Topography regulates soil extracellular enzyme activities through soil properties, and affects microbial resource...

Arid riparian forests are vital yet vulnerable transition zones linking aquatic and terrestrial ecosystems, representing key areas for biodiversity enrichment in arid regions. However, they are affected by multiple environmental stressors that influence soil microbial functionality. Soil enzyme activities and microbial resource limitation have been widely reported to reflect the nutrient demands and metabolic activities of microorganisms and affect microbial biosynthesis processes, but little is known about their characteristics in riparian forest ecosystems. The complex topography of this region results in diverse habitat types and environmental spatial heterogeneity, with piedmont plain areas being subjected to more environmental stress. Here, we sampled in the valley and piedmont plain riparian forests of the Irtysh River Basin measured soil extracellular enzyme activities and quantified microbial resource limitations by vector analysis. The results showed that topography significantly influenced extracellular enzyme activities, with enzyme activities related to carbon (β-1,4-glucosidase, 1444.30 ± 120.00 nmol g^-1 h^-1; β-D-1,4-cellobiosidase, 5962.77 ± 539.82 nmol g^-1 h^-1), nitrogen (β-1,4-N-acetylglucosaminidase, 697.66 ± 69.96 nmol g^-1 h^-1; leucine aminopeptidase, 467.37 ± 46.39 nmol g^-1 h^-1) and phosphorus (alkaline or acid phosphatase, 123.23 ± 24.71 nmol g^-1 h^-1) acquisition being more intensive in valley riparian forest. Microbial community metabolism in riparian forest ecosystems was limited by nitrogen, with a vector angle of <45°. Additionally, topography indirectly affected C-, N-, and P-acquiring enzyme activities primarily by regulating soil physicochemical properties (direct effect = 0.78, 0.50 and 0.40), while climatic factors largely controlled microbial relative C limitation (direct effect = -0.88, total effect = -0.91) and N limitation (direct effect = -0.56; total effect = -0.77). These findings contribute to enhancing the understanding of the key drivers of enzyme activities and microbial resource limitation in the soil of arid riparian forests, and provide theoretical support for improving soil quality and addressing degradation issues in the Irtysh River Basin. Thus, we suggest that the protection of arid riparian forests, especially plain riparian forests, should be improved and N fertilizer application should be considered for the restoration of riparian forests ecosystems in the Irtysh River Basin.