Interplay between nitrogen deposition and grazing causes habitat degradation

Increased atmospheric nitrogen (N) deposition has been held responsible for the large-scale invasion of graminoids (grasses, sedges and rushes) in a wide range of habitats from forests to upland heaths, causing dramatic changes in plant species composition. Concurrently with an increase in N deposition over the last century, livestock grazing has intensified in many parts of the world following policy reform, leading to large-scale degradation of natural and seminatural ecosystems. On the basis of a series of experiments conducted in a Scottish montane ecosystem, we discovered that grazing and N deposition do not operate independently, and the interplay between them is leading to the replacement of valuable moss-dominated habitat by grasses and sedges. Our study indicates that in setting 'critical loads' of N, widely used to minimize habitat degradation, it is necessary to account for substantial amplification of N-deposition effects by grazing.