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

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Effects of Dry-Season N Input on the Productivity and N Storage of Mediterranean-Type Shrublands Ecosystems (2009) 12: 473–488 DOI: 10.1007/s10021-009-9236-6 � 2009 Springer Science+Business Media, LLC Effects of Dry-Season N Input on the Productivity and N Storage of Mediterranean-Type Shrublands George L. Vourlitis,1* Sarah C. Pasquini,1,2 and Robert Mustard1 1Department of Biological Sciences, California State University, San Marcos, California 92096, USA; 2Department of Botany and Plant Sciences, University of California, Riverside, California 92521, USA ABSTRACT Anthropogenic nitrogen (N) deposition is a globally leaching in chaparral but not CSS. Nitrogen addi­ important source of N that is expected to increase tion also lead to an increase in litter and tissue N with population growth. In southern California, N concentration and a decline in the C:N ratio, but input from dry deposition accumulates on vegeta­ failed to alter the ecosystem productivity and N tion and soil surfaces of chaparral and coastal sage storage of the chaparral and CSS shrublands over scrub (CSS) ecosystems during the summer and fall the 4-year study period. The reasons for the lack of and becomes available as a pulse following winter a treatment response are unknown; however, it is rainfall. Presumably, N input will act to stimulate possible that these semi-arid shrublands are not N the productivity and N storage of these Mediterra­ limited, cannot respond rapidly enough to capture nean-type, semi-arid shrublands because these the ephemeral N pulse, are limited by other nutri­ ecosystems are thought to be N limited. To assess ents, or the N response is dependent on the amount whether dry-season N inputs alter ecosystem pro­ and/or distribution of rainfall. These results have ductivity and N storage, a field experiment was important implications for understanding the po­ conducted over a 4-year period where plots were tential effects of anthropogenic N deposition on the exposed to either ambient N deposition (control) or C and N cycling and storage of Mediterranean-type, ambient + 50 kgN ha -1 y -1 (added N) that was semi-arid shrublands. added as NH4NO3 during the fall dry-season of each year. Plots exposed to added N had significantly Key words: Adenostoma fasciculatum; Artemisia cal­ higher accumulation of NH4 and NO3 on ion ifornica; biogeochemistry; Ceanothus greggii; chap­ exchange resins that was due in part to direct fer­ arral; coastal sage scrub; global change; Salvia tilization and N mineralization, and the increase in mellifera; semi-arid ecosystems. N availability lead to a significant increase in NO3 INTRODUCTION Anthropogenic nitrogen (N) deposition is a signif­ Received 11 January 2008; accepted 16 January 2009; icant input of N into many southern Californian published online 21 February 2009 semi-arid ecosystems (Bytnerowicz and Fenn 1996; Author Contributions: GLV conceived or designed study, performed Fenn and others 2003a). Nitrogen deposition in research, analyzed data, contributed new methods or models, and wrote polluted urban shrublands and woodlands is esti­ the article. SCP performed research, analyzed data, and contributed to the -1 -1 writing of the article. RM performed research, analyzed data, and con­ mated to be 20–45 kgN ha y (Riggan and tributed to the writing of the article. others 1985; Bytnerowicz and Fenn 1996; Meixner *Corresponding author; e-mail: [email protected] 473 474 G. L. Vourlitis and others and Fenn 2004); however, some more exposed 1 year of N addition and it may take several years locales can receive up to 145 kgN ha -1 y -1 (Fenn for a treatment effect to be discernible in slow- and Poth 2004). Approximately, 85–95% of N growing, woody ecosystems (Oechel and Vourlitis deposition is in the form of dry deposition that is 1994; Milchunas and Lauenroth 1995). composed of oxidized and particulate N (By­ Assuming that the productivity of chaparral and tnerowicz and Fenn 1996; Padgett and others CSS shrublands is N limited, we hypothesized that 1999). Deposited N accumulates on shrub and soil repeated dry-season N addition would significantly surfaces during the summer and fall when atmo­ increase the productivity and N storage of chaparral spheric inversion traps pollutants within urban and CSS shrublands. To test this hypothesis we basins, and becomes available as a large and conducted a field experiment in chaparral and CSS, ephemeral pulse after the first rainfall event (By­ where N was added at the end of the dry season tnerowicz and Fenn 1996; Padgett and others 1999; (fall) each year. The experiment is ongoing and Fenn and others 2003a and b). part of a long-term research effort designed to Chaparral and coastal sage scrub (CSS) shrub- quantify the effects of chronic, dry-season N addi­ lands are major recipients of anthropogenic N in tion on ecosystem structure and function. Here we southern California, and represent over 70% of the report the response of aboveground biomass pro­ natural vegetation of coastal, interior, and moun­ duction and N storage after 4 years of fertilization. tain regions (Westman 1981). Chaparral is com­ posed of evergreen shrubs and is distributed in mid- to high-elevation foothill and mountain regions, MATERIALS AND METHODS whereas CSS is composed of semi-deciduous shrubs Site Description and Experimental and is distributed in coastal and low-elevation re­ Design gions (Westman 1981; Keeley 2000). Both shrub- lands tolerate intense summer drought (Poole and Field experiments were conducted between Sep­ Miller 1975; Gill and Mahall 1986; Kolb and Davis tember 2003 and 2007 at the Santa Margarita 1994), have similar rates of annual net primary Ecological Reserve (SMER: 33°29’N:117°09’W) and productivity (Gray and Schlesinger 1981, 1983), the Sky Oaks Field Station (SOFS: 33°21’N: and are subject to fire that has an average return 116°34’W). SMER is a CSS stand located in SW interval of 20–30 years (Keeley 2000). Chaparral Riverside County, California, USA at an elevation of and CSS have high biodiversity (Cowling and oth­ 338 m on a 9–11o S-SW facing slope. The site burned ers 1996) and large numbers of endangered species approximately 35 years ago and is dominated by the (Dobson and others 1997), and are relevant models semi-deciduous shrubs Artemisia californica Less. and for many semi-arid ecosystems worldwide, includ­ Salvia mellifera Greene (nomenclature according to ing Chilean mattoral, Spanish maquis, South Afri­ Munz 1974). Herbaceous species comprise 1–2% of can fynbos and thorn-scrub, and Australian the total plant cover, and are only found during the kwongan/mallee (DiCastri 1991), which share spring rainy season. Soil is a sandy clay loam of the similar adaptations to drought (Cody and Mooney Las Posas Series derived of igneous and weathered 1978), fire (Bond and van Wilgen 1996), and Gabbro material (Knecht 1971) with a bulk density nutrient-poor soils (Canadell and Zedler 1995). of 1.22 g/cm3. SMER receives an average of 36 cm Anthropogenic N inputs have the potential to of rainfall annually, most of which occurs between increase net primary productivity because the December and April. SOFS is a chaparral stand growth of chaparral and CSS shrubs is thought to located in NE San Diego County, California, USA at be N limited (McMaster and others 1982; Kum­ an elevation of 1418 m on a 4–10o SE-SW facing merow and others 1982; Gray and Schlesinger slope. The stand burned in July 2003, and unfortu­ 1983; Padgett and Allen 1999). However, drought nately this unplanned perturbation limits the that develops over the summer and fall reduces potential for direct comparison to the mature CSS physiological activity (Poole and Miller 1975; Gray stand. Before fire the site was a monoculture of the and Schlesinger 1981; Oechel and others 1981), evergreen shrub Adenostoma fasciculatum H. & A., and presumably, the ability of plants to utilize whereas after fire the stand was dominated by ephemeral pulses of anthropogenic N (Fenn and A. fasciculatum, but Ceanothus greggii A. Gray became others 2003b; Meixner and Fenn 2004). Previous a sub-dominant toward the end of 2005. The site research (Vourlitis and others 2007a) indicates that receives an average of 53 cm of precipitation dry-season N input significantly increases tissue annually consisting of rain with occasional snow and litter N concentration but not ecosystem C that occurs in November–April. The soil is an Ultic storage. However, these results were after only Haploxeroll derived of micaceous schist (Moreno Dry-Season N Addition Effects on Ecosystem C and N 475 and Oechel 1992) with a sandy loam texture and a resins consisting of 15 g anion (USF-A244B) and bulk density of 1.34 g/cm3 . 15 g cation (USF-C211) exchange resin (US Filter, The experimental layout at each site consisted of Rockford, Illinois, USA) mixed within a 6.25 9 a completely randomized design where four­ 15.0 cm 160-mesh nylon bag (n = 4 per plot). 10 9 10 m plots received 50 kgN ha -1 y -1 as Resin bags were deployed for approximately granular NH4NO3 (added N) and an additional 3 months and the NH4–N and NO3–N that accu­ four-10 9 10 m plots served as un-manipulated mulated on the mixed-resins over the 3-month controls. N deposition estimated from a high-reso­ period was extracted with 2 M KCl and analyzed lution (4 km) model suggest that both sites receive using an auto-analyzer (Quikchem 3000, Lachat 6–8 kgN ha -1 y -1 (Tonnesen and others 2007); Instruments, Milwaukee, Wisconsin, USA). thus, control plots received 6–8 kgN ha -1 y -1 , Surface organic matter (litter pool) was collected whereas plots exposed to added N received 56– seasonally within a 25 cm 9 12.5 cm rectangular 58 kgN ha -1 y -1.
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