Much of the runoff from agricultural fields in the southeastern Coastal Plain is carried to a stream channel system in a shallow phreatic aquifer. This subsurface runoff often passes through a band of riparian forest before becoming streamflow. It is hypothesized that the riparian ecosystem acts as a nutrient sink and reduces the concentrations and loads of nutrients in the shallow aquifer before the nutrients reach the stream channel. Concentrations and loads (kg/m2) of NO3‐N, NH4‐N, organic N, dissolved molybdate‐reactive P1 total P, Ca, Mg, K, Cl, and SO4‐S were measured in shallow phreatic wells at 37 locations on an agricultural watershed near Tifton, Ga. Total water volume moving off the watershed in subsurface flow was < 1% of streamflow with corresponding small amounts of nutrients. Nitrate‐N, Ca, and Mg had significantly higher concentrations in field wells than in forest or streamside wells. Concentrations of Cl were not reduced as water moved from field to forest. Processes within the riparian zone apparently converted primarily inorganic N from fields (76% NO3‐N, 6% NH4‐N, 18% organic N) into primarily organic N in streamside wells (10% NO3‐N, 14% NH4‐N, and 76% organic N). Concentration differences between field and forest wells indicated the riparian forest's ability to act as a sink for NO3‐N, Ca, Mg, K, and SO4‐S. Due to their role as nutrient sinks, riparian forests are important in maintaining stream water quality on agricultural watersheds.
The effectiveness of a grassed waterway in decreasing 2,4‐D [(2,4‐dichlorophenoxy) acetic acid] content in surface runoff was investigated. Corn (Zea mays L.) plots were treated with 2,4‐D (0.56 kg/ha) and runoff produced by applying simulated rain was directed through a 24.4‐m‐long grassed waterway. The 2,4‐D concentrations were measured under wet and dry antecedent waterway and plot conditions. Reduction in 2,4‐D load in waterways results from water loss by infiltration, sediment loss, and by attachment‐absorption on vegetative and organic matter. Of the simulated rainfall applied 1 day after application of 2,4‐D, 50% of the water ran off the plots under dry antecedent soil conditions, and 78% ran off under wet conditions. Infiltration reduced runoff flowing down the waterway an additional 25% under dry conditions and 2% under wet conditions. Suspended sediment reduction in the waterway was 98 and 94% of the total amount moving from the plot for the dry and wet waterway conditions, respectively. The total loss (on sediment and in solution) of the applied 2,4‐D from the plot in the dry and wet states was 2.5 and 10.3%, respectively. Of the 2,4‐D lost from the plots and entering the 24.4‐m waterway, approximately 30% reached the end of the waterway, regardless of antecedent soil moisture.
Watershed—level agroecosystem studies are essential to relate land management to the external environmental effects produced by agricultural nutrients and to enhance our understanding of agricultural nutrient cycles. Inputs and outputs of N, P, K. Ca, Mg, and Cl were determined for four subwatersheds of the Little River in the Georgia Coastal Plain from 1979 through 1981. The four watersheds had 40, 36, 54, and 50%, respectively, of their land in agricultural uses (row crop and pasture). Prescipitation inputs and streamflow outputs were determined by field sampling of water volumes and nutrient concentrations. Agronomic inputs (from fertilizer and symbiotic N—fixation) and outputs in harvested material were estimated from land use data; countrywide averages of fertilizer applications and crop yield, and plot studies on peanuts and soybeans. All elements except Cl had greater inputs than outputs on each watershed each year. The general order of streamflow loads was Cl > Ca > K > Mg> N > P. Fertilizer inputs exceeded precipitation inputs for all elements on all watersheds. Outputs on N, P, and K in harvest generally exceeded streamflow loads, but harvest outputs of Ca, Mg, and Cl were generally lower than streamflow loads. The two watersheds with more agricultural land had consistently higher loads of N, K, Ca, Mg, and Cl in streamflow and had No3—N loads 1.5 to 4.4 times higher than loads from the less agricultural watersheds. Streamflow loads on the Little River watersheds were similar to those on other Coastal Plain agricultural watersheds with comparable land use and discharge volumes. Budgets for the upland portion of one of the watersheds indicated that large amounts of N, P, K, Ca, and Mg were not accounted for. about 56 kg°ha—1@?yr—1 of N were retained or lost to gaseous emissions from the uplands. Apparently, a large percentage of the nutrients applied to these watersheds was being retained somewhere in the watershed or being lost in some unquantified way.
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