Effects of Disturbance and Environmental Gradients on Soil Microbial Diversity and Community Structure

Abstract

Microorganisms are the major component of global biodiversity, and it is found in the various habitats of terrestrial ecosystems. However, their ecological roles in nature and the patterns of microbial diversity are still poorly understood. Also the dominant factors regulating soil microbial community composition and diversity variation within these ecosystems are still unknown. In this study, the extent of soil microbial diversity was investigated in both experimental and Mountain ecosystem and the way microbial communities are affected by disturbance and environmental gradients, as well as the extent to which ecological processes or other environmental factors contribute to structuring the soil microbial communities. Firstly, I tested fungal community responses to disturbance gradients in a laboratory environment (Microcosms). Although disturbance is thought to be important in many ecological processes, responses of fungal communities to soil disturbance have been experimentally little studied and remained unknown about the responsiveness of soil fungal community structure to disturbance although there is a long history of the effects of disturbance on community structure in larger organisms. I subjected a soil microcosm to physical disturbance, at a range of frequencies designed to simulate ecological disturbance events. A soil microcosm is subjected to physical disturbance, sterilizing 90% of the soil volume each time, at a range of frequencies. The fungal community structure was analyzed using Illumina HiSeq sequencing of the ITS1 region. It was found that fungal diversity decline with the increasing disturbance frequencies, with no sign of the humpback pattern found in many studies of larger sedentary organisms. There is thus no evidence of an effect of release from competition resulting from moderate disturbance – which suggests that competition and niche overlap may not be important in limiting soil fungal diversity. Changing disturbance frequency also led to consistent differences in community composition. There were clear differences in OTU-level composition, with different disturbance treatments each having distinct fungal communities. The functional profile of fungal groups (guilds) was changed by the level of disturbance frequency. These predictable differences in community composition suggest that soil fungi can possess different niches in relation to disturbance frequency, or time since last disturbance. Fungi appear to be most abundant relative to bacteria at intermediate disturbance frequencies, on the time scale we studied here. Also, bacterial community responses to environmental gradients were tested in Mt.Norikura, Japan. Little is known about the factors affecting the relative influence of stochastic and deterministic processes on environmental gradients. The investigation on the community assembly, phylogenetic diversity and the relative role of both deterministic (niche-based) process and stochastic process may play in delimiting the bacterial phylogenetic community structure was conducted. Soil DNA from samples collected at a range of elevations was sequenced using Illumina MiSeq of the 16S rRNA gene. Mt. Norikura showed no increase in phylogenetic clustering in upper elevations, suggesting that this may not be a general pattern in elevational systems, no greater role of stochasticity towards upper elevations. However, the strength of phylogenetic clustering and the role of stochasticity was strongly related pH, with structuring and determinism being strongest at lower pH. This pattern follows that found in an earlier study of successional environments, where pH also dominates community structuring. The possibility that pH is a dominant factor in bacterial community structure, as well as in diversity, should be considered.