Controls on the dynamics of dissolved organic carbon and nitrogen in a Central European deciduous forest

Despite growing attention concerning the role of dissolved organic matter ( DOM) in element cycling of forest ecosystems, the controls of concentration s and fluxes of both dissolved organic carbon (DOC) and nitrogen (DON) unde r field conditions in forest soils remain only poorly understood. The goal of this project is to measure the concentrations and fluxes of DON, NH4+, N O3- and DOC in bulk precipitation, throughfall, forest floor leachates and soil solutions of a deciduous stand in the Steigerwald region (northern Bav aria, Germany). The DOC and DON concentrations and fluxes were highest in l eachates originating from the Oa layer of the forest floor (73 mg C L-1, 2. 3 mg N L-1 and about 200-350 kg C, 8-10 kg N ha(-1) yr(-1)). They were obse rved to be highly variable over time and decreased in the mineral topsoil ( 17 mg C L-1, 0.6 mg N L-1 and about 50-90 kg C, 2.0 to 2.4 kg N ha(-1) yr(- 1)). The annual variability of DOC and DON concentrations and subsequential DOC/DON ratios was substantial in all solutions. The DOC and DON concentra tions in throughfall were positively correlated with temperature. The DOC a nd DON concentrations did not show seasonality in the forest floor and mine ral soil. Concentrations were not related to litterfall dynamics but did co rrespond in part to the input of DOC and DON from throughfall. The throughf all contribution to the overall element fluxes was higher for DON than for DOC. Concentrations and fluxes of DON were significantly correlated to DOC in throughfall and the Oi layer. However, the correlation was weak in Oa le achates. In addition, seasonal and annual variation of DOC/DON ratios indic ated different mechanisms and release rates from the forest floor for both components. The concentrations of DOC and DON in forest floor leachates wer e in most cases dependent neither on the pH value or ionic strength of the solution, nor on the water flux or temperature changes. As a consequence, t he DOC and DON fluxes from the forest floor into the mineral soil were larg ely dependent on the water flux if annual and biweekly time scales are cons idered.