Enhanced Efficiency Nitrogen Fertilizers for Rice Systems: Meta-Analysis of Effects On Yield and Nitrogen Uptake.

Nitrogen is deficient in most soils and is applied in the greatest quantities of all nutrients. Given its high potential for loss, efficient fertilizer N management has both economic and environmental consequences. Enhanced efficiency nitrogen fertilizers (EENF) have been developed to decrease N losses and improve N use efficiency. However, studies evaluating the effectiveness of EENF products in rice systems show mixed results. The objective of this meta-analysis was to quantify the benefits of EENF (i.e., nitrification and urease inhibitors, neem, and slow release fertilizers) in terms of yield and N uptake and to determine under what conditions EENF are most effective. The analysis included 32 field studies (178 observations) for the effects of EENF on crop yield and 14 studies (82 observations) on N uptake. Overall, the use of EENF led to a 5.7% (95% CI = 3.9 to 7.7%) increase in yield and an 8.0% (95% CI = 5.2 to 10.7%) increase in N uptake. Soil pH (pH of dry soil) had a significant impact on EENF effectiveness. In acidic soils (pH ≤6.0) the application of EENF did not significantly affect yield or N uptake; however the yield response to EENF increased to 10.2% (95% CI = 5.3 to 16.6%) in alkaline soils (pH ≥8.0). There was no difference among the classes of EENF when separated by their mode of action (i.e., urease inhibitors, nitrification inhibitors or slow release). When EENF products were analyzed separately, NBPT [N-(n-butyl) phosphoric triamide] and neem proved effective in increasing yield, while PPD (phenyl phosphorodiamidate) and DCD (dicyandiamide) were not effective. The EENF effectiveness was not dependent on N rate, method of first N application (incorporated, surface applied, or applied into water), timing of first N application in relation to a permanent flood being established, and how water was managed during the season (permanent flood vs. intermittent wet and dry). Overall, this meta-analysis suggests that certain EENF products can increase yield and N uptake but the average increase is modest.