Global meta-analysis and three-year field experiment shows that deep placement of fertilizer can enhance crop productivity and decrease gaseous nitrogen losses

There is an urgent need to address the contradiction between food security and climate change, and achieve the highest crop productivity at the lowest environmental cost. Scientific fertilization is the key to solving this problem. Deep placement of fertilizer (DPF) is an effective fertilization strategy for improving crop productivity and reducing gaseous N losses, but its effectiveness varies among agricultural systems and appropriate fertilization depths have not yet been determined. In this study, we synthesized 99 global studies (120 locations) and conducted a three-year field experiment to evaluate the effects of DPF on the crop yield, nitrogen use efficiency (NUE), and nitrous oxide (N2O), and ammonia (NH3) emissions, and explored their responses to different climates, field management practices, and environmental factors. DPF could increase the grain yield by 13.5%, and NUE by 33.8%, and reduce N2O emission by 16.2%, and NH3 emissions by 86.6%, respectively. Random forest analysis showed that the fertilization depth and nitrogen application rate were the most important factors that determined the impacts of DPF on the crop yield, NUE, and N2O and NH3 emissions, but their effects also depended on the climate conditions, field management practices, and soil environmental factors. Meta-regression modeling showed that the effects of DPF on the crop yield and NUE increased but then decreased as the fertilizer application depth and nitrogen application rate increased, and the effects on N2O and NH3 emissions tended to increase gradually. The meta-analysis also demonstrated that under DPF, the emission factors for N2O and NH3 decreased by 35.8% and 84.5%, respectively. According to the meta-analysis, the global N2O and NH3 emissions under DPF can be reduced by 0.23 and 5.46 Tg N·year–1 respectively. The results obtained based on global meta-analysis and a three-year experiment demonstrated that DPF obtained higher grain production with lower environmental costs, and the most suitable fertilization depth was identified as 15–25 cm. The results obtained in this study provide valuable insights into food security and environmental costs under effective fertilization management.