Soil aggregate size distribution and stability of farmland as affected by dry and wet sieving methods

Soil aggregate has a vital role in improving soil structure and enhancing soil aeration.Dry-wet cycle is an important factor controlling potential changes in soil aggregate size distribution and stability.In order to investigate responses of soil aggregate size distribution and stability to dry-wet cycles, samples of undisturbed soil (in the depth range of 0-100 cm) at 20 cm layer intervals from long-term cultivated farmland in the Huanghuaihai Plain, China were collected and subjected to different levels of dry-wet cycles: 0, 1, 3 and 5 cycles, noted as DW0, DW1, DW3 and DW5, respectively.The soil is classified as fluvo-aquic with a silty texture, which has been cultivated with winter wheat and summer maize rotation for more than 50 years.The soil aggregate size distribution, mean weight diameter (MWD) and geometric mean diameter (GMD) were analysed using the dry sieving and wet sieving methods.The soil aggregates were dominated by aggregates >0.25 mm, and the proportion of soil aggregates >0.25 mm was more than 85% under dry sieving method, while the proportion of <0.25 mm soil aggregates was higher under wet sieving method.The percentage of aggregate destruction (PAD) was increased by DW1, but DW3 and DW5 had little effect on it.Treatments DW1 and DW5 significantly decreased the proportion of >5 mm soil aggregates under wet sieving method.The MWD values were generally higher in treatment DW3 under both sieving methods.GMD values in treatment DW1 significantly decreased under wet sieving method, but in treatments DW3 and DW5 they varied with soil layers.Differently, dry-wet cycles generally increased GMD values under dry sieving method across soil layers.The soil aggregate stability under dry sieving method was higher than that under wet sieving method, suggesting that water-stable aggregates dominated by a small proportion in this given soil.