Long-term consequences of disturbance on nitrogen dynamics in an arid ecosystem

Anthropogenic activity is causing dramatic changes in the nitrogen (N) cycl e in many ecosystems. Most research has focused on the increase in N input caused by atmospheric deposition and invasion of N-fixing species, and on t heir effects on resource availability and species composition. However, in contrast to many ecosystems experiencing large increases in N input, many a rid ecosystems are experiencing loss of nutrients due to land-use change. A n important component of many arid ecosystems on a worldwide basis is the m icrobiotic crust, a biological soil crust composed of lichens, cyanobacteri a, mosses, and algae. Nitrogen fixation by lichens and cyanobacteria compri sing the crust is the primary source of N input in many of these ecosystems . We quantified the long-term consequences of surface disturbance in an ari d ecosystem on the Colorado Plateau by comparing pristine sites with those of known disturbance history. Disturbance caused an increase in the abundan ce of cyanobacteria and a decrease in lichens within the microbiotic crust. Carbon isotope composition (delta(13)C) of the crust reflects this shift i n species composition; values for disturbed sites were 4.5 parts per thousa nd higher than undisturbed sites. Nitrogen isotope composition (delta(15)N) of the microbiotic crust was 1.5-2.2 parts per thousand higher for disturb ed sites, probably resulting from relatively greater gaseous N loss from th e crust. Historic disturbance has caused a long-term decrease in rates of N fixation by the microbiotic crust; nitrogenase activity in pristine sites was 250% greater than sites intermittently disturbed 30 yr ago. The decreas e in N input from fixation and continued gaseous N loss has caused a 25-75% decrease in soil N content. Altering relative rates of N input and loss, c oupled with input of N from microbiotic crusts with relatively higher delta (15)N, has caused an increase in soil and plant delta(15)N at disturbed sit es. This decrease in soil N caused by disturbance will likely cause changes in species composition similar to those observed in ecosystems that have b een disrupted by excess N input from atmospheric deposition.