Spatial heterogeneity of soil carbon exchanges and their drivers in a boreal forest

Boreal forests have a large impact on the global greenhouse gas balance and their soils constitute an important carbon (C) reservoir. Mature boreal forests are typically a net CO₂ sink, but there are also examples of boreal forests that are persistent CO₂ sources. The reasons remain often unknown, presumably due to a lack of understanding of how biotic and abiotic drivers interact to determine the microbial respiration of soil organic matter (SOM). This study aimed at identifying the main drivers of microbial SOM respiration and CO₂ and CH₄ soil chamber-fluxes within dry and wet sampling areas at the mature boreal forest of Norunda, Sweden, a persistent net CO₂ source. The spatial heterogeneity of the drivers was assessed with a geostatistical approach combined with stepwise multiple regression. We found that heterotrophic soil respiration increased with SOM content and nitrogen (N) availability, while the SOM reactivity, i.e., SOM specific respiration, was determined by soil moisture and N availability. The latter suggests that microbial activity was N rather than C limited and that microbial N mining might be driving old-SOM decomposition, which was observed through a positive correlation between soil respiration and its δ¹³C values. SOM specific heterotrophic respiration was lower in wet than in dry areas, while no such dependencies were found for chamber-based soil CO₂ fluxes, implying that oxygen depletion resulted in lower SOM reactivity. The chamber-based soil CH₄ flux differed significantly between the wet and dry areas. In the wet area, we observed net CH₄ emission that was positively related to soil moisture and NH₄⁺-N content. Taken together, our findings suggest that N availability has a strong regulatory effect on soil CO₂ and CH₄ emissions at Norunda, and that microbial decomposition of old-SOM to release bioavailable N might be partly responsible for the net CO₂ emission at the site.