The extra nitrogen can increase the growth of some species such as rough grasses and nettles, which out-compete other species such as herb species which have lower nitrogen requirements.
Gall y nitrogen ychwanegol gynyddu twf rhai rhywogaethau megis glaswellt garw a danadl, sy'n drech na rhywogaethau eraill fel rhywogaethau perlysiau sydd angen llai o nitrogen.