Mitigating Nitrate Levels in Agricultural Runoff with Barrier Crops

Agricultural runoff water is the leading cause of non-point source pollution in the United States. In addition, excess nitrogen fertilizer can be a potential source for groundwater contamination. The inclusion of a “bio-filter” in any crop management system could optimize nutrient uptake and minimize leaching. In a study aimed at assessing quality of agricultural runoff and leachate which would ultimately be discharged into the Salton Sea, California, we compared the bio-filtering capability of two forages. The experimental design consisted of alternating beds of Elephant grass (EG) and Bermuda grass (BG) subjected to three fertigation rates (T1, T2 and T3) with four replicates. Nitrate (NO3-N), total nitrogen and total suspended solids were determined in the source and runoff waters during each irrigation event to evaluate the effectiveness of the two forages as barrier crops. Additionally, soil water quality at 60 cm and 120 cm was monitored using suction lysimeters to assess solute movement through the soil profile and determine the role of the grasses in reducing nitrate contamination below the root zone. Forage samples were taken after each harvest to compare the nitrogen uptake capability of the two grasses. Results to date indicate the EG appears to be a better barrier crop than the BG for controlling nitrate levels in agricultural runoff. For example, irrigation water with average NO3-N levels of 3.9 ppm (T1), 4.6 ppm (T2) and 11.3 ppm (T3) resulted in NO3-N levels in runoff ranging from 0.1 ppm to 1.5 ppm for the EG, and from 0.6 ppm to 5.7 ppm for BG. For the forages harvested at the end of the corresponding irrigation cycle, NO3-N levels ranged from 4,030 ppm to 14,800 ppm for the EG, and from 730 ppm to 5,020 ppm for the BG in fields subjected to the lowest and highest fertigation rates, respectively.