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Soil-Applied Atrazine Alters Blue Grama Physiology and Indirectly Influences Soil Nitrogen

Earth Sciences Group, 5217 Mail Creek Lane, Ft. Collins, CO 80525
USDA-ARS, Rangeland Resources Res. Unit, Crops Res. Lab., 1701 Center Ave., Ft. Collins, CO 80526
USDA-ARSS, Soil-Plant Nutrient Res. Unit, P.O. Box E, Ft. Collins, CO 80522
USDA-ARS, Rangeland Resources Res. Unit, Crops Res. Lab., 1701 Center Ave., Ft. Collins, CO 80526

^* Corresponding author

Atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] applied at sublethal concentrations to rangelands, has resulted in herbage production and/or enhanced plant N concentration of desirable warm-season grasses beyond that expected from weed control alone. This investigation was conducted to determine whether this is a metabolically regulated plant response, the result of enhanced soil N availability via microbially mediated processes, or both. Aerobic and anaerobic laboratory incubation studies with an atrazine-amended, rangeland soil indicated no direct effect of atrazine on soil-mediated N transformations. Applications of atrazine to the same soil type in column-lysimeters containing blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Steud.] plants resulted in temporarily elevated soil NO₃ concentrations and reduced NH₄ concentrations in proportion to the different rates of atrazine applied (0, 0.5, 1.0, and 2.0 mg column^–1). Applications of atrazine to soil containing blue grama plants initially inhibited photosynthesis and plant growth, and resulted in temporarily elevated soil NO₅ concentrations and reduced NH4 concentrations. A strong inverse relationship was observed across the four atrazine treatments between soil available N and shoot N content, strongly suggesting that the dominant plant-mediated effect of atrazine on soil N pools was elicited through atrazine's initial inhibitory effect on plant growth. A strong negative correlation between leaf photosynthesis rate and tissue NO₃ levels across all four atrazine rates suggests that in addition to its negative effects on blue grama photosynthesis and growth, atrazine also impairs blue grama's ability to reduce NO₃, enhancing the accumulation of plant tissue NO₅ levels. We conclude that temporary changes in soil NO₃ and NH₄ concentrations from atrazine resulted indirectly from changes in plant growth and metabolism, and not from alterations in soil microbiological processes.

Received for publication May 4, 1992.