Abstract. Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate ( ) loss to groundwater Ϫ

A series of 15 N isotope tracer experiments showed that Nitrosomonas europaea produces nitrous oxide only under oxygen-limiting conditions and that the labeled N from nitrite, but not nitrate, is incorporated into nitrous oxide, indicating the presence of the “denitrifying enzyme” nitrite reductase. A kinetic analysis of the m/z 44, 45, and 46 nitrous oxide produced by washed cell suspensions of N. europaea when incubated with 4 mM ammonium (99% 14 N) and 0.4 mM nitrite (99% 15 N) was performed. No labeled nitrite was reduced to ammonium. All labeled material added was accounted for as either nitrite or nitrous oxide. The hypothesis that nitrous oxide is produced directly from nitrification was rejected since (i) it does not allow for the large amounts of double-labeled ( m/z 46) nitrous oxide observed; (ii) the observed patterns of m/z 44, 45, and 46 nitrous oxide were completely consistent with a kinetic analysis based on denitrification as the sole mechanism of nitrous oxide production but not with a kinetic analysis based on both mechanisms; (iii) the asymptotic ratio of m/z 45 to m/z 46 nitrous oxide was consistent with denitrification kinetics but inconsistent with nitrification kinetics, which predicted no limit to m/z 45 production. It is concluded that N. europaea is a denitrifier which, under conditions of oxygen stress, uses nitrite as a terminal electron acceptor and produces nitrous oxide.

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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.

Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate (NO3−) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO3−, and low soil C:N ratios. Recent nitrogen‐fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greater mortality. Long‐term fertilization at Mount Ascutney, Vermont, suggests that declining and slow N‐cycling coniferous stands may be replaced by fast‐growing and fast N‐cycling deciduous forests. Symptoms of N saturation are particularly severe in high‐elevation, nonaggrading spruce–fir ecosystems in the Appalachian Mountains and in eastern hardwood watersheds at the Fernow Experimental Forest near Parsons, West Virginia. In the Los Angeles Air Basin, mixed conifer forests and chaparral watersheds with high smog exposure are N saturated and exhibit the highest streamwater NO3− concentrations for wildlands in North America. High‐elevation alpine watersheds in the Colorado Front Range and a deciduous forest in Ontario, Canada, are N saturated, although N deposition is moderate (∼8 kg·ha−1·yr−1). In contrast, the Harvard Forest hardwood stand in Massachusetts has absorbed >900 kg N/ha during 8 yr of N amendment studies without significant NO3− leaching, illustrating that ecosystems vary widely in the capacity to retain N inputs. Overly mature forests with high N deposition, high soil N stores, and low soil C:N ratios are prone to N saturation and NO3− leaching. Additional characteristics favoring low N retention capacity include a short growing season (reduced plant N demand) and reduced contact time between drainage water and soil (i.e., porous coarse‐textured soils, exposed bedrock or talus). Temporal patterns of hydrologic fluxes interact with biotic uptake and internal cycling patterns in determining ecosystem N retention. Soils are the largest storage pool for N inputs, although vegetation uptake is also important. Recent studies indicate that nitrification may be widespread in undisturbed ecosystems, and that microbial assimilation of NO3− may be a significant N retention mechanism, contrary to previous assumptions. Further studies are needed to elucidate the sites, forms, and mechanisms of N retention and incorporation into soil organic matter, and to test potential management options for mitigating N losses f...

Conventional throughfall collection methods are labor intensive and analytically expensive to implement at broad scales. This study was conducted to test an alternative approach requiring infrequent sample collection and a greatly reduced number of chemical analyses. The major objective of the study was to determine the feasibility of using ion exchange resin (IER) to measure N deposition in throughfall with field deployment periods of 3 to 12 mo. Nitrogen deposition measurements in bulk throughfall collected under pine (Pinus sp.) canopies and in forest clearings were compared between co-located conventional throughfall solution collectors and IER throughfall collectors using mixed bed IER columns. Deposition data were collected for 1 yr at a high deposition site (Camp Paivika, CP) and a relatively low one (Barton Flats, BF) in the San Bernardino Mountains in southern California: Annual throughfall deposition values (kg ha(-1) of NH(4)-N + NO(3)-N) under large ponderosa pine trees (Pinus ponderosa Laws.) were 145.8 and 143.9 at CP and 17.0 and 15.0 at BF according to the IER and conventional methods, respectively. Analogous values for bulk deposition in forest clearings were 15.6 and 12.3 at CP and 4.0 and 3.3 at BF. It was concluded that the IER collectors can be used for routine monitoring of deposition in throughfall and bulk deposition, provided that field blanks are used to account for background levels of N in the IER columns, which at times are slightly elevated, possibly from slow release of amine groups from the anion exchange resin during field exposures.

Administration of dehydroepiandrosterone (DHEA) appears to have physiological effects opposing those of glucocorticoids in several animal models. Recently, immunomodulatory effects of treatment with DHEA have been described. This paper reports the effects of DHEA treatment on splenocyte blastogenic responses as well as thymic and spleen weights in C3H/HeN mice. Pretreatment of mice with sc DHEA (60 mg/kg.day) for 3 days in vivo antagonized the profound suppression of in vitro blastogenic responses seen in T- and B-lymphocytes after a single injection of dexamethasone (DEX; 60 mg/kg). Pretreatment with DHEA also significantly reduced dexamethasone-induced thymic and splenic atrophy. Splenic lymphocytes from DHEA-treated mice were markedly more resistant to in in vitro suppression of blastogenesis by DEX at 10(-6)-10(-8) M compared to lymphocytes from control mice. However, DHEA added to lymphocyte cultures in vitro over a concentration range from 10(-7)-10(-8) M failed to protect against suppression of mitogenic responses caused by addition of DEX to cultures. In summary, DHEA given in vivo antagonizes the suppressive actions of DEX on lymphoid target tissues in mice.

Among all global ecosystems, tropical savannas are the most severely and extensively affected by anthropogenic burning. Frequency of fire in cerrado, a type of tropical savanna covering 25% of Brazil, is 2 to 4 years. In 1992 we measured soil fluxes of NO, N2O, CH4, and CO2 from cerrado sites that had been burned within the previous 2 days, 30 days, 1 year, and from a control site last burned in 1976. NO and N2O fluxes responded dramatically to fire with the highest fluxes observed from newly burned soils after addition of water. Emissions of N‐trace gases after burning were of similar magnitude to estimated emissions during combustion. NO fluxes immediately after burning are among the highest observed for any ecosystem studied to date. These rates declined with time after burning and had returned to control levels 1 year after the burn. An assessment of our data suggested that tropical savanna, burned or unburned, is a major source of NO to the troposphere. Cerrado appeared to be a minor source of N2O and a sink for atmospheric CH4. Burning also elevated CO2 fluxes, which remained detectably elevated 1 year later.