Composition of organic matter in sandy relict and cultivated heathlands as examined by pyrolysis-field ionization MS

Abstract

Unusually high SOC levels have been reported for sandy cropland soils in North-Western Europe. A potential link with their general heathland land-use history was investigated by comparing two soil pairs of relict heathland and cultivated former heathland in the Belgian sandy region. A sequential chemical fractionation yielded similar sizes in corresponding SOM fractions between the heathland and cropland soils (i.e. NaOCl resistant: 12.3-15.0 g C kg(-1) and NaOCl + HF resistant: 2.6-5.3 g C kg(-1)). Higher amounts of clay sized N in the cropland plots can be attributed to N additions from mineral fertilizers and animal manure. Temperature resolved Pyrolysis Field Ionization Mass Spectroscopy analysis showed that the composition of both relict heathland and cultivated soils was surprisingly similar, in spite of over 60 years of intense cropland management. The mass spectra of SOM in both heathland-cropland soil pairs investigated was dominated by signals from lipids, alkylaromatics and sterols. The accumulation of this SOM rich in aliphatics was logically linked to the high input of lipids, long-chain aliphatics and sterols from heathland vegetation and the low soil pH and microbial activity. Based on the relatively high OC surface loadings of HF-extractable OM (13-44 mg C m(-2) Fe and 1.2-2.3 mg C m(-2) clay), direct organo-mineral bonds between OM and Fe-oxides or clay minerals seem to be only partly involved as a stabilization mechanism in these soils. The distinct bimodal shape of the thermograms indicates that OM-crosslinking could furthermore contribute substantially to SOM stabilization in these soils. This study therefore corroborates the previously proposed view that lipids may be bound in networks of alkylaromatics, the structural building blocks of OM macromolecules. We hypothesize that such binding is able to explain the measured retention of these OM components, even under several decades of cropland management

Total, hot water extractable, and oxidation-resistant carbon in sandy hydromorphic soils-analysis of a 220-year chronosequence

“…2). Compared to the TOC concentrations of sandy long-term cropland soils from many regions of Northwest Europe, which have been reported to exhibit about 20 g kg −1 TOC (Sleutel et al 2008;Springob et al 2001;Strebel et al 1988), the TOC concentrations of the soils of this study, which have been used as cropland for 46 years, are very low. Probably, the reason for the high TOC contents in other sandy long-term cropland soils is a former use of the sites as heathland and the input of recalcitrant Callunaderived organic matter (Sleutel et al 2008;Springob and Kirchmann 2003).…”

“…The NaOCl-resistant C fraction in this study represented 8 to 53% of the initial TOC concentration for the chronosequence sites. Sleutel et al (2008) found that the NaOCl-resistant C fraction of sandy soils with recent or historical heathland use ranged between 49 and 78% of the TOC. This significant difference shows that the soil organic matter of the Gleyic Podzols and Haplic Gleysols under cropland is more labile than that of the sites investigated by Sleutel et al (2008).…”

Soil‐organic‐matter stability in sandy cropland soils is related to land‐use history

“…As discussed by Sleutel et al (2008), because of the low content of both crystalline and poorly crystalline Fe and Al (Tab. 1), binding between the latter and SOM plays only a limited role in SOM stabilization in these soils.…”

Hot regions of labile and stable soil organic carbon in Germany – Spatial variability and driving factors

“…Calluna litter contains very stable SOC due to high contents of lipids, 397 long-chain aliphatics and sterols, and may persist in the light fraction of soil for decades or even 398 centuries (Sleutel et al, 2008). 399…”