Molecular Insight into Soil Organic Matter Formation

To determine the compound specific isotope ratios, either individual compounds or their breakdown products are extracted from organic matter by solvents or heat, respectively (Hayes et al, 1990; Gleixner and Schmidt, 1998). The isotopic content of soluble compounds is determined directly, i.e., alkanes, or after derivatization of polar groups, i.e., phospholipid fatty acids, in a gas chromatograph coupled via a combustion unit to an isotope ratio mass spectrometer (GC-C-IRMS) (Hilkert et al, 1999). Alternatively, molecular fragments are produced by heat from non-soluble compounds, like proteins, carbohydrates or lignin and transferred on-line to a GC-C-IRMS system (Gleixner et al, 1999). Under pyrolysis conditions intramolecular water release and intermolecular bond cleavage from specific volatile breakdown products, e.g., from carbohydrates derivatives of furane and pyrane and from lignin derivatives of phenol, are analyzed for their isotopic content. In combination with vegetation change experiments, the label of individual compounds can be evaluated (i.e., squares in Fig. 3.8 are already completely labeled with the isotopic signal of the new vegetation whereas circles are not labeled at all).

In spite of our current knowledge of SOM stability, we were able to demonstrate turnover times shorter than 1 year for the major plant-derived molecules like 'stable' lignin and cellulose (Gleixner et al, 1999; Gleixner et al, 2001b) indicating that plant-derived carbon skeletons are neither chemically nor physically stabilized in soil. No indication for specific, recoverable molecules with turnover times in the millenium range, as suggested by soil carbon models, could be found (Gleixner et al, 2002). Moreover, pyrolysis products of carbohydrates and proteins that were only present in soil samples but not in plant samples had unexpectedly long turnover times between 20 and 100 years (Fig. 3.10). These turnover times are in agreement with turnover times of the bulk soil. However, carbohydrates and proteins are known to be unstable in soil (Trojanowski et al, 1984). At the same time they are also known to be a major part of soil microorganisms. Consequently carbon turnover might be controlled by soil organisms. Moreover, the composition of the belowground food web might be of higher importance than plants for providing the major carbon source for storage in SOM.

Therefore, we focused on the role of the soil microbiota in the storage of soil carbon using labile PLFAs to trace the flow of labeled carbon into the microbial carbon pool. PFLAs were extracted from soils of a 40-year-old C3 to C4 vegetation change experiment in Halle, Germany. In spite of our present knowledge suggesting a preferential flow of carbon from labile plant material over labile carbon in organisms to stable carbon in soil, we detected organisms feeding on all available carbon sources in soil (Fig. 3.11). After vegetation change some organisms are completely labeled by the new vegetation, while others are only labeled according to the change in the isotopic content of the soil organic matter or less. As soil organisms have only short life cycles and PLFAs are unstable in soil, the soil organisms were obviously feeding on carbon, like soil organic matter, that was not

3. Stable Isotope Composition of Soil Organic Matter 0.8 ■

Lignin Protein

Polysaccharide Unknown

20 40 60

Turnover time (years)

100 120

Figure 3.10 Turnover time and relative peak area of individual pyrolysis products for bulk soil submitted to vegetation change from Cg plants to C4 plants. From Gleixner et al. (2002).

Figure 3.10 Turnover time and relative peak area of individual pyrolysis products for bulk soil submitted to vegetation change from Cg plants to C4 plants. From Gleixner et al. (2002).

iit3C-value (%°)v-pdb phospholipid fatty acids from maize cultivated soil

Figure 3.11 Isotopic difference between phospholipid fatty acids extracted from soil under maize (C4) and those of soil under continuous wheat (C3) cropping.

labeled even 40 years after the start of the experiment. This suggests that soil carbon is not 'stable' in soil but is constantly under ongoing reuse. Every process that keeps individual carbon atoms in this recycling process possibly increases the carbon storage in soils. This assumption is strongly supported by results from the Long Term Ecological Research site at the Niwot Ridge, Colorado (Neff et al., 2002). The addition of nitrogen increased both the primary production and the species richness in this N-limited system.

However, neither the amount of carbon in the soil nor the 14C content of soil organic matter changed significantly. Using compound-specific isotope ratios we were able to demonstrate that 'young' plant-derived carbon from cellulose and lignin was completely degraded through the addition of nitrogen. At the same time the turnover of the mineral-associated carbon was accelerated and new carbon entered this slower pool.

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