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-sum mer cultivation, hand-hoeing and two herbicide treatments) were applie d to each of two cropping systems (maize and asparagus) near Hamilton, New Zealand. Assessments of the response of microbial activity and bi omass were made over an entire annual cropping cycle (from August 1990 to October 199 1). 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, bu t the other treatments did not exert any strong consistent effects. Us e of the selective inhibitor technique demonstrated temporary stimulat ory effects of mulching, cultivation and (occasionally) herbicide appl ication 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 c ontrol values were not subtracted. Most of the effects detected occurr ed in the top 5 cm of the mineral soil. Placement of litter-bags on th e surface and at 10 cm depth indicated that litter decomposition was o ften most rapid in the sawdust-mulched plots, probably as a result of enhanced abiotic decomposition. Soil respiration and SIR were also gre atest for the litter placed on the mulched plots, over most of the ann ual cropping cycle. We concluded that weed management strategies which influence soil moisture contents are likely to induce the most signif icant responses by the soil microflora,