Effect of urease inhibitor and biofertilizer on nitrous oxide emission

Conventional agricultural practices are heavily dependent on nitrogen fertilizers, which can have negative impacts on the environment through ammonia volatilization and nitrous oxide emission. Previous studies have shown that the use of urease inhibitors or biofertilizers may help reduce such impacts.

A field experiment was established by the Joint FAO/IAEA Centre at the experimental station of the University of Natural Resources and Life Sciences (BOKU) located east of Vienna (Austria) to determine the effect of urease inhibitor and biofertilizer on nitrous oxide (N₂O) emission, in wheat cropping systems. A randomized complete block design including five treatments and four replicates was used in this study. The treatments were: T₁ (control treatment - without N fertilizer), T₂ (Urea only), T₃ (Urea+Urease Inhibitor (UI)), T₄ (Urea+Biofertilizer), T₅ (Urea+UI+Biofertilizer). All treatments received 50 kg N ha^-1 at tillering stage (GS 31), except T₁. In this study N-(n-butyl) thiophosphoric triamide (nBTPT) or “Agrotain” was used as UI and Azotobacter chroococcum (“AZOTOHELP”) was applied as biofertilizer.

Soil N₂O gas fluxes were measured using the static chamber method, eight times between 3 to 84 days after fertilizer application. Gas sampling was performed at the same time each day of measurement, between 8:00 and 10:00 h, to minimize diurnal variation and better represent the mean daily fluxes. A PVC chamber (24 cm height and 24 cm diameter) was inserted into the soil 5 cm deep. The chamber was composed of two separate parts joined together with an airtight rubber. Gas samples were taken at 0 and 30 minutes after closing the chambers using a 500 mL syringe. The gas sample was then immediately transferred from the syringe to a pre-evacuated 1L gas sampling bag with multi-layer foil. Nitrous oxide in the gas samples was analysed using off-axis integrated cavity output spectroscopy (ICOS, Los Gatos).

The statistical analysis showed that UI and biofertilizer had a clear and significant effect on nitrous oxide emission. However, this effect was only observed during the first week after the fertilizer application. Further, the results showed that the highest N₂O emission, within this week after adding urea fertilizer, was under the U+UI treatment, which was significantly higher by about 139, 91,79% compared to the Urea+Biofertilizer, Urea, Urea+UI+Biofertilizer treatments, respectively. No significant difference was observed between the other Urea, Urea+Biofertilizer and Urea+UI+Biofertilizer treatments in this period. Although not significantly (p < 0.05), N₂O emission was higher in Urea+UI+Biofertilizer treatment compared to the Urea+Biofertilizer treatment.

Due to the ability of UI to reduce ammonia volatilization, we assume that pollution swapping from ammonia volatilization to nitrous oxide emission occurred, explaining the stimulus of UI on nitrous oxide emission. The lower N₂O emission in the treatments receiving biofertilizer, compared to the one with no biofertilizer, may be caused by the ability of Azotobacter to reduce N₂O emission by N₂O-fixation, N₂ fixation and reduction of N₂O to N₂.