Abstract
Abstract. The mean residence times (MRT) of different compound classes of soil organic matter (SOM) do not match their inherent recalcitrance to decomposition. One reason for this is the stabilization within the soil matrix, but recycling, i.e. the reuse of "old" organic material to form new biomass may also play a role as it uncouples the residence times of organic matter from the lifetime of discrete molecules in soil. We analysed soil sugar dynamics in a natural 30-year old labelling experiment after a wheat-maize vegetation change to determine the extent of recycling and stabilization by assessing differences in turnover dynamics between plant and microbial-derived sugars: while plant-derived sugars are only affected by stabilization processes, microbial sugars may be subject to both, stabilization and recycling. To disentangle the dynamics of soil sugars, we separated different density fractions (free particulate organic matter (fPOM), light occluded particulate organic matter (≤ 1.6 g cm−3; oPOM1.6), dense occluded particulate organic matter (≤ 2 g cm−3; oPOM2) and mineral-associated organic matter (> 2 g cm−3; mineral)) of a silty loam under long-term wheat and maize cultivation. The isotopic signature of neutral sugars was measured by high pressure liquid chromatography coupled to isotope ratio mass spectrometry (HPLC/IRMS), after hydrolysis with 4 M Trifluoroacetic acid. While apparent MRT of sugars were comparable to total organic carbon in the bulk soil and mineral fraction, the apparent MRT of sugar carbon in the oPOM fractions were considerably lower than those of the total carbon of these fractions. This indicates that oPOM formation was fuelled by microbial activity feeding on new plant input. In the bulk soil, MRT of the mainly plant-derived xylose were significantly lower than those of mainly microbial-derived sugars like galactose, rhamnose, fucose, indicating that recycling of organic matter is an important factor regulating organic matter dynamics in soil.
Highlights
For several decades, it was assumed that the molecular structure accounts for the rate of decomposition of different organic compounds in soils, i.e. compounds of high chemical recalcitrance were assumed to be selectively preserved (Stevenson, 1994)
Less carbon was found in the oPOM1.6 fractions and the free particulate organic matter
This supports the concept of Golchin et al (1994a), who suggest that the fresh, carbohydrate rich POM is utilized by microorganisms with concurrent increase of organo-mineral associations (→ oPOM2) and the formation of aggregates
Summary
It was assumed that the molecular structure accounts for the rate of decomposition of different organic compounds in soils, i.e. compounds of high chemical recalcitrance were assumed to be selectively preserved (Stevenson, 1994). A. Basler et al.: A C3-C4 vegetation change field labelling experiment chemical recalcitrance (Six et al, 2002; Sollins et al, 1996; von Lützow et al, 2006), and microbial recycling on the other, i.e. the reuse of “old” organic compounds by microorganisms (Gleixner et al, 2002; Sauheitl et al, 2005). Basler et al.: A C3-C4 vegetation change field labelling experiment chemical recalcitrance (Six et al, 2002; Sollins et al, 1996; von Lützow et al, 2006), and microbial recycling on the other, i.e. the reuse of “old” organic compounds by microorganisms (Gleixner et al, 2002; Sauheitl et al, 2005) The latter leads to an underestimation of the actual turnover dynamics but overestimates the persistence of single molecules as a whole within the SOM. Assessing the importance of stabilization and recycling for the persistence of organic matter in soil will improve the understanding of the carbon cycle and close an important knowledge gap
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