Release and Properties of Dissolved Organic Matter Related to Soil Organic Matter Quality

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1 Release and Properties of Dissolved Organic Matter Related to Soil Organic Matter Quality Kerstin Michel 1, Egbert Matzner 2, Marie-France Dignac 3, Ingrid Kögel-Knabner 4 1 Department of Environmental Chemistry, University of Kassel, Nordbahnhofstr. 1a, D Witzenhausen, Germany, kmichel@uni-kassel.de 2 Department of Soil Ecology, Bayreuth Institute for Terrestrial Ecosystem Research (BITÖK), University of Bayreuth, Dr. Hans-Frisch-Str. 1-3, D Bayreuth, Germany, egbert.matzner@bitoek.uni-bayreuth.de 3 UPMC, Laboratoire de Biogéochimie Isotopique, FR Paris, France 4 Wissenschaftszentrum Weihenstephan, Technische Universität München, D Freising- Weihenstephan, Germany, koegel@wzw.tum.de As a consequence of chronically high N depositions in forest ecosystems, the C-to-N ratio of forest floors has narrowed in many forest ecosystems. This might affect the sequestration of soil C, the partitioning of C during decomposition and the properties of dissolved organic matter (DOM). We investigated samples from Oa layers of 15 different forest floors under Norway spruce (Picea abies Karst. L.) with varying C-to-N ratios in respect to soil respiration and release of dissolved organic carbon (DOC) under standardized laboratory conditions. Samples were incubated aerobically at 15 C and water holding capacity over a period of 10 months. Aliquots of the Oa material samples were characterized by solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analysis of lignin. The obtained data were related to DOC release and respiration rates of bulk samples. Composition of DOM was characterized using different spectroscopic methods: 13 C NMR spectra of freezedried percolates were recorded for six selected Oa samples, UV absorption at 280 nm to estimate aromaticity and two humification indices derived from fluorescence spectra indicating the degree of humification of DOM were determined for DOM leached from the 15 Oa material samples. These parameters were related to the chemical composition of soil organic matter (SOM), to DOC release and to respiration rates of bulk samples. Soil respiration and DOC release decreased significantly with decreasing C-to-N ratio. The chemical composition of SOM determined by 13 C NMR could not be related to the C-to-N ratio, DOC release or the respiration rates. Composition of DOM as revealed by specific UV absorption and fluorescence spectra indicated that DOM leached from SOM with wide C-to-N ratio is more aromatic and complex than DOM from Oa material characterized by low C-to-N ratio. Our results show that composition of DOM is related to SOM properties. Furthermore the hypothesis is supported that low C-to-N ratios in later stages of decomposition stabilize SOM. 1

2 1. Introduction In many central European forest ecosystems, chronically high N inputs from atmospheric deposition have caused increased N contents in needles and leaves (BML, 2001) as well as in forest floors as indicated by decreasing C-to-N ratios (McNulty et al., 1991; Wolff and Riek, 1998). The C originating from plant litter might take different pathways during decomposition in the forest floor and be either incorporated in stabilized (remaining) soil organic matter (SOM), released as CO 2 or transported to the mineral soil as dissolved organic C (DOC). The increased N contents may influence the C pathways during decomposition. Due to increasing anthropogenic N emissions, deposition has become the most important N source in many forest ecosystems. In central Europe the annual inputs range from 15 to 50 kg ha -1 (Berg and Matzner, 1997). Nitrogen is a key element for microbial activity, which is considered to be a major source of DOC (Guggenberger, 1992; Qualls and Haines, 1992). Therefore, it is often hypothesized that release of DOC is influenced by changes in N availability (Aber, 1992; Guggenberger, 1992; Gundersen et al., 1998). On the one hand, it is suggested that higher N inputs to forest ecosystems may increase DOC production by repression of lignin-degrading white-rot fungi by mineral N leading to an increased release of water-soluble soft-rot products (Guggenberger, 1992) or by stimulating microbial activity and hence SOM decomposition (Guggenberger, 1992; Zech et al., 1994). On the other hand, DOC is thought to be a C source for microbial growth. As a consequence, a decrease in DOC release due to an increased C demand for N immobilization by microorganisms was hypothesized (Aber, 1992; Gundersen et al., 1998). Instead of altered DOM production rates, high N inputs to forest ecosystems may lead to changes in the structure and composition of DOM by influencing the humification process. The acceleration or retardation of the humification process would be expected to alter the DOM interaction with ionic constituents in solution and with mineral surfaces. Recent studies, using spectroscopic methods such as UV and fluorescence spectroscopy to characterize DOM properties, indicate that N inputs may alter the aromaticity of DOM and the complexity, i.e. the degree of condensation of the molecules (Park et al., 2002; Hagedorn et al., 2002). The objective of this work was to study the effects of internal N in SOM on the C dynamics in late stages of decomposition by investigating forest floor samples from Oa layers with different C-to-N ratios. The following hypotheses were tested: (1) with increasing N content the rates of C mineralization decrease; (2) increasing N contents and the subsequent decreased decomposition rates result in an increasing release of DOC. Furthermore we hypothesized that 2

3 (3) differences in the composition, especially C-to-N ratio, of SOM are reflected in the properties of released DOM. 2. Materials and methods 2.1 Sampling sites Samples of Oa layers were taken in August and September 1999 from 15 different Norway spruce (Picea abies Karst. L.) sites. Eight of these sites were located in southern and eastern Bavaria, Germany, and one (Spanbeck) in Lower Saxony, Germany. Samples were also taken from the control (V1) and the N fertilized (V3) plot in the Villingen catchment as well as from the control plot in the Schluchsee catchment both located in the German Black Forest area. Additional samples were obtained from the Klosterhede site in Denmark, from the Birkenes catchment in southern Norway and from Nordmoen, located in central Norway. Sites were selected by the C-to-N ratio of the forest floor, such that a wide range of C-to-N ratios was obtained (Table 1). Tab. 1 C-to-N ratio, respiration rates and release of dissolved organic matter (DOC) of the Oa material of 15 Norway spruce Oa horizons Site C-to-N ratio Respiration (µg C g C -1 d -1) DOC release µg C g C -1 d -1 Bodenmais ± 0.3 Fall ± 0.3 Garmisch-P ± 10.4 Fichtelberg ± 0.5 Waldstein ± 0.3 Villingen V ± 1.7 Schluchsee ± 2.9 Spanbeck ± 0.5 Villingen V ± 1.4 Klosterhede ± 2.0 Starnberg ± 2.0 Goldkronach ± 4.2 Birkenes ± 2.5 Kelheim ± 1.9 Nordmoen ± 5.1 3

4 2.2 Experimental design and treatment Thirty-five to 50 g of each Oa horizon sample were incubated in 100 ml plastic syringes. Moisture was adjusted by distilled water to field capacity. For each site six replicates were used. All samples were conditioned for 2 weeks at 15 C (Michel and Matzner, 1999). To minimize the effect of initial disturbance and to study the long term decomposition rates, samples were incubated for 10 months. To avoid accumulation of soluble organic and inorganic compounds all samples were leached every fourth week with artifical throughfall solution (soil-to-solution ratio 1-to-2). An aliquot of each sample from the 6th percolation was frozen for measurement of UV spectra, synchronous fluorescence, and emission fluorescence spectra as described below. Samples from the 6th to the 9th percolation were collected and freeze-dried for recording NMR spectra. Respiration was measured weekly for 3 to 4 h in a closed system with constant air flow. Carbon dioxide was detected in the air flow by microprocessor controlled non-dispersive infrared photometers (NDIR; BINOS 100-4P, Fisher- Rosemount, Hanau, Germany). Because CO 2 production was rather low, the three subsamples of each site were measured simultaneously. During the first 2 months, respiration rates showed wide fluctuations, which were followed by a strong decrease. In the last 9 weeks, respiration rates were almost constant and mean hourly respiration rates were calculated using the values of this period. It was assumed that these reflect decomposition of humus best. Structure and composition of SOM were characterized by 13 C NMR spectroscopy and cupric oxide (CuO) oxidation (Dignac et al., 2002). Composition of DOM was characterized using different spectroscopic methods: 13 C NMR spectra of freeze-dried percolates were recorded, specific UV absorption, i.e. UV absorption at 280 nm (A 280) (UVIKON 930, BIO-TEK Instrument, Neufahrn, Germany) was measured to estimate the aromaticity of DOM (Chin et al., 1994). The values were normalized to 1 mg C L -1. Synchronous fluorescence spectra were recorded (SFM 25, BIO-TEK Instrument, Neufahrn, Germany) applying a wavelength range from 300 to 550 with a scan speed of 100 nm min -1, a constant wavelength interval ( λ of 18 nm) (Miano and Senesi, 1992), and a 1 cm cuvette. On base of synchronous fluorescence spectra, a humification index (HIX syn ) of DOM was calculated by dividing the intensity at a band of a longer wavelength (460 nm) by that of a shorter wavelength (345 nm) (Kalbitz et al., 1999). Emission fluorescence spectra (excitation wavelength 254 nm, emission range nm, scan speed 100 nm min -1 ) were also obtained with a SFM 25 spectrometer and a 1 cm cuvette. Based on these spectra, a second humification index (HIX em ) was calculated by dividing the peak area between 435 to 480 nm to the peak area between 300 to 345 nm (Zsolnay et al., 1999). 4

5 3. Results Mean daily CO 2 emissions ranged from 25.3 to µg C g C -1 d -1 (Tab. 1) indicating considerable differences between the sites in respect to C mineralization and microbial activity. The release of CO 2 was positively correlated to C-to-N ratios (R 2 = 0.49, P < 0.01; Fig. 1). The release of DOC from the forest floor samples differed largely: Between 0.0 and 58.6 µg C g C -1 d -1 were released as DOC. The extremes were Fichtelberg which had no net release of DOC during the 10-month-incubation and Nordmoen. Mean daily DOC production increased significantly with increasing C-to-N ratio (R 2 = 0.52, P < 0.01; Tab. 2). DOC release showed a highly significant, positive correlation to respiration rates (R 2 = 0.77, P < 0.001; Tab. 2). 120 mean respiration [µg C g C -1 d -1 ] R² = 0.49 P < C-to-N ratio Fig. 1 Relationship between C-to-N ratio and mean daily respiration rates of the material of 15 Oa horizons UV absorption at 280 nm of DOM leached from Oa material of different spruce sites differed only slightly (data not shown). Only one of the calculated humification indices (synchronous mode) was related to SOM parameters and respiration (Table 2). In general, DOM leached from Oa samples with wide C-to-N ratio had a significantly higher HIX syn and A 280 than DOM released from N-rich samples. Specific UV absorption and HIX syn tended to increase with increasing content of aromatic C determined by 13 C NMR spectroscopy. The correlation between aromatic C and HIX syn was significant (R 2 = 0.31, P < 0.01). 5

6 Tab. 2 Pearson correlation coefficients (r-values) between the composition of soil organic matter (SOM), respiration rates of the bulk samples and release and spectroscopic properties of dissolved organic matter (DOM) (Oa horizons) DOC release A 280 HIX syn HIX em µg C g C -1 d -1 mg C L -1 cm -1 C-to-N ratio 0.72 ** 0.64 * 0.65 ** 0.22 Carboxyl C, % of C org Aromatic C, % of C org * 0.12 O-alkyl C, % of C org Alkyl-C, % of C org Alkyl C-to-O-alkyl C ratio a Σ VSC, g kg C (ac/al) v *** * ** Respiration, µg C g C -1 d ** 0.64 * 0.75 ** 0.43 a Spearman R (data not normally distributed) *, **, and *** represent P < 0.05, P < 0.01, and P < 0.001; n = 15 Abbreviations: ΣVSC, sum of vanillyl, syringyl, and cinnamyl cupric oxide (CuO) oxidation products; (ac/al) v, acid-to-aldehyde ratio of the vanillyl units in CuO oxidation products 4. Discussion Release of organic C as DOC significantly decreased with both decreasing C-to-N ratio (R² = 0.52, P < 0.01; Tab. 2) and increasing (ac/al) v ratio (R² = 0.67, P < 0.001; Tab. 2). The respiration rates of the investigated Oa horizon samples were significantly and positively correlated to C-to-N ratio (R² = 0.49, P < 0.01; Fig. 1). Possible reasons for the observed decrease in CO 2 evolution and DOC release with increasing C-to-N ratio were discussed by Michel and Matzner (2002). Briefly, possible explanations would be the chemical stabilization of organic matter by N (Nömmik and Vahtras, 1982), the repression of lignolytic enzyme production by white-rot fungi (Eriksson et al., 1990) and, for DOC only, an enhanced mineralization due to higher N availability (Aber, 1992; Gundersen et al. 1998). It is unlikely that decreasing DOC release with decreasing C-to-N ratio is due to repressed lignolytic enzyme activity. Besides the fact that enzyme activities (cellulose, polyphenol oxidase activity) were not correlated with DOC release (Michel, 2002), the significantly negative relationship between DOC re- 6

7 lease and (ac/al) v ratio (Tab. 2) clearly contradicts this explanation. The (ac/al) v ratio reflects the degree of oxidation of lignin-derived compounds. An increase in (ac/al) v ratios indicates an increased side-chain alteration by white-rot fungi (Hedges et al., 1988). Lower CO 2 and DOC release from samples with low C-to-N and high (ac/al) v ratios might thus be due to the fact, that SOM with low C-to-N ratio is more decomposed and older than SOM with wide C-to-N ratio. However, this assumption is not supported by the results of 13 C NMR spectroscopy. The ongoing degradation of plant residues is indicated by the relative decrease of O-alkyl C and the relative increase of alkyl C in forest soils, resulting in an increase of the alkyl C-to-O-alkyl C ratio (Kögel-Knabner et al., 1988). The alkyl C-to-O-alkyl C ratios of Oa samples were not related to DOC release (Tab. 2), respiration rates, C-to-N or (ac/al) v ratio (Michel, 2002). Kalbitz et al. (2003) showed that UV absorption was positively related to the content of aromatic compounds, i.e. the portion of aromatic H measured by liquid-state 1 H-NMR spectroscopy. The A 280 is hence a rough measure for the aromatic content of DOM (Chin et al., 1994). The fluorescence-based HIX is well-suited for determining the extent of humification (Ohno, 2002), i.e. the conversion of specific, lower molecular weight organic compounds derived from plant and animal products to more condensed, higher molecular weight polymers (Stevenson, 1994). Low HIX values correspond to maximal fluorescence intensity at short wavelengths. This can be ascribed to relatively non-humified material and thus simple structural components characterized by low molecular weight and low degree of aromatic polycondensation. A shift in the maximal fluorescence intensity toward the red region (higher HIX values) is associated with the presence of more complex and complicated molecules like condensed aromatic rings and compounds with high molecular weight which are typical for humic materials (Senesi et al. 1991; Miano and Senesi, 1992). Humification indices thus describe not only the aromatic content like A 280, but also the structure of the DOM molecules. In the present study, C-to-N ratio correlated significantly and positively to A 280 and HIX syn of DOM and aromatic C content of SOM determined by 13 C NMR spectroscopy to HIX syn. This points to a higher degree of complexity of the molecules and to higher aromaticity of DOM released from N-poor SOM and samples with high content of aromatic C. The significantly negative correlation between A 280 and HIX syn and the (ac/al) v ratio of SOM might be explained by more oxidized lignin-derived, dissolved compounds that are smaller and less complex than at lower degrees of oxidation. The extent of humification is linked to the stability of organic matter. As stated above, DOM released from Oa horizon material with wide C-to-N ratio was characterized by higher HIX syn 7

8 and A 280. It might be therefore more recalcitrant than DOM released from N-rich Oa material due to its higher degree of polycondensation and its higher aromatic content. In forest ecosystems, a higher recalcitrance of DOM against microbial decomposition might lead to an enhanced C storage in the mineral soil, since C is transported into the mineral soil primarily as DOC (Fröberg et al., 2003). In summary, DOM properties were related to SOM composition: Wide C-to-N ratios and a high aromaticity of SOM were linked to high C losses as both CO 2 and DOC. Dissolved organic matter released from samples with wide C-to-N ratio was characterized by high HIX and A 280 which is typical of relatively humified organic matter with a high degree of aromatic polycondensation. Thus, it is likely that the investigated DOM was the result of microbial decomposition of simple, relatively non-humified, low-complex compounds, i.e. compounds that are characterized by low molecular weight and a low degree of aromatic polycondensation. References Aber, J.D Nitrogen cycling and nitrogen saturation in temperate forest ecosystems. Trends in Ecology & Evolution 7, Berg, B. and Matzner, E., Effect of N deposition on plant litter and soil organic matter in forest systems. Ecological Reviews 5, BML, Bericht über den Zustand des Waldes Ergebnisse des forstlichen Umweltmonitoring. Bundeministerium für Ernährung Forsten und Verbraucherschutz, Bonn. Chin, Y.-P., Aiken, G. and Loughlin, E.O., Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances. Environmental Science and Technolology 28, Dignac, M.-F., Kögel-Knabner, I., Michel, K., Matzner, E. and Knicker, H., Chemistry of soil organic matter related to C : N in Norway spruce forest (Picea abies (L.) Karst.) floors and mineral soils. Journal of Plant Nutrition and Soil Science 165, Eriksson, K.-E., Blanchette, R.A. and Ander, P., Microbial and Enzymatic Degradation of Wood and Wood Components. Springer, Berlin. Fröberg, M., Berggren, D., Bergkvist, B., Bryant, C. and Knicker, H., Contributions of Oi, Oe and Oa horizons to dissolved organic matter in forest floor leachates. Geoderma 113, Guggenberger, G., Dynamik und Eigenschaften gelöster organischer Substanzen (DOM) in unterschiedlich immisionsbelasteten Fichtenstandorten. Bayreuther Bodenkundliche Berichte 26,

9 Gundersen, P., Emmett, B.A., Kjønaas, O.J., Koopmans, C.J. and Tietema, A., Impact of nitrogen deposition on nitrogen cycling in forests: a synthesis of NITREX data. Forest Ecology and Management 101, Hagedorn, F., Blaser, P. and Siegwolf, R., Elevated atmospheric CO 2 and increased N deposition effects on dissolved organic carbon clues from δ 13 C signature. Soil Biology & Biochemistry 34, Hedges, J.I., Blanchette, R.A., Weliky, K. and Devol, A.H., Effects of fungal degradation on the CuO oxidation products of lignin: a controlled laboratory study. Geochimica Cosmochima Acta 52, Kalbitz, K., Geyer, W. and Geyer, S., Spectroscopic properties of dissolved humic substances a reflection of land use history in a fen area. Biogeochemistry 47, Kalbitz, K., Schmerwitz, J., Schwesig, D. and Matzner, E., Biodegradation of soilderived dissolved organic matter as related to its properties. Geoderma 113, Kögel-Knabner, I., Zech, W. and Hatcher, P., Chemical composition of organic matter in forest soils: the humus layer. Zeitschrift für Planzenernährung und Bodenkunde 151, McNulty, S.G., Aber, J.D. and Boone, R.D., Spatial changes in forest floor and foliar chemistry of spruce-fir forests across New England. Biogeochemistry 14, Miano, T.M., and Senesi, N., Synchronous excitation fluorescence spectroscopy applied to soil humic substances chemistry. The Science of the Total Environment 117/118, Michel, K., Nitrogen as a factor of soil organic matter stability in forest soils. Bayreuther Forum Ökologie 98, Michel, K., and Matzner, E., Release of dissolved organic carbon and nitrogen from forest floors in relation to solid phase properties, respiration and N-mineralization. Journal of Plant Nutrition and Soil Science 162, Michel, K., and Matzner, E., Nitrogen content of forest floor Oa layers affects carbon pathways and nitrogen mineralization. Soil Biology & Biochemistry 34, Nömmik, H., and Vahtras, K., Retention and fixation of ammonium and ammonia in soils. In: F. Stevenson (Editor), Nitrogen in agricultural soils. Agronomy 22, Madison, WI, pp Ohno, T., Fluorescence inner-filtering correction for determining the humification index of dissolved organic matter. Environmental Science and Technolology 36, Park, J.-H., Kalbitz, K. and Matzner, E., Resource control on the production of dissolved organic carbon and nitrogen in a deciduous forest floor. Soil Biology & Biochemistry 34,

10 Qualls, R.G., and Haines, B.L., Biodegradability of dissolved organic matter in forest throughfall, soil solution, and stream water. Soil Science Society of America Journal 56, Senesi, N., Miano, T.M., Provenzano, M.R. and Brunetti, G., Characterization, differentiation, and classification of humic substances by fluorescence spectroscopy. Soil Science 152, Stevenson, F.J., 1994., Humus Chemistry. 2 nd edition, John Wiley, New York. Wolff, B. and Riek, W., Chemischer Waldbodenzustand in Deutschland. AFZ/Der Wald Nr. 10, Zech, W., Guggenberger, G. and Schulten, H.-R., Budgets and chemistry of dissolved organic carbon in forest soils: effects of anthropogenic soil acidification. The Science of the Total Environment 152, Zsolnay, A., Baigar, E., Jimenez, M., Steinweg, B. and Saccomandi, F., Differentiating with fluorescence spectroscopy the sources of dissolved organic matter in soils subjected to drying. Chemosphere 38,

Aromaticity and humification of dissolved organic matter (lysimetric experiment)

Aromaticity and humification of dissolved organic matter (lysimetric experiment) Elizaveta Karavanova 1, Evgeny Milanovskiy 2 1 LomonosovMoscowStateUniversity, Faculty of Soil Science,Department of soil