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

A preliminary study was conducted using the stable isotope C-13 to pul se label the cover crop phacelia (Phacelia tanacetifolia) to examine i ts decomposition in soil, under field conditions. Plants were grown, i n 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 wa s placed over the pots, and (CO2)-C-13 was generated, inside the chamb er, by injecting lactic acid into sodium carbonate (99 atom% C-13). Fo r calculating the quantity of Na2CO3 required, a target enrichment of 5 atom% C-13 within the shoots of plants, assuming no respiration loss es, 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 p lant was divided into leaves and stem from three sections of equal len gth. Ninety-three percent of this plant's dry matter had a C-13 enrich ment 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 so il (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) en hancing the uniformity of labelling with C-13 within plants, acid (ii) minimising variability in the recovery of C-13 from soil are suggeste d.