Coffee intercropping reduces soil N₂O and N₂ emissions: Empirical test and meta-analysis

With the expansion of global coffee plantations, clarifying the patterns of soil nitrous oxide (N₂O) and nitrogen (N₂) emissions in coffee plantations is significant to understanding and estimating soil gaseous nitrogen losses in tropical regions. Coffee plantations promote nitrogen mineralization through sustained inputs of nitrogen-rich litter, root exudates that enhance microbial activity, and targeted management practices. This process generates bioavailable NH₄⁺ and NO₃^-, which subsequently undergo nitrification and denitrification to produce N₂O and N₂. However, the sensitivity of N₂O and N₂ emissions to coffee-based land use changes, such as coffee monoculture (CM) and coffee intercropping (CI), is a major uncertainty and lacks studies. In this study, three land uses, namely, natural forest, CM, and CI were selected to investigate the effects of coffee plantations and stand ages on soil N₂O and N₂ emissions under different oxygen conditions. Results showed that decades of coffee intercropping significantly decreased N₂O and N₂ by 11-57 % and 25-56.1 %, respectively, compared with coffee monoculture. Low-oxygen conditions amplified the N₂O and N₂ emissions, and the cumulative N₂O emissions of CM and CI plantations were 4.23-4.62 and 1.27-4.28 times higher than that of natural forests, respectively. Coffee stand age of 34 years amplified the cumulative N₂O emissions (CM-34 and CI-34) by 78 -254 % compared with the stand age of 25 years (CM-25 and CI-25). Further microbial analysis indicated that the abundance of nirK and nosZ-I denitrification genes and the stimulation of nitrogen mineralization were promoted by the coffee plantations, providing substantial substrates for denitrification and contributing to N₂O and N₂ emissions. To verify the generality of our findings, a meta-analysis was conducted to identify the quantitative effects of coffee-based land use change (LUC) and fertilizer on N₂O emissions and the results were consistent with our experimental observation. Overall, the conversion of forest to coffee plantations significantly increased soil N₂O and N₂ emissions, but the introduction of shade tree intercropping alleviated soil N₂O emissions.