Effect of N content and soil texture on the decomposition of organic matter in forest soils as revealed by solid-state CPMAS NMR spectroscopy
Introduction
Soil organic matter (SOM) is a heterogeneous mixture of organic material originating from plant, animal and microbial residues. These residues are intimately mixed with inorganic soil components and are subjected to biodegradation processes. The efficiency of the degradation varies depending on the chemical structure of the substrate, leading to a continuum from fresh organic matter (OM) to strongly humified material (Kögel-Knabner, 1993). The stability of organic matter in forest soil can be the result of the chemical recalcitrance of organic molecules against microbial attack and/or the result of limited accessibility of OM present in organo-mineral associations to microbes and enzymes due to physical and chemical protection (Sollins et al., 1996).
Decomposition of litter is influenced by the physical and chemical conditions of the environment, among which the nutrient supply is an important parameter. The amount of N that is available for the microbial biomass is one of the important factors controlling the biodegradation of OM in forest soils (Berg et al., 1995). In forest systems, most nutrients are supplied through soil weathering, while N is added through the fixation of N2 from the atmosphere, which is a relatively slow process. Since the end of the 19th century, N concentration in forest soils has increased due to atmospheric N deposition caused by anthropogenic N emissions, such as the use of N-fertilizers in agriculture and deposition after combustion of fossil fuels for energy production. Michel and Matzner (in press) have recently shown that the decomposition rate of Oh material under Norway spruce is decreasing with decreasing C/N ratios. Linear relationships have been found between the concentrations of lignin and the C/N ratios of Norway spruce sites (Dignac et al., 2002) indicating a negative effect of N on lignin degradation only in the mineral soil and not in the humus layer. It has been suggested that lignin biodegradation by fungi is reduced by high N levels (Keysere et al., 1978, Fog, 1988). Thus, the controlling effect of N on litter biodegradation has major implications for C and N cycling in soils, especially in forest soils with high N loads caused by atmospheric deposition (Berg and Matzner, 1997).
The quality of the OM in the soil horizons is controlled by the litter and root input, and by the contrasting degradation and stabilization processes. The biodegradation of litter residues leads to a preferential loss of polysaccharides and an accumulation of alkyl-C (Kögel-Knabner et al., 1988). On the other hand, the particle-size distribution in the mineral soil (A horizon) may influence the stabilization of degradation residues. Humified and aliphatic OM was suggested to be mainly associated with the clay fraction while the more labile plant residues, mainly containing polysaccharides, are susceptible to be present in the sand fraction (Oades, 1995).
In the study presented here, the OM of Oh and A horizons of different forest sites was characterized by nuclear magnetic resonance (NMR) spectroscopy in order to follow changes in OM chemical composition during humification. The structural changes were related to C and N contents of the soil horizons and to the particle size distribution of the mineral soils (A horizons) in order to elucidate the impact of those parameters on the stabilization of OM in forest soils.
Section snippets
Samples
The Oh and A horizons of 13 Norway spruce [Picea abies (L.) Karst.] sites were selected for sampling (Michel and Matzner, 2000). Nine sites are located in Bavaria (SW Germany). Schluchsee (site 9) and Villingen (site 12) are located in the German Black Forest area, Nordmoen (site 8) in Norway and Klosterhede (site 7) in Denmark. These sites have been selected for their differences in N concentrations, covering a wide range of C/N ratios. In addition, brown needles from Norway spruce trees were
Carbon and nitrogen contents, soil texture
The N contents and C/N ratios of the Oh horizons (Table 2) are expected to give an indication of the N exposure and N cycling experienced by the soils in the different sites. Two groups of sites can be distinguished according to the C/N ratios of the Oh horizons. The sites 5 to 12 have a C/N ratio ranging from 26.1 to 38.1 (Table 2), which may indicate a low N deposition on these sites. Nitrogen enrichment in the Oh horizons of sites 1–4 is reflected by consistently lower C/N ratios (ranging
Conclusions
The accumulation of OM in forest soils was studied according to the amount and quality of the litter and to the contrasting importance of the decomposition and stabilization processes. N content, an important factor that controls litter biodegradation, and the mineral soil texture, that influences the stabilization processes were tentatively related to the composition of SOM, as followed by spectroscopic methods. We investigated the Oh and A horizons of Norway spruce sites that have different
Acknowledgements
The first author wishes to thank the Alexander von Humboldt Foundation (Bonn) for allowance of a research grant. This work was part of a project supported by the Deutsche Forschung Gemeinschaft (DFG project KO 1035/14-1). The authors wish to thank Pr E. Matzner and K. Michel (BITÖK, Bayreuth) for providing the soil samples.
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