Temporal fluctuations in biochemical properties of soil under pasture. I. Respiratory activity and microbial biomass

Abstract

Temporal fluctuations in respiratory activity (CO2 production) and two indices of microbial biomass (biomass carbon and mineral-nitrogen flush contents) were determined in topsoil, predominantly a Typic Haplaquoll, from a site under grazed pastures in the Wairarapa area. Samples, with organic carbon contents averaging 6.7 and 3.6%, were taken from two separate plots at c. 4-weekly intervals for over a year. Biomass indices were estimated by the chloroform fumigation technique. The suitability of a physiological procedure for indicating biomass fluctuations was also investigated. Correlations between properties were calculated with plot effects removed. Rates of CO2 production by field-moist soil, and soil at a standardized water potential, were lowest in samples taken at the driest time of the year and correlated significantly with field-moisture content. In contrast, biomass carbon estimates were generally highest in late summer and autumn, and lowest in winter, and were correlated negatively with soil moisture content. Mineral-nitrogen flush fluctuations were less marked, and not significantly related to soil moisture or biomass carbon content. In the physiological procedure, using field-moist soil, neither rates of CO2 production by soil + glucose, nor net glucose response values, were correlated significantly with biomass carbon estimated by the fumigation technique. This procedure therefore appears unsuitable for estimating temporal fluctuations in the biomass of an individual soil under pasture.