Evidence for nitrogen saturation in the San Bernardino Mountains in southern California

Abstract

Elevated N deposition has occurred in the Los Angeles Basin in southern California for at least the last 40 years. Elevated streamwater NO; fluxes and high nitric oxide (NO) fluxes from soil, indicators of N saturation, have recently been reported for chaparral watersheds exposed to chronic N deposition in the San Gabriel Mountains north/northeast of Los Angeles. A number of nutritional and edaphic parameters across a deposition gradient in the San Bernardino Mountains (SBM) support the hypothesis that the mixed conifer forest in the western end of the range is also N saturated. Concentrations of NO; in the soil solution or in soil extracts during the summer months were 14 to 44 times higher at Camp Paivika (CP), a western high N deposition site, than at Camp Osceola (CAO) or Barton Flats (BF), eastern low-pollution sites. Accumulation of NO; in foliage of bracken fern (Pteridium aquilinum var. pubescens Underw.) and overstory species was also much greater at CP than at CA0 and a site near BF. Nitric oxide fluxes in mid-August from relatively dry soil at CP were ca. 20 times higher than for typical forests in North America. Nitrous oxide (N,O) emissions, on the other hand, were low in the SBM sites. However, emissions of NO and N,O were several-fold higher at CP than at BF, a relatively low-pollution site. High NO emissions from otherwise undisturbed and well-drained forest soils of the western US may prove useful as a diagnostic indicator of N saturation. Nitrogen mineralization was greater at CP and Dogwood (high-pollution sites) than at CA0 and Heartbar (low-pollution sites). Additional indicators of N enrichment at CP compared with the low N deposition sites include: low C : N ratios in soil and foliage, high foliar N : P ratios, higher nitrification rates and high soil acidity. Lower pH and base saturation were observed in soil from two high-pollution sites compared with two low-pollution sites. In summary, high NO emissions and elevated NO; concentrations in the soil solution and in foliage, and high foliar N : P ratios at CP, indicate N in excess of biotic demand, with potential above-normal loss of N from the ecosystem -and thus, a N-saturated condition. Model outputs from the nutrient cycling model (NuCM) agreed well with field data from the SBM on elevated soil solution NO; concentrations, reduced soil base saturation, and lack of a growth response to increasing N deposition.

Ecological issues related to N deposition to natural ecosystems: research needs

“…Because water is limited, the N cycle is different than in humid regions and not dominated by surface flows or wet deposition. Research conducted in the forests of southern California, a Mediterranean climate, suggests that dry deposition of N is 20 times as great as that fiom wet deposition, and is sufficient to lead to N saturation (Fenn et al, 1996). Effects of elevated N on coastal sage scrub and chaparral ecosystems have also been documented (Allen et al, 1998;Riggan et al, 1994).…”

Decadal-scale Dynamics of Water, Carbon and Nitrogen in a California Chaparral Ecosystem: DAYCENT Modeling Results

“…The effects of chronic nitrogen (N) deposition in areas of the western United States include increased greenhouse gas emissions (Fenn et al 1996(Fenn et al , 2003b, higher N concentrations in plant tissues (Fenn et al 2003a), and increased nitrification rates and nitrate (NO 3 À ) levels in soil, streams, and lakes (Riggan et al 1985;Williams et al 1996;Fenn and Poth 1999;Sickman et al 2002). N deposition has caused some environmental problems such as reduced drinking water quality and eutrophication of recipient water bodies, NO 3 À -induced changes in plant community composition, disruption, and contributions to regional haze and impaired visibility Fenn et al (2003a, b).…”

“…The long-term dynamics of water, carbon (C) and N in chaparral ecosystems under the effects of N deposition are not well known. Field studies of C and N biogeochemistry in chaparral ecosystems over the last three decades have included biomass and net primary production (NPP) (Mooney and Rundel 1979;Schlesinger and Gill 1980;Parsons and Stohlgren 1986), nutrient cycling (Christensen and Muller 1975;Rundel and Parsons 1980;Gary and Schlesinger 1983;Graham and Wood 1991;Ulery et al 1995;Fenn et al 1996), ecological effects of N deposition (Riggan et al 1985(Riggan et al , 1994Fenn et al 1996Fenn et al , 2003a, greenhouse gaseous N emissions (Anderson and Poth 1989;Fenn et al 1996), and stream NO 3 À export (Davis 1989;Fenn et al 2003c). Our understanding of ecological processes in chaparral ecosystems is derived from short-term studies of less than a few years at a limited set of intensively studied sites.…”

“…, in the San Bernardino Mountains of southern California (Fenn et al 1996), with increasing N deposition levels. Our modeled net N mineralization in the chaparral zone of the Sierra Nevada was 6.3 g N m À2 yr À1 .…”

Effects of nitrogen addition on leaf traits of common species in natural Pinus tabuliformis forests in Taiyue Mountain, Shanxi Province, China

“…Chinese Journal of Plant Ecology, 40, 686-701. doi: 10.17521/cjpe.2015Ecology, 40, 686-701. doi: 10.17521/cjpe. .1043 自工业革命以来, 大气氮沉降呈现急剧增加的 趋势 (Holland et al, 1999;Zheng et al, 2002;Adams et al, 2004;Suding et al, 2005), 这直接影响到人类 和生态系统的健康、温室气体的平衡及生物的多样 性, 引起了科学家们的广泛关注与担忧 (Vitousek et al, 1997;Richter et al, 2005;Clark & Tilman, 2008; (Aber et al, 1989;Tamm, 1990), 但长期、连 续、过量的氮输入会降低植物的净光合速率和净初 级生产力, 引起森林生产力的下降和森林生态系统 功能的退化 (Ulrich, 1995;Fenn et al, 1996;Magill et al, 2004;方运霆等, 2005), 这被认为是森林衰退的 主要原因 (Nihlgard, 1985;Bobbink et al, 1998 (Comelissen et al, 2003), 因此成为探索植物 与环境之间关系的重要纽带, 也为预测在全球变化 背景下的生态系统响应提供了方法和依据 (Wright et al, 2005)。其中, 叶片性状对气候变化十分敏感, 能较为准确地反映植物对气候变化的响应与适应机 制 (Royer et al, 2010;Scoffoni et al, 2011)。研究表…”