Relevance of substrate quality and nutrients for microbial C-turnover in top- and subsoil of a Dystric Cambisol

Abstract

The generally high apparent 14C age of soil organic carbon (SOC) in subsoils suggests a high stability against microbial degradation. However, the SOC decomposition in subsoils may also be limited by easily available substrates and nutrients, such as N and P. In topsoils, priming effects in response to substrate additions are largely determined by substrate quality which also differently affects the microbial community, while the knowledge for deep soil layers is very scarce. In order to gain further insight into processes controlling SOC decomposition in subsoils, we conducted a laboratory incubation experiment for 105days to investigate the impact of different substrates and mineral N and P on SOC mineralization in a Dystric Cambisol sampled at 2–12, 35–65 and 135–165cm. We studied the impacts of 14C-labeled citric, vanillic and palmitic acid and of N or P alone and in combination with two of the substrates on changes in SOC mineralization. The choice of substrates was based on their nominal oxidation state of carbon, which reflects the energy yield and the biogeochemical reactivity of a compound. Further, the impact of the treatments on the activity of six extracellular enzymes involved in C-, N-, P- and S-acquisition, of peroxidase and phenoloxidase and of the intracellular dehydrogenase was investigated to clarify if substrate qualities and nutrients differently affect the decomposition potential and nutrient demand of the microbial community. Our results show that microbial metabolism is limited by N in both subsoil layers, although the upper subsoil (35–65cm) may have become P limited during the later stage of incubation. Even after 105days, C-cycling enzymes and dehydrogenase activity were highly elevated in the lower subsoil (135–165cm) in response to N additions which indicates a sustained higher decomposition potential and activity of the microbial community once the N-limitation is overcome. In the upper subsoil (35–65cm), we found high amounts of labile C, indicating a high proportion of fast cycling SOC. Furthermore, all added substrates induced negative priming effects, while positive priming effects were only induced with N addition which was related to N limiting metabolism. The suppressed SOC mineralization likely occurred because the present microbial community was adapted to more labile carbon compounds, resulting in preferential substrate utilization. In the lower subsoil (135–165cm), real positive priming was induced by vanillic acid and palmitic acid, while citric acid had no effect on SOC mineralization. This clearly reflects that substrate quality matters for inducing positive priming in this subsoil. Altogether, this study evidently shows that in consequence of altered substrate and N input deep SOC storage is destabilized in forest soils.