Temporal fluctuations in biochemical properties of soil under pasture. II. Nitrogen mineralization and enzyme activities

Abstract

Temporal fluctuations in rates of nitrogen mineralization and the activities of six enzymes were measured in topsoil, predominantly a Typic Haplaquoll, from two plots that contained pastures of different age in the Wairarapa area. Samples were taken at c. 4-weekly intervals for over one year. Organic carbon contents averaged 6.7 and 3.6% in soil from the older and younger pastures respectively. Net mineral-nitrogen production at 25°C was higher in soil at a standardized water potential (-3 and -4 kPa for soil from the older and younger pastures respectively) than at field moisture content. It was initially higher in soil from the younger than from the older pasture. Generally, distinct seasonal patterns were not apparent. All of the enzyme activities showed significant temporal fluctuations. Amylase and cellulase activities fluctuated more than invertase activity, but all three carbohydrase activities were generally high in wet spring samples. When plot effects were removed, only the fluctuations in amylase activity were related positively and significantly to soil moisture content. Fluctuations in cellulase, urease, phosphatase and sulphatase activities were correlated negatively with soil moisture content. The temporal fluctuations in enzyme activities were, when plot effects were removed, mainly independent of the small variations that occurred in soil organic carbon and total nitrogen contents. Interrelationships of these biochemical properties, and relationships with rates of CO2 production and indices of microbial biomass, are discussed.