹. DOC also showed reservoir- and segment-specific seasonality: concentrations were generally higher in BHT than in WDD and often exceeded 130 mg kg⁻¹, but the wet season did not uniformly reduce DOC, with elevated values observed at RM in BHT. Aggregate-size fractions shifted moderately between seasons in BHT, with macroaggregates averaging about 20 % in the dry season and 18 % in the wet season, while microaggregates remained similar across both periods; aggregate composition in WDD was comparatively stable.