ENVIRONMENTAL IMPACTS OF DRYLAND RESIDUE MANAGEMENT-SYSTEMS IN THE SOUTHERN HIGH-PLAINS

Increased use of agricultural chemicals with no-tillage (NT) may negat ively impact the environment through chemical or nutrient loss in runo ff or by leaching below the root zone. We compared environmental impac ts of NT and the lower chemical input stubble mulch (SM) management me thods for dryland crop production on a clay loam soil in a semiarid en vironment. We measured runoff volume and sediment, nutrient, and triaz ine concentrations in runoff for 9 yr from seven field-sized watershed s (2-5 ha each) cropped in a dryland winter wheat (Triticum aestivum L .)-grain sorghum [Sorghum bicolor (L.) Moench]-fallow sequence with SM or NT management with no N or P fertilizer added. We analyzed soil co res to a 6-m depth for NO3--N and to a 3-m depth for atrazine and prop azine content. Adoption of NT management reduced sediment loss by 54% compared with SM, although annual runoff was 15 mm yr(-1) greater from NT. Nutrient concentrations and losses (NO3--N, NH4+-N, TN, soluble P , biologically available P, TP) in runoff were extremely small from bo th tillage systems (losses <3 kg N and <1 kg P ha(-1) yr(-1)) on these unfertilized watersheds. Atrazine thyl-N'-(1methylethyl)-1,3,5-triazi ne-2,4-diamine] did not accumulate in the soil or leach below the root zone. Maximal losses of atrazine and propazine in runoff were 0.26 an d 1.5% of total application, respectively. Propazine ,N'-bis(1methylet hyl)-1,3,5-triazine-2,4-diamine], applied to both NT and SM sorghum wh en runoff is probable, appears to have a greater potential for negativ ely impacting the environment under semiarid conditions than does atra zine, which is applied when runoff probability is small. Propazine acc umulated in the soil profile but was undetected below 0.6 m. Perhaps t he most detrimental impact of adopting NT management was increased lea ching of NO3--N to depths below the plant root zone as a result of wet ter soil and improved water conservation with NT in the semiarid envir onment.