The Performance of Controlled Drainage and inline Denitrifying Woodchip Bioreactor for Reducing Nutrient losses from Subsurface Drained Grassland Receiving Liquid Swine Lagoon Effluen

Over application of livestock manure has become a principal nutrient source in groundwater and surface water. Controlled drainage (CD) and denitrifying bioreactors have been used to reduce nutrient losses from artificially drained agricultural land to surface waters. The overall goal of this research was to evaluate the performance of CD and denitrifying woodchip bioreactors for reducing nutrient losses from a subsurface drained grass field receiving liquid swine lagoon effluent (SLE). A four-year field experiment was conducted on a 1.25 ha pasture in eastern North Carolina. Eight subsurface drains (1.0 m depth and 12.5 m spacing), including four experimental drains and four guard drains, were installed in the naturally poorly drained field. Four drains were managed in CD mode with drain outlet set at 36 cm below surface while the remaining four drains were managed in free drainage (FD) mode. Denitrifying bioreactors were installed at the edge of the four experimental drains.;Compared to FD, CD reduced annual subsurface drainage volume by 88% to 98% and raised the mean daily water table by 15 cm. The DRAINMOD model was used to simulate the hydrology of the drained field under both FD and CD scenarios and predict the main components of the water balance. Statistical performance measures indicated acceptable to excellent agreement between predicted and measured water table depth and daily drainage. Results showed clearly that seepage was a significant component of the water balance for the CD plots.;Compared to FD, CD reduced annual load of total nitrogen (TN) in subsurface drainage by 87% to 95%. The estimated population mean (EPM) of nitrate and TN concentrations in drainage water for CD treatment (4.10 and 6.95 mg L -1, respectively) were significantly lower than that from FD treatment (7.52 and 9.06 mg L-1, respectively). The EPM of nitrate concentration in groundwater at three depths (75--225 cm) in CD plots were significantly lower than that from FD plots. Annual load reduction of total phosphorus (TP) through subsurface drain lines in CD treatment ranged from 76% to 95%. The EPM of TP concentration in drainage water for CD plots was 0.18 mg L-1, which was significantly higher than that for FD plots (0.1 mg L-1). The difference of P concentration between CD and FD plots was mainly due to the significant difference of particulate P concentration. Reduced drainage volume, enhanced denitrification, and to a far lesser extent increased grass uptake of N and P during dry growing condition contributed to the observed reduction in N and P loading via subsurface drainage under CD treatment.;All bioreactors significantly reduced nitrate concentrations. Yearly percent nitrate reduction for CD-bioreactor (CDB) and FD-bioreactor (FDB) systems during study period ranged from 48 +/- 22% to 87 +/- 6% and 21 +/- 8% to 51+/- 8%, respectively. Nitrate removal rates increased with water flow rate, initial nitrate concentration, hydraulic retention time (HRT), and temperature; however, the temperature effect was not as strong as the other factors. Longer than needed HRT would also negatively affect nitrate removal rate of bioreactors. Percent nitrate load reduction was affected by the volume of flow that passes through bioreactors rather than bypass pipes. The portion of the water flowing through bioreactors (three out of four) decreased from 2012 to 2014 due to decreasing of estimated saturated hydraulic conductivity of the bioreactor systems. Upflow in-line bioreactors showed sufficient removal of nitrogen loading from drained pasture lands. The practicality of bioreactors was not only related to carbon consumption longevity, but also related to proper maintenance of anaerobic condition, suitable hydraulic conductivity, and appropriate HRT, as well as the management of the whole field drainage system.