Effects of organic matter characteristics on soil aggregate turnover by using rare earth oxides as tracers in a red clay soil

Abstract

Organic materials input is remarkably essential for soil aggregates formation and breakdown processes. Which characteristics of organic materials control soil aggregate turnover is still largely unknown. Eleven organic materials were characterized in terms of nutrient stoichiometry, biochemical features and carbon (C) functional groups. The effects of organic matter characteristics on soil aggregate turnover were investigated by using rare earth oxides (REOs) as tracers. REOs concentrations in four aggregate fractions were measured on 0, 14, 28, and 56 d of incubation to calculate the aggregates transformation paths and turnover time. Our results exhibited that aggregate turnover time was reduced considerably with the addition of organic materials in order of easily decomposed residues (ED) < moderately decomposed residues (MD) < slowly decomposed manures (SD) but increased within aggregate fractions in sequence of silt and clay fractions < macroaggregates < microaggregates, such effects attenuated over time (P < 0.05). Nutrient stoichiometry had no impacts on relative changes and turnover time of aggregates. Soluble sugars increased the formation of large macroaggregates at early stage of incubation, but laid no impacts on aggregate turnover time. Lignin reduced soil aggregates formation but increased aggregate turnover time in the first four weeks. C functional groups showed short-term effects on relative changes of aggregates while these characteristics did not explain aggregate turnover time except aromatic carbon. Under ED treatments, the relative formation of 0.053–0.25 mm aggregates increased with the accelerating breakdown of macroaggregates, suggesting the formation of stable microaggregates in the mid-to-late incubation time. With MD and SD application, the relative formation was increased with the decrease of aggregate breakdown over time. We proposed the pathways of soil aggregates turnover, in which the stable microaggregates were released with the breakdown of stable macroaggregates in ED treatments, while such transformation was not observed in MD or SD treatments during the incubation time. Our results demonstrate that aggregate turnover depends on the initial characteristics of incorporated organic matters defined by biochemical features and C functional groups.