Leaf litter additions enhance stream metabolism, denitrification, and restoration prospects for agricultural catchments

Globally intensive agriculture has both increased nitrogen pollution in adjacent waterways and decreased availability of terrestrially derived carbon frequently used by stream heterotrophs in nitrogen cycling. We tested the potential for carbon additions via leaf litter from riparian restoration plantings to act as a tool for enhancing denitrification in agricultural streams with relatively high concentrations of nitrate (1.3–8.1 mg/L) in Canterbury, New Zealand. Experimental additions of leaf packs (N   =   200, mass   =   350 g each) were carried out in 200‐m reaches of three randomly selected treatment streams and compared to three control streams receiving no additional leaf carbon. Litter additions increased ecosystem respiration in treatment streams compared to control streams but did not affect gross primary production, indicating the carbon addition boosted heterotrophic activity, a useful gauge of the activities of microbes involved in denitrification. Bench‐top assays with denitrifying enzymes using acetylene inhibition techniques also suggested that the coarse particulate organic matter added from leaf packs would have provided substrates suitable for high rates of denitrification. Quantifying denitrification directly in experimental reaches by open‐channel methods based on membrane inlet mass spectrophotometry indicated that denitrification was around three times higher in treatment streams where litter was added compared to control streams. We further assessed the potential for riparian plantings to reduce large‐scale downstream nitrogen losses through increasing in‐stream denitrification by modeling the effects of increasing riparian vegetation cover on nitrogen fluxes. Here, we combined estimates of in‐stream ecosystem processes derived from our experiment with a network model of catchment‐scale nitrogen retention and removal based on empirical measurements of nitrogen flux in this typical agricultural catchment. Our model indicated leaf inputs associated with increased riparian cover had the potential to double the catchment level rate of denitrification, offering a promising way to mitigate nitrate pollution in agricultural streams. Altogether, our study indicates that overcoming carbon limitation and boosting heterotrophic processes will be important for reducing nitrogen pollution in agricultural streams and that combining empirical approaches for predictions suggests there are large potential benefits from riparian re‐vegetation efforts at catchment scales.