Cover Crop Effects on the Effectiveness of Urease Inhibitors.

Urease inhibitors continued to be introduced in the agricultural market, and thus new studies with these products are warranted. The objective of this project was to examine the utility of these inhibitors for reducing ammonia (NH₃) losses from soil (Pacolet fine sandy loam (clayey, kaolinitic, thermic, Typic Hapludults)) to which cotton (Gossypium hirsutum L.) had been cropped. For this laboratory experiment intact (15-cm diam., 4-cm deep) cores were removed from selected plots of the Old Rotation, with cover residue treatments of either none (continuous cotton since 1896) or winter cover (since 1896, winter cover of either hairy vetch (Vicia villosa Roth) or crimson clover (Trifolium incarnatum L.). Harvested cores were immediately removed to the laboratory and placed into glass jars for use in a 7 day experiment where emitted ammonia was trapped in boric acid, with levels measured daily. Specific treatments were: 1) no residue via winter cover, or, winter cover crop residue, and, 2) possible urease inhibitors, including various formulations of NBPT (N-(n-butyl) thiophosphoric triamide) and maleic–itaconic acid copolymer. Treatments were arranged as a 2 x 4 factorial of residue cover and urea with/without urease inhibitors (surface applied), with 4 replications of each. The entire experiment was repeated in time. Statistical analyses revealed a residue by inhibitor interaction at almost every sampling date. This was because any product with NBPT reduced ammonia volatilization regardless of the presence of residue, while ammonia losses from urea-treated and urea plus maleic-itaconic acid copolymer were higher from soil containing crop residue, when compared to soil with little crop residue. Results from this laboratory study reveal that use of NBPT as a urease inhibitor may have utility in high residue cropping situations.