Organic matter stabilization in young calcareous soils as revealed by density fractionation and analysis of lignin-derived constituents

Abstract

The mechanisms of organic matter (OM) stabilization and possible organic–mineral interactions in alkaline soils rich in Ca (calcareous soils) were investigated. We sampled soils that developed over a period of 70 years under natural succession from homogeneous calcareous and organic-free substrate resulting from industrial production. We took advantage of a shift from non-woody to woody angiosperm vegetation with increasing soil age and tracked lignin as an indicator of the fate of plant residues. Using density separation in combination with ultrasonic dispersion, four fractions were obtained: (1) free particulate OM (FPOM), (2) OM occluded within soil aggregates (OPOM), (3) heavy fraction with a density of 1.6–2.2 g cm−3 and (4) heavy fraction with a density >2.2 g cm−3. The fractions were analysed for C, N, CuO oxidation products and mineral composition. Accumulation of OM by occlusion within aggregates contributed little (4–16%) to the total organic C in the soils. The dominant portion of organic C was in the fractions >1.6 g cm−3. Lignin phenols in the fractions >1.6 g cm−3 showed the most intensive diagenetic alteration as reflected in small yields of vanillic, syringic and cinnamic (VSC) lignin and stronger oxidation of the side chains of vanillyl phenols. In contrast to the POM fractions, the fractions >1.6 g cm−3 from the older sites conserved the non-woody angiosperm lignin signature of the former vegetation, indicating slower OM turnover. The mineral composition of the two heavy fractions clearly differed, with layered double hydroxides (i.e., hydrotalcite and hydrocalumite) dominating the fractions >1.6 g cm−3 and calcite dominating the fractions >2.2 g cm−3. We conclude that partly degraded lignin, enriched in acidic groups, accumulated in the fraction 1.6–2.2 g cm−3, due to preferential adsorption to the positively charged layered double hydroxides. Although representing only a small portion of the soil mineral assemblage, layered double hydroxides seem to be of significant relevance for OM accumulation. Over the years, the fraction 1.6–2.2 g cm−3 comprised the largest single pool of organic C. Compared with the 1.6–2.2 g cm−3 fraction, the >2.2 g cm−3 fraction had a much smaller carbon concentrations. The lignin in that fraction showed by far the strongest oxidative alteration of all the fractions. We assume that partly degraded lignin components, not adsorbed to layered double hydroxides, undergo further microbial oxidation until they form complexes with calcite. Consequently, we conclude that in these calcareous soils accumulation of OM is mainly due to the attachment of partly oxidized OM to positively charged clays, resulting in stabilization against further decomposition, thus slowing down turnover. To a minor extent, calcite is also involved in the stabilization of the plant residues.