Forest conversion to conifers induces a regime shift in soil process domain affecting carbon stability

Abstract

A substantial part of forests worldwide is located on acidic soils. Acidification processes are typically characterized by non-linear responses of soils to external drivers. Acid buffer ranges and thresholds in soils are widely acknowledged, yet these non-linearities are rarely incorporated into our understanding of soil carbon dynamics. Here, we studied the effect of conversion of broadleaved mixed forest to Norway spruce monocultures on different functional compartments of the belowground carbon cycle, i.e. litter layers, soil fauna and soil micro-organisms, and examined how in turn they affect soil biochemical characteristics and ultimately, soil carbon stability. By studying this effect chain along a soil buffering gradient, we were able to evaluate the relative significance of forest management versus edaphic constraints on soil carbon processing. The effects of conversion are extensive and change trajectories are larger for forests that shifted from one buffering domain to another upon conversion. In the latter, conversion leads to a larger buildup of the litter layer, significantly stronger acidification, and a decrease in microbial functional diversity and earthworm biomass in the mineral soil, mirrored by a collapse of the base saturation. Although topsoil total carbon concentrations increased under spruce, this soil carbon is stored in less stable carbon pools compared to broadleaved forests. These are carbon pools where carbon is not occluded in aggregates or bound to soil minerals and therefore more vulnerable to environmental factors. Our findings show that changes in forest composition can, depending on the initial distance from a threshold in acid buffering, cascade through different compartments of the soil carbon cycle and eventually alter the way carbon is stored.