Soil warming accelerates above-ground litter decomposition and soil organic carbon turnover

The terrestrial soil organic matter (SOM) pool size depends on the balance between SOM formation and stabilization of decomposing plant litter relative to mineralization as CO₂.  Decomposition and mineralization processes are to large extents mediated by microbial decomposer communities. In addition, labile fractions released by decomposing litter can be stabilized in mineral associations (MAOM). High latitude ecosystems are particularly affected by global warming. Increasing temperatures can stimulate litter decomposition and increase nutrient mineralization, thereby increasing nutrient (e.g. nitrogen) availability for plants allowing higher productivity and subsequent plant organic matter inputs. On the other hand, if warming accelerates SOM decomposition stronger than formation processes it can cause large carbon and nutrient losses.

Tracing ¹³C/¹⁵N labelled above-ground litter we aimed to disentangle if soil warming changes the balance between litter derived SOM formation through microbial communities, over particulate organic matter (POM) and MAOM stabilization across a decadal geothermal soil warming gradient (from ambient up to +5 °C) in a grassland in Iceland. In addition, we added three levels of inorganic N to disentangle potential direct warming from indirect (over plant feedbacks) effects of increased warming on SOM dynamics.  We found that over the course of two years warming accelerated litter decomposition rates and litter-derived carbon turnover by the microbial community,  and we could recover more litter-derived carbon recovered in particulate organic matter (POM) nor MAOM. Nitrogen additions triggered a faster decomposition of structural above-ground litter compounds, but did not influence carbon turnover in different soil fractions. On the other, hand we found that overall the absolute amount of soil carbon decreased in response to warming. We therefore conclude that direct warming not only increased SOM formation, but also decomposition and mineralization processes, and – at least in the studied grassland – plant inputs may not have counterbalanced warming induced losses.