While it is well known that litter chemistry can vary greatly among plants, the extent to which these differences are reflected in the chemical composition of the soil organic matter (SOM) is unknown. If microbial decomposition of litter results in similar product formation, SOM chemistry should converge independently of the plant community. Alternatively, SOM chemistry may reflect litter chemistry if, for example, recalcitrant leaf litters lead to the build-up of chemically recalcitrant SOM. Finally, SOM chemistry may diverge owing to differences in the functional attributes of decomposer communities. For example, if labile leaf litter is used more efficiently used by soil microbes, a greater fraction of microbial products may become SOM. To address these alternative hypotheses, we used pyrolysis gas chromatography/mass spectroscopy to characterize SOM chemistry in the upper soils (0-5cm depth) of forest plots dominated by arbuscular (AM)- or ectomycorrhizal (ECM)-associated trees in south-central Indiana. AM trees at the site have leaf and root litters with lower C:N and lignin:N ratios than ECM litters.
Results/Conclusions
PCA revealed that the chemical composition of pyrolysis products differed significantly between AM and ECM soils (p<0.01), indicating a lack of chemical convergence. Broadly, ECM soils contained relatively more lipid- and phenol-derived pyrolysis products while AM soils contained relatively more N-bearing and protein-derived pyrolysis products. Furthermore, both contained the same relative amount of lignin (average: 18.6±1.5%; p=0.54) and had similar lignin:polysaccharide ratios (average: 0.69±0.09; p=0.64), indicating SOM chemical recalcitrance did not reflect initial differences in the chemical recalcitrance of plant inputs. However, ECM soils were richer in plant-derived pyrolysis products (e.g., levoglusinone derived from cellulose and n-nonacosane derived from plant waxes) while AM soils were richer in microbe-derived pyrolysis products (e.g., chitin-derived pyridines and bacterial-derived n-pentadecane). In addition, the ratio of protein:lignin+phenolics, an indicator of decomposition extent, was greater in AM soils than in ECM soils (AM: 0.9±0.1; ECM: 0.5±0.1; p<0.01). Taken together, these results support our third scenario: high quality litter is rapidly decomposed and incorporated into microbial products. This may drive the common observation that ECM soils have wider C:N ratios than AM soils: in AM soils, C is locked up in N-rich compounds owing to a microbial origin of SOM, whereas C:N ratios are wide in ECM stands owing to the selective preservation of plant inputs.