Interactions between soil organic matter status, cropping history, method of quantification and sample pretreatment and their effects on measured aggregate stability

 The effects of sample pretreatment (field-moist, air-dried or tension rewetted) on aggregate stability measured by wet sieving or turbidimetry were compared for a group of soil samples ranging in organic C content from 20 to 40 g C kg–1. Concentrations of total N, total and hot-water-extractable carbohydrate and microbial biomass C were linearly related to those of organic C. Aggregate stability measured by wet sieving using air-dried or field-moist samples and that measured by turbidimetry, regardless of sample pretreatment, increased curvilinearly with increasing soil organic C content. However, when tension-rewetted samples were used for wet sieving, aggregate stability was essentially unaffected by soil organic C content. Measurements of aggregate stability (apart from wet sieving using rewetted soils) were closely correlated with one another and with organic C, total and extractable carbohydrate and microbial biomass C content of the soils. The short-term effects of aggregate stability were also studied. Soils from under long-term arable management and those under long-term arable followed by 1 or 3 years under pasture had similar organic C contents, but aggregate stability measured by turbidimetry and by wet sieving using air-dried or field-moist samples increased with increasing years under pasture. Light fraction C, microbial biomass and hot-water-extractable carbohydrate concentrations also increased. It was concluded that both total and labile soil organic C content are important in relation to water-stable aggregation and that the use of tension-rewetted samples to measure stability by wet sieving is unsatisfactory since little separation of values is achieved.