Nitrous oxide emissions associated with ammonia-oxidizing bacteria abundance in fields of switchgrass with and...

The nitrogen (N) fertilizer required to supply a bioenergy industry with sufficient feedstocks is associated with adverse environmental impacts, including loss of oxidized reactive nitrogen through leaching and the production of the greenhouse gas nitrous oxide (N₂ O). We examined effects on crop yield, N fate and the response of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) to conventional fertilizer application or intercropping with N-fixing alfalfa, for N delivery to switchgrass (Panicum virgatum), a potential bioenergy crop. Replicated field plots in Prosser, WA, were sampled over two seasons for reactive nitrogen, N₂ O gas emissions, and bacterial and archaeal ammonia monooxygenase gene (amoA) counts. Intercropping with alfalfa (70:30, switchgrass:alfalfa) resulted in reduced dry matter yields compared to fertilized plots, but three times lower N₂ O fluxes (≤ 4 g N₂ O-N ha^-1 d^-1 ) than fertilized plots (12.5 g N₂ O-N ha^-1 d^-1 ). In the fertilized switchgrass plots, AOA abundance was greater than AOB abundance, but only AOB abundance was positively correlated with N₂ O emissions, implicating AOB as the major producer of N₂ O emissions. A life cycle analysis of N₂ O emissions suggested the greenhouse gas emissions from cellulosic ethanol produced from switchgrass intercropped with alfalfa cultivation would be 94% lower than emissions from equivalent gasoline usage.