Vegetation changes following forest disturbance affect soil carbon and nitrogen cycles through microbial communities

Increasing forest disturbance is among the most profound impacts of climate change on terrestrial ecosystems. Insect outbreaks, storms, or wildfires can destroy the whole tree layer, with serious consequences for biogeochemical cycles until succession returns the ecosystem back to a forested state. However, tree regeneration is often inhibited by ungulate herbivory and herbaceous competition, and disturbed ecosystems remain in non-forested states for decades. The impact of such vegetation changes on soil carbon (C) and nitrogen (N) cycles is highly unknown, because a multitude of plant-soil feedbacks are involved, and underlying processes have hardly been investigated. Here, we studied soil microbial community structure, gene abundance of bacteria, fungi, and N cycling microorganisms, soil enzymes, and C-N dynamics across a disturbed forest landscape in Central Europe, covering a range of successional stages after storm damage and bark-beetle attacks. We used a chronosequence-approach including disturbed sites regrown with Picea abies stands, and disturbed sites dominated by herbaceous pioneer plants, particularly Calamagrostis grasses. Soil C and N stocks increased under a prolonged herbaceous cover. Three decades after disturbance the stocks were ca. 45% higher than those of regrown forest stands. Beside C inputs from herbaceous fine roots, we link this increase to changes in the structure and functioning of the microbial community, which reduces the decomposition of organic matter. With a prolonged herbaceous cover, decreasing fungal abundances coincided with declining activities of phenol oxidase and of hydrolytic enzymes used to acquire nutrients. Since ectomycorrhizal fungi were almost absent compared to regrowing forest stands, this may be linked to reduced ectomycorrhizal mining for organic N. Moreover, ammonia-oxidising (amoA) gene abundances increased along with ammonium and nitrate concentrations, pointing towards an accelerated inorganic N cycle under a prolonged herbaceous cover. A surplus of inorganic N and grass-rhizodeposits renders it also likely that saprotrophs are less dependent on organic matter-bound C and N. Taken together, we found strong evidence for a linkage between above- and belowground communities following forest disturbance. We suggest a prolonged cover of herbaceous pioneer plants opens the nitrogen cycle through microbial communities which reduces mining for organic N and thus, increases soil C storage.