Nitrogen fertilizer increases nitrous oxide, but not dinitrogen, emissions from moist tropical forest soils in Puerto Rico

Abstract:

Nitrogen (N) deposition in tropical forests may increase substantially in coming decades, stimulating a concomitant increase of soil N gas emissions (dinitrogen (N₂), nitrous (N₂O) and nitric oxides). How N deposition might alter the relative emissions of these gases is unclear, and has ramifications for the global climate since N₂O is a potent greenhouse gas. We used a small-scale fertilization study in the Luquillo Experimental Forest (LEF) of Puerto Rico to simulate the effects of N deposition on N gas emissions. Fertilizer was applied by placing mesh bags filled with ammonium saturated weak cation exchange resin directly on the mineral soil for two months. At that time, intact soil cores (0-10cm) were taken from below the bags. The cores were shipped to the Cary Institute of Ecosystem Studies, where they were incubated in a helium headspace with either 0 or 20% oxygen (O₂), and analyzed for N₂ and N₂O emissions. N fertilization increased N₂O emissions fourfold (p=0.03). N₂O production was positively correlated with field soil moisture (r=0.45, p=0.002), and was higher under 20% than 0% atmospheric O₂ (p=0.003). With the exception of a handful of samples, we detected no measureable N₂ production from these soils, and fertilization did not influence N₂ production. This may have been because our experiment occurred during a drought that reduced soil moisture in the field by ∼20%. We have found that N₂ emissions correlate with soil moisture elsewhere in the LEF. While we conclude that N deposition may not influence the N₂O:N₂ of soil emissions under such conditions, it is still unclear whether this result would hold under higher rainfall.