Dissolved and particulate carbon fluxes in forest ecosystems

Abstract

Forest provides the most significant ecosystem services among the terrestrial ecosystems, and plays an important role in sequestering carbon (C) and mitigating the ongoing global climate change. Dissolved carbon (DC) and particulate carbon (PC) fluxes are important components in forest C budgets. Quantifying fluxes of DC and PC in forest ecosystems is essential to better understanding the C cycling processes and improving the assessment and prediction of C sequestration capacity in the ecosystems. However, the budgets of DC and PC have often been ignored in forest C cycling studies largely due to knowledge shortage and difficulty in taking measurements. Here, we reviewed the recent progress in DC and PC studies in forest ecosystems. We discussed the composition and function of DC and PC, associated ecological processes and influencing factors, and finally pointed out future research priorities in this field. The DC and PC in a forest ecosystem mainly consist of dissolved organic C (DOC), dissolved inorganic C (DIC), particulate organic C (POC), black C and carbonate C, which fundamentally originate from net primary production of the ecosystem. DOC, an important active pool of C and nutrients, is mainly composed of carbohydrates, long-chain aliphatic compounds, aromatic compounds, proteins, and low molecular weight organic acids. The DOC content is correlated positively with concentrations of proteins and carbohydrates, but negatively with concentrations of carboxyl C. The DOC in soil solution is mainly derived from plant litter, soil humus, microbial debris, root exudates, and ectomycorrhizal mycelium. DIC consists of dissolved inorganic carbonate, dissolved bicarbonate, dissolved carbonate and dissolved carbon dioxide. It activates soil nutrient elements and thus enhances plant growth. POC is the major form of PC in forest soils that includes polymeric gel, organic fiber, cell fragments, microbial biomass (such as algae and bacteria), and polymerized DOC. Both DC and PC are labile C pools in the forest soil system, which enter the ecosystem from atmospheric deposition, throughfall and litterfall, and leave the ecosystem via soil respiration, lateral transport and seepage. At a small local scale, fluxes of DC and PC are mainly influenced by such biotic factors as root exudates, fine root decomposition and microbial turnover. At a large regional scale, they are jointly affected by soil and vegetation characteristics, coupling effects of ecological processes, climatic conditions and global changes. Future research priorities in this field should be: (1) to explore the factors controlling fluxes of DC and PC in forest ecosystems on various spatio-temporal scales, their potential coupling relationships and underlying mechanisms; (2) to understand interactions and transformation between DC/PC and other C fluxes in forest ecosystems, and explore potential stoichiometric relationships between DC/PC and other nutrients in the systems and (3) to explore effects of global changes on fluxes of DC and PC in forest ecosystems, particularly focusing on the effects of extreme disturbances, such as forest management, wildfire, drought, flooding, ice and frozen, and freeze-thaw cycles.