Showing papers by "Karsten Kalbitz published in 2005"

Stabilization of dissolved organic matter by sorption to the mineral soil

Abstract: The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We quantified mineralized C by frequent CO2 measurements in the headspace of the incubation vessels and calculated mean residence times by a double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence) and isotope (13C, 14C) measurements after hot-water extraction of OM. The fraction of sorbed organic C mineralized during the incubation was only one-third to one-sixth of that mineralized in solution. The mean residence time of the most stable OM sample was estimated to increase from 28 years in solution to 91 years after sorption. For highly degradable DOM samples, the portion of stable C calculated by a double exponential model nearly doubled upon sorption. With less degradable DOM the stability increased by only 20% after sorption. Therefore, the increase in stability due to sorption is large for labile DOM high in carbohydrates and relatively small for stable DOM high in aromatic and complex molecules. Nevertheless, in terms of stability the rank order of OM types after sorption was the same as in solution. Furthermore, the extent of sorption of recalcitrant compounds was much larger than sorption of labile compounds. Thus, sorptive stabilization of this stable DOM sample was four times larger than for the labile ones. We conclude that stabilization of OM by sorption depends on the intrinsic stability of organic compounds sorbed. We propose that the main stabilization processes are selective sorption of intrinsically stable compounds and strong chemical bonds to the mineral soil and/or a physical inaccessibility of OM to microorganisms. The UV, fluorescence and 13C measurements indicated that aromatic and complex compounds, probably derived from lignin, were preferentially stabilized by sorption of DOM. The 13C and 14C data showed that degradation of the indigenous OM in the mineral soil decreased after sorption of DOM. We estimated DOM sorption stabilizes about 24 Mg C ha−1 highlighting the importance of sorption for accumulation and preservation of OM in soil.

Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Abstract: Litter is one of the main sources of dissolved organic carbon (DOC) in forest soils and litter decomposition is an important control of carbon storage and DOC dynamics. The aim of our study was to evaluate (i) effects of tree species on DOC production and (ii) relationships between litter decomposition and the amount and quality of DOC. Five different types of leaves and needles were exposed in litterbags at two neighboring forest sites. Within 12 months we sampled the litterbags five times and leached aliquots of field moist litter in the laboratory. In the collected litter percolates we measured DOC concentrations and recorded UV and fluorescence spectra in order to estimate the aromaticity and complexity of the organic molecules. Furthermore, we investigated the biodegradability of DOC from fresh and decomposed litter during 6 weeks incubations. Fresh sycamore maple litter released the largest amounts of DOC reaching about 6.2% of litter C after applying precipitation of 94 mm. We leached 3.9, 1.6, 1.0 and 3.3% carbon from fresh mountain ash, beech, spruce and pine litter, respectively. In the initial phase of litter decomposition significantly decreasing DOC amounts were released with increasing litter mass loss. However, after mass loss exceeds 20% DOC production from needle litter tended to increase. UV and fluorescence spectra of percolates from pine and spruce litter indicated an increasing degree of aromaticity and complexity with increasing mass loss as often described for decomposing litter. However, for deciduous litter the relationship was less obvious. We assume that during litter decomposition the source of produced DOC in coniferous litter tended toward a larger contribution from lignin-derived compounds. Biodegradability of DOC from fresh litter was very high, ranging from 30 to 95% mineralized C. DOC from degraded litter was on average 34% less mineralizable than DOC from fresh litter. Taking into account the large DOC production from decomposed needles we can assume there is an important role for DOC in the accumulation of organic matter in soils during litter decomposition particularly in coniferous forests.

Short‐term response on the quantity and quality of rhizo‐deposited carbon from Norway spruce exposed to low and high N inputs

Abstract: The amount and composition of rhizo-deposition as an important component of dissolved organic carbon (DOC) in forest soils is largely unknown. The objective of our study was to examine one component of DOC production in forest soils, namely rhizo-deposition, under conditions of low, moderate, and high inorganic-nitrogen (N) input. Two-year-old Picea abies saplings were incubated in hydroponic solution for a period of 2 weeks, comprising an adaptation and an experimental period of each 1 week. Treatments included 0 (zero N), 1, 10 (moderate N), and 100 (high N) mg N L–1 nitrogen addition split equally between nitrate and ammonium. Quantity and quality of dissolved-organic-carbon (DOC) production were investigated. Cumulative DOC production ranged between 3.7 and 8.8 mg C (g root dry weight)–1 (7 d)–1 and was significantly reduced in zero- and high-N treatments. In turn, the specific UV absorbance as a measure of the aromaticity increased significantly in zero- and high-N treatments, indicating a reduced contribution of microbial products to rhizo-deposited C. Mineralization of rhizo-deposited DOC was not significantly different between N treatments at day 1 of the experimental period, but by day 7, 69% of rhizo-deposition in zero-N hydroponic solution was mineralizable compared to 37%–49% for the moderate- and high-N treatments. Increasing N input tended to result in decreased degradability. We conclude from this short-term experiment that rhizo-deposited DOC from Norway spruce displays a fast response to changes in inorganic N. Kurzfristige Wirkungen niedriger und hoher Stickstoffgaben auf Menge und Eigenschaften der C-Rhizodeposition von Fichte Die Menge und Zusammensetzung der Rhizodeposition als einem wichtigen Bestandteil der gelosten organischen Substanz (DOC) in Boden ist noch weitgehend unbekannt. In dieser Arbeit widmeten wir uns diesem Prozess der DOC-Produktion und untersuchten die Rhizodeposition bei niedrigen, mittleren und hohen anorganischen Stickstoff(N)-Gaben. Wir inkubierten zwei Jahre alte Fichten-Schosslinge in Hydrokultur uber einen Zeitraum von 2 Wochen. Dabei wurde eine Woche fur die Adaption der Schosslinge genutzt. In den Losungen wurden durch Zugabe von Ammoniumnitrat folgende N-Konzentrationen eingestellt: 0 mg L–1, 1 mg L–1, 10 mg L–1, 100 mg L–1. Menge und Eigenschaften des gelosten organischen Kohlenstoffs (DOC) in den Losungen wurden quantifiziert. Die kumulative DOC-Produktion lag zwischen 3.7 and 8.8 mg C pro g Wurzeltrockenmasse in 7 Tagen und war signifikant niedriger in den Varianten mit 0 und mit 100 mg L–1 N. In diesen Varianten stieg die spezifische UV-Absorption als ein Mas fur die Aromatizitat des DOC gegenuber den anderen Varianten mit mittlerer N-Konzentration (1 und 10 mg L–1) signifikant an. Dies deutet auf einen verringerten Anteil mikrobieller Produkte am DOC hin. Die Mineralisierung des DOC aus Rhizodeposition unterschied sich am ersten Tag der Experimentierperiode nicht. Allerdings wurden am 7. Tag der Experimentierperiode 69 % des DOC in der Variante ohne Stickstoff mineralisiert, wahrend in den anderen Varianten (mittlere und hohe N-Konzentration) 37 %–49 % des DOC mineralisiert wurden. Dies lasst vermuten, dass ansteigender N-Eintrag zu einer Verminderung der Abbaubarkeit des DOC fuhrt. Wir schlussfolgern aus unserem Versuch, dass DOC aus Rhizodeposition sehr schnell auf Veranderungen im Angebot an mineralischem Stickstoff reagiert.