Petroleum prices impact cotton nitrogen (N) fertilization cost. A field study was conducted from 2005 to 2007 to assess the interactions of cover crop (none, Austrian winter pea (Pisum sativum spp. arvense) or hairy vetch (Vicia villosa Roth)) and N fertilization (0, 67 or 134？kg N/ha applied at planting) on N availability and cotton yield under reduced-tillage management. Nitrogen content in desiccated residues averaged 49, 220, and 183？kg N/ha, in no cover crop, Austrian winter pea, and hairy vetch, respectively. Seventy percent of N in the above ground cover crop was derived from biological N fixation. In 2005, cover crops decreased cotton yield, while fertilizer N had no effect. In 2006, cover crops did not affect yield, but yield was positively correlated with N rate. In 2007, in no N plots, cotton yields were 65% higher in cover crops than in no cover crop. However, yield from N fertilized cover crop plots were similar to N fertilized no cover plots. These results indicate that leguminous cover crops can provide over 150？kg N/ha, but this N may not be as effective as fertilizer N for lack of synchronization between cotton N requirements and N release from residues. 1. Introduction Cotton (Gossypium hirsutum L.) is a major row crop in the Mississippi Delta region of the southern US, and its production under reduced and no-tillage systems has increased in this region with the introduction of glyphosate-resistant varieties. Sustainable, low-input cropping systems, that is, reduced tillage, no-tillage, and fall seeded leguminous cover crops, are favored by economic pressures including energy, fertilizer, and agrochemical costs. Moreover, crop production systems that combine minimum and (or) reduced tillage with fall seeded cover crops offer environmental advantages such as improved soil tilth, weed suppression, reduced erosion, and off-site agrochemical transport [1]. In some areas of the southern US, high residue cereal cover crops, such as rye, were successfully integrated into cotton cropping systems. Cereal cover crops, however, require additional N for adequate plant residue production which may lead to N immobilization and reduced cotton yield [2]. However, as legumes can provide additional N for the following crop, leguminous cover crops may provide a source of sustainable N input. Mississippi Delta field trials indicate some benefits of a hairy vetch cover crop in corn production [3], as did similar trials in Missouri [4]. Leguminous cover crops provide an additional benefit of N fixation in the fallow period which can supply N to the
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