Differential influence of plant litter quality on the molecular identity of mineral-associated soil carbon pools in invaded ecosystems
Plant litter is the precursor of most soil organic matter (SOM), and the major energy source that sustain heterotrophic soil biota that actively facilitate SOM formation. Invasive plant species that input substantial quantities of litter that are chemically distinct from native species could thus greatly influence SOM formation is invaded ecosystems. We studied the influence of invasion of two noxious exotic species on the SOM quality in two native ecosystems – Polygonum cuspidatum (Japanese knotweed) that inputs recalcitrant litter invading into a native old-field that produces labile litter, and Pueraria lobata (kudzu) that inputs labile litter invading native pine forest that produces recalcitrant litter. Within each ecosystems, the quantity and molecular composition of the organic C in aggregates (53-250μm) and silt-clay (<53μm) fractions of invaded and adjacent non-invaded soils were compared using biomarker analysis, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and solution-state 13C nuclear magnetic resonance (13C NMR) spectroscopy.
The C content in aggregate and silt-clay fraction of knotweed invaded soils was similar to the adjacent non-invaded old-field soils, which indicates that the differences in bulk soil C observed in previous study might be contributed by non-mineral associated particulate organic matter. Despite this similar total soil C, the DRIFT analysis indicated that aggregates under knotweed invasion retained higher concentration of alkyl-C. The identity of this alkyl-C was further confirmed to be plant-derived using biomarker analysis that showed an accumulation of plant derived polymers in aggregates of knotweed invaded soils. Influence of invasion on soil C quantity was evident under kudzu invasion, where mineral fractions in kudzu invaded soils had 30% lower organic carbon than non-invaded pine soil. Mineral fractions of kudzu invaded soils reflected a corresponding three-fold decrease in alkyl-C. Despite this decrease in total C, compared to adjacent non-invaded pine soils, the silt-clay fraction of kudzu invaded soil had a similar concentration of plant biomarkers and short-chain fatty acid (mainly microbe-derived), but higher lignin concentration. Overall, across both ecosystems, the increase in bulk soil carbon associated with input of recalcitrant litter seems to be driven by the accumulation of plant derived polymers in aggregates or as particulate organic matter. Labile inputs appeared to reduce the carbon-pool even in the mineral associated fraction potentially by fueling microbial activity. The accumulation of residual plant polymers along with short-chain lipid on silt-clay implied that high microbial activity could potentially facilitate the stabilization of organic carbon under input of labile litter.