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

Abstract

. 2017. Leaf litter additions enhance stream metabolism, denitrification, and restoration prospects for agricultural catchments. Ecosphere 8(11):e02018. 10. 1002/ecs2.2018 Abstract. 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.

Carbon sequestration in riparian forests: A global synthesis and meta‐analysis

“…Riparian forest restoration is already widely recognized for providing multiple benefits, including (1) creating transition zones between water channels and adjacent land uses that can improve water quality and store flood water (Daigneault et al, 2017;Naiman et al, 2010;O'Brien et al, 2017); (2) enhancing fish and wildlife habitat (Dybala, Engilis, Trochet, Engilis, & Truan, 2018;Golet et al, 2008;Jansen & Robertson, 2001); and (3) providing recreational opportunities such as wildlife watching, fishing, and hunting that can help support local economies (Carver, 2013;Carver & Caudill, 2013;Golet et al, 2006). Our results demonstrate that riparian forests have a strong potential to contribute to carbon sequestration, which should be considered an additional co-benefit of riparian restoration.…”

“…Reference rates of carbon stock accumulation have been compiled for many forest types (e.g., IPCC, 2006), but these do not typically distinguish between riparian and upland forests. Further, riparian ecosystems are widely recognized to provide numerous ecosystem services (Daigneault, Eppink, & Lee, 2017;Naiman et al, 2010;O'Brien et al, 2017), having the potential to mitigate the effects of climate change (Capon et al, 2013), and being biodiversity hotspots that provide critical habitat for fish and wildlife (Knopf, Johnson, Rich, Samson, & Szaro, 1988;Naiman et al, 2010). Further, riparian ecosystems are widely recognized to provide numerous ecosystem services (Daigneault, Eppink, & Lee, 2017;Naiman et al, 2010;O'Brien et al, 2017), having the potential to mitigate the effects of climate change (Capon et al, 2013), and being biodiversity hotspots that provide critical habitat for fish and wildlife (Knopf, Johnson, Rich, Samson, & Szaro, 1988;Naiman et al, 2010).…”

“…Despite their relatively small spatial footprint, riparian forests will usually have more favorable growing conditions (e.g., soil moisture), and they may accumulate carbon stocks at a greater rate than upland forests (Matzek, Stella, & Ropion, 2018;Naiman, Decamps, & McClain, 2010;Sutfin, Wohl, & Dwire, 2016), contributing more to rapid carbon sequestration in the short-term. Further, riparian ecosystems are widely recognized to provide numerous ecosystem services (Daigneault, Eppink, & Lee, 2017;Naiman et al, 2010;O'Brien et al, 2017), having the potential to mitigate the effects of climate change (Capon et al, 2013), and being biodiversity hotspots that provide critical habitat for fish and wildlife (Knopf, Johnson, Rich, Samson, & Szaro, 1988;Naiman et al, 2010). Because riparian ecosystems have been severely degraded worldwide (Nilsson & Berggren, 2000;Perry, Andersen, Reynolds, Nelson, & Shafroth, 2012;Zedler & Kercher, 2005), riparian forest restoration may be a valuable strategy for providing both rapid carbon sequestration value and long-term ecosystem services returns.…”

Leaf litter identity alters the timing of lotic nutrient dynamics

“…Although our results line up with predictions based on both terrestrial and aquatic literature surrounding shifts in microbial decomposer nutrient demands (Cheever et al, 2012;García-Palacios et al, 2017;Manzoni et al, 2010;Melillo et al, 1984), not all of the uptake in our mesocosms can be attributed to immobilisation and mineralisation in added litter, as is clear from the cumulative net immobilisation estimates in comparison to cumulative uptake of added nutrients. For example, litter quality and quantity can drive denitrification (O'Brien et al, 2017;Stelzer, Scott, Bartsch, & Parr, ). Non-litter associated processes should also influence our observed uptake rates.…”

Net Ecosystem Production of a River Relying on Hydrology, Hydrodynamics and Water Quality Monitoring Stations

“…Dissolved oxygen is also closely related to water mixing, gas exchanges at the air-water interface, water temperature, flow, velocity and irradiance [27]. In addition, dissolved oxygen is subjected to spatial variability according to specifications of land use and local inflows [33][34][35][36][37]. Therefore, our study uses high-frequency data at various locations along the river as a way to contribute to the analysis of spatiotemporal impact of physicochemical properties of water on NEP.…”