Soil microorganisms and biogeochemical cycles in a grazing ecosystem: interactions between producers, consumers, and decomposers

Abstract

Microorganisms are the unseen majority that determines ecosystem processes, they perform biogeochemical functions that translate into essential services, and regulate global climate. In grazing ecosystems, which 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 some implications for biogeochemical cycles and sustainability under climate change. In particular, I asked two questions, (1) whether land-use change alter the magnitude and heterogeneity of decomposer functions, and (2) whether changes in decomposer biomass follows functional heterogeneity. I found 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 vii 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. Overall, this thesis explores and explains how soil microbes contribute to the functioning of grazing ecosystems