Tree species effects on the release of dissolved organic carbon and nitrogen from decomposing logs

Besides carbon dioxide emissions, the leaching of dissolved organic carbon (DOC) and nitrogen (DON) is the most important process contributing to the mass loss of coarse woody debris (CWD comprising all dead woody material with at least >7 cm in diameter). Increased DOC and nitrogen (N) inputs into the forest floor can be expected from CWD. Since the fate of leached DOC depends on its biodegradability, the latter might play a substantial role for carbon sequestration in forest soils. The general goal of this study was the investigation of tree species effects, forest management type and site conditions on the release and quality of dissolved organic C and N from CWD. Within the Biodiversity Exploratories, a long-term and large scale biodiversity project funded by the DFG, logs of 13 different coniferous and deciduous European tree species were exposed to decomposition on the soil under natural environmental conditions in winter 2008/2009. Logs were exposed at three geographically distinct Exploratory sites in Germany (Schorfheide/Brandenburg, Hainich/Thuringia, Swabian Alb/Baden-Württemberg) underneath different forest management types. From June 2011 until November 2012, CWD runoff from 120 logs was collected periodically at the three Exploratories. After filtration (0.45 µm) the amount of DOC, DON, NO3 and NH was measured to calculate C and N budgets. Quality of DOC was measured in runoff and in incubation solutions, namely the content of phenols, hydrolysable carbohydrates, spectroscopic properties as SUVA280nm, HIXem and FTIR. Furthermore initial parameters of wood and bark were determined. In order to quantify DOC biodegradation an incubation experiment lasting 64 days was carried out. As proxy for biodegradation the CO2 production was adapted to a 2-phase first order kinetic model. DOC concentrations in CWD log runoff were 3-10 times higher than in throughfall for all 13 tree species. The highest concentrations and accordingly the largest net release were found for Prunus and Quercus (56 resp. 60 g DOC m-2 yr-1), the lowest for Fraxinus (14.8 g m-2 yr-1). On monthly to annual scale, the amount of precipitation had only minor influence on DOC net release, but a clear seasonality resulted in a higher net release during the growing season. Carbohydrate concentrations in CWD runoff ranged from 2.5-9.4 mg L-1 and phenols from 2.7-9.5 mg L-1. The spectroscopic measurements indicated microbial modification of the leached DOC compared to DOC extracted initially from CWD. The most dominant N form in runoff from CWD was DON and a net release of N was found for all tree species, even though there was a high variation. The net N release was not related to the initial C/N ratio of bark and sapwood. The NH4-N net release was larger in the dormant than in the growing season. For NO3-N the seasonality was opposite to NH4-N. No seasonality was found for DON net release. Throughfall amount as well as temperature had only minor influence on the total release of N from CWD pointing to other key drivers. The linear mixed effect model based on single sampling dates revealed management effects on DOC and DON net release, whereas effects of Exploratory were only observed for DON. However these factors were not identified for the cumulative net release of C and N. The biodegradation of DOC was quite similar for all tree species, ranging from 14 to 29% of the initial DOC in 64 days and no significant differences in the mean residence times (MRT) were found between coniferous and deciduous species. As a conclusion, CWD-derived DOC is tree species specific and causes large C inputs to the soil underneath CWD. Depending on time scales, DOC derived from CWD has the potential to increase soil organic matter (SOM) pools beneath CWD. Furthermore it was shown, that CWD provides a source for solute N, even in the early phase of decomposition. The general goal of this study was the investigation of tree species effects, forest management type and site conditions on the release and quality of dissolved organic C and N from CWD. Within the Biodiversity Exploratories, a long-term and large scale biodiversity project funded by the DFG, logs of 13 different coniferous and deciduous European tree species were exposed to decomposition on the soil under natural environmental conditions in winter 2008/2009. Logs were exposed at three geographically distinct Exploratory sites in Germany (Schorfheide/Brandenburg, Hainich/Thuringia, Swabian Alb/Baden-Württemberg) underneath different forest management types. From June 2011 until November 2012, CWD runoff from 120 logs was collected periodically at the three Exploratories. After filtration (0.45 µm) the amount of DOC, DON, NO3 and NH was measured to calculate C and N budgets. Quality of DOC was measured in runoff and in incubation solutions, namely the content of phenols, hydrolysable carbohydrates, spectroscopic properties as SUVA280nm, HIXem and FTIR. Furthermore initial parameters of wood and bark were determined. In order to quantify DOC biodegradation an incubation experiment lasting 64 days was carried out. As proxy for biodegradation the CO2 production was adapted to a 2-phase first order kinetic model. DOC concentrations in CWD log runoff were 3-10 times higher than in throughfall for all 13 tree species. The highest concentrations and accordingly the largest net release were found for Prunus and Quercus (56 resp. 60 g DOC m-2 yr-1), the lowest for Fraxinus (14.8 g m-2 yr-1). On monthly to annual scale, the amount of precipitation had only minor influence on DOC net release, but a clear seasonality resulted in a higher net release during the growing season. Carbohydrate concentrations in CWD runoff ranged from 2.5-9.4 mg L-1 and phenols from 2.7-9.5 mg L-1. The spectroscopic measurements indicated microbial modification of the leached DOC compared to DOC extracted initially from CWD. The most dominant N form in runoff from CWD was DON and a net release of N was found for all tree species, even though there was a high variation. The net N release was not related to the initial C/N ratio of bark and sapwood. The NH4-N net release was larger in the dormant than in the growing season. For NO3-N the seasonality was opposite to NH4-N. No seasonality was found for DON net release. Throughfall amount as well as temperature had only minor influence on the total release of N from CWD pointing to other key drivers. The linear mixed effect model based on single sampling dates revealed management effects on DOC and DON net release, whereas effects of Exploratory were only observed for DON. However these factors were not identified for the cumulative net release of C and N. The biodegradation of DOC was quite similar for all tree species, ranging from 14 to 29% of the initial DOC in 64 days and no significant differences in the mean residence times (MRT) were found between coniferous and deciduous species. As a conclusion, CWD-derived DOC is tree species specific and causes large C inputs to the soil underneath CWD. Depending on time scales, DOC derived from CWD has the potential to increase soil organic matter (SOM) pools beneath CWD. Furthermore it was shown, that CWD provides a source for solute N, even in the early phase of decomposition.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Bantle, Andreas

Contributors dc:contributor

• Matzner, Egbert

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2149/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2149