Increased Duration of Drying–Rewetting Cycles Increases Nitrate Removal in Woodchip Bioreactors

Abstract

D enitrifying bioreactors (also known as woodchip bioreactors) are an agricultural beneficial management practice (BMP) used for the removal of nitrate (NO 3) in water discharged from farms (Schipper et al., 2010; Addy et al., 2016). Denitrifying bioreactors have been shown to be effective for reducing NO 3. Previous applications of woodchip bioreactors include subsurface drainage water from crop (Woli et al., 2010; Christianson et al., 2012; David et al., 2016), dairy (Schipper et al., 2010) and hog farm fields (Liu, 2017), aquaculture units (Lepine et al., 2016), and hydroponics operations (Warneke et al., 2011), with reported volumetric removal rates in the range of 0.3 to 16.0 g N m-3 d-1 (Addy et al., 2016). A previously reported study conclusively showed that exposing woodchip bioreactors to weekly, 8-h drying-rewetting (DRW) cycles increased NO 3 removal rates by 30 to 80% relative to constantly saturated conditions (Maxwell et al., 2018). Greater dissolved organic carbon (DOC) in the effluent likely explained removal increases, since aerobic breakdown should produce more labile C to be used during denitrification. The previous study only considered an 8-h duration DRW cycle. If periods in which woodchips are unsaturated increase subsequent release of labile C and NO 3 removal once woodchips are rewetted, longer periods of aerobic conditions should increase C release and NO 3 removal rates. The objectives of this study were (i) to determine if the duration of unsaturated conditions affects NO 3 removal rates and (ii) to qualify the relationship of DOC with NO 3 removal rates and its role in increased rates following DRW cycles. These research objectives were explored through a 105-d column experiment as a follow-up experiment to the previously published study (Maxwell et al., 2018).