Environmental Impacts of Dryland Residue Management Systems in the Southern High Plains

Increased use of agricultural chemicals with no-tillage (NT) may negatively impact the environment through chemical or nutrient loss in runoff or by leaching below the root zone. We compared environmental impacts of NT and the lower chemical input stubble mulch (SM) management methods for dryland crop production on a clay loam soil in a semiarid environment. We measured runoff volume and sediment, nutrient, and triazine concentrations in runoff for 9 yr from seven field-sized watersheds (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 cores to a 6-m depth for NO⁻₃-N and to a 3-m depth for atrazine and propazine 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 (NO⁻₃-N, NH⁺₄-N, TN, soluble P, biologically available P, TP) in runoff were extremely small from both tillage systems (losses <3 kg N and <1 kg P ha⁻¹ yr⁻¹) on these unfertilized watersheds. Atrazine [6-chloro-N-ethyl-N′-(1methylethyl)-1,3,5-triazine-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 and 1.5% of total application, respectively. Propazine [6-chloro-N,N′-bis(1-methylethyl)-1,3,5-triazine-2,4-diamine], applied to both NT and SM sorghum when runoff is probable, appears to have a greater potential for negatively impacting the environment under semiarid conditions than does atrazine, which is applied when runoff probability is small. Propazine accumulated in the soil profile but was undetected below 0.6 m. Perhaps the most detrimental impact of adopting NT management was increased leaching of NO⁻₃-N to depths below the plant root zone as a result of wetter soil and improved water conservation with NT in the semiarid environment.