BURNING IN A NEW-ZEALAND SNOW-TUSSOCK GRASSLAND - EFFECTS ON SOIL MICROBIAL BIOMASS AND NITROGEN AND PHOSPHORUS AVAILABILITY

Fire has been an important management tool in the pastoral use of New Zealand tussock grasslands. The effects of a farm-scale pastoral fire and subsequent grazing by sheep on soil biochemical properties in tuss ock grasslands dominated by the narrow-leaved snow tussock (Chionochlo a rigida ssp. rigida) were investigated, 1.5 and 2.5 years after the f ire event, in 0-2 cm depth mineral soil at a site at 975 m altitude in Central Otago, New Zealand. The nitrogen (N) and phosphorus (P) conce ntrations of C. rigida leaves were also measured. Comparisons were mad e with soil and tussock leaves from an adjacent unburned site. At both samplings, values of total soil organic carbon (C), extractable C, mi crobial biomass C, and basal respiratory activity were, on average, 14 %, 18%, 23%, and 40%, respectively, lower at the burned than at the un burned site. In contrast, microbial N values were roughly similar at b oth sites, while microbial P values were 42% higher at the burned site after 1.5 years. Phosphomonoesterase and phosphodiesterase activities were then also similar at both sites, whereas invertase activity was higher at the burned site. The greater availability of N and P at the burned site was confirmed by the higher concentrations of N and P in C . rigida leaves sampled 2 years after the fire. Ratios of microbial C: microbial N and microbial C:microbial P were significantly lower at bo th samplings at the burned site, and emphasise the importance of the s oil microbial biomass in conserving N and P after pastoral burning in a grassland ecosystem.