THE EFFECT OF HERBICIDES (ATRAZINE AND BALANCE) AND GROUND COVERS ON MICROBIAL BIOMASS CARBON AND NITRATE REDUCTION

Authors: LIN, C - CENTER FOR AGROFORESTRY; Lerch, Robert; GARRETT, H - SCHOOL OF NATURAL RES; GEORGE, M - UNIVERSITY OF MISSOURI

Submitted to: North American Agroforestry Conference
Publication Type: Proceedings
Publication Acceptance Date: 10/30/2003
Publication Date: 12/22/2003
Citation: Lin, C.H., Lerch, R.N., Garrett, H.E., George, M.F. 2003. The effect of herbicides (Atrazine and Balance) and ground covers on microbial biomass carbon and nitrate reduction. North American Agroforestry Conference. p. 182-195.

Interpretive Summary: Grass buffers can reduce nutrient levels in soils and water. However, optimizing the effectiveness of grass buffers requires that we know which grass species reduces nutrient levels in soils the most. This ability is important because lower nutrient levels in soils will mean less off-site transport of nutrients to surface or ground water. In addition, grasses used in buffers need to be reasonably resistant to the presence of herbicides (i.e., weed killers) commonly used in agricultural production. Our tests showed that nitrate in soil and groundwater was reduced to some extent by all five grasses tested. Switchgrass, tall fescue, orchardgrass and smooth bromegrass were all found to be highly effective while timothy was the least effective. Switchgrass was most effective at reducing phosphate in groundwater. Two corn herbicides we tested had no effect on the ability of the grasses to reduce nutrient levels in soil or ground water. Nutrient reductions in soil and water were primarily caused by plant uptake of nutrients and water and enhanced conversion of nitrate to various nitrogen gases (a process known as denitrification). This research will benefit growers and state and federal resource managers by providing needed information to improve grass buffer designs for reducing nutrient transport from agricultural fields.

Technical Abstract: Six different forage treatments (orchardgrass, tall fescue, timothy, smooth bromegrass, switchgrass, and bare ground control) were established on 1 m wide X 0.5 m deep lysimeters to evaluate their capacity for reducing nutrients in leachate and soils. Nitrate was uniformly applied to each lysimeter by irrigating with 3 L of solutions containing 50 mg/L of NO3-N with either atrazine (ATR; 500 µg/L) or Balance (isoxaflutole, IXF) (80 µg/L). The leachate from each lysimeter was collected after major rainfall events during a 25-day period. NO3-N in leachate from switchgrass, tall fescue, orchardgrass, and smooth bromegrass treatments was reduced 98.2 to 99.7% compared to bare ground during the experimental period. Similarly, timothy reduced NO3-N from 74.5 to 82.3%. For both herbicide treatments, switchgrass showed the highest capacity to remove the PO4-P from leachate. It reduced PO4-P from 60.0 to 74.2%. Relative to the soil NO3-N level in the bare ground controls, tall fescue, smooth bromegrass, switchgrass, timothy and orchardgrass reduced total NO3-N in soils by 91.2, 89.8, 89.3, 79.0, and 40.9%, respectively. The difference in removal rates between ATR and Balance treatments was not significant. The results of the microbial denitrification work suggested that the denitrification rate was significantly enhanced by the presence of some species. Denitrification capacity was found to be greatest in soil collected from switchgrass and smooth bromegrass lysimeters. Dissipation rates of NO3-N were similar between ATR- and Balance-treated soils. For ATR-treated lysimeters, the maximum denitrification rate strongly correlated with elevated microbial biomass carbon in forage treatments. However, this correlation was poor in the soil collected from Balance-treated lysimeters. Switchgrass, smooth bromegrass, and tall fescue would be suitable candidates to be incorporated into a riparian buffer design to remove nitrate from subsurface flow.