The response of dissolved organic carbon (DOC) and the ecosystem carbon balance to experimental drought in a temperate shrubland

Climate change has been proposed as a driver of carbon (C) loss from the large pool of C held in soils. Aqueous (dissolved organic carbon, DOC) and gaseous (soil respiration or net ecosystem CO(2) exchange) forms of C loss from soils have been considered. Under some climate change scenarios, gaseous emissions of C have been predicted to result in terrestrial ecosystems becoming a net source of C by 2050. Indeed, both forms of C loss have been linked to climate-related changes, such as warming and/or changes in precipitation. In our field-based drought manipulation experiment on an upland moorland in northeast Wales, we have carried out an annual drought treatment for 8 years, reducing levels of annual rainfall by 23% on average (1999-2007) through the use of automated roofs, which prevent rain falling on experimental plots between June and September annually. Following 5 years of repeated summer drought, there was a 26% increase in concentrations of DOC in soil water in the mineral soil in the drought plots and this further increased to 52% after 8 years. A similar pattern was not observed in the organic soil horizons. Despite higher DOC concentrations in the mineral soil horizon, decreased drainage of water from the drought-treated soils resulted in an overall decrease of 9% in total DOC export. Calculating the carbon (C) balance for the below-ground component of the ecosystem reveals that DOC represents 3% of gross C export. Previous studies at the site have demonstrated large increases in soil respiration resulting from the repeated drought treatment. By including data presented here with other C fluxes and pool measurements from the site, we demonstrate that soil carbon is accumulating by 126 g C m(-2) year(-1) in the control plots, but decreasing by 18 g C m(-2) year(-1) in the drought plots. The repeated drought treatment has thus resulted in the ecosystem switching from a net sink for C into a net source.