Nitrous oxide emissions associated with ammonia‐oxidizing bacteria abundance in fields of switchgrass with and without intercropped alfalfa

Summary 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 (N 2 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, N 2 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 N 2 O fluxes (≤ 4 g N 2 O‐N ha −1 d −1 ) than fertilized plots (12.5 g N 2 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 N 2 O emissions, implicating AOB as the major producer of N 2 O emissions. A life cycle analysis of N 2 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.