Pulse-labelling a cover crop with 13C to follow its decomposition in soil under field conditions

A preliminary study was conducted using the stable isotope ¹³C 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 ¹³CO₂ six times, at 1–2 week intervals. A single chamber was placed over the pots, and ¹³CO₂ was generated, inside the chamber, by injecting lactic acid into sodium carbonate (99 atom % ¹³C). For calculating the quantity of Na₂CO₃ required, a target enrichment of 5 atom% ¹³C 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% ¹³C, or 1.9654 atom% excess ¹³C. 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 ¹³C enrichment within 20 % of the weighted mean. At a field site with sandy soil, ¹³C 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 ¹³C 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 ¹³C within plants, and (ii) minimising variability in the recovery of ¹³C from soil are suggested. ei]R Merckx