Sediment, nitrogen, and phosphorus runoff with conventional-and conservation-tillage cotton in a small watershed

ABSTRACT:

Research on watershed runoff losses from cotton (Gossypium hirsutum L.) cropping systems in limestone soil regions is limited. Runoff of water, sediment, total N, NH₄-N, NO₃-N, and solution and particulate P were measured from a 3-8-ha (9.4-ac) watershed during three years of conventional tillage (CvT) cotton, followed by three years of conservation-tillage (CsT) cotton. The study was conducted from 1984 through 1989 in the Limestone Valley region of northern Alabama, on slopes of 1-6 percent and Decatur (Rhodic Paleudults) and Emory (Fluventic Um-bric Dystrochrepts) soils. Although CsT resulted in a higher proportion of annual rainfall as runoff than CvT, about twice as much sediment was discharged from the watershed with CvT than with CsT [average of 2,979 vs. 1,311 kg ha⁻¹ yr⁻¹, (2,660 vs. 1,170 lbs ac⁻¹ yr⁻¹) respectively]. A few intense storms during late winter through early spring, before full cotton canopy, contributed to most of the erosion losses in CvT. Annual mean concentrations of NO3-N in runoff were equally low for both tillage systems, ranging from 1.3 to 2.2 mg L⁻¹ during the six years. Winter rye was very effective in diminishing NO₃-N concentrations in runoff from January to spring fertilization. A temporary period of elevated NO₃-N and P concentrations occurred in runoff sampled shortly after surface application of NP fertilizer in April, especially with CsT. In our study, most of the runoff P loss was associated with the solution rather than the particulate phase, and more P runoff occurred with CsT than with CvT In balance, however, CsT is more environmentally acceptable than CvT for cotton production, assuming prudent NP fertilizer management.