The optimal ammonium-nitrate ratio for various crops : A Meta-analysis

Abstract

Contexts or ProblemNitrogen (N) is an essential nutrient for crop growth, and its different forms have a significant impact on crop uptake and various physiological processes that underpin yield and quality. Although many studies suggesting that a combination of ammonium (NH4+) and nitrate (NO3-) can enhance crop outcomes, the optimal ammonium-nitrate ratio (ANR) for different crops remains uncertain due to a lack of extensive validation with large datasets. Objective and MethodThis paper collected 151 peer-reviewed studies (with 2417 observations) on the effects of ANR on crop growth published from 1972 to 2022. The comprehensive effects of different ANRs on crop yield and quality (protein, vitamin C, starch etc.) of various crops were evaluated via meta-analysis to provide an optimal N management strategy. ResultsOur analysis identified rice as an ammonium-preferred crop, and the ANR of 75:25 shows the most effective improvement in total biomass and photosynthetic characteristics at seedling age, with a 26.0% increase in the biomass and a 21.9% boost in chlorophyll content compared to pure ammonium treatment. Wheat and tobacco can be considered ammonium-nitrate-balanced crops, and a balanced ANR of 50:50 resulted in significant synergistic improvement in both yield and quality, i.e., wheat biomass and protein increased by 13.3% and 7.1%, respectively, and tobacco yield significantly increased by 21.9%, compared to pure nitrate supply. In contrast, maize, soybeans, and vegetables were identified as nitrate-preferred crops, applying pure ammonium or an excess of ammonium-N over nitrate led to a significant reduction in yield and quality, with the most pronounced inhibitory effect observed in soybeans. However, a slight supplement of ammonium can also promote the growth of these crops. Furthermore, soil pH significantly influences the preference for ammonium and nitrate. In soils with an acidic environment, crops tend to efficiently utilize nitrate, whereas in alkaline conditions, there is a preference for an increased presence of ammonium. ConclusionThe combination of ammonium and nitrate in appropriate ratio can improve multiple agronomic characteristics both for ammonium-preferred and nitrate-preferred crops, which may due to complementary physiological functions of two forms of N. Among various soil factors, soil pH showed significant influence on determining the optimal ANR and should be taken into consideration. Implications or significanceThese findings reveal that the appropriate ANR is crucial for both crop yield and quality improvement and should be fully valued in future nutrient management.