Different factors control organic matter degradation in bulk and rhizosphere soil from the top- and subsoils of three forest stands

The input of substrates and nutrients into deeper soil layers is limited to small regions of the bulk soil through living roots and preferential flow paths which can stimulate a more abundant and diverse microbial community. The aim of this study was to elucidate if SOC mineralization in bulk and rhizosphere soil is differentially limited by nutrients and substrates and to what extent this is site-specific. An 82-day incubation study was conducted with bulk and rhizosphere soil samples from the topsoil (0–10 cm) and subsoil (30–50 cm) from three beech forest sites (Fagus sylvatica L.) in northern Germany. All sites are within the same climatic region and share the same land use history and a similar old tree population but differ in parent material (Pleistocene sand = PS, Triassic sandstone = TS and Loess = LS). To identify the response of the microbial activity to changing nutritional conditions, 14C-substrate and SOC mineralization, microbial activity, enzyme activities from the C-, N-, and P-cycle, dehydrogenase and peroxidase activity were determined after substrate- and nutrient-additions. We found high amounts of labile C in the subsoil at the LS and PS sites and a generally higher SOC and faster substrate mineralization in the rhizosphere samples compared to the respective bulk soil in both, the topsoil (29–296%) and subsoil (73–563%) compared to the respective bulk soil. The addition of glucose caused positive priming effect only in samples from one site (TS), while effects were negative (PS) or not significant (LS) at the other sites. While glucose mineralization was similar in all samples, reaching 50–65% after 82 days, the mineralization of palmitic acid showed a differential pattern across the three sampling sites. In samples from two sites (PS, LS), the mineralization of palmitic acid was delayed by 10–30 days and had no effects on SOC turnover, while it showed no lag phase at the TS site and additionally stimulated SOC turnover. N additions generally and in some cases also P additions decreased SOC mineralization compared to the control samples. The results therefore show that labile and more complex substrates induce SOC mineralization differently between the investigated sample sites, with further differentiation with depth, sampling type and soil type, indicating that microorganisms in bulk and rhizosphere soil undergoes different adaptation processes based on their unique community composition.