Biological and physical effects of non-native earthworms on nitrogen cycling in riparian soils

Global nitrogen cycling is being altered by anthropogenic disturbances including invasion by non-native species. European and Asian earthworms have invaded northern temperate forests in North America with dramatic consequences for litter thickness, forest floor plant diversity, and soil nitrogen cycling. Invasive earthworms present at the boundary of terrestrial and aquatic ecosystems (i.e., riparian zones) may alter the flux of nitrogen into adjacent aquatic ecosystems. We examined how nitrogen cycling in riparian soil responds to amendments of invasive earthworms or artificial earthworm burrows. In earthworm-free riparian plots (0.25 m2), we established treatments of invasive earthworms (60 g fresh mass·m−2), artificial burrows (120 m−2), or control plots and sampled the plots after 30 days. Before and after treatment application we measured major soil characteristics (water-filled pore space, organic matter, and pH), nitrogen pools (exchangeable NH4+ and NO3−), and nitrogen transformation rates (net N-mineralization, net nitrification, and denitrification). Exchangeable NH4+ and NO3− changed through time but did not differ among treatments. Net N-mineralization and net nitrification rates did not change through time and were similar across all treatments. However, denitrification rates in plots with added earthworms were 4 times greater than rates in control and burrow-only plots, which represents a large rapid increase in gaseous nitrogen flux out of these riparian soils. For all response variables, artificial burrows responded similarly to control plots, suggesting that earthworm biological activity (i.e., feeding, excretion, and mucus production) rather than physical effects (i.e., burrowing and soil aeration) drove the changes in nitrogen cycling. Examination of soil nitrogen pool and flux measurements suggest that this increase in denitrification was coupled with NH4+ consumption by nitrifying bacteria, but future studies are needed to confirm this hypothesis. We conclude that the activity of invasive earthworms in riparian zones can increase the flux of N out of riparian zones, but the hydrologic context of the riparian zone (e.g., pore-water residence time) ultimately controls whether denitrification or nitrate leaching is the dominant flux of N.