Effects of Dry-Season N Input on the Productivity and N Storage of Mediterranean-Type Shrublands

Anthropogenic nitrogen (N) deposition is a globally important source of N that is expected to increase with population growth. In southern California, N input from dry deposition accumulates on vegetation and soil surfaces of chaparral and coastal sage scrub (CSS) ecosystems during the summer and fall and becomes available as a pulse following winter rainfall. Presumably, N input will act to stimulate the productivity and N storage of these Mediterranean-type, semi-arid shrublands because these ecosystems are thought to be N limited. To assess whether dry-season N inputs alter ecosystem productivity and N storage, a field experiment was conducted over a 4-year period where plots were exposed to either ambient N deposition (control) or ambient + 50 kgN ha⁻¹ y⁻¹ (added N) that was added as NH₄NO₃ during the fall dry-season of each year. Plots exposed to added N had significantly higher accumulation of NH₄ and NO₃ on ion exchange resins that was due in part to direct fertilization and N mineralization, and the increase in N availability lead to a significant increase in NO₃ leaching in chaparral but not CSS. Nitrogen addition also lead to an increase in litter and tissue N concentration and a decline in the C:N ratio, but failed to alter the ecosystem productivity and N storage of the chaparral and CSS shrublands over the 4-year study period. The reasons for the lack of a treatment response are unknown; however, it is possible that these semi-arid shrublands are not N limited, cannot respond rapidly enough to capture the ephemeral N pulse, are limited by other nutrients, or the N response is dependent on the amount and/or distribution of rainfall. These results have important implications for understanding the potential effects of anthropogenic N deposition on the C and N cycling and storage of Mediterranean-type, semi-arid shrublands. Author Contributions  GLV conceived or designed study, performed research, analyzed data, contributed new methods or models, and wrote the article. SCP performed research, analyzed data, and contributed to the writing of the article. RM performed research, analyzed data, and contributed to the writing of the article.