Nitrate removal and greenhouse gas production of woodchip denitrification walls under a humid subtropical climate

Abstract

Denitrification walls are a low-cost technology with the capability to reduce nitrogen (N) loading in shallow groundwater beneath agricultural systems. The aims of this study were to quantify the effect of different carbon (C) substrates on nitrate removal rate (NRR) and greenhouse gas (GHG) production in two soil-capped denitrification walls (volume ≈ 27 m3) under subtropical climate conditions. The relative performance of softwood and hardwood woodchips to promote denitrification was tested over a 2-year program of weekly monitoring, during which water samples were collected for nitrate (NO3−) and dissolved GHG analysis. Both the softwood and the hardwood wall had similar average NRR (2.0 and 1.6 g N m−3 d−1, respectively) but were NO3− limited, and acted as a sink for nitrous oxide (N2O) produced in the walls and dissolved in the aquifer. Both walls produced carbon dioxide (CO2) and methane (CH4), with the hardwood producing respectively 3-fold and 2.5-fold higher fluxes compared to the softwood. Calculation of the Global Warming Potential (GWP) permitted a comparison of the GHG emissions within the walls in terms of CO2 equivalents (CO2-eq). Both the walls emitted CO2-eq lower than natural environments, with the softwood producing null emissions and the hardwood emitting 65-fold higher than softwood. The results of the present study suggest that woodchip bioreactors may be used to reduce nutrient loading from agricultural areas into surrounding aquatic environments as well as to decrease GHG emissions under subtropical climates, with softwood being a preferable substrate.