Linking extracellular enzyme activities in soil aggregates to carbon stabilization along an elevational gradient in alpine forest and grassland ecosystems

Abstract

Soil extracellular enzymes play a crucial role in the cycling of carbon (C), nitrogen (N), and phosphorus (P) in alpine ecosystems, and they are sensitive to variations in plant inputs, climate, soil, and microbial properties occurring across short elevation gradients. However, the dynamics of soil enzyme activities in alpine ecosystems and their relationship to soil C stabilization remain uncertain. Here, we investigated the regulating factors driving the activities of hydrolyzing enzymes responsible for C (β-1,4-glucosidase, α-1,4-glucosidase, β-_D-1,4 cellobiohydrolase, and β-1,4-xylosidase), N (β-1,4-acetylglucosaminidase and _L-leucine aminopeptidase), and P (acid phosphatase) cycling, as well as the ¹³C natural abundance (δ¹³C) of soil aggregates at two depths (0–10 cm and 10–20 cm) within alpine forest and grassland soils along an elevational gradient in the Yulong Mountains of Southwest China. Soil enzymatic C, N, and P activities increased significantly with increasing elevation but decreased with increasing soil depth, and we observed higher levels of enzymatic activity in grassland soil than in forest soil. Soil enzymatic C and N activities in the silt + clay fraction (<53 µm) were higher compared to the larger aggregates (>53 µm), likely due to the higher soil C and N substrate availability in small fractions. The δ¹³C values of the aggregates relative to bulk soils showed an increasing trend of ¹³C enrichment with decreasing aggregate size classes and were correlated with the corresponding enzyme activities. The soil C potentially flowed from macro- to microaggregates in the order of large macroaggregate (>2000 µm) → small macroaggregate (250–2000 µm) → microaggergate (53–250 µm) → silt + clay fractions, suggesting the predominant formation of recent C inputs in the large macroaggregate and its stabilization in the smallest fraction. Overall, our findings provided valuable insights into soil C stabilization and microbial processing within aggregate fractions along elevational gradients in alpine ecosystems.