Why not all productivity leads to carbon sequestration: the role of plant carbon surplus, allocation, and the Gadgil effect

Abstract Increasing plant productivity is considered as the way to maximize carbon sequestration potential of forests. However, the changes in the vegetation and soil carbon stores often exhibit a puzzling divergence. Greater productivity is often associated with smaller rather than larger soil carbon pool. In the present report, I review plant and microbial processes that elucidate this phenomenon. Different growth-limiting factors affect carbon allocation (the relative sink strength of plants, symbiotic fungi, and free-living saprotrophs), biomass chemical composition, its carbon to nitrogen stoichiometry, and decomposition differently. The decomposition appears coordinated through detritus chemistry, asymmetric competition for carbohydrates between mycorrhizal and free-living saprotrophic fungi, and suppression of some extracellular enzymes by soil acidification under nitrogen addition (that increases sink strength) but not under elevated carbon dioxide (that increases carbon supply). The net effect of these interactions is a positive correlation between plant productivity and soil carbon accumulation in some environments and a negative one in others.