Amount and quality of dissolved organic carbon from coarse woody debris of different tree species in the initial phase of decomposition
P 3.4 in Research Yields: Ideas Pursued to the End
The release of dissolved organic carbon (DOC) from decomposing coarse woody debris (CWD) contributes to its mass loss and results in large DOC inputs to the forest soil underneath CWD The project is guided by the following hypotheses: Quality and amount of net DOC release is:
(i) tree species specific, (ii) increasing with precipitation and (iii) depends on forest management type.
Our study is part of the BeLongDead-initiative within the DFG Biodiversity Exploratories. Since 2009 logs of 13 different tree species were exposed to decomposition at sites (Hainich, Schorfheide and Schwäbische Alb) under natural environmental conditions determining effects of tree species, forest management system and precipitation on the release of DOC derived from CWD. We collect runoff samples from the logs since June 2011 in intervals of 2 - 6 weeks, depending on precipitation. At the Schorfheide and Schwäbische Alb and Hainich we sampled beech, oak and spruce logs under different management systems and additional at the Hainich Exploratory all 13 tree species at 3 mixed plenter Fagus forest sites. The management systems were differentiated in “age class Fagus”, “age class conifers” (Pinus and Picea) and “unmanaged Fagus” forest with 3 plots each. After filtration (0.45µm) we determined the concentration of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), nitrate and ammonia, pH and the electrical conductivity. The net release of DOC was calculated by subtracting the throughfall fluxes from the runoff fluxes at each log. For 4 sampling dates the quality of DOC was identified measuring the content of hydrolysable carbohydrates and phenols in runoff. Furthermore spectroscopic properties of bark, sapwood extracts and runoff were determined and related to DOC net release.
The concentrations of DOC in runoff from logs ranged from 5-548 mg L-1 and generally exceed those in throughfall by far. Highest DOC net release was found underneath Quercus and Prunus logs and lowest under Fraxinus trees ranging from 20 - 80 g m-2 projected log area cumulative for the sampling period (June 2011 - November 2012). On a monthly to annual scale, the amount of precipitation during the sampling period did not correlate with the concentration of DOC in log runoff. Tree species also influenced the quality of DOC: A humification index calculated from the emission spectra at 280nm ranged from 5 for Pinus up to 9.3 in Quercus log runoff. Concentration of hydrolysable carbohydrates varied from 2.4 mg L-1 for Fraxinus up to 9.4 mg L-1 for Prunus. Our data show a rather low mass loss of the logs by DOC (0.02 - 0.06 % per year) related to the initial bulk mass for the initial phase of decomposition.
Based on our results we conclude that tree species and management has a strong influence on DOC dynamics of decomposing logs. Surprisingly the net DOC release did not correlate with the precipitation amount as hypothesized. Management effects we found for beech logs being exposed to the “age class Fagus” forest management type. For the DOC net release data seasonal pattern were visible with maxima during summer months and minima in winter. The seasonality was most pronounced for Quercus log runoff.