Soil microbial biomass, C and N mineralization, and enzyme-activities in a hill pasture - Influence of grazing management

Abstract

Grazing and fertilizer management practices are of prime importance for maintaining summer-moist hill pastures of introduced grasses and clovers 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 investigated in a Typic Dystrochrept soil from unfertilized and fertilized (rock phosphate and elemental S) low-fertility pastures at a temperate 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 concentration and CO2-C production were enhanced at the completion of the fallow. Increases in net N mineralization (14-56 days incubation), following 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 concentrations of total C and N or microbial C and P, or on invertase, phosphodiesterase 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 fallowing giving a short-term increase in pools of readily decomposable soil organic matter. Generally, the changes that did occur in these soil biochemical properties are, with the partial exception of increased N availability, unlikely to have had any pronounced impact on subsequent pasture performance.