Organic Matter in Clay Density Fractions from Sandy Cropland Soils with Differing Land‐Use History

Abstract

Land‐use history is often overlooked when assessing soil fertility of intensive cropland production systems. The unusually high organic carbon (OC) content of many sandy cropland soils in Northwestern Europe is unexpected given their general low clay content (3–8%) and organic matter (OM) input typical of cropland, but appears to be related to historical heathland land‐use. Clay fraction OM composition was compared between two groups of sandy cropland soils with (HC) or without (CC) a history of heathland/forest land‐use. Light (1.6–2.2 g cm−3) and heavy (>2.2 g cm−3) clay fractions in HC soils were nearly twice as rich in OC (on average 199 g kg−1) compared with those of CC soils (on average 109 g kg−1). The hypothesized preferential presence of stable heathland derived OM in light soil fractions, was not supported by our data. Pyrolysis‐field ionization mass spectrometry of the clay fractions revealed a more decomposed character of OM in the CC soils and lasting long‐term influence of land‐use history on SOM composition. This could be concluded from higher proportions of lipids and sterols, a lower thermostability in the HC compared with the CC soils, and enrichment of alkylaromatics and heterocyclic N‐containing compounds in the latter. The density fractionation methodology separated organic‐mineral particles with similar OM loadings but lower proportions of sterols and medium to long‐chained lipids in the heavy compared with the light clay fraction. Given the very high clay OC loadings (6–16 mg C m−2) and low binding capacity of the quartz/kaolinite/mica dominated clays, we hypothesize that OM–OM interactions are involved as an OM stabilization mechanism. However, contrary to our hypothesis high clay OC loading (and hence thick OM layering) were found in all sandy croplands regardless of land‐use history or density fraction.