Leaf litter of two riparian tree species has contrasting effects on nutrients leaching from soil during large rainfall events

Background and aims: One goal of revegetating riparian zones is to mitigate nutrient movement into waterways, however release of dissolved nutrients and organic compounds from decaying leaf litter during rainfall may partially offset these benefits. We aim to understand how litter-derived nutrients and organics affect nutrient leaching from riparian soil, and how tree species affect these processes. Methods: In the laboratory we simulated large rainfall events through the leaf litter of two riparian woody tree species, Eucalyptus tereticornis and Casuarina cunninghamiana, with and without subsequent leaching through soil, as well as soil alone. We compared the forms and loads of nitrogen and phosphorus in leachate. Results: Litter of both tree species stimulated additional nitrate release from soil (~100–200%) compared to bare soil. More ammonium and dissolved organic nitrogen also leached from soil with E. tereticornis litter, whereas less leached with C. cunninghamiana litter. More phosphate leached from soil with either species litter compared to bare soil. Increased nitrogen releases from soil with litter were greater than inputs from litter, suggesting they are driven by positive priming of soil microbial processes, whereas litter inputs increased phosphorus leaching. Conclusions: This information could be combined with rates of other riparian nutrient processes (e.g. plant uptake and denitrification) to quantify the net impact of trees on nutrient retention in riparian zones.