Comparison of soil organic matter composition after incubation with maize leaves, roots, and stems

As plants are major contributors to soil organic matter (OM), the chemical composition of plant tissues entering the soil should be related to the soil OM composition. We examined the OM composition of maize leaves, stems, and roots, to determine if differences in plant tissue composition altered the soil OM composition during a 36-week degradation experiment. Lignin phenols were measured by gas chromatography/mass spectrometry (GC/MS), and soil OM and humic substances were characterized using solid-state 13C and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy respectively. Lignin phenol composition, oxidation, and depletion relative to cutin-derived OH-acids changed less in leaf- compared to stem- and root-amended soils over time, and may be due to greater vanillyl concentrations in leaves. Soil amended with stems had higher concentrations of carbohydrates in soil OM. Humic substances from leaf-amended soils had higher concentrations of aliphatic components, likely due to higher concentrations of aliphatic compounds in leaf tissues, which suggests that compounds derived from leaves are potential contributors to the stable pool of soil OM. After 36-weeks of incubation, the contribution of microbial-derived OM was greatest in humic extracts from root-amended soils, and increased contribution from these compounds was detected earlier in these soils than stem- and leaf-amended soils. This indicates that root amendment may enhance contributions from microbial-derived OM. Our study suggests that changes in soil OM composition over time was related to the chemical composition of the plant tissue, and demonstrates the important link between plant chemistry and soil OM turnover.