SOIL MICROBIAL BIOMASS, C AND N MINERALIZATION, AND ENZYME-ACTIVITIESIN A HILL PASTURE - INFLUENCE OF GRAZING MANAGEMENT

Grazing and fertilizer management practices are of prime importance fo r maintaining summer-moist hill pastures of introduced grasses and clo vers in New Zealand for sheep and cattle production. The influence of withholding grazing (a pastoral fallow) from spring to late summer on microbial biomass, C and N mineralization, and enzyme activities was i nvestigated in a Typic Dystrochrept soil from unfertilized and fertili zed (rock phosphate and elemental S) low-fertility pastures at a tempe rate hill site. The fallow increased pasture but not legume growth in the following year in the unfertilized treatment, but had no effect on pasture or legume growth in fertilized plots. High background levels of the biochemical propel-ties examined, and very variable rates of N mineralization, complicated data interpretation. Extractable-C concent ration and CO2-C production were enhanced at the completion of the fal low. Increases in net N mineralization (14-56 days incubation), follow ing initial immobilization, after the fallow were clearly indicated in the unfertilized treatment, but were less distinct, in the fertilized treatment. The fallow had no detectable influence on the concentratio ns of total C and N or microbial C and P, or on invertase, phosphodies terase and sulfatase activities. Some small changes in microbial N and an increased proportion of bacteria in the microbial population were, however, suggested. Results are consistent with the concept of fallow ing giving a short-term increase in pools of readily decomposable soil organic matter. Generally, the changes that did occur in these soil b iochemical properties are, with the partial exception of increased N a vailability, unlikely to have had any pronounced impact on subsequent pasture performance.