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 13 C 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.