PULSE-LABELLING A COVER CROP WITH C-13 TO FOLLOW ITS DECOMPOSITION IN SOIL UNDER FIELD CONDITIONS

摘要

A preliminary study was conducted using the stable isotope C-13 to pulse label the cover crop phacelia (Phacelia tanacetifolia) to examine its decomposition in soil, under field conditions. Plants were grown, in pots, in the greenhouse and after four weeks of growth were labelled with (CO2)-C-13 six times, at 1-2 week intervals. A single chamber was placed over the pots, and (CO2)-C-13 was generated, inside the chamber, by injecting lactic acid into sodium carbonate (99 atom% C-13). For calculating the quantity of Na2CO3 required, a target enrichment of 5 atom% C-13 within the shoots of plants, assuming no respiration losses, was used. When harvested, at flowering, the mean enrichment of the shoot material was 3.0466 atom% C-13, or 1.9654 atom% excess C-13. To assess uniformity of labelling within plants, the shoot of a single plant was divided into leaves and stem from three sections of equal length. Ninety-three percent of this plant's dry matter had a C-13 enrichment within 20 % of the weighted mean. At a held site with sandy soil, C-13 labelled shoot and root material were combined and mixed with soil (0-15 cm). The soil was sampled 16 and 179 days later to determine the recovery of the added excess C-13 in soil total C. The recoveries in soil (0-30 cm) were, respectively, 78 and 40 % at 16 and 179 days; there was appreciable variation associated with the recovery data from day 16, much less so at day 179. Methodological procedures for (i) enhancing the uniformity of labelling with C-13 within plants, acid (ii) minimising variability in the recovery of C-13 from soil are suggested.