Manure application magnifies methane emission from drainage ditch sediments

Abstract

Agricultural drainage ditches have been recently identified as hotspots of methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) emissions. Ditches often experience inputs of fertilizers from adjacent agricultural fields, which increases the availability of organic carbon and nutrients in ditch sediments, thereby fueling greenhouse gas (GHG) production. Here, we quantify the effects of two types of fertilizers (manure and artificial fertilizer) on GHG dynamics from agricultural drainage ditch sediments subjected to oxic and anoxic conditions. We first measured rates of potential sediment N₂O production, CH₄ production, and aerobic CH₄ oxidation under different fertilizer doses. We observed that manure additions (expressed here as ammonium (NH₄⁺) dose, referring to the resulting NH₄⁺ concentration in a given bottle after addition) strongly stimulated CH₄ and N₂O production. Methane production rates increased approximately threefold as a result of manure additions, ranging from 18.4 to 61.7 µmol CH₄ gDW⁻¹ d⁻¹, whereas N₂O production rates increased approximately 16-fold, and varied from 0.1 to 1.6 µmol N₂O gDW⁻¹ d⁻¹ across manure doses. Aerobic CH₄ oxidation was also stimulated by manure addition, while at resulting NH₄⁺ concentrations above 2 mmol L⁻¹, oxidation rates declined. In contrast, artificial fertilizer caused immediate inhibition of CH₄ and N₂O production and aerobic CH₄ oxidation, even at the lowest NH₄⁺ concentration tested (0.05 mmol L⁻¹). Focusing on the effects of manure on GHG emissions, we observed that under anoxic conditions, sediment cores receiving high manure inputs emitted approximately 3.5 times more GHGs (in CO₂-equivalents) than anoxic controls (no manure) and about 5.5 times more than oxic controls. Methane was the dominant driver of increased emissions at higher manure doses. As fertilizer use continues to rise globally, these results highlight the importance of implementing climate-smart water and nutrient management strategies in ditches and adjacent grasslands to mitigate climate trade-offs.