Abstract:

With global warming, drought has become a serious issue in the world. Drought can significantly affect the soil carbon accumulation and transformation in forest ecosystems. Forest soil organic carbon (SOC) is the largest carbon pool in terrestrial ecosystems, and its dynamic changes significantly affect the global carbon cycle. Drought stress affects all processes of soil organic carbon dynamics, including the input of organic carbon from the above-ground and belowground litter and the transformation and decomposition of litter, and then causes changes in the soil carbon pool. Here, we reviewed the research progress of the effects of drought on soil organic carbon in forests. The results show that (1) short-term drought increases forest litterfall by promoting leaf shedding in advance, while long-term drought affects forest plant growth, reduces forest primary productivity, and thus decreases plant litterfall. To compensate for water loss, plants in mild and moderate drought increase fine root biomass to maintain plant vitality. In severe drought, plants lose self-repair ability, resulting in reduced fine root biomass and increased fine root mortality. On average, drought decreases forest litter (1.9%)and fine-root biomass(8.7%) worldwide, ultimately reducing the input of plant organic carbon to soil. (2) Drought can reduce the decomposition rate of litter(10% to 70%) by changing its chemical properties and stressing the soil animals and microorganisms responsible for its decomposition. Drought changes the carbon and nitrogen concentration of litters; causes accumulation of secondary metabolites such as cellulose, lignin and tannin; and changes the chemical components of root exudates, thus affecting the decomposition of litter. Moreover, drought results in the decrease of fungal biomass and abundance of soil fauna in decomposers, increases the predation pressure of decomposer animals, and decreases the activities of related microorganisms and enzymes, resulting in the decrease of litter decomposition rate.(3) Drought-driven changes in microbial community composition (the ratio of fungi to bacteria and of Gram-positive to Gram-negative-bacteria increased) result in the decrease of microbial biomass and activity, and drought reduces the feeding and enzyme activities of scavengers, which eventually lead to the decrease of soil organic carbon mineralization rate(10% to 50%).(4) The effects of drought on different components of soil organic carbon are different. Drought produces a smaller and more sensitive soil microbial biomass carbon (MBC) pool(2% to 30%), resulting in the accumulation of dissolved organic carbon (DOC) in surface soil(30%-60%). However, the effects of drought on SOC accumulation vary in different regions of the world. In subtropical forests, the effects of drought on SOC accumulation are mostly negative, while in tropical forests, the effects are positive. In general, drought may have little effect on the forest SOC pool, but reduces soil carbon turnover. Additionally, forest SOC turnover is not affected only by drought; temperature, species, and other factors work together in the turnover of SOC and accumulation. The simple superposition simulation may be single-or multi-factor in the real-world impact on soil carbon flux, which has certain differences. For example, the effect of increasing temperature and drought produce antagonism; the effect of their interaction on soil carbon input is lower than the superposition effect on soil carbon input, and the interaction effect of the two on soil carbon loss is not significant. We propose that it is important to conduct long-term observations, prolonging control experiment through simulating native environments and multi-factor comprehensive influence, to understand the effects of drought on the dynamic process and mechanism of soil organic carbon.

Key words: forest soil, organic carbon input, plant litter, root litter, soil microbes, organic carbon mineralization, soil respiration