Effects of climate warming and nitrogen fixing bacterial inoculants on nitrous oxide emissions

Abstract

The application of synthetic nitrogen (N) fertilizer to cropping systems leads to reactive N pollution including nitrous oxide (N2O) emissions. Nitrous oxide is an ozone-depleting substance and an important long-lived greenhouse gas. Potential mitigation solutions, including bacterial inoculants (BI) and enhanced efficiency fertilizers (EEF), have been proposed to reduce reactive N pollution. Nitrogen-fixing BIs supply the crop with N so that less synthetic N fertilizer must be added to maintain agricultural crop productivity. Enhanced efficiency fertilizers include controlled-release fertilizers and inhibitors to reduce N2O emissions and N leaching. Here, we examined the efficacy of using a nitrogen-fixing BI to lower N2O emissions over seven growing seasons across a broad range of fertilizer rates (FR). Further, we examined the efficacy of using a dual-inhibitor EEF to lower N2O emissions over four growing seasons and assessed if warmer soil temperature treatments (+ 2℃ reduced the EEF mitigation potential for climate change. The results indicated that the BI treatment effect alone did not have any significant effects on any of the environmental variables tested (e.g. N2O emissions, soil N concentrations, crop yield). Increasing FR increased cumulative N2O emissions, enhanced soil N concentrations, and decreased N use efficiency. Fertilizer rate had a polynomial relationship with leachate N, crop yield, and grain N. The interaction between FR and the BI treatment was significant for decreasing soil nitrate (NO3-) concentrations. This result indicated that soil NO3- concentrations increased at a faster rate per FR without a BI than when treated with a BI. Finally, N2O emissions increased linearly (0.012 kg N2O-N ha-1 per kg N ha-1 applied) with increasing FR from 81 to 404 kg N ha-1. The EEF experiments indicated that the growing season and temperature interaction effect was significant on cumulative N2O emissions and emission factors. The growing season effect was significant for most of the environmental variables that were tested. The conventional urea had significantly higher values of initial total soil N concentrations compared to the EEF. The conventional urea treatments and the effects of climate warming (i.e. heated soil temperatures) when using the EEF led to significantly less leachate N mass loss than the unheated EEF treatments. Additionally, the conventional urea treatments had lower leachate N concentrations than the unheated EEF treatments. The heated EEF and conventional urea treatments also had significantly lower plant N concentrations compared to the unheated EEF treatments. The results imply that there was better plant N acquisition in lower soil temperatures across growing seasons and between temperature treatments. Fertilizer rate had the dominant effect on cumulative N2O emissions, soil N concentrations, N use efficiency, crop yield, leachate N loss, and grain N uptake when compared to the effect of BIs. Soil warming has potential to reduce leachate N mass loss and plant N uptake. Using EEFs rather than conventional urea has potential to enhance leachate N loss as well as increase plant N uptake from improved synchronization between crop N demand and supply. Overall, N cycling was highly dependent on the mixed effects of FR, N source, and climate.