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

Dissolved organic carbon (DOC) and nitrogen (DON) represent an important pa rt of the C and N cycles in forest ecosystems. Little is known about the co ntrols on fluxes and concentrations of these compounds in soils under field conditions. Here we compiled published data on concentrations and fluxes o f DOC and DON from 42 case studies in forest ecosystems of the temperate zo ne in order to evaluate controls on a larger temporal and spatial scale. Th e focus was on annual fluxes and concentrations in throughfall, forest floo r leachates and soil solutions. In all compartments considered, concentrati ons and fluxes differed widely between the sites. Highest concentrations of DOC and DON were generally observed in forest floor leachates and in A hor izons. Highest fluxes occurred in forest floor leachates. The fluxes of DOC and DON in forest floor leachates increased with increasing annual precipi tation and were also positively related to DOC and DON fluxes with throughf all. Variation in throughfall fluxes could explain 46% and 65% of the varia tion in DOC and DON fluxes from the forest floor, respectively. No general difference in DOC and DON concentrations and fluxes in forest floor leachat es was found when comparing coniferous and hardwood sites. Concentrations o f 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 calcul ated from fluxes from the forest floor were independent of the amount of an nual precipitation, pointing to a similar response of DOC and DON to precip itation conditions. A decrease in the ratio of DOC to DON with soil depth a s observed on a plot-scale, was not confirmed by data analysis on a large s cale. The controls observed on annual fluxes and concentrations of DON and DOC at regional scale differed from those reported for smaller time and spa ce scales.