Field incorporation of straw and its effects on soil microbial biomass and soil inorganic N

The fumigation-extraction method was used to measure changes in soil microbial biomass in a field experiment following incorporation of 10 t ha-1 wheat straw, with and without 100 kg N ha-1 (as NH4NO3), into a silty clay loam soil in autumn 1987. The first measurements were 7 days after straw incorporation and then periodically for ca 1 yr. The amount of biomass (measured as biomass C, N and ninhydrin-reactive N) roughly doubled (from initial levels of ca 340 kg C, 76 kg N and 17 kg ninhydrin-N ha-1) within 7 days of straw incorporation, remained constant for the next 27 days and then slowly declined. The increase in biomass was similar when N was incorporated with the straw, although maximal amounts were a little greater (by ca 30 kg N ha-1) and attained a little later (14 days after incorporation). At the last sampling (363 days after incorporation) the biomass had declined to about half of its size shortly after straw addition; this was still ca 20% more than in the unamended soil. Initial soil inorganic N contents were small (< 10 kg N ha-1) so that the rapid increase in biomass in the soil amended with straw alone could not be explained by immobilization of inorganic N. Similarly, in the soil receiving straw and 100 kg ha-1 inorganic N, nearly all of this N was still present in inorganic form 7 days after straw incorporation, yet by this time the biomass had increased by ca 50 kg N ha-1. In both treatments, most of the newly synthesized biomass probably came from N already present in the straw. Although straw incorporation increased the size of the biomass, there were no significant differences in biomass C to N ratios, or in biomass C to ninhydrin-N ratios between treatments and at different times. The mean ratios were biomass C/N = 4.7; biomass C/ninhydrin-N = 21.4; biomass N/ninhydrin-N = 4.6. A feature of this work is the extraction of relatively large samples of soil for biomass measurements with minimal sample preparation; a procedure that gave reproducible results even with the very heterogeneous mixtures of soil and straw encountered.