The carbon we do not see—the impact of low molecular weight compounds on carbon dynamics and respiration in forest soils: a review

Dissolved organic matter (DOM), typically quantified as dissolved organic carbon (DOC), has been hypothesized to play many roles in pedogenesis and soil biogeochemical cycles, however, most research to date concerning forest soils has focussed on the high molecular weight (HMW) components of this DOM. This review aims to assess the role of low molecular weight (LMW) DOM compounds in the C dynamics of temperate and boreal forest soils focussing in particular on organic acids, amino acids and sugars. The current knowledge of concentrations, mineralization kinetics and production rates and sources in soil are summarised. We conclude that although these LMW compounds are typically maintained at very low concentrations in the soil solution (<50 μM), the flux through this pool is extremely rapid (mean residence time 1–10 h) due to continued microbial removal. Due to this rapid flux through the soil solution pool and mineralization to CO2, we calculate that the turnover of these LMW compounds may contribute substantially to the total CO2 efflux from the soil. Moreover, the production rates of these soluble transitory compounds could exceed HMW DOM production. The possible impact of climate change on the behaviour of LMW compounds in soil is also discussed.