Effects of lime and phosphate additions on changes in enzyme activities, microbial biomass and levels of extractable nitrogen, sulphur and phosphorus in an acid soil

Summary

The effects of adding lime and/or phosphate to an acid, phosphate-deficient soil on microbial activity, enzyme activities and levels of biomass and extractable N, S and P were studied under laboratory conditions. Following rewetting there was, as expected, an initial flush in microbial growth and activity, as shown by large increases in CO₂ evolution, in levels of biomass N, S and P and by accumulation of extractable mineral N and sulphate in the soil. Following rewetting, additions of lime and phosphate further stimulated mineralization of C, N and S. In the first 4 weeks of incubation, the mineralized N accumulated in the soil as ammonium N and there was a concomitant rise in soil pH. After this initial period, nitrification increased substantially and soil pH decreased again. Additions of lime generally increased protease and sulphatase activities but decreased phosphatase activity. Additions of phosphate decreased the activities of all three enzymes. The positive effect of liming on protease and sulphatase activities persisted for the duration of the experiment while accumulation of mineral N and sulphate effectively ceased after about 4 weeks. Furthermore, although phosphate additions decreased the activities of protease and sulphatase they increased the accumulation of mineral N and sulphate. Thus, protease and sulphatase activities were not reliable indicators of the relative amounts of mineral N and sulphate accumulated in the soil during incubation. Some uncertainty surrounded the validity of biomass S and P values estimated by the chloroform fumigation technique because differing proportions of the sulphate and phosphate released from the lysed cells may have been extracted from the different treatments.