MEASUREMENT OF SOIL MICROBIAL BIOMASS PROVIDES AN EARLY INDICATION OF CHANGES IN TOTAL SOIL ORGANIC-MATTER DUE TO STRAW INCORPORATION

The straw and stubble of spring barley (Hordeum vulgare; 5t dry matter ha-1) were either burned or incorporated into soil annually for 18 yr in two field experiments in Denmark. Both experiments were on light soils situated at Studsgaard (loamy sand) and Ronhave (sandy loam). At both sites 18 yr of annual straw incorporation increased total soil organic C by only 5% and total N by about 10% but produced large increases in microbial biomass measured by the CHCl3-fumigation method. The increases in biomass C were 45 and 37% at Studsgaard and Ronhave, respectively; the corresponding increases in biomass N were 50 and 46%. Biomass measurements thus gave an early indication of slow changes in organic matter content long before these could be measured accurately against the background of organic matter already present in the soils. Increases in biomass P due to straw incorporation appeared to be even greater. However, the amounts of P released by CHCl3 were small so the measurements of biomass P were less accurate than those of biomass C or N. During a 60-day laboratory incubation at 25.degree.C, evolution of CO2-C was 55-79% greater in soil from straw incorporated plots than in soil from burned plots. Mineralization of N was 40-50% greater where straw had been incorporated, indicating that the long-term incorporation of straw had increased the quantity of mineralizable N in soil.