Global Warming Potential of CH ₄ and N ₂ O Sources Is Offset by CO ₂ Sinks Across an Experimental Pond Trophic Gradient

Abstract Nutrient fertilization of freshwater systems can influence greenhouse gases (GHGs) by altering carbon and nutrient cycling. Increases in productivity and organic matter availability may drive CO 2 and CH 4 , while nitrogen (N) availability may impact N 2 O concentrations and emissions. While the impact of nutrient concentrations on GHGs individually in lakes and ponds has been well described, we have less information on how increases in nutrient concentration impact the relative influence of CO 2 , CH 4 , and N 2 O on total GHG concentrations and emissions and the total impact on global warming potential (GWP) from these systems. We used experimental mesocosms (limnocorrals) to establish replicated treatments of increasing N:phosphorus (P) across a wide gradient of N loads and measured the concentration and emissions of CO 2 , CH 4 , and N 2 O as well as GWP in total CO 2 equivalents. With increasing TN concentrations, we observed declines in emissions and concentrations of CO 2 (−17.01 vs. 11.31 mm m −2 d −1 with greater N load treatment) and increases in N 2 O (mean of 56.77 vs. −0.36 μmol m −2 d −1 with greater N load treatment). We did not observe any systematic pattern between nutrient concentrations and CH 4 . Interestingly, nutrient loads did not alter GWP due to greater CO 2 uptake offsetting increased N 2 O under high TN concentrations and variable CH 4 across the treatments. Our study suggests that increases in N 2 O from even extreme N loading may not necessarily increase the total warming impact from these systems, as higher productivity and greater CO 2 influx may override increased N 2 O emissions as a result of fertilization.