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

Abstract

Carbon isotopic fractionations in plant materials and those occurringduring decomposition have direct implications in studies of short-andlonger-term soil organic matter dynamics, Thus the products ofdecomposition, the evolved CO2 and the newly formed soil organic matter,may vary in their C-13 signature from that of the original plantmaterial, To evaluate the importance of such fractionation processes,the variations in C-13 signatures between and within plant parts of atropical grass (Brachiaria humidicola) and tropical legume (Desmodiumovalifolium) were measured and the changes in delta(13)C content(signatures) during decomposition were monitored over a period of fourmonths. 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 (1.5parts per thousand) depleted in C-13 compared with the leaves. Plantlignin-C was strongly depleted in C-13 compared with the bulk materialby up to 2.5 parts per thousand in the legume and up to 4.7 parts perthousand in the grass. Plant materials were subsequently incubated in asand/nutrient-solution/microbial inoculum mixture. The respirationproduct CO2 was trapped in NaOH and precipitated as CaCO3, suitable foranalysis using an automated C/N analyser coupled to an isotope ratiomass spectrometer, Significant depletion in C-13 Of the evolved CO2 wasobserved during the initial stages of decomposition probably as a resultof microbial fractionation as it was not associated with the C-13signatures of the measured more decomposable fractions (non-aciddetergent fibre and cellulose). While the cumulative CO2-C-13 signaturesof legume materials became slightly enriched with ongoing decomposition,the CO2-C of the grass materials remained depleted in C-13, Associatedisotopic fractionation correction factors for source identification ofCO2-C varied with time and suggested errors of 2-19% in the estimationof the plant-derived C at 119 days of incubation in a soil of anintermediate (-20.0 parts per thousand) C-13 signature. Analysis of theresidual material after 119 days of incubation showed little or nochange in the C-13 signature partly due to the incomplete decompositionat the time of harvesting. Copyright (C) 1999 John Wiley & Sons, Ltd.