Cover Crop Effects on Urease Inhibitors

Abstract

Urease inhibitors continue to be introduced in the agricultural market, and thus new studies with these products are warranted. The objective of this research 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. Another soil was examined (Compass loamy sand [coarse-loamy, siliceous, thermic, Plinthic Paleudults]) to which soybeans (Glycine max L. Merr.) had been cropped with cover crops. For these laboratory experiments, intact (15-cm diam., 4-cm deep) cores were removed from selected plots of the Old Rotation (Auburn, AL.) and E.V. Smith Experiment Station (Shorter, AL.). Cover crop residue treatments were either none, or winter cover by either hairy vetch (Vicia villosa v Roth), crimson clover (Trifolium incarnatum L.), or cereal rye (Secale cereal L.). Cores were immediately removed to the laboratory and placed into glass jars for use in a 7 day incubation 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) untreated urea, and urea with possible urease inhibitors, including various formulations of NBPT (N-(n-butyl) thiophosphoric triamide) and maleic–itaconic acid copolymer (MIP). 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, with two experiments using soil from the Old Rotation (Auburn, AL.), and three experiments with soil from E.V. Smith Research Center (Shorter, AL). When soil was used from the Old Rotation, statistical analyses revealed a cover by inhibitor interaction on almost every sampling date. This was due to soils with a long history of green manures and 20+ years of cover cropping greatly increasing ammonia volatilization as a result of higher OM and total C levels, along with the urease inhibitor NBPT significantly reducing N losses via volatilization. When soils were used with no cover cropping history, ammonia volatilization decreased across all inhibitor treatments, but ammonia volatilization losses from those containing NBPT were still significantly lower as compared to untreated urea and urea + MIP. Experiments using soil from E.V. Smith Research Center had different results, in that the presence of a cover crop or lack thereof, did not always affect ammonia loss like at the Old Rotation. However, a significant cover by inhibitor interaction was still significant across many sampling days. In all three experiments conducted with soil from vi E.V. Smith, the inhibitor source was still highly significant (P < 0.10), and the use of NBPT as a urease inhibitor was effective in delaying and reducing ammonia volatilization in Days 1-2. However, cumulative N losses in Experiment 1 revealed no statistical difference across inhibitor treatments. This was most likely due to the increased soil moisture in Experiment 1- Run 1. Experiment 2 showed no significant effects from the presence of a cover crop, due to comparable soil moisture between those treatments. Use of NBPT delayed volatilization through Day 2, but cumulative losses were only significantly reduced in treatments with urea + NBPT/NPPT. In Experiment 3, average losses were greatest in the first 4 days in treatments where NBPT was not present, as compared to treatments containing NBPT. Also, cumulative losses from treatments with NBPT were only significantly lower than untreated urea when no cover was present. Ammonia losses from urea-treated and urea + MIP were often higher from soil containing crop residue, when compared to soil with little crop residue. Use of NBPT was effective in delaying volatilization, regardless of the presence of a cover crop. Results from this laboratory study reveal that use of NBPT as a urease inhibitor may have utility in high residue cropping situations.