Dissolved nitrogen release from coarse woody debris of different tree species in the early phase of decomposition

In forest ecosystems, coarse woody debris (CWD) can represent a large stock of organic matter that contributes to the C and N cycling in forest ecosystems. Here we investigated the net release of dissolved organic N (DON), NH4 and NO3 from CWD of different tree species in the early phase of decomposition. Logs of 13 tree species were exposed in the winter 2008/2009 on the soil in a temperate Fagus sylvatica L forest in Germany. Runoff solutions were periodically collected underneath logs for 17 months from June 2011 to November 2012 in the third to fourth year of decomposition. The net release of N was calculated for each log on an annual scale as difference of N fluxes in runoff and throughfall. Nitrogen was net released from CWD of all tree species with DON as the dominant N form, but leaching of NH4 and NO3 was also observed. Differences in total N release between tree species were not statistically significant due to high intra-specific variation. The net release of N ranged from 0.42 (Pseudotsuga) to 1.39 (Carpinus) g N m(-2) log projection area yr(-1) The variation in net release of N was not related to the initial C/N ratio of bark and sapwood of the different species. The DOC/DON ratios in runoff were positively related to the initial C/N ratio of bark and sapwood. Net release of NH4 was larger in the dormant than in the growing season, opposite to the release of NO3, but no seasonality was found for DON release. Average ratios of NH4/NO3 in runoff from CWD did not significantly differ between species. Precipitation amount and temperature had only minor effect on the total N release, suggesting other dominating drivers. Our results demonstrate that CWD is a source of solute N, even in the early phase of decomposition. (C) 2014 Elsevier B.V. All rights reserved.