Groundwater nitrogen and phosphorus dynamics under cattle grazing and row crop management in two contrasting soils in Missouri
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Nutrient pollution of water resources has been a major environmental problem for decades. Excessive use of chemical fertilizer and livestock management results in agricultural non-point source pollution. The regional characterization of nutrient contamination in groundwater is important for the identification of the groundwater zones receiving heavy nutrient loads from the surface. The objectives of this study were, (1) To quantify landscape and buffer effects on NO3-N and TN in groundwater under rotational cattle grazing, (2) To quantify landscape and buffer effects on PO4-P and TP transport in groundwater under rotational cattle grazing, and (3) To access the spatial and temporal variations of NO3-N, TN, PO4-P and TP concentrations in groundwater under row crop management. The experiment was conducted in two different watersheds. (1) Horticulture and Agroforestry Research Center (HARC) near New Franklin, Missouri and (2) The Romine experimental watershed within the Good Water Creek Experimenatl Watershed (GCEW) near Centralia, Missouri. The experimental design at HARC consisted of two rotationally grazed watersheds with a grass buffer (GB) and an agroforestry buffer (AB). The predominant soil within the treatment watersheds is Menfro silt loam. The grazed portion of both watersheds had common forage species; tall fescue [Schedonorus phoenix (Scop.) Holub], red clover (Trifolium pratense L.), and lespedeza (Lespedeza Michx)., and the AB treatment had four rows of poplar trees (Populus tremula L.) at the footslope. A transect of three wells was installed at summit, backslope, and footslope positions in both watersheds. Wells were identified by the buffer type and landscape position as AB summit, AB backslope, AB footslslope, GB summit, GB backslope and GB footslope. The Romine experimental watershed consisted of crop rotation (corn, wheat, soybeans, and sorghum), grassland and woodland. The watershed is under conservation tillage. The soils within the watershed comprise naturally formed argillic horizon located between 0.15 and 0.3 m below the soil surface. Three groundwater monitoring wells were installed in strategic locations around the resurgent flow area and named as Romine north (RN), Romine south (RS) and Romine west (RW). A series of shallow piezometers were also installed across a catena sequence from summit to the footslope and numbers were given 1-7. Out of all the piezometers four piezometers had no water during the study period. The productive piezometers were identified as PZ 4, PZ 6 and PZ 7. Weekly groundwater samples were collected from both study sites and analyzed for NO3-N, TN, PO4-P and TP concentrations from December 2014 to December 2016. At HARC both NO3 --N and TN concentrations were significantly lower at the footslope than at the summit and backslope (p<0.001). The median concentrations of NO3-N and TN were 0 mg L-1 and 0.24 mg L-1 at the footslope, compared to the median concentrations >1.4 mg L-1 at the summit and backslope. In addition, the NO3 --N/Clˉ ratio was significantly lower at the footslope than at the summit and backslope, suggesting that denitrification was the main process for the very low NO3-N concentrations at the footslope. Seasonal differences in TN and NO3-N were most apparent in the footslope wells, and the seasonal pattern in concentrations was different between the two watershed treatments. Overall, the results showed that landscape position most affected TN and NO3-N concentrations in groundwater and denitrification was the major cause of the very low NO3-N concentrations at the footslope. The presence of trees in the AB treatment further reduced NO3-N concentrations at the footslope. In the same study site, median 2015 AB (0.14 mg L-1) and GB (0.2 mg L-1) footslope TP were also significantly greater (p<0.001) than the concentrations in 2016 (AB 0.04 mg L-1 and GB 0.08 mg L-1). Median PO4-P and TP at the footslope were significantly lower (p<0.001) than the concentrations of <0.1 mg L-1 PO4-P and <0.1 mg L-1 TP at the summit and backslope in 2016 with low precipitation. Median PO4-P and TP concentrations at footslope of AB and GB were not significantly different (p=0.20). Orthophosphate showed a significant positive correlation with NH4-N, suggesting that reduced conditions promoted dissolution of Fe and Mn phosphates which increased the release of P to groundwater. The results demonstrated that significant P transport to groundwater occurred in these loess soils following long-term cattle grazing, and P concentrations in groundwater increased with greater precipitation and low redox conditions. At the Romine experimental site, median NO3-N concentrations were significantly greater at Romine north (RN>8.15 mg L-1) and Romine west (RW>9.18 mg L-1) (NO3-N, p<0.001; TN, p<0.001) compared to Romine south (RS<0 mg L-1) well. TN concentration also followed the same pattern but with elevated concentrations (RN and RW>8.5 mg L-1, RS<0.17 mg L-1). Significantly greater median PO4-P (0.079 mg L-1, p<0.001) and TP (0.096 mg L-1, p<0.001) concentrations were observed at RS as compared to concentrations at RN and RW (<0.005 mg L-1 PO4-P and < 0.006 mg L-1 TP). Out of three piezometers backslope piezometer (PZ 6) showed greater NO3-N, TN, PO4-P and TP than summit (PZ 4) and footslope (PZ 7) piezometers., but with relatively low concentrations. Overall results showed that preferential flow through the soil and hydraulic conductivity of the subsurface strata controlled NO3-N transport in this claypan watershed and the PO4-P concentrations may have mediated by redox conditions of the well. The general groundwater nutrient pattern between two study sites showed greater N (NO3-N and TN) and P (PO4-P and TP) concentrations in crop cultivated site with claypan soils (GCEW) than the cattle managed site with loess soils. Denitrification was observed in both watersheds under low redox conditions which caused to decrease NO3- N concentration in groundwater. However, the low redox conditions also have caused to increase the PO4-P concentration in groundwater in both study watersheds.
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