||Nitrogen (N) deposition may drive plant invasions and threaten biodiversity. To investigate the impacts of deposition on plant community composition, I conducted three studies. In 2008, I examined how native and non-native plant diversity related to existing variations in soil N availability. Native and non-native diversity were negatively correlated. Native diversity was negatively related to soil N while non-native diversity did not change. In an experiment in 2009, I added various N forms simulating components of deposition. Organic N addition decreased native diversity more than inorganic N, but non-native diversity was not affected by N addition. In a companion greenhouse experiment, a non-native grass tended to outcompete native grass regardless of N treatment. These results indicate that (1) diverse native ecosystems are capable of resisting invasion, (2) increasing soil N is detrimental to native diversity, and (3) native diversity may be more negatively impacted by organic than inorganic N deposition.