Microbial Activity Indicators & Drivers

Field: Environmental Science (Physical Sciences)

Biota involved in the carbon (C) cycle play a vital role in C sequestration and allocation in the soil food web. Though, the decomposition of soil organics strongly depends on the availability of soil organic matter (SOM) to microorganisms and enzymes. As rhizo-sphere and detritusphere are biospheres with very high availability of C, leading to high abundance, species diversity and contrasting functions of microbial communities, they are worth to be called hot spots of microbial functioning. During the last decades, the interest on research on microbial activity in soil increased. However, there is a lack of comprehensive understanding of the indicators and drivers of microbial activity in rhizosphere and detritusphere. This is especially due to the objective necessity for more than a single measure to determine these factors. For that reason, this thesis aims to investigate microbial activity, its indicators and its drivers. An arable field experiment with different substrate input (corn rhizodeposits vs. corn litter) was conducted to determine microbial activity by various activity indicators, such as microbial respiration, microbial biomass and enzyme kinetics. To account for decreasing amount and quality of substrates in rhizosphere and detritusphere with soil depth, we exhibited depth gradients of microbial activity indicators. Special attention was paid to several indices of multiple enzymes and their activity (e.g. specific activity, catalytic efficiency and ratios between C- and N-cycling enzymes) down the soil profile. Not only in the field, but also along a climatic gradient we investigated microbial activity. Therefore, the RNA:dsDNA ratio was determined in order to identify the metabolic status of microbes in contrasting soil types. Alongside with the major field and climatic gradient experiment laboratory and greenhouse studies were performed. These addressed the drivers, such as root hairs and protozoan predation, which were identified by isotopic labelling approaches and several indicators of microbial activity. Isotope technologies facilitate the tracing of e.g. C- and N-fluxes to achieve their central pur-pose of understanding the linkages of biota in terrestrial soil food webs. Soil zymography was used to exhibit the spatial distribution of enzyme activity in the rhizosphere in situ. Substrates with contrasting availability changed functional properties of the soil microbial community and induced a shift in enzymatic systems. In particular, the rooted surface layer showed increased microbial activity compared to litter-amended and bare fallow soil. Rhizodeposits are an important primary source of C and energy for soil microorganisms, stimulating their growth and activity. Accounted by the availability of rhizodeposits, hot spots in the surface layer provided preferable habitats for microbes. Most microbial indicators were affected by the substrate input only in the topsoil. Microbial indicators included the catalytic efficiency of enzymes, which decreased by 2- to 20-fold from top- (< 40 cm) to subsoil (> 40 cm), irrespective of the substrate input. This suggests that the limited amount and quality of substrates at depth is an important constraint on microbial activity. The RNA:dsDNA ratios towards the indication of the metabolic status of soil microbial communities was subjected to biased RNA quantity due to high clay contents in Chernozems, whereas at intermediate and low clay contents the RNA reflected reliable results. Protozoa are assumed to be key-players in the C flux from bacteria to higher trophic levels, thereby affecting soil microbial activity. A triple-labelling experiment was conducted to investigate the effects of grazing by Acanthamoeba on C and N fluxes and microbial activity indicators in the rhizosphere and detritusphere. C fluxes and enzyme activities were driven by substrate input and quality and further stimulated by faunal grazing. This revealed that Acanthamoeba grazing contributes to microbial stimulation, especially in the rhizosphere. To assess the influence of root hairs on microbial activity and rhizosphere priming, a continuous 13C labelling experiment was conducted in a greenhouse. Root hairs induced positive priming during tillering. Without root hairs SOM decomposition was sup-pressed. Chitinase and ß-xylosidase activities increased during positive priming, indicat-ing decomposition of stable SOM. This clearly showed the strong influence of root hairs on microbial activity during the early stages of plant growth, whereas at later stages the root hairs were a less important driver of microbial activity. In summary, this thesis extends the understanding of factors affecting microbial activity in soil. It demonstrates that microbial activity can be meaningfully characterized by a careful selection of indicators. The chosen set of indicator is applicable at the landscape scale as well as for process-based investigations at the root scale.