The effect of nitrification inhibitors on soil ammonia emissions in nitrogen managed soils: a meta-analysis

Nitrification inhibitors (NI) retain nitrogen (N) in the ammonium (NH4 (+)) form longer in soil providing more time for plant uptake of NH4 (+). They can also reduce production of the greenhouse gas nitrous oxide (N2O) by inhibiting nitrification and subsequent denitrification processes. However, this extended retention of N in the NH4 (+) form in the soils treated with NI can increase ammonia (NH3) emission. Studies conducted so far provide conflicting results on the effect of NI treatment on NH3 emissions. Here we have collated results available to date from peer-reviewed literature (46 data set from 21 studies from 1970 to 2010) and categorized the reported results into three groups-increase, no change, and decrease in % applied N lost as NH3 (hereafter NH3 loss) in NI treatments. Significant increase in NH3 loss in NI treatment was observed in both pasture and cropping soils and from both applied urine and urea with NI (e.g., dicyandiamide (DCD), ATC [4-amino- 1.2,4-triazole]). This increase in NH3 loss was between 0.3 and 25.0 % (n = 26, mean 6.7 +/- A standard error 1.3 %). No change in NH3 loss with DCD was also observed in some soils (n = 14), while a small number of studies reported a decrease which was between -0.3 and -4.1 % (n = 6, -1.3 +/- A 0.6 %). Overall, the soils with higher pH and lower cation exchange capacity (CEC) lost more NH3 with NIs irrespective of land use and type of N input. The combined addition of both NI and urease inhibitor reduced NH3 loss compared to sole NI application (n = 4, -5.9 +/- A 1.3 %). Collectively, the analysed results from the small number of available data sets reported suggest that NH3 loss significantly increases with NI application, depending on soil properties such as soil pH and CEC. More studies are needed both to quantitatively determine the effect of NIs on NH3 loss and to mitigate the loss.