HR: 0800h
AN: B11A-1018    [Abstracts]
TI: Variable Contribution of Soil and Plant Derived Carbon to Dissolved Organic Matter
AU: * Steinbeiss, S
EM: sstein@bgc-jena.mpg.de
AF: Max Planck Institute for Biogeochemistry, Hans-Knoell-Str.10, Jena, 07745 Germany
AU: Gleixner, G
EM: ggleix@bgc-jena.mpg.de
AF: Max Planck Institute for Biogeochemistry, Hans-Knoell-Str.10, Jena, 07745 Germany
AB: The seasonal variation in the amount and sources of dissolved organic matter (DOM) in soil profiles was investigated. In general DOM in soil solution can evolve from the decomposition and mobilization of soil organic matter (SOM), dissolution of dead microbial cells or from the input of plant material such as root exudates or decomposing litter. Here we used vegetation change from C3 to C4 plants to quantify the plant derived carbon in DOM. In 2002 an agricultural field was converted to an experimental grass land. The average carbon isotope value of SOM was -26.5 per mill (sd = 0.2) for the plough horizon. On two independent plots, each 10 x 20 m, we used Amaranthus retroflexus as C4 plant with a carbon isotope label of 13.0 per mill to distinguish unlabeled SOM and plant derived carbon sources. To quantify the contribution of litter input on DOM formation we applied a split plot design. One half had no litter and the other half double amount of above ground litter. Soil water was collected in 10, 20 and 30 cm depth biweekly and DOM concentrations in solution and carbon isotope ratios of the freeze dried and decarbonized material were investigated. During winter uniform concentrations of DOM of about 7 mg/l were measured throughout all depth and treatments. In spring when soil temperatures increase and water availability decreases DOM concentrations increased with similar rates in all depth. Even in the second year of Amaranth growth the carbon isotope ratios of DOM in winter and spring had no C4 signal. The carbon isotope ratios of -26 to -27 per mill suggest SOM as carbon source and contradict a contribution of root exudates to the DOM pool. During summer almost no soil solution was collected. After rewetting in fall DOM concentrations up to 50 mg/l in 10 cm depth and up to 35 mg/l in deeper layers were found. These high concentrations held carbon isotope signals from -25 to -26.5 per mill contradicting carbon input from plant material. With ongoing wetting of the soil the carbon isotope ratios suddenly increased up to -21.7 per mill on the double litter plots and to -24 per mill on no litter plots. However, this signal was not detected in 30 cm depth. Keeling plots proved that the major part of the DOM comes from SOM. In fall and early winter only 36 % and 19 % of plant derived carbon were found in the double litter and no litter plots, respectively. Our results suggest that carbon of the SOM pool is the major source for carbon in DOM. In the spring season root exudates seem to be completely respired by soil organisms suggesting that root and rhizosphere respiration are the same respiratory pool. Only in fall the decomposition of plant litter contributed to carbon in DOM. However, this carbon source is already exhausted in the next spring. In consequence our results may indicate that stored soil carbon is more active than thought and that DOM transport might be a key process to understand carbon sequestration.
DE: 0400 BIOGEOSCIENCES
DE: 0410 Biodiversity
DE: 0428 Carbon cycling (4806)
DE: 0438 Diel, seasonal, and annual cycles (4227)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
SC: Biogeosciences [B]
MN: Fall Meeting 2005