Microbial-induced soil aggregate stability under different crop rotations

Changes in the quantity and quality of soil organic carbon, and their effec t on soil aggregate stability as a result of growing different crops in rot ation with wheat, were investigated on a red earth (Oxic Paleustalf) in Wag ga Wagga, New South Wales, Australia. After two cycles of the 1:1 rotation, while the total organic carbon in the 0-5cm soil depth was similar (15.1 g /kg), significant differences in water stable aggregation were observed in the order: wheat/lupin = wheat/barley > wheat/canola > wheat/field pea. Usi ng a selective extraction technique, significant differences in the quality (composition) of the soil organic carbon were detected in the soils from t he different rotations. Soil from the lupin rotation had the highest salt- and acid-extractable carbon whereas that from the barley rotation had the h ighest level of hot-water-extractable carbon and microbial biomass carbon. Rather than total carbon or other extractable fractions, the observed diffe rences in aggregate stability were only significantly (P<0.05) related to m icrobial biomass carbon, which made up only 1.3-1.7% of the total carbon po ol. Multiple linear regression analysis indicated that with the exception o f salt-extractable carbon, inclusion of any other of the less labile fracti ons failed to improve the correlation relationship. The labile nature of th e microbial biomass carbon therefore accounted for the transient existence of the differences in aggregate stability under different rotation crops. T he latter was found to be transient and disappeared at the end of the subse quent wheat crop.