SEDIMENT, NITROGEN, AND PHOSPHORUS RUNOFF WITH CONVENTIONAL-TILLAGE AND CONSERVATION-TILLAGE COTTON IN A SMALL WATERSHED

Research on watershed runoff losses from cotton (Gossypium hirsutum L. ) cropping systems in limestone soil regions is limited. Runoff of wat er, sediment, total N, NH4-N, NO3-N, and solution and particulate P er e measured from a 3.8-ba (9.4-ac) watershed during three years of conv entional 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 p ercent and Decatur (Rhodic Paleudults) and Emory (Fluventic Umbric Dys trochrepts) soils. Although CsT resulted in a higher proportion of ann ual rainfall as runoff than CvT about twice as much sediment was disch arged from the watershed with CvT than with CsT [average of 2,979 vs. 1,311 kg ba-1 yr-1, (2,660 vs. 1,170 lbs ac-1 yr-1) respectively]. A f ew intense storms during late winter through early spring, before full cotton canopy, contributed to most of the erosion losses in CvT Annua l mean concentrations of No3-N in runoff were equally low for both til lage systems, ranging from 1.3 to 2.2 mg L-1 during the six years. Win ter rye was very effective in diminishing NO3-N concentrations in runo ff from january to spring fertilization. A temporary period of elevate d 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 accept able than CvT for cotton production, assuming prudent NP fertilizer ma nagement.