Microbial decomposers are the unseen majority that determine ecosystem processes, and perform biogeochemical functions which translate into essential services, and regulate global climate. In grazing ecosystems that represent over 40% of the terrestrial realm, soil microbes respond to aboveground interactions between plants and herbivores. In this thesis, I analyse different aspects of soil microbial functions in the high-altitude grazing ecosystem of the Trans-Himalaya, and quantify the implications for biogeochemical cycles and sustainability under climate change.

In the first chapter, I explore how large mammalian herbivores alter foraging strategies of soil microbial decomposers. Soil microbes forage by releasing extracellular enzymes (EE) into their environment to break down organic matter. Long-term herbivore-exclusion experiment revealed that herbivores improve quality of biomass-input to soil. This reduced microbial deployment of generic-depolymerizing EE relative to specialised-EE that release assimilable end-products. I validated the underlying role of quality of detrital input to soil by a reciprocal transplant experiment using laboratory incubations. I synthesised 860 soil-EE profiles from across the world to establish that this response to quality of detrital-input to is soil is both widespread and general. These observations provide evidence of a continuum between herbivores and decomposers that is relevant to global nutrient cycles and can also explain how microbes control soil-C sequestration in grazing ecosystems. In other words, soil microbial decomposers forage more efficiently in the presence of large mammalian herbivores.

In the second chapter, I explored how microbes alter the stability of the soil-carbon pool when humans replace wild-herbivores with livestock in grazing ecosystems. I found microbial-respiration was lower in soils under grazing by wild-herbivores than under livestock, with corresponding differences in fungal:bacterial ratio, microbial-biomass, and metabolic-quotient. Direction and magnitude of these inter-related microbial responses were driven by reduced soil microbial carbon use efficiency (CUE) under livestock. Since CUE is a fundamental microbial trait, wild-herbivores sequestered twice the soil-C than livestock despite comparable ecological settings. This implies that investments in wildlife conservation in multiple-use landscapes can help decarbonise the atmosphere to mitigate some of the negative environmental impacts of livestock-production.

In the third chapter, I addressed the relationship between functional diversity of decomposer functions in soil and the stability of ecosystem processes under land-use change. Unlike producers and consumers, decomposer functions are common between natural and agroecosystems. I take advantage of alternative land-use in the Trans-Himalaya where the native reference state with wild plants and wild herbivores is repurposed into two distinct agroecosystems—to grow livestock, or crops. I find that the extent of human-alteration of the reference state is reflected in the degree of homogenization of decomposer functions. Relative to the native state, magnitude of individual functions was often higher under crops but remained unchanged under livestock, such that land-use had no net effect on multifunctionality. However, univariate and multivariate measures of functional heterogeneity were lower under crops but were unaffected under livestock. Stability of decomposer biomass, measured as invariance through time, was comparable across land-use types. These results show that previous knowledge on diversity-relationships in producers and consumers are not easily extended to decomposers, and there are fundamental differences. Although agroecosystems in the Trans-Himalaya show remarkably high degree of ecological resistance, homogenization of their decomposer functions can make them susceptible to environmental fluctuations, such as those foreseen by future climate projections.

In summary, this thesis explains how soil microbes contribute to the functioning of grazing ecosystems.