In-drain denitrifying woodchip bioreactors for reducing nitrogen runoff from sugarcane

Denitrifying woodchip bioreactors offer an opportunity to intercept and reduce nitrogen loads between agricultural fields and downstream aquatic ecosystems. Here we assessed the performance of three in-drain bioreactor beds installed in open drains receiving runoff and shallow ground water from rain-fed sugarcane production in the Wet Tropics of Australia. Drain nitrate-N concentrations were generally low, with a mean of 0.2 mg L−1 and maximum of 3.3 mg L−1, which may have been partially due to denitrification in the contributing soils. Bioreactors reduced the concentration of nitrate-N in intercepted waters (average 41% reduction). However, removal rates were often limited by nitrate-N availability. Load reduction over the 2018/19 season was just 0.11 kg N ha−1 yr−1. This limited performance was in large part due to the dynamic nature of nitrogen loads in this system. Specifically, a high proportion of the annual nitrogen load occurring during ‘first-flush’ events immediately after fertilization (i.e. 72% during a 10-day period in 2018/19) resulting in considerable bypass flow (i.e. low interception). Our study is the first in the Australian Wet Tropics to assess the annualized performance and total load reductions of in-drain denitrifying bioreactor beds. Although showing their potential, our results highlight the fact that denitrifying bioreactor performance is determined by the hydraulic context of the catchment, and the proportion of the annual nitrate load that can be successfully intercepted.