Fluxes and concentrations of dissolved organic carbon and nitrogen - a synthesis for temperate forests

Dissolved organic carbon (DOC) and nitrogen (DON) represent an important part of the C and N cycles in forest ecosystems. Little is known about the controls on fluxes and concentrations of these compounds in soils under field conditions. Here we compiled published data on concentrations and fluxes of DOC and DON from 42 case studies in forest ecosystems of the temperate zone in order to evaluate controls on a larger temporal and spatial scale. The focus was on annual fluxes and concentrations in throughfall, forest floor leachates and soil solutions. In all compartments considered, concentrations and fluxes differed widely between the sites. Highest concentrations of DOC and DON were generally observed in forest floor leachates and in A horizons. Highest fluxes occurred in forest floor leachates. The fluxes of DOC and DON in forest floor leachates increased with increasing annual precipitation and were also positively related to DOC and DON fluxes with throughfall. Variation in throughfall fluxes could explain 46% and 65% of the variation in DOC and DON fluxes from the forest floor, respectively. No general difference in DOC and DON concentrations and fluxes in forest floor leachates was found when comparing coniferous and hardwood sites. Concentrations of DOC in forest floor leachates were positively correlated to the pH of the forest floor. Furthermore, there was no relationship between organic C and N stocks, soil C/N, litterfall or mineral N inputs and concentrations and fluxes of DOC and DON in forest floor leachates. Including all compartments, fluxes of DOC and DON were highly correlated. Ratios of DOC to DON calculated from fluxes from the forest floor were independent of the amount of annual precipitation, pointing to a similar response of DOC and DON to precipitation conditions. A decrease in the ratio of DOC to DON with soil depth as observed on a plot-scale, was not confirmed by data analysis on a large scale. The controls observed on annual fluxes and concentrations of DON and DOC at regional scale differed from those reported for smaller time and space scales.