David P. Huber

Ecosystem response to nutrient enrichment across an urban airshed in the Sonoran Desert

Rates of nitrogen (N) deposition have increased in arid and semiarid ecosystems, but few studies have examined the impacts of long-term N enrichment on ecological processes in deserts. We conducted a multiyear, nutrient-addition study within 15 Sonoran Desert sites across the rapidly growing metropolitan area of Phoenix, Arizona (USA). We hypothesized that desert plants and soils would be sensitive to N enrichment, but that these effects would vary among functional groups that differ in terms of physiological responsiveness, proximity to surface N sources, and magnitude of carbon (C) or water limitation. Inorganic N additions augmented net potential nitrification in soils, moreso than net potential N mineralization, highlighting the important role of nitrifying microorganisms in the nitrate economy of drylands. Winter annual plants were also responsive to nutrient additions, exhibiting a climate-driven cascade of resource limitation, from little to no production in seasons of low rainfall (winter 2006 and 2007), to moderate N limitation with average precipitation (winter 2009), to limitation by both N and P in a season of above-normal rainfall (winter 2008). Herbaceous production is a potentially important mechanism of N retention in arid ecosystems, capable of immobilizing an amount equal to or greater than that deposited annually to soils in this urban airshed. However, interannual variability in precipitation and abiotic processes that limit the incorporation of detrital organic matter into soil pools may limit this role over the long term. In contrast, despite large experimental additions of N and P over four years, growth of Larrea tridentata, the dominant perennial plant of the Sonoran Desert, was unresponsive to nutrient enrichment, even during wet years. Finally, there did not appear to be strong ecological interactions between nutrient addition and location relative to the city, despite the nearby activity of nearly four million people, perhaps due to loss or transfer pathways that limit long-term N enrichment of ecosystems by the urban atmosphere.

Charge sign dependence of cosmic ray modulation near a rigidity of 1 GV

New observations of electron fluxes made in 1997 and 1998 extend our ongoing investigation of the relative modulation of positively and negatively charged particles. We compare electron fluxes measured on high-altitude balloon flights with continuing observations of helium fluxes from the IMP 8 spacecraft and present new measurements of the primary cosmic ray positron abundance in 1997 and 1998. Electron fluxes during the 1984 -1990 period show a flat topped distribution, whereas the positively charged He fluxes show a peaked distribution, with the peak in 1987. This is expected from modulation theory, including the role of drifts when the northern heliospheric magnetic field is inward, and the southern heliospheric field is outward. From 1990 to 1999, data are consistent with an inverse relationship, but electron data are too sparse to allow a definitive statement. Near a rigidity of 1 GV the relative abundance of electrons and helium nuclei is a weak function of the tilt angle of the heliospheric current sheet.

Cosmic electron spectra 1987–1994

We report cosmic electron spectra obtained on balloon flights in August of the years 1987, 1990, 1992, and 1994 in an extension of the series of observations with the same instrument begun in 1968. Electron fluxes at approximately 6 GeV have returned to levels seen in the decade of the 1980s in spite of the reversal of solar magnetic polarity in 1990, which in some models should have produced a return to the much lower fluxes observed in the 1970s. We know of no satisfactory theoretical interpretation of these results.