Deep fertilization improves rice productivity and reduces ammonia emissions from rice fields in China; a meta-analysis

Deep fertilization (DF) has been introduced to rice production systems in China to improve rice productivity with minimum nitrogen (N) losses, however, the effects of DF on rice yield and ammonia (NH3) emissions were never analyzed systematically. In the present study, we evaluated the impact of DF on rice productivity and NH3 emissions via a meta-analysis based on 1023 observations collected from 144 studies. Data interpretation showed that DF can significantly increase yield by 8.6% (95% confidence interval (CI): 7.3–10.0%), N use efficiency by 16.7% (95% CI: 14.5–18.9%) and reduce NH3 emissions by 78.1% (95% CI: 71.3–81.3%), respectively compared to broadcast fertilizer (BF). Variation in the effects of DF was mainly due to climate factors, soil properties, and agronomic practices whereas optimization of agronomic practices can improve the rice productivity under DF. The optimal depth of DF ranges between 7 and 13 cm, which coincides with the distribution of the majority of the rice root system. Reducing the fertilizer application rate to the range of 150–250 kg/ha coupled with split fertilization can improve rice yield and reduce NH3 emissions. Importantly, the DF could significantly increase the total output and net economic benefits compared to the BF. In conclusion, the DF can significantly increase rice productivity and reduce ammonia emissions, however, large-scale adoption of the DF in rice systems are constrained by the availability of well-equipped machinery, good fertilizer products, and skilled farmers and labor.