Mulch derived organic carbon stimulates high denitrification fluxes from agricultural ditch sediments

Authors: Nifong, Rachel; Taylor, Jason; Moore, Matthew

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2018
Publication Date: 2/14/2019
Citation: Nifong, R.L., Taylor, J.M., Moore, M.T. 2019. Mulch derived organic carbon stimulates high denitrification fluxes from agricultural ditch sediments. Journal of Environmental Quality. https://doi.org/10.2134/jeq2018.09.0341.
DOI: https://doi.org/10.2134/jeq2018.09.0341

Interpretive Summary: Nitrogen fertilizer supports high agricultural production. Yet runoff of excess fertilizer from agricultural fields into water bodies has unintended negative outcomes including major algal blooms in our nation’s estuaries and so called dead zones that impact other industries such as tourism and fisheries. One approach to dealing with this issue is to stimulate a process called denitrification which transforms excess nitrogen into unreactive nitrogen gas. This process is conducted by bacteria which require sources of carbon to facilitate denitrification. We conducted a study comparing the ability of low cost agricultural amendments to increase denitrification in order to establish effective solutions to improve ecosystem outcomes while maintaining agricultural production. Results indicate that overlying hardwood mulch additions either as mulch alone or mulch and gypsum significantly enhance the capacity of agricultural ditch sediments to remove nitrogen from runoff and reduce negative agricultural impacts. The addition of an overlying hardwood mulch layer to aquatic agricultural ditch sediments could be an effective, low-cost tool to reduce impacts of nitrogen pollution while maintaining agricultural production. These results should be of interest to farming communities and natural resource managers who are seeking to improve long-term sustainability and maintain robust agricultural economies.

Technical Abstract: Reactive nitrogen (N) is an essential input for healthy, vibrant crop production; yet excess N is often transported off-field via agricultural ditches to downstream receiving ecosystems where it can cause negative impacts to human health, biodiversity loss, as well as eutrophication and resultant hypoxia. Denitrification, the transformation of reactive N to unreactive N2 gas, within agricultural ditches has potential to reduce impacts to downstream ecosystems but requires significant carbon (C) substrates. We utilized a flow-through intact core experiment to test the effects of low-cost management options including a common agricultural amendment, gypsum, and an overlying hardwood mulch layer, on promoting denitrification within agricultural ditch sediments. We found significantly higher denitrification potentials in mulch and mulch/gypsum cores than in gypsum or control cores. Higher denitrification rates corresponded with high dissolved organic carbon (DOC) fluxes within the mulch and mulch/gypsum treatments and were ultimately able to remove 65-69% of N loads. Results indicate organic carbon from overlying mulch additions to agricultural ditches significantly increase denitrification in intact cores and suggest the addition of DOC sources in agricultural ditches may contribute a simple, low-cost option to reduce reactive N export and improve ecological outcomes within aquatic agroecosystems.