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SOIL CHEMISTRY

Tillage Management Alters Surface Soil Organic Matter Composition: A Pyrolysis Mass Spectroscopy Study

^a Dep. of Soil Management and Soil Care, Ghent Univ., Coupure Links 653, 9000 Gent, Belgium
^b Institute for Land Use, Univ. of Rostock, Justus von Liebig Weg 6, 18059 Rostock, Germany
^c Dep. of Soil Management and Soil Care, Ghent Univ., Coupure Links 653, 9000 Gent, Belgium

^* Corresponding author (steven.sleutel{at}UGent.be).

We examined the effects of 10 yr of reduced tillage (RT) management, compared with conventional tillage (CT), on the chemical composition of soil organic matter (SOM) using a combination of physical fractionation and pyrolysis–field ionization mass spectroscopy (Py–FIMS). Surface soil samples (0–10 cm), which were collected from two adjacent CT and RT managed fields, were separated into particulate organic matter (POM) fractions and sand, silt, and clay size separates. Higher amounts of C (77%) and N (64%) were observed in all size separates of the RT soil and, in particular, the proportion of intramicroaggregate POM was two times as large as in the CT soil. The Py–FIMS spectra showed that larger proportions of free fatty acids, sterols, and N-containing compounds were present in the RT soil's sand fraction than in the CT soil. Similar observations were also made for both free and intramicroaggregate POM fractions. In the silt and clay size fractions, no substantial differences in SOM composition were measured between tillage treatments, except that the RT soil's clay fraction had a higher proportion of N-containing compounds and a lower proportion of alkylaromatics. Ratios of carbohydrate hexose to pentose subunits and of lignin monomers + phenols to lignin dimers confirmed a less decomposed character of the SOM in the silt and clay size fractions of the RT soil. These observations can be logically linked to the reduced mixing of crop residues and manure in the soil profile and higher surface soil microbial biomass under RT.

Abbreviations: CT, conventional tillage • fPOM, free particulate organic matter • iPOM, intramicroaggregate particulate organic matter • LF, light fraction • NMR, nuclear magnetic resonance • OC, organic carbon • OM, organic matter • Py-FIMS, pyrolysis–field ionization mass spectroscopy • RT, reduced tillage • SOC, soil organic carbon • SOM, soil organic matter • TII, total ion intensity