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

1999; Wiley; Volume: 13; Issue: 13 Linguagem: Inglês

10.1002/(sici)1097-0231(19990715)13

1097-0231

Marina Schweizer, John M. Fear, Georg Cadisch,

Isotope Analysis in Ecology

Carbon isotopic fractionations in plant materials and those occurring during decomposition have direct implications in studies of short-and longer-term soil organic matter dynamics. Thus the products of decomposition, the evolved CO2 and the newly formed soil organic matter, may vary in their 13C signature from that of the original plant material. To evaluate the importance of such fractionation processes, the variations in 13C signatures between and within plant parts of a tropical grass (Brachiaria humidicola) and tropical legume (Desmodium ovalifolium) were measured and the changes in 13C content (signatures) during decomposition were monitored over a period of four months. As expected the grass materials were less depleted in 13C (−11.4 to −11.9‰) than those of the legume (−27.3 to −25.8‰). Root materials of the legume were less (1.5‰) depleted in 13C compared with the leaves. Plant lignin-C was strongly depleted in 13C compared with the bulk material by up to 2.5‰ in the legume and up to 4.7‰ in the grass. Plant materials were subsequently incubated in a sand/nutrient–solution/microbial inoculum mixture. The respiration product CO2 was trapped in NaOH and precipitated as CaCO3, suitable for analysis using an automated C/N analyser coupled to an isotope ratio mass spectrometer. Significant depletion in 13C of the evolved CO2 was observed during the initial stages of decomposition probably as a result of microbial fractionation as it was not associated with the 13C signatures of the measured more decomposable fractions (non-acid detergent fibre and cellulose). While the cumulative CO2-13C signatures of legume materials became slightly enriched with ongoing decomposition, the CO2-C of the grass materials remained depleted in 13C. Associated isotopic fractionation correction factors for source identification of CO2−C varied with time and suggested errors of 2–19% in the estimation of the plant-derived C at 119 days of incubation in a soil of an intermediate (−20.0‰) 13C signature. Analysis of the residual material after 119 days of incubation showed little or no change in the 13C signature partly due to the incomplete decomposition at the time of harvesting. Copyright © 1999 John Wiley & Sons, Ltd.