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

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 11.9 gN m−2 y−1, respectively. Soil extractable N increased significantly after 7–10 g/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.

► We tested for N enrichment as a function of ecosystem N uptake capacity.
► Soil extractable N and acidification increased when N uptake capacity was exceeded.
► Plant tissue responded to added N on a similar time scale as soil extractable N.
► Litter N enrichment required longer cumulative N exposure than tissue N.
► N exposure resulted in a decline in the C:N ratio of tissue and litter.