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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]

Abstract: Fire and atmospheric N deposition have the capacity to alter the N and C cycling and storage of semi-arid shrublands. Thus, we measured the pre- and post-fire soil and tissue C and N dynamics of three southern California chaparral stands exposed to varying levels of N deposition. Total soil N was positively correlated with N deposition exposure before and after fire, indicating that fire did little to alter patterns of soil N enrichment from atmospheric N deposition. Fire caused a significant increase in soil extractable N, and this increase was positively correlated with N deposition indicating that fire enhanced soil N availability more at sites exposed to high N deposition. Fire also caused the tissue N concentration of regenerating Adenostoma fasciculatum (chamise) shrubs to be significantly correlated with N deposition; however, the correlation was negative during the first year of fire recovery (r 2=0.62) and positive during the second (r 2=0.94) indicating complex and transient dynamics in shrub growth and N concentration. Overall, our results suggest that periodic fire in chaparral may not reduce the potential for N enrichment that develops over decades of N deposition and have important implications for the propensity of these semi-arid shrublands to become “N-saturated”. [Copyright &y& Elsevier]

Abstract: Semi-arid shrublands of southern California, including chaparral and coastal sage, are found in widely varying elevation and microclimatic regimes and are subjected to disturbance such as fire and atmospheric N deposition that have the capacity to alter soil and litter C and N storage. Here we present a case study where soil and litter C and N were measured over 19 months in post-fire chaparral and mature coastal sage stands to assess whether differences in soil and litter C and N between these diverse shrublands could be attributed to differences in elevation, stand age, rainfall, and/or estimated N deposition exposure. Our results indicate that atmospheric N deposition exposure, either alone or in conjunction with other environmental variables (elevation, rainfall, and/or stand age), was the most frequent predictor of the spatial pattern in the soil and litter N and C variables observed. These results are consistent with those reported for high-elevation coniferous forests arrayed along an N deposition gradient in southern California, suggesting that N deposition may affect the soil N and C storage of semiarid shrublands and woodlands in a qualitatively similar manner. [Copyright &y& Elsevier]