Effects of resource availability and quality on soil microorganisms and their carbon assimilation

Soil microorganisms play a pivotal role in decomposition processes and therefore influence nutrient cycling and ecosystem function. Availability and quality of resources determines activity, growth and identity of substrate users. In agricultural systems, availability of resources is dependent on, for example, crop type, management, season, and depth. At depth substrate availability and microbial biomass decrease. However, there remain gaps in our understanding of C turnover in subsoil and how processes in the topsoil may influence abundance, activity, and function of microorganisms in deeper soil layers. With respect to substrate quality it is thought that bacteria are the dominant users of high quality substrates and more labile components whereas fungi are more important for the degradation of low quality and more recalcitrant substrates (i.e. cellulose, lignin). Therefore, this thesis was designed to increase our understanding of C turnover and the influence of both availability and quality of substrates on microorganisms in an agricultural soil.\nIn the first and second studies, a recently established C3-C4 plant exchange field experiment was used to investigate the C flow from belowground (root) and aboveground (shoot litter) resources into the belowground food web. Maize plants were cultivated to introduce a C4 signal into the soil both by plant growth (belowground / root channel) and also by applying shoot litter (aboveground litter channel). To separate C flow from the shoot litter versus the root channel, maize litter was applied on wheat cultivated plots, while on half of the maize planted plots no maize litter was returned. Wheat cultivated plots without additional maize litter application served as a reference for the calculation of incorporated maize-C into different soil pools. Soil samplings took place in two consecutive years in summer, autumn and winter. Three depths were considered (0-10 cm: topsoil, 40-50 cm: rooted zone beneath the plough layer, 60-70 cm: unrooted zone). In the third study a microcosm experiment with substrates of different recalcitrance and complexity was carried out to identify primary decomposers of different plant litter materials (leaves and roots) during early stages of decomposition (duration of 32 days) and to follow the C flow into the next higher trophic level (protozoa).\n