Soil microbial biomass, C and N mineralization and enzyme activities in a hill pasture: Influence of season and slow-release P and S fertilizer

Hill pastures are widely used in New Zealand for sheep and cattle production. The influence of season and added slow-release fertilizer (rock phosphate and elemental S) on soil microbial pools and biochemical activities in a low-fertility hill pasture was investigated during 2 y. The soil was a Typic Dystrochrept. Specifically determined kec-and ken-factors were used in the fumigation-extraction procedures for measuring microbial C and N. An appropriate factor for converting substrate-induced respiration values to microbial C was also determined. Fertilizer tended to increase dry matter production and the legume component of the sward in the second year of the trial, but variability was high and differences were generally non-significant. Fertilizer increased extractable soil inorganic P and S, but had no consistent, if any, effect on soil biochemical properties. Soil moisture content was generally high at all sampling times. Amounts of microbial biomass, and the percentages of microbial C and N in total C and N, respectively, were similar to those in more fertile lowland pastures. Temporal fluctuations in microbial C, N and P concentrations were small and mainly non-significant, and were related to variability in total C and N concentrations. Temporal fluctuations were also mainly non-significant for invertase and sulphatase activities, but were very marked for phosphodiesterase activity. CO2C production under standardized conditions was highest in spring samples and significantly correlated with extractable C concentration. Min-N was generally immobilzed initially on incubation at 25°C, but net N mineralization increased over 14–56 d with maxima also in spring; NH4+N predominated throughout. Overall, potential rates of C and N mineralization were more susceptible to seasonal effects than were the microbial biomass pools.