Greenhouse gas emissions and extracellular enzyme activity variability during decomposition of native versus invasive riparian tree litter

Invasive plants alter riparian vegetation communities and shift biogeochemical processes by changing decomposition rates and the soil chemical environment created by leaf litter. It is unclear if this mechanism shifts nutrient dynamics favoring invasive dominance; riparian areas in the Southwestern USA invaded by salt cedar and Russian olive often still host mixed stands of native plants. To test the hypothesis that invasive plant success is related to altered litter inputs, microbial activity and nutrient cycling, we performed laboratory incubations examining greenhouse gas emissions and microbial extracellular enzyme activity (EEA). The responses of GHG flux and EAA were measured from decomposing translocated litter from native and invasive woody perennials between soils where they were growing. Litter decomposition from two invasive species (salt cedar and Russian olive) and two native trees (coyote willow and Fremont cottonwood) were tracked for 3 months. Soil respiration, carbon content and EEA were all more closely related to soil origin than leaf litter species. The highest decomposition rate was from willow soil. Soil nitrate at the end of the experiment was highest for soils collected under cottonwood. Nitrous oxide (N₂O) emissions were significantly greater from Russian olive litter than other species, on all soil types. Patterns observed here suggest that (1) plant influences on local soil properties over the lifetime of a plant have a greater control on decomposition processes than short-term litter input source, (2) EEA is strongly related to available C resources, and (3) the invasive shrub Russian olive may be responsible for previously undocumented large N₂O emissions in riparian systems in the USA.