Smooth brome changes gross soil nitrogen cycling processes during invasion of a rough fescue grassland

Invasive plants have variable effects on net nitrogen cycling, but how invasion alters gross N cycling is poorly understood. We examine how Bromus inermis (smooth brome) invasion affects gross N cycling rates and investigate potential mechanisms for the changes including relationships between smooth brome and ammonia-oxidizing bacteria (AOB) and archaea (AOA), plant community productivity, and litter quality. Gross nitrogen cycling rates, AOA and AOB population sizes, and plant community productivity were examined in native and invaded plots in smooth brome-invaded rough fescue grassland in central Saskatchewan, Canada. Despite no changes in inorganic nitrogen between invaded and native grassland soils, gross nitrogen mineralization rates and total soil nitrogen were higher in invaded soils. Invaded areas had greater plant productivity and litter production, which likely stimulated microbial activity and higher gross mineralization rates. Nitrification rates did not differ between invaded and native soils. Smooth brome had a weak positive effect on AOA in the B horizon but not in the A horizon, and AOB responded positively in both horizons. These results demonstrate that the full effects of plant invasion on soil N cycling may be masked in net N cycling rate measures.