Influence of soil mineral nitrogen content on soil respiratory activity and measurements of microbial carbon and nitrogen by fumigation-incubation procedures

Abstract

The influence of soil mineral-N (min-N) content on rates of respiratory activity (CO2 production), and measurements of microbial biomass carbon and nitrogen by fumigation-incubation procedures, was investigated with three fertile soils and one low-fertility soil. These soils were sampled at each of the four seasons from pastures in which intensive grazing can result in high levels of min-N. Values of CO2-C flush and thin-N flush [the difference between CO2-C and min-N produced by fumigated samples and an unfumigated contl ol (or, for CO2-C only, fumigated control)] were used as indices of biomass C and N. Soil min-N content was adjusted by the addition of ammonium sulfate (approx. 50 µg NH4+-Ng-1 soil). In the low fertility soil and two of the fertile soils, added min-N had no significant effect on CO2 production or CO2-C flush values. In the other soil (Castlepoint), the added min-N usually lowered CO2 production in unfumigated samples, and increased CO2-C flush values when an unfumigated control was used; CO2-C flush values were not affected when a fumigated control was used. Use of a fumigated control for estimating biomass C in these grassland soils is recommended. Added min-N had few significant effects on the min-N flush values of the three fertile soils. In the low-fertility soil (Pomare), the min-N flush values of summer and autumn samples were appreciably higher in the presence of added min-N, with the results suggesting that the min-N flush values of the samples without added min-N were erroneously low because of N immobilization. Overall, min-N flush measurements appear to provide a satisfactory index of microbial biomass in fertile soils under pasture, but care in the interpretation of min-N flush values from low-fertility grassland soils seems advisable.