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 arabic soils (a sandy loam and a coarse sandy soil) was examined periodically for a full year after field incorporation of 0, 4 or 8 t dry mass ha-1 of oilseed rape straw in late summer. Both unlabelled and 15N-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 treatments than in the unamended treatment at both sites during the first 6-8 wk, but the general temporal pattern was mainly controlled by soil temperature and soil water content. Microbial biomass C and N increased 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 +/- 13-22% of the mean at both sites and in all straw treatments over the 1 y period. Microbial biomass C-to-N ratios were not significantly different between straw treatments and were relatively constant over time. Extractable organic C and N were slightly higher in the straw-amended treatments 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% of the initial increase in microbial biomass N was derived from the straw N, with up to 27% of the straw N being incorporated into the microbial biomass. Rapid immobilization of soil mineral N occurred simultaneously 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. Significant differences in microbial biomass C and N between the straw treatments were still found after nearly 1 y and the decay constant of straw-derived microbial biomass N was estimated to be ca. 0.26 y-1.