TEMPORAL VARIATION OF C AND N MINERALIZATION, MICROBIAL BIOMASS AND EXTRACTABLE ORGANIC POOLS IN SOIL AFTER OILSEED RAPE STRAW INCORPORATION IN THE FIELD

The temporal variation of soil microbial biomass C and N, extractable organic C and N, mineral N and soil-surface CO2 flux in situ in two ar able soils (a sandy loam and a coarse sandy soil) was examined periodi cally for a full year after field incorporation of 0, 4 or 8 t dry mas s ha(-1) of oilseed rape straw in late summer. Both unlabelled and N-1 5-labelled straw were applied. Soil-surface CO2 flux, used as an index of soil respiration, was up to 2-fold higher in the straw-amended tre atments than in the unamended treatment at both sites during the first 6-8 wk, but the general temporal pattern was mainly controlled by soi l temperature and soil water content. Microbial biomass C and N increa sed very rapidly after the straw amendments and the 31-49% difference from the unamended treatment persisted throughout the winter. Temporal variations in soil microbial biomass C and N were only within +/-13-2 2% of the mean at both sites and in all straw treatments over the ly p eriod. Microbial biomass C-to-N ratios were not significantly differen t between straw treatments and were relatively constant over time. Ext ractable organic C and N were slightly higher in the straw-amended tre atments and were higher in spring and summer than in autumn and winter . More than 90% of the added straw N could be accounted for initially and there was no loss of straw N over the winter period, in spite of a winter rainfall that was twice the 25 y average. Between 52 and 80% o f the initial increase in microbial biomass N was derived from the str aw N, with up to 27% of the straw N being incorporated into the microb ial biomass. Rapid immobilization of soil mineral N occurred simultane ously and the sum of this and the straw-derived microbial biomass N on day 7 exceeded the total increase in microbial biomass N, indicating a very rapid turnover of microbial biomass in the first few days. Sign ificant differences in microbial biomass C and N between the straw tre atments were still found after nearly ly and the decay constant of str aw-derived microbial biomass N was estimated to be ca. 0.26 y(-1). (C) 1997 Elsevier Science Ltd.