Spatial and temporal variations of microorganisms in grassland soils : influences of land-use intensity, plants and soil properties

Field: Environmental Science (Physical Sciences)

Grassland ecosystems provide a wide range of services to human societies (Allan et al., 2015) and plants and soil microorganisms have been identified as key drivers of ecosystem functioning (Soliveres et al., 2016). Therefore, understanding soil microbial distributions and processes in agricultural grassland soils is crucial for characterizing these ecosystems and for predicting how they may shift in a changing environment. Yet we are only beginning to understand these complex ecosystems, which account for about 26% of the worlds terrestrial surface (FAOSTATS, 2018), making it especially urgent to gain better insights into the effects of land-use intensity on soil microbial properties and plant-microbe interactions. This thesis was conducted to evaluate the impact land-use intensity has on soil microbial biogeography of grasslands with respect to both spatial patterns and temporal changes in soil microbial abundance, function (in terms of enzyme activities), and community composition. It also investigated the relationships between plants and the spatial and temporal distributions of soil microorganisms. Thereby both, land-use intensity effects and plant-microbe interactions, were assessed in light of ecological niche and neutral theory. This thesis is based on three observational studies conducted on from one to 150 continuously farmed, un-manipulated grassland sites in three regions of Germany within the Biodiversity Exploratories project (DFG priority program 1374). \nThe first study assessed the effects of land-use intensity and physico-chemical soil properties on the spatial biogeography of soil microbial abundance and function in 18 grasslands sites from two of the three regions, sampled at one time point. The second study analyzed spatial and temporal distributions of alpha- and beta-diversity of arbuscular mycorrhizal fungi in a low land-use intensity grassland with six sampling time points across one season. The third study investigated both legacy and short-term change effects of land-use intensity, soil physico-chemical properties, plant functional traits, and plant biomass properties on temporal changes in soil microbial abundance, function, and community composition in 150 grassland sites across three regions, with particular regard to direct and indirect land-use intensity effects. \nAlthough the three studies used different approaches and assessed different soil microbial properties, general patterns were detectable. Abiotic soil properties, namely pH, nitrogen content, texture, and bulk density played fundamental roles for spatial and temporal microbial biogeography. Since these factors were specific and unique for each investigated site, they formed the background based on which other processes occurred. In addition to abiotic soil properties, impacts of land-use intensity and plants were detected, though to various degrees in the three studies. Land-use intensity played a much smaller role than anticipated in the first and third study. No influence on the spatial distribution of soil microbial abundance and function could be detected in the first study. In the third study, short-term changes in and legacy effects of land-use intensity played a minor role with respect to short-term changes in soil microbial abundance, function, and community composition. Where detected, changes in land-use intensity had a direct and negative effect on soil microbial properties in structural equation modelling; i.e., increases in land-use intensity reduced, e.g., soil microbial enzyme activities, while legacy effects of land-use intensity were shown to act both directly and indirectly on soil microbial properties. Thereby indirect legacy effects were mediated via plant functional traits. Only one of the three studies detected minor plant diversity effects on soil microbial properties. Instead, functional properties of the plant communities, i.e., plant functional traits, biomass, and nutritional quality, were significantly related to spatial and temporal distributions of soil microorganisms. Finally, the findings of the three studies suggest that processes related to niche and neutral theory both drive spatial and temporal patterns of soil microbial properties at the investigated plot scale (up to 50 m × 50 m).\nThis thesis concluded that in order to gain deeper insights into the complex functions and processes occurring in grassland ecosystems, a multidisciplinary approach investigating fundamental physico-chemical site characteristics, microbial soil properties, and plants is necessary. The results of the thesis suggest that focus be turned to functional properties of plant and microbial communities, as they are closely intermingled, provide more detailed insights into plant-microbe interactions, and are able to reflect effects of human impacts on grassland soils better than diversity measures.\n