Future Forests - Chapter 5: Mitigation potential of forests: challenges to carbon accrual in the ecosystem

Growing forests has been proposed as a cornerstone of natural climate solutions, and some national scale policies facilitating this have been in place for decades. However, not all carbon that is assimilated remains sequestered in biological pools for equal length of time. Carbon retention in ecosystems depends on the balance of inputs, outputs, and turnover, which in turn depends on the chemical composition of the biomass produced, its lifetime as part of a living tissue, its exposure to different decomposer communities and extracellular enzymes upon death, the biological and physicochemical environment, and disturbances. That is, carbon retention in the long term is an ecosystem property, the result of the interacting properties of plants, microbes, and the environment, which all affect the different steps of carbon assimilation, retention, and decomposition. Attempts to increase C sequestration in forests must consider not only productivity, but also decomposition and stabilization. Recent evidence suggests that carbon is retained in the soil when the decomposer community (free-living saprotrophs) is resource-limited and unable to produce the necessary enzymes. This limitation is imposed by vigorous mycorrhizal community that thrives under overall nutrient limitation. High nutrient inputs typical for production-oriented forests reduce that competition and promote the production of carbon mining enzymes by saprotrophs. Furthermore, plant biomass produced in nutrient-rich conditions tends to be lower in secondary compounds and therefore easier to decompose. Thus, the production-oriented cultivation practices appear contradictory to what is required for high carbon retention and long-term sequestration. Here we review the latest literature about plant-microbe and microbe-microbe interactions in coordinating the interlinked carbon, nutrient, and water cycles, and we estimate how these may respond to the key global change factors of CO2, temperature, and precipitation.