Chemical and carbon isotopic dynamics of grass organic matter during litter decompositions: A litterbag experiment

A litterbag method was used for studying the variability in chemical and carbon isotopic compositions of four grasses during litter decomposition. After the 300 d degradation, > 90% of litter mass was lost for three C4 species (Setaria viridis, Eleusine indica, Amaranthus retroflexus) and one C3 species (Erigeron speciosus). The solid state 13C NMR spectra showed that mean proportion of aromatic and alkyl carbon increased from ca. 10% to 15% and ca. 10% to 20%, respectively, whereas that of O-alkyl carbon substantially decreased from ca. 70% to 50%. The carbon preference index and average chain length of n-alkanes remained relatively constant, whereas the carbon isotopic compositions of long chain n-alkanes varied < 2‰. Our results demonstrate that the degradation of litters alone does not significantly change the n-alkane chemical and carbon isotopic proxies. Compared to open plant-soil systems, our litterbag experiments present much less variability in chemical and carbon isotopic compositions of n-alkanes. Based on these facts, we recommend a combined measurement of chemical and carbon isotopic properties in evaluation of carbon sources, dynamics and paleoenvironments.