Stability of soil aggregates in relation to organic constituents and soil water content

Abstract

SUMMARY: The effects of soil organic matter content, soil water content and duration of wet‐sieving on aggregate stability of soils with contrasting cropping histories were investigated. Long‐term pasture samples had a greater aggregate stability than long‐term arable samples. However, air‐drying aggregates before wet‐sieving increased the aggregate stability of long‐term pasture samples, but decreased that of long‐term arable samples. With increasing duration of wet‐sieving, the proportion of water‐stable aggregates declined until a near‐constant value was reached for each sample. Thus, within a sample there are aggregates possessing a wide range of stabilities; with increasing time under arable cropping there is an increase in the proportion of unstable aggregates present, and the measured aggregate stability, therefore, declines. Unstable aggregates (defined as those dispersed after wet‐sieving for 1 min) generally had lower organic matter content than stable ones (those still intact after sieving for 15 min). The aggregate stability of a regrassed site (13 years of arable plus 2 years of pasture) was markedly higher than that of a corresponding site from 15 years of arable cropping. Nonetheless, levels of organic matter (organic C, total N and hydrolysable carbohydrate) were almost identical at the two sites. However, aggregates from the regrassed site did have a higher biomass C and water‐extractable carbohydrate content than those from the 15‐year arable site. For a group of soils with varying cropping histories, aggregate stability was significantly more closely correlated with hot water‐extractable carbohydrate content than with organic C or hydrolysable carbohydrate content. It is suggested that the hot water‐extractable carbohydrate fraction may represent a pool of carbohydrate involved in the formation of stable aggregates.