Atrazine impacts on shortgrass prairie microcosms

Alterations in shortgrass ecosystem structure and function following long-term use of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3, 5-triazine,2,4-diamine] for increased secondary productivity raised concerns about sustainability of the practice. A microcosm approach was designed to 1) model the direction and temporal features of blue grama [Bouteloua gracilis H.B.K.) Lag. ex Steud.] biomass accumulation and tissue N, nitrate reductase activity, chlorophyll, total nonstructural carbohydrate, and phosphorus (P) levels following atrazine applications of 0.84 and 2.24 kg ha-1 and hydroxyatrazine (6-hydroxy-N-ethyl-N'-(1-methylethyl)-1,3, 5-triazine,2,4,-diamine) at 1.12 kg ha-1, and 2) to relate plant growth and metabolism changes to possible short- and long-term modifications of soil microbial abundance and activities associated with C and N transformations. Atrazine applied to dormant plant-soil microcosms reduced below-ground (crown plus root) biomass and shoot total nonstructural carbohydrate levels during regrowth. Atrazine application increased shoot chlorophyll content, N levels, and nitrate reductase activity, but not total plant N content. Decreased below-ground biomass accumulation, and increased shoot N and nitrate reductase activity levels were linked to decreased total nonstructural carbohydrate availability. Total plant P levels were highest at the intermediate atrazine rate. Differences in soil microbial biomass and activities, and chemical properties resulted primarily from presence of blue grama and duration of plant regrowth. Soil nitrifying activity was depressed in soil previously exposed to atrazine whether or not blue grama was present. Hydroxyatrazine was not identified as an important factor in observed plant or soil changes. Atrazine may alter shortgrass system structure and function by immediate impacts on primary producers and long-term impacts on soil microbial processes.