Invaded ecosystems as a model to elucidate the influence of plant litter chemistry in facilitating soil organic matter formation

Background/Question/Methods

Plant litter is the precursor of most soil organic matter (SOM) and the major energy source for sustaining most of the heterotrophic soil biota that actively facilitates SOM formation. Invasive plant species that input disproportionate quantities of chemically distinct litter could greatly influence SOM formation.  Yet the impact of the chemistry of plant litter inputs in SOM formation is widely debated. Since the SOM formation is an ecosystem property that develops and flourishes over decadal time scales, the influence of plant identity on SOM composition and stability could be better captured in invaded-ecosystems where a single species dominates and regulates ecosystem processes over decadal time scales. We studied the impact of litter chemistries of two noxious invasive species – Polygonum cuspidatum (Japanese knotweed) that produces recalcitrant litter, and Pueraria lobata (kudzu) that produces labile litter, on the quantity, molecular composition, and stability of organic C in the soils they invade. Soils were size fractionated and the concentration and composition of organic C were analyzed using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy. 

Results/Conclusions

Compared to the adjacent, non-invaded soils inhabited by grasses and forbs, the soils under knotweed had 35 and 69% higher organic C content in silt-clay and micro-aggregate fractions, respectively. Despite receiving a 25% higher biomass input as kudzu invates established Pinus forest, the invaded soils exhibited lower organic C than the adjacent, non-invaded Pinus soils in silt-clay and microaggregate fraction (by 61%). The spectroscopic analysis revealed a lower accumulation of cutin and wax in the silt-clay fraction of knotweed invaded soils compared to the non-invaded soils. The silt-clay fractions in kudzu invaded soils had 52% lower content of cutin and waxes compared to non-invaded Pinus soils.   Our results suggest that the knotweed derived, recalcitrant OM is assimilated more in micro-aggregates and are less associated with silt-clay fraction, which could explain lower proportion of oxidative resistant OC in these soils observed previously. Lower organic carbon quantity in silt-clay fraction under kudzu invasion is likely the consequence of mineralization of Pinus derived C thorough microbial priming.