Slow-release fertilizer deep placement increased rice yield and reduced the ecological and environmental impact in Southeast China: A life-cycle perspective

An unreasonable application of nitrogen fertilizer leads to increased agricultural greenhouse gas (GHG) emissions and carbon footprint (CF). To solve this issue, slow-release fertilizer and deep placement of nitrogen fertilizer are recommended, and have been proven effective in improving nitrogen utilization efficiency and reducing nitrogen loss. However, the deep placement of fertilizer increases the cost of mechanical fuel and labor, especially in the hilly and mountainous areas of southeast China and the comprehensive effects throughout the life cycle of crop production, including economic benefits, energy flow, and environmental and ecological advantages, resulting from different application strategies for nitrogen fertilizer, are still largely unknown. In this work, a two-year field trial was conducted in Fujian Province, China, from 2021 to 2022. The main objectives of the study were to investigate the yield and GHG emissions resulting from the deep placement of slow-release fertilizer (DSK) and to assess the carbon footprint, energy use efficiency, and net economic and ecological benefits from a life-cycle perspective. The results showed that compared with deep placement of urea (DCK), slow-release fertilizer broadcasting (SK), and the traditional fertilization mode practiced by farmers (CK), the average annual yield of rice under DSK treatment significantly increased by 8.78%, 15.18%, and 23.54% respectively. Compared with CK, the contents of indole-3-acetic acid (IAA) and trans-Zeatin Riboside (t-ZR) under DSK were significantly increased and the dry matter translocation amount, Non-Structural Carbohydrates (NSC) translocation amount and Nitrogen translocation amount were significantly increased by 22.79%, 49.60% and 41.47% respectively. GHG emissions were indirectly reduced by the transport of above-ground substances to grains. The average cumulative CH4 emissions were significantly reduced by 25.34% in the DSK treatment, compared to the CK treatment, primarily as a result of decreased methanogenic gene abundance of mcra gene. Additionally, N2O emissions decreased significantly by 48.97%, compared to the CK, mainly due to a decrease in nirK gene abundance and an increase in nosZ gene abundance in denitrifying bacteria. Furthermore, the carbon footprint of the DSK treatment significantly decreased by 38.74% compared to the CK treatment, while net energy income and net ecological and economic benefits increased by 43.83% and 76.43%, respectively. Comprehensive analysis showed that deep placement of slow-release fertilizer is a sustainable nitrogen fertilizer management mode that helps to balance grain yield, environmental footprint, and economic benefits in southeast China.