|Michel, K: Nitrogen as a factor of soil organic matter stability in forest soils in Bayreuther Institut für Terrestrische Ökosystemforschung (BITÖK): Bayreuther Forum Ökologie, Selbstverlag, 98, 1-114 (2002)
It is suggested that increasing N inputs to forest ecosystems will result in an accumulation of soil organic matter (SOM). This may be caused by decreased decomposition of SOM in late stages. The objectives of this study were to investigate C dynamics (soil respiration, DOC release), N mineralization and microbial parameters (microbial biomass, extracellular enzyme activities) in dependence on internal N concentration and external N inputs. Furthermore, structure and chemical composition related to SOM with different C-to-N ratios were investigated. Spectroscopic properties of dissolved organic matter (DOM) released from SOM varying in C-to-N ratio and the effect of mineral N on composition of DOM were also determined. Three incubation experiments under standardized laboratory conditions were carried out: (1) the influence of internal N concentrations on C dynamics and N mineralization in late stages of decomposition was investigated using samples from Oa layers of 21 different forest floors under Norway spruce (Picea abies) with varying C-to-N ratios; (2) the influence of periodically added mineral N on the spectroscopic properties of DOM was studied. Oa material percolates of eight selected sites was investigated; (3) was carried out to evaluate the effect of external N inputs on C dynamics, N mineralization and microbial parameters by treating Oa samples of four spruce sites with different concentrations of NH4NO3. Furthermore, the chemical composition of SOM was determined using SOM from Oa and A horizons of 15 Norway spruce sites covering a wide range of C-to-N ratios. Samples were analyzed with solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, along with chemolytic analyses of lignin. There is clear evidence that increasing N inputs to forest ecosystems may result in accumulation of organic matter in forest floors. Decomposition of SOM in late stages is apparently reduced by N as indicated by lower C and N mineralization rates. It appears to be likely that reduced mineralization rates at high concentrations of N were due to changes in the activity and composition of the microbial community and a less efficient use of N.
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