Isotopic (C-13) fractionation during plant residue decomposition and its implications for soil organic matter studies

Carbon isotopic fractionations in plant materials and those occurring durin g decomposition have direct implications in studies of short-and longer-ter m soil organic matter dynamics, Thus the products of decomposition, the evo lved CO2 and the newly formed soil organic matter, may vary in their C-13 s ignature from that of the original plant material, To evaluate the importan ce of such fractionation processes, the variations in C-13 signatures betwe en and within plant parts of a tropical grass (Brachiaria humidicola) and t ropical legume (Desmodium ovalifolium) were measured and the changes in del ta(13)C content (signatures) during decomposition were monitored over a per iod of four months. As expected the grass materials were less depleted in C -13 (-11.4 to -11.9 parts per thousand) than those of the legume (-27.3 to -25.8 parts per thousand). Root materials of the legume were less (1.5 part s per thousand) depleted in C-13 compared with the leaves. Plant lignin-C w as strongly depleted in C-13 compared with the bulk material by up to 2.5 p arts per thousand in the legume and up to 4.7 parts per thousand in the gra ss. Plant materials were subsequently incubated in a sand/nutrient-solution /microbial inoculum mixture. The respiration product CO2 was trapped in NaO H and precipitated as CaCO3, suitable for analysis using an automated C/N a nalyser coupled to an isotope ratio mass spectrometer, Significant depletio n in C-13 Of the evolved CO2 was observed during the initial stages of deco mposition probably as a result of microbial fractionation as it was not ass ociated with the C-13 signatures of the measured more decomposable fraction s (non-acid detergent fibre and cellulose). While the cumulative CO2-C-13 s ignatures of legume materials became slightly enriched with ongoing decompo sition, the CO2-C of the grass materials remained depleted in C-13, Associa ted isotopic fractionation correction factors for source identification of CO2-C varied with time and suggested errors of 2-19% in the estimation of t he plant-derived C at 119 days of incubation in a soil of an intermediate ( -20.0 parts per thousand) C-13 signature. Analysis of the residual material after 119 days of incubation showed little or no change in the C-13 signat ure partly due to the incomplete decomposition at the time of harvesting. C opyright (C) 1999 John Wiley & Sons, Ltd.