Molecular characterization of soil organic carbon in water-stable aggregate fractions during the early pedogenesis from parent material of Mollisols

Abstract

The aims of this study were to investigate how short-term agricultural practices varying in vegetation cover, tillage, and organic amendments affected the chemistry of aggregate-associated C and particulate organic matter (POM) after 8-year soil restoration from parent material (PM) of a Mollisol. The research allowed indicating the effects of perennialization and organic amendments on the chemical compositions of soil organic carbon (SOC) in both aggregates and POM in short-term and thereof helped to unveil the mechanisms of C sequestration and stabilization. The field surface soils (0–20 cm) under 8-year natural perennials and an arable soil with organic amendments were collected. Soil aggregate fractionation method was applied to obtain three aggregate-sized fractions: macroaggregate (> 250 μm), microaggregate (53–250 μm), and silt-clay unit ( 250 μm), microaggregate (53–250 μm), and silt-clay units within macroaggregate, the fine POM (fPOM, 53–250 μm), and silt-clay units within microaggregate were further separated. In all, 10 subgroups of aggregates were obtained to determine organic carbon content and chemical structure using 13C-NMR spectroscopy technique. The PM for the establishment of the experiment was also used. The chemical composition of SOC differed among aggregate sizes, representing different organic compositions with different decomposition stages. Compared with PM, soil restoration under natural perennial and organic amendments increased SOC in both POM and aggregate fractions; higher proportion of POM with larger proportion of plant-derived O-alkyl C was observed in arable soil than in perennials. Meanwhile, the silt-clay units, sequestrating the most of the organic C, contained larger proportions of alkyl C, aromatic C, and carbonyl C, probably due to the microbial-derived by-products. The data partially supported our hypothesis that continuous organic amendments to PM likely promoted aggregate formation and subsequently affected the chemical composition of C therein. Perennialization and organic amendments increased not only the total SOC stocks in bulk soils and aggregate fractions but also changed the SOM chemistry by varying quantity and quality of plant residues. The silt-clay units contributed largely to the stock and stability of SOC during the soil development stage from PM of a Mollisol. The main mechanisms of plant-derived C sequestrated and SOC accrued were controlled probably not only by the physiochemical protection of soil aggregates but also by the microbial C sequestration capacity in silt-clay fraction at the initial stage of soil formation.