Cover crops can increase ammonia volatilization and reduce the efficacy of urease inhibitors

Surface application of urea can result in high nitrogen (N) losses through ammonia (NH3) volatilization. While management practices aim to increase the efficiency of nutrient cycling and prevent N loss, it is unknown whether the combination of multiple practices will have a synergistic or antagonistic effect. Therefore, laboratory volatilization studies were conducted to determine the effect of five cover crop treatments (surface clover [Trifolium incarnatum L.] and rye [Secale cereale L.], incorporated clover and rye, and bare soil), three N application timings (2, 4, and 8 wk after cover crop addition), and two N sources (untreated and treated urea) on the effectiveness of a urease inhibitor. Soils were incubated according to N application timing treatment, amended with the appropriate N source, and placed in chambers which captured NH3 over 7 d. There were significant interactions between cover crop treatment and N source and N source and N application timing on cumulative NH3 loss, ranging from 29 to 174 kg N ha(-1). Losses were highest from treated urea when applied 2 wk after residue addition (75.9 kg N ha(-1)) or on top of surface residues (85.8 kg N ha(-1)). There was no significant effect of application timing on cumulative NH3 loss from untreated urea. However, inhibitor effectiveness did increase when residue was applied eight weeks after residue addition (77%) as compared with 2 wk after residue addition (45%). Future research should focus on alternate dosing or application timing to overcome high residue scenarios in these systems.

Automated summary

The application of urea on the soil surface can lead to high nitrogen losses through ammonia volatilization. The effectiveness of a urease inhibitor varies depending on factors such as cover crop treatment, nitrogen source, and timing of nitrogen application. Further research is needed to explore alternate dosing or application timing strategies to minimize nitrogen losses in these systems.