Changes in the soil, litter, and vegetation nitrogen and carbon concentrations of semiarid shrublands in response to chronic dry season nitrogen input

Abstract: Southern Californian chaparral and coastal sage scrub (CSS) shrublands are exposed to high-levels of dry-atmospheric nitrogen (N) deposition. A field experiment was conducted over a 6-year period to assess the effects of cumulative, dry-season N inputs on soil and vegetation carbon (C) and nitrogen (N). We hypothesized that plots exposed to exogenous N should exhibit signs of N enrichment once a cumulative N exposure threshold was reached and the N immobilization capacity was exceeded. Using mass-balance we found that chaparral and CSS have the capacity to immobilize 6.2 and 11.9gNm−2 y−1, respectively. Soil extractable N increased significantly after 7–10g/m2 of cumulative N exposure, resulting in a simultaneous increase in the N concentration and a decline in the C:N ratio of shrub tissue. Similar results were observed for the surface litter pool and litter production but at a higher cumulative N exposure. While we cannot tease out the potential for lagged responses to affect our estimate of the cumulative N threshold, these results indicate that the C and N cycling of semi-arid shrublands is significantly altered once the ecosystem N uptake capacity is exceeded. [Copyright &y& Elsevier]