No abstract

Denitrifying bioreactors can be effective for removing nitrate from agricultural tile drainage; however, questions about cold springtime performance persist. The objective of this study was to improve the nitrate removal rate (NRR) of denitrifying bioreactors at warm and cold temperatures using agriculturally derived media rather than wood chips (WC). Corn ( L.) cobs (CC), corn stover (CS), barley ( L.) straw (BS), WC, and CC followed by a compartment of WC (CC+WC) were tested in laboratory columns for 5 mo at a 12-h hydraulic residence time in separate experiments at 15.5 and 1.5°C. Nitrate-N removal rates ranged from 35 to 1.4 at 15.5°C and from 7.4 to 1.6 g N m d at 1.5°C, respectively; NRRs were ranked CC > CC+WC > BS = CS > WC and CC ≥ CC+WC = CS ≥ BS > WC for 15.5 and 1.5°C, respectively. Although NRRs for CC were increased relative to WC, CC released greater amounts of carbon. Greater abundance of nitrous oxide (NO) reductase gene () was supported by crop residues than WC at 15.5°C, and CS and BS supported greater abundance than WC at 1.5°C. Production of NO relative to nitrate removal (NO) was consistently greater at 1.5°C (7.5% of nitrate removed) than at 15.5°C (1.9%). The NO was lowest in CC (1.1%) and CC-WC (0.9%) and greatest in WC (9.7%). Using a compartment of agricultural residue media in series before wood chips has the potential to improve denitrifying bioreactor nitrate removal rates, but field-scale verification is needed.

The watershed-scale effects of agricultural conservation practices are not well understood. A baseline calibration and an input parameter sensitivity analysis were conducted for simulation of watershed-scale hydrology in the Little River Experimental Watershed (LREW) in the Coastal Plain near Tifton, Georgia. The Soil and Water Assessment Tool (SWAT) was manually calibrated to simulate the hydrologic budget components measured for the 16.9 km 2 subwatershed K of the LREW from 1995 to 2004. A local sensitivity analysis was performed on 16 input variables. The sum of squares of the differences between observed and simulated annual averages for baseflow, stormflow, evapotranspiration, and deep percolation was 19 mm 2 ; average annual precipitation was 1136 mm. The monthly Nash-Sutcliffe model efficiency (NSE) for total water yield (TWYLD) was 0.79 for the ten-year period. Daily NSE for TWYLD was 0.42. The monthly NSE for three years with above-average rainfall was 0.89, while monthly NSE was 0.59 for seven years with below annual average rainfall, indicating that SWAT's predictive capabilities are less well-suited for drier conditions. Monthly average TWYLD for the high-flow winter to early spring season was underpredicted, while the low-flow late summer to autumn TWYLD was overpredicted. Results were negatively influenced when seasonal tropical storms occurred during a dry year. The most sensitive parameters for TWYLD were curve number for crop land (CN2(crop)), soil available water content (SOL_AWC), and soil evaporation compensation factor (ESCO). The most sensitive parameters for stormflow were CN2(crop), curve number for forested land (CN2(forest)), soil bulk density (SOL_BD), and SOL_AWC. The most sensitive parameters for baseflow were CN2(crop), CN2(forest), ESCO, and SOL_AWC. Identification of the sensitive SWAT parameters in the LREW provides modelers in the Coastal Plain physiographic region with focus for SWAT calibration.

Phosphorus (P) losses from agricultural landscapes arise from the interaction of hydrologic, edaphic, and management factors, complicated by their spatial and temporal variability. We monitored sites along two agricultural hillslopes to assess the effects of field management and hydrology on P transfers in surface runoff at different landscape positions. Surface runoff varied by landscape position, with saturation excess runoff accounting for 19 times the volume of infiltration excess runoff at the north footslope position, but infiltration excess runoff dominated at upslope landscape positions. Runoff differed significantly between south and north footslopes, coinciding with the extent of upslope soil underlain by a fragipan. Phosphorus in runoff was predominantly in dissolved reactive form (70%), with the highest concentrations associated with upper landscape positions closest to fields serving as major sources of P. However, the largest loads of P were from the north footslope, where runoff volumes were 24 times larger than from all other sites combined. Loads of P from the north footslope appeared to be primarily chronic transfers of desorbed soil P. Although runoff from the footslope likely contributed directly to stream flow and hence to stream water quality, 27% of runoff P from the upslope sites did not connect directly with stream flow. Findings of this study will be useful for evaluating the critical source area concept and metrics such as the P-Index.