Content and composition of free and occluded particulate organic matter in a differently textured arable Cambisol as revealed by solid‐state ¹³C NMR spectroscopy

Abstract

Summary ± ZusammenfassungThe composition of functional light soil organic matter pools of arable Cambisols with a gradient in clay content was investigated. Soil texture differences originate from increasing loess admixture to the parent material (coarse-grained tertiary sediments). Using density fractionation in combination with ultrasonic dispersion, two types of particulate organic matter (POM) were obtained: (1) free POM and (2) POM occluded in soil aggregates. Both POM fractions were analyzed by elemental analysis (C, N) and CPMAS 13 C NMR spectroscopy. With increasing clay content the amount of organic carbon stored in the occluded POM fraction increased considerably, whereas the amounts of free POM were not related to the soil clay content. With increasing soil clay contents increasing proportions of O-alkyl C and decreasing proportions of aryl C were found for both POM fractions. The occluded POM fraction showed a higher degree of degradation as indicated by lower amounts in O-alkyl carbon. A lower degree of POM degradation was associated with higher clay contents. Higher soil clay contents promoted the conservation of POM with a low degree of alteration. This effect of soil texture was found to be highly significant when the aryl C : O-alkyl C ratio was used as indicator for POM decomposition rather than the alkyl C : O-alkyl C ratio.Menge und Zusammensetzung freier und okkludierter partikulärer organischer Substanz in ackerbaulich genutzten Braunerden mit unterschiedlicher Bodentextur, aufgezeigt mittels Festkörper-13C-NMRSpektroskopie Key words: soil texture / density fractionation / particulate organic matter / organic carbon and nitrogen / CP MAS 13 C NMR spectroscopy PNSS P118/5B

Alpine grassland soils contain large proportion of labile carbon but indicate long turnover times

“…The trend towards increasing degree of transformation from root + litter → fPOM → oPOM → mOM both in C/N ratios and in alkyl-C/O-alkyl-C ratios is consistent with findings from temperate soils (Golchin et al, 1994a,b;Baisden et al, 2002). The results are also in line with systematic differences in composition between fPOM and oPOM reported for soils in climatically different regions (Golchin, 1994a;Kölbl and Kögel-Knabner, 2004). In subtropical soils, Golchin et al (1994b) reported mean alkyl-C/O-alkyl-C ratios of 0.43 (fPOM) and 0.92 (oPOM).…”

“…In subtropical soils, Golchin et al (1994b) reported mean alkyl-C/O-alkyl-C ratios of 0.43 (fPOM) and 0.92 (oPOM). Ratios reported for POM in temperate soils indicated POM to be less decomposed and to range from 0.20-0.32 in fPOM, 0.28-0.36 in oPOM (Kölbl and Kögel-Knabner, 2004), 0.37 in mineral associated SOM and 0.44-0.50 in fine earth . In our alpine soils, alkyl-C/O-alkyl-C ratios of the root + litter fraction were similar to those reported previously for agricultural crops including grass-clover roots (Leifeld and Kögel-Knabner, 2005).…”

“…Initial breakdown of plant polysaccharides decreases O-alkyl-C and increases alkyl-C (Kölbl and Kögel-Knabner, 2004). Thus, the alkyl-C/O-alkyl-C ratio, which increases from light/coarse to fine/heavy soil fractions, can be used as an indicator of the degree of decomposition of SOM fractions (Golchin et al, 1994a;.…”

DRIFT spectroscopy combined with sodium hypochlorite oxidation reveals different organic matter characteristics in density-size fractions of organically managed soils

“…They confirmed from DRIFT spectroscopy that the oPOM fraction contained more aliphatic compounds than the fPOM fraction. Similar trends were also observed in other studies (Kölbl and Kögel-Knabner 2004;Marín-Spiotta et al 2008;Wagai et al 2008). The higher proportion of aliphatic compounds in the oPOM fraction could have resulted from the selective preservation of recalcitrant aliphatic compounds.…”

Rock Fragments Control Size and Saturation of Organic Carbon Pools in Agricultural Topsoil

“…Also the potential for SOC storage by micro-aggregation is signifi cantly aff ected by clay and silt contents (Mayer et al, 2004), turning texture into key factor of C saturation concepts. Results of Kölbl and Kögel-Knabner (2004) showed that even at fi eld scale, the amount of SOC stored as occluded POM may reveal a signifi cant spatial variability, depending on the spatial variation of the clay content. Results of Bornemann et al (2010) confi rmed that the amount of coarse POM in FE can be highly heterogeneous throughout a single agricultural site which was homogeneously managed but also contained variable amounts of RF.…”