Soil horizon development and altitudinal-vegetational zonation decouple microbial biomass from bulk soil nutrient availability across a Himalayan treeline ecotone

Abstract. Alpine treeline ecotones are environmentally sensitive transition zones where vegetation structure, soil development, and nutrient cycling interact over short spatial scales. Despite increasing attention to Himalayan treeline dynamics, the distribution of soil microbial biomass and its stoichiometric relationships across vegetation and soil horizon gradient remains poorly understood. We quantified microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP), together with soil organic carbon (SOC) and total nitrogen (TN), across four altitudinal–vegetational zones and four genetic soil horizons in the Rolwaling Himalayan treeline ecotone, Nepal. Linear mixed-effects models were used to evaluate the effects of soil horizon development, vegetation zonation, and slope aspect on microbial biomass concentrations and stoichiometric relationships. Soil horizon development was the dominant control on microbial biomass distribution. MBC, MBN, and MBP declined significantly with depth (p < 0.001), with the surface O-horizon accounting for 55.7 %, 62.0 %, and 62.8 % of the cumulative horizon-wise concentrations of MBC, MBN, and MBP, respectively. Altitudinal-vegetational zonation generated pronounced non-linear patterns. SOC and TN concentrations were highest in the lower krummholz zone dominated by Rhododendron campanulatum, whereas MBC and MBN concentrations peaked in lower dwarf shrub heath zone, demonstrating a clear decoupling between bulk soil nutrient concentrations and microbial biomass. Microbial phosphorus concentrations exceeded microbial nitrogen concentrations by 3.9-fold across the study area, yielding an overall microbial biomass stoichiometry of 3.4:0.3:1 (MBC:MBN:MBP), substantially different from globally synthesised microbial biomass ratios, indicating plasticity in microbial biomass stoichiometry in stress environments in treeline ecotones. Our results indicate that microbial biomass distribution across the Rolwaling treeline ecotone is shaped by both soil horizon differentiation and altitudinal-vegetational zonation. The contrasting spatial patterns of microbial biomass and bulk soil nutrient concentrations suggest that measurements of total soil nutrients alone may not fully capture biologically active nutrient dynamics in high-elevation ecosystems.