Can Leguminous Cover Crops Replace Fertilizer Nitrogen in Mississippi Delta Cotton Production?.

The instability of petroleum supply has impacted nitrogen fertilization costs of crops such as cotton. A three year field study was conducted on a Dundee silt loam to assess the interactions of leguminous cover crops Austrian winter field pea (Pisium sativum L.) or hairy vetch (Vicia villosa Roth) on nitrogen availability and cotton yield grown under reduced-tillage management. Cover crops were seeded in October and desiccated using paraquat in early April. At planting nitrogen fertilizer was applied at 0, 67 or 134 kg ha^-1. Plant residues before desiccation averaged 4.0, 8.3, and 8.0 Mg ha^-1 in no cover crop, Austrian pea, and hairy vetch plots, respectively.  Nitrogen present in residues averaged 49, 220, and 183 kg N ha^-1, in no cover crop, Austrian peas, and hairy vetch plots, respectively. Based on isotope discrimination mass spectroscopy, approximately 80% of the nitrogen present in cover crop residues was due to biological nitrogen fixation. In the first year of the study, both cover crops decreased cotton yield, with no significant effect of fertilizer N. In the second year, cover crop had no effect on cotton yield and the highest yield was with N applied at 134 kg ha^-1.  In the third year, in no N plots, cotton yields were 65% higher in both cover crops than no cover crop. These results indicate that either legume can contribute over 150 kg N ha^-1. However, lack of synchronization between N release and plant uptake may depress cotton yield in a fertile Mississippi Delta soil.