Dissolved organic carbon in terrestrial ecosystems: Synthesis and a model

The movement of dissolved organic carbon (DOC) through soils is an importan t process for the transport of carbon within ecosystems and the formation o f soil organic matter. In some cases, DOC fluxes may also contribute to the carbon balance of terrestrial ecosystems; in most ecosystems, they are an important source of energy, carbon, and nutrient transfers from terrestrial to aquatic ecosystems. Despite their importance for terrestrial and aquati c biogeochemistry, these fluxes are rarely represented in conceptual or num erical models of terrestrial biogeochemistry, In part, this is due to the l ack of a comprehensive understanding of the suite of processes that control DOC dynamics in soils. In this article, we synthesize information on the g eochemical and biological factors that control DOC fluxes through soils. We focus on conceptual issues and quantitative evaluations of key process rat es to present a general numerical model of DOC dynamics. We then test the s ensitivity of the model to variation in the controlling parameters to highl ight both the significance of DOC fluxes to terrestrial carbon processes an d the key uncertainties that require additional experiments and data. Simul ation model results indicate the importance of representing both root carbo n inputs and soluble carbon fluxes to predict the quantity and distribution of soil carbon in soil layers. For a test case in a temperate forest, DOC contributed 25% of the total soil profile carbon, whereas roots provided th e remainder. The analysis also shows that physical factors-most notably, so rption dynamics and hydrology-play the dominant role in regulating DOC loss es from terrestrial ecosystems but that interactions between hydrology and microbial-DOC relationships are important in regulating the fluxes of DOC i n the litter and surface soil horizons. The model also indicates that DOC f luxes to deeper soil layers can support a large fraction (up to 30%) of mic robial activity below 40 cm.