Impact of Phosphorus Fertilization Rates on Nitrous Oxide Emissions in Switchgrass: Nonlinear Response Reveals Emission Reduction at Low Phosphorus Rates

Abstract Aims Phosphorus (P) fertilization can play a critical role in increasing switchgrass biomass yields for bioenergy production. However, applying mineral P to switchgrass can stimulate nitrous oxide (N2O) emissions, offsetting its climate mitigation benefits. The effect of P fertilization on N2O emissions is not well understood, with previous studies producing conflicting results. Moreover, studies evaluating the effect of P fertilizer on nitrogen (N) dynamics and its contribution to N2O emissions under switchgrass are lacking. Methods A 43-day study was conducted in a controlled-environment to evaluate the effect of different P fertilizer rates on N2O emissions in N-fertilized switchgrass. Four different fertilizer rates [(i) 60 kg N ha–1 (60N0P); (ii) 60 kg N and 20 kg P ha–1 (60N20P); (iii) 60 kg N and 40 kg P ha–1 (60N40P); (iv) 60 kg N and 60kg P ha–1 (60N60P)] and a treatment with no fertilizer (0N0P) were evaluated. Results While switchgrass biomass yield was less responsive to N and P fertilization, P fertilization had a nonlinear effect on N2O emissions. Applying P at 20 kg ha–1 resulted in lower emissions compared to N-only treatment. In contrast, increasing the P rate to 40 kg ha–1 led to the highest N2O emissions, surpassing those of N-only treatment. At 60 kg P ha–1, emissions were comparable to the N-only treatment. Conclusions These findings suggest that while moderate P application can reduce N2O emissions, higher P rates may increase emissions, offsetting the climate benefits of switchgrass as a bioenergy crop.