Assessing the Effect of Stubble Removal On Soil Microbial Biomass and Activity.

Soil organic matter is important for the sustained function of agro-ecosystems as it influences soil properties. Management practices involving stubble retention increase carbon (C) input in soil systems and may be beneficial for C sequestration by balancing its input and decomposition. However, growers may remove stubble to control disease, weeds and pests. We investigated the potential effect of stubble removal on soil microbial biomass and activity in relation to C storage in stubble retention vs removal plots at two sites in South Australia which had been under no-till management practice for 20 years (where seed is directly drilled into previously untilled soil). Stubble removal started in 2008 at both sites. We examined Wirrabara (Xerult [USDA]; Chromosol [Isbell, 2002]) and Maitland (Calcic Xeralf [USDA]; Calcarosols [Isbell, 2002]) soils collected from three depths (0-2.5, 2.5-5 and 5-10 cm) for microbial activity, organic carbon (OC) and C fractions. The sites were under mono-cropping with wheat-canola-malt barley-faba bean (Wirrabara) and malt barley-lentil-canola-wheat (Maitland) with stubble loads of ca. 6 t ha^-1 y^-1. The OC and labile C fraction, microbial biomass carbon and microbial activity as measured by basal respiration (BR) and substrate-induced respiration (SIR) were all higher in stubble retention than stubble removal soils at both sites (P <0.01). However, the difference between SIR and BR was greater in stubble removal plots. This result indicates that C may be the limiting factor in influencing microbial activity in the stubble removal system. Our data suggest that stubble retention not only provides carbon input but may also retard the decomposition of soil OC.