Changes in litter chemistry associated with global change-driven forest succession resulted in time-decoupled responses of soil carbon and nitrogen cycles
Highlights: Forest succession from pine to oak results in decoupled soil nutrient dynamics. The different sensitivities of microbial functional groups trigger this decoupling. The vegetation shift produces changes in microbial community structure and ecology. Successional changes led to a significant loss of the soil carbon sink capacity. There is a substantial gain in available mineral nitrogen in oak soils. Global change-driven forest succession may modify key soil processes with potentially important impacts over carbon (C) and nutrient cycling. We studied how changes in litter throughout the replacement of Pinus sylvestris by Quercus pyrenaica influence the structure and functioning of soil microbial communities and the capacity of soils to sequester C and retain nitrogen (N). We designed a microcosm experiment to simulate the chronological sequence from pine to oak forest conversion in Central Spain, using mixtures of senescent litter (oak leaves, pine needles and an equal mixture of needles:leaves) and soils (from pure oak, mixed and pure pine stands). We investigated changing patterns of soil C and N contents, microbial community structure (PLFA) and greenhouse gas fluxes (CO2, CH4, N2O) across the chronosequence. The succession from pine to oak forest was associated with substantial changes in microbial community structure and functioning. Soil-C sink capacity was reduced, although soil-N availability was enhanced. We further show how effects of secondary succession on the C cycle were mismatched with N dynamics in response to two chronologically decoupled facts. First, there was an acceleration in soil organic matter (SOM) turnover after microbial –especially bacterial– growth ceased to be so intensely inhibited by needle litter (ecotone soils), resulting in lower fungal to bacterial ratios; Keywords: Greenhouse gas fluxes ; Carbon sequestration ; Carbon-use efficiency ; Nitrogen availability ; Microbial community structure ; Chronological sequence
