Response of soil microbial biomass and plant litter decomposition to weed management strategies in maize and asparagus cropping systems

Five weed management strategies (sawdust mulching, repeated spring-summer cultivation, hand-hoeing and two herbicide treatments) were applied to each of two cropping systems (maize and asparagus) near Hamilton, New Zealand. Assessments of the response of microbial activity and biomass were made over an entire annual cropping cycle (from August 1990 to October 1991). Soil respiration and substrate-induced respiration (SIR) were strongly stimulated by sawdust mulch over the experimental period, probably as a result of the enhanced soil moisture status, but the other treatments did not exert any strong consistent effects. Use of the selective inhibitor technique demonstrated temporary stimulatory effects of mulching, cultivation and (occasionally) herbicide application on both the bacterial and fungal components of the soil system. The fumigation-incubation technique also suggested that mulching had stimulatory effects on microbial activity and biomass but only when control values were not subtracted. Most of the effects detected occurred in the top 5 cm of the mineral soil. Placement of litter-bags on the surface and at 10cm depth indicated that litter decomposition was often most rapid in the sawdustmulched plots, probably as a result of enhanced abiotic decomposition. Soil respiration and SIR were also greatest for the litter placed on the mulched plots, over most of the annual cropping cycle. We concluded that weed management strategies which influence soil moisture contents are likely to induce the most significant responses by the soil microflora.