EFFECT OF ORGANIC MULCHES AND A RESIDUAL HERBICIDE ON SOIL BIOACTIVITY IN AN APPLE ORCHARD

In New Zealand orchards, weed control is usually achieved with a combi nation of herbicides and mowing or cultivation. Alternative strategies (e.g., organic mulches) have been called for, on the grounds that per sistent herbicides may have undesirable effects on non-target organism s and may leach into the ground water. We compared the effects of sawd ust, straw, compost, and wooldust mulches and the residual herbicide t erbuthylazine on soil respiration, cellulose degradation activity, and bacterial and fungal biomass in an apple orchard in Hawke's Bay, New Zealand. The control received no residual herbicide or mulch but weeds were checked by paraquat/diquat or glyphosate. Measurements were made overtwo growing seasons. Terbuthylazine application had no detectable effects on CO2 emission or cellulose degradation activity. Over both seasons, total CO2 emission in the field was increased by compost. In the summer immediately following mulch application, sawdust, and straw mulches suppressed CO2 emission from the soil, but total emission ove r both seasons was not different from the control. Soil biomass measur ements were only taken from the herbicide, sawdust, and wooldust plots and the grass inter-row. In general, bacterial and fungal biomasses i n the soil decreased in the order grass > sawdust mulch greater than o r equal to residual herbicide > wooldust mulch. Cellulose degradation activity was increased by compost and straw but depressed by wooldust. There was no simple relationship between respiration and cellulose de gradation activity across treatments.