Bacteria on leaves: a previously unrecognised source of N 2 O in grazed pastures

Nitrous oxide (N 2 O) emissions from grazed pastures are a product of microbial transformations of nitrogen and the prevailing view is that these only occur in the soil. Here we show this is not the case. We have found ammonia-oxidising bacteria (AOB) are present on plant leaves where they produce N 2 O just as in soil. AOB ( Nitrosospira sp. predominantly) on the pasture grass Lolium perenne converted 0.02–0.42% (mean 0.12%) of the oxidised ammonia to N 2 O. As we have found AOB to be ubiquitous on grasses sampled from urine patches, we propose a ‘plant’ source of N 2 O may be a feature of grazed grassland.