Air pollution, principally in the form of photochemical ozone and deposition of nitrogen compounds, has significantly affected mixed conifer forests in the mountains of southern California. Foliar injury, premature needle abscission, crown thinning, and reduced growth and vigor have been well documented, particularly for ponderosa (Pinus ponderosa Laws.) and Jeffrey (P. jeffreyi Grev. and Balf.) pines on the western side of the pollution deposition gradient in the San Bernardino Mountains. Tree mortality of the more ozone-susceptible ponderosa and Jeffrey pines has led to alterations in stand composition, in favor of increased dominance by more ozone-resistant species such as incense cedar (Calocedrus decurrens (Torr.) Florin), white fir (Abies concolor (Gord. & Glend.) Lindl.), and sugar pine (P. lambertiana Doug.). Increased rates of litter deposition, alterations in C/N ratios in litter and soil, and reductions in fine root biomass of trees have also altered the dynamics of biogeochemical processing in stands impacted by ozone and excess N deposition. Research into the effects of atmospheric deposition across the mixed conifer forests of the San Bernardino Mountains continues to provide insights into the complex interactions among anthropogenic and natural stresses in a forest ecosystem.

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.

Before Euro-American settlement fire was a common process in the forests of the Lake Tahoe Basin. The combination of drought, fire suppression, and past harvesting has produced ecosystems that are susceptible to high-severity wildfires. Consequently, a program of prescribed fire has been recommended but there is incomplete understanding of the ecological effects of fuels treatments, especially with regard to how treatments will affect the flow of nutrients to Lake Tahoe. Nitrogen and phosphorus are the most important nutrients affecting algal growth, and thus lake clarity. Existing data demonstrate a long-term shift from a co-limitation by both nitrogen and phosphorus to phosphorus limitation. Two high-consumption, moderate-intensity prescribed fires were conducted to determine their effects on soil and stream water chemistry. Stream water calcium concentrations increased in burned watersheds whereas soluble reactive phosphorus concentrations were not significantly different. Prescribed fires released calcium and raised soil pH and this may have resulted in the incorporation of phosphorus into insoluble forms. Stream monitoring data indicates water quality effects last for ~3 months. Prescribed fires did not significantly increase the amount of soluble reactive phosphorus in stream waters. However, additional research is needed to determine if prescribed fire increases erosion or movement of particulate P, particularly in areas with steep slopes.

The greatest air pollution impacts in forests of California are the physiological disturbances imposed on trees as a result of the combined effects of excess N and phytotoxic ozone exposure (Takemoto et al., 2001). In highly-polluted stands in the San Bernardino Mountains in southern California, fine root biomass is greatly reduced and C cycling within the tree and within the ecosystem is also significantly altered. Air pollution effects appear to be more subtle over most of the Sierra Nevada. Individual trees with significant amounts of ozone injury in the southern and western edge of the Sierra have been identified in previous surveys. Additional significant environmental impacts of N deposition in southern California forest and chaparral ecosystems include high NO3− concentrations in streamwater and groundwater and increased greenhouse gas emissions from soil. Nitrogen deposition in the Sierra Nevada does not appear to be sufficiently high to cause major physiological impacts or widespread deterioration of water quality, although it is possible that chronic N deposition may be at least partially offsetting the depressive growth effects of ozone in the southern Sierra. However, unusually high nitrate concentrations frequently occur in a chaparral catchment with high N deposition inputs in Sequoia National Park. Preliminary results from N deposition measurements, streamwater analyses for NO3−, and soil and plant indicators of N enrichment suggest that N cycling in the mixed conifer forests in the Mountain Home State Park region in the southwestern Sierra Nevada is being altered by N deposition to a greater extent than similar forests in Sequoia National Park. Ozone and N deposition levels are relatively low in high-elevation ecosystems of the Sierra Nevada and do not appear to have severe impacts, although N deposition in the southern Sierra may contribute to the natural peak in nitrate in runoff during early snowmelt. In forests throughout California, periodic droughts and stand densification from long-term fire suppression are major risk factors responsible for reduced tree vigor, greater mortality and predisposition to disease and insect attack; the latter a common ultimate cause of tree mortality. Current land management plans for the Sierra Nevada focus on decreasing overstocking of stands and reducing fuel loads and wildfire risk.

Measurement of ionic deposition in throughfall is a widely used method for measuring deposition inputs to the forest floor. Many studies have been published, providing a large database of throughfall deposition inputs to forests. However, throughfall collection and analysis is labor intensive and expensive because of the large number of replicate collectors needed and because sample collection and chemical analyses are required on a stochastic precipitation event-based schedule. Therefore we developed and tested a throughfall collector system using a mixed bed ion exchange resin column. We anticipate that this method will typically require only one to three samplings per year. With this method, bulk deposition and bulk throughfall are collected by a funnel or snow tube and ions are retained as the solution percolates through the resin column. Ions retained by the resin are then extracted in the same column with 2N KCl and analyzed for nitrate and ammonium. Deposition values in throughfall from conventional throughfall solution collectors and colocated ion exchange samplers were not significantly different during consecutive 3- and 4-month exposure periods at a high (Camp Paivika; >35 kg N ha-1 year-1) and a low deposition (Barton Flats; 5–9 kg N ha-1 year-1) site in the San Bernardino Mountains in southern California. N deposition in throughfall under mature pine trees at Camp Paivika after 7 months of exposure was extremely high (87 and 92 kg ha-1 based on the two collector types) compared to Barton Flats (11 and 13 kg ha-1). A large proportion of the N deposited in throughfall at Camp Paivika occurred as fog drip, demonstrating the importance of fog deposition as an input source of N at this site. By comparison, bulk deposition rates in open areas were 5.1 and 5.4 kg ha-1 at Camp Paivika based on the two collector types, and 1.9 and 3.0 kg ha-1 at Barton Flats.

Virtually complete nitrification of the available ammonium in soil and nitrification activity in the forest floor are important factors predisposing forests in the San Bernardino Mountains of southern California to nitrogen (N) saturation. As a result, inorganic N in the soil solution is dominated by nitrate. High nitrification rates also generate elevated nitric oxide (NO) emissions from soil. High-base cation saturation of these soils means that soil calcium depletion or effects associated with soil acidification are not an immediate risk for forest health as has been postulated for mesic forests in the eastern U.S. Physiological disturbance (e.g., altered carbon [C] cycling, reduced fine root biomass, premature needle abscission) of ozone-sensitive ponderosa pine trees exposed to high N deposition and high ozone levels appear to be the greater threat to forest sustainability. However, N deposition appears to offset the aboveground growth depression effects of ozone exposure. High nitrification activity reported for many western ecosystems suggests that with chronic N inputs these systems are prone to N saturation and hydrologic and gaseous losses of N. High runoff during the winter wet season in California forests under a Mediterranean climate may further predispose these watersheds to high nitrate leachate losses. After 4 years of N fertilization at a severely N saturated site in the San Bernardino Mountains, bole growth unexpectedly increased. Reduced C allocation below- ground at this site, presumably in response to ozone or N or both pollutants, may enhance the bole growth response to added N.

This study utilized isotope analyses to contrast nitrogen and carbon dynamics at four sites located along an air pollution gradient in the San Bernardino National Forest in southern California. Natural 15N and 13C abundances along with nutritional and edaphic properties were determined in soil, litter, and vegetation samples. Mean bulk nitrogen δ15N values of soil and vegetation at Camp Paivika (CP), the most polluted site, were at least 1.7‰ more enriched than the other, less polluted sites. Mean soil δ15NH4+ was also significantly enriched in 15N at CP compared to Barton Flats (BF), the least polluted site, by 3.8‰. Soil δ15NO3− signatures were not statistically different among sites. The litter δ15NH4+ values followed a trend similar to that of the soil. Furthermore, the litter δ15NO3− at CP was significantly depleted in 15N compared to the other sites. The isotopic discrimination for the eventual production of nitrate from organic nitrogen in soil and litter was maximized at CP and minimized at BF. A stable carbon isotopic gradient of decreasing soil, litter, and foliar δ13C was also observed with increasing site pollution level. These results support the hypothesis that chronic atmospheric deposition has enhanced nitrogen cycling processes and has affected carbon metabolism at CP.

Tropical ecosystems play an important role in production or consumption of atmospheric trace gases including nitric oxide (NO), nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4). Here we describe field and laboratory experiments, performed during 1994, to determine the influence of fire on processes responsible for fluxes of gases from cerrado sites burned 17 and 45 d earlier, and a control site, last burned in 1974. Burning stimulated gross N mineralization but depressed nitrification rates; however, rates were sufficient to support NO fluxes observed in a 1992 study at the same site. Extractable nutrients and fluxes of NO and N2O from wetted and dry soils were measured prior to and for a 3-d period following burning. Over this period NO2- declined to undetectable levels; NH4+ increased, and NO fluxes remained relatively constant, suggesting that nitrifiers replaced the NO2- reduced to NO. Soils at burned and unburned sites exhibited CH4 uptake, which was inhibited by CH3F, thereby converting soils from a strong sink to a weak source of CH4. Carbon dioxide fluxes did not increase, and there were no detectable fluxes of N2O following burning. In lab studies NO and N2O emissions were inhibited by autoclaving, suggesting that nitrification was key to their production. However, addition of NO2- to autoclaved soil resulted in large fluxes of NO but no detectable N2O, suggesting that chemodenitrification may have been responsible for NO but not N2O production. Further research is needed to determine whether NO is produced directly by nitrifier NO2- reduction or indirectly by chemodenitrification of NO2- produced by nitrifiers.

Nitric oxide (NO), nitrous oxide (N 2 O), and carbon dioxide (CO 2 ) measurements were made at unburned and burned sites within a semiarid chaparral ecosystem located in southern California. Some sites were burned during the dry season (July); others at the start of the wet season (December). This report primarily describes measurements of gas fluxes and soil parameters at unburned sites and sites burned in December. Burning greatly increased the concentrations of ammonium (NH 4 + ), nitrate (NO 3 − ) and potentially mineralizable nitrogen in soil. Nitrate concentrations at sites burned in December were much higher than those at sites burned in July, indicating active nitrification and decreased water stress. Irrigation usually produced immediate but short‐lived increases in NO flux; however, these increases were more moderate than those observed in July. Diurnal studies performed at irrigated and nonirrigated burned sites both in July and December indicated a high correlation of NO flux with temperature (r 2 > 0.81) at most sites, provided that the soil moisture was greater than 30%. Results of diurnal studies were used to normalize daily fluxes and to calculate semiannual losses of nitrogen from both burned and unburned sites. Over a six‐month period, 0.3 g N m −2 was lost as NO from burned chaparral, representing 75% of the exchangeable nitrogen lost over the same period and 3.9% of the potentially mineralizable nitrogen present in the soil. From unburned soil, 0.1 g N m −2 was lost as NO, representing 33% of the exchangeable nitrogen and 3.3% of the potentially mineralizable nitrogen in soil. These studies indicate that over a six‐month period following a burn, much of the NH 4 + produced by the burning of aboveground vegetation is lost to the atmosphere as NO. These emissions may be expected to persist for periods in excess of six months because of the increased mineralizable nitrogen available in soil as a result of the burn. Losses of nitrogen both during and following a burn may impact soil fertility, contribute to both air and water pollution, and yield significant inputs to the global atmospheric budgets of both NO and N 2 O.

The San Dimas Experimental Forest serves as a field laboratory for studies of chaparral and related ecosystems, and has been recognized by national and international organizations. It covers 6,945 ha (17,153 acres) in the foothills of the San Gabriel Mountains northeast of Los Angeles, and has a typical Mediterranean-type climate. The Forest encompasses the San Dimas and Big Dalton watersheds, which have vegetation typical of southern California, are separated by deep canyons from the rest of the San Gabriel Mountains, have small tributaries suitable for study, and are harnessed by flood control dams. Unique physical features and a broad database covering over 50 years of research make the Forest an irreplaceable resource. Over the years data has been collected on water (precipitation, streamflow, etc.), soils and slope stability, effects of fire, vegetation management, chaparral ecology and physiology, vegetation classification, litter decomposition, and community structure of fauna. On-going studies include these: investigations into erosion processes; sediment movement in streams; particle size shifts with burning; air pollution impacts on vegetation, soil and water; denitrification in streams and nitrogen fixation; regeneration of oaks, postfire vegetation composition changes, dynamics of seed populations in soil; long term changes in site quality including Ceanothus dieback; and wildlife interactions.

Recent measurements indicate significantly enhanced biogenic soil emissions of both nitric oxide (NO) and nitrous oxide (N2O) following surface burning. These enhanced fluxes persisted for at least 6 months following the burn. Simultaneous measurements indicate enhanced levels of exchangeable ammonium in the soil following the burn. Biomass burning is known to be an instantaneous source of NO and N2O resulting from high‐temperature combustion. Now we find that biomass burning also results in significantly enhanced biogenic emissions of these gases, which persist for months following the burn.

Rhizosphere microbes play a significant role in nutrient cycling and in plant health. Biochar soil amendment can enhance carbon sequestration and alter soil edaphic features (e.g., increase pH and water holding capacity) in terrestrial ecosystems. However, the effect of biochar amendment on rhizosphere microbiome is still an open question, especially for calcareous soils, which generally have neutral or alkaline pH. We investigated the impacts of biochar amendment on tomato rhizosphere bacterial community composition and diversity in alkaline calcareous soil. To characterize the bacterial communities that consume root-exudates, we grew tomato plants with and without the biochar amendment in plant growth chambers supplied with and without 13CO2 labeled gas for 35 days. Biochar amendment increased soil pH and total carbon (C) concentration but had little impacts on soil NO3–-N, NH4+-N, OlsenP concentrations, and total vegetative tomato biomass compared with unamended soil. Furthermore, the tomato rhizosphere bacterial community composition did not significantly change after biochar amendment. We observed 76 and 82 13C-labeled amplicon sequence variants (ASVs) in the control and biochar treated rhizospheres, respectively, and 60 of the 98 ASVs were shared between the two treatment samples. The bacterial taxonomic composition of the 13C-labeled ASVs was similar between control and biochar treatments, and mainly belonged to the phylum Proteobacteria (e.g., Acinetobacter and Kaistia). Additionally, the relative abundance of 13C-labeled bacteria only accounted for approximately 2% of the total rhizosphere bacterial communities for both treatments. These results demonstrate that biochar amendment may have limited impacts on rhizosphere bacterial community composition and their utilization of plant-derived carbon in alkaline calcareous soils. Our study furthers our understanding of the association of plant and bacterial communities in response to biochar amendment in agricultural systems.

The coastal sage scrub (CSS) vegetation of southern California is rapidly converting to annual grasslands, perhaps in part because of air pollution. By contrast, chaparral and coniferous forest are subject to equally high levels of air pollution but are relatively stable. A comparative analysis of ozone and nitrogen deposition on plants of CSS, exotic annual grassland, chaparral, and coniferous forest shows these vegetation types have different susceptibilities to each pollutant. Historically high concentrations of ozone in the local mountains weakened pines, contributing to tree mortality. Native shrub seedlings had decreased growth in chambers with current-day levels of 150 ppb ozone. Under natural field conditions the shrubs may escape ozone injury by being physiologically active early in the season when ozone concentrations are below phytotoxic levels. Summer-active pines are more susceptible to ozone than summer-deciduous CSS shrubs and senescent annual grasses. Nitrogen deposition has different impacts from ozone because N accumulates on leaf and soil surfaces during the summer. Conifers are more susceptible to leaf-deposited nitric acid because they are physiologically active in summer, while chaparral may be less so because of thick cuticles and reduced summertime activity. Deciduous CSS and senescent grasses are less susceptible to direct leaf damage. However, N becomes available for root uptake after the first fall rains. Soil accumulations up to 87 µg/g extractable N have been measured in surface soil of CSS shrubland, levels that have caused mortality in the greenhouse. Grasses may escape the deleterious effects of high soil N levels because of their annual habit. Coniferous forest may have a higher threshold for N damage because of high stand biomass, high N immobilization in soil organic matter, and watershed N runoff. The resistance of chaparral to high N is less well understood, but may be due to higher biomass and slower growth rates than CSS and also high leachate losses of N.

deposition fluxes for the array of dry-deposited gaseous and particulate forms of N, in addition to wet deposition Conventional throughfall collection methods are labor intensive and fog or cloudwater deposition of N. Determining the andanalyticallyexpensivetoimplementatbroadscales.Thisstudywas conducted to test an alternative approach requiring infrequent sample deposition of these many compounds to complex recipi- collection and a greatly reduced number of chemical analyses. The ent ecosystem surfaces under dynamic meteorological major objective of the study was to determine the feasibility of using conditions is a daunting task on the local scale and par- ion exchange resin (IER) to measure N deposition in throughfall with ticularly impractical and cost prohibitive over extensive field deployment periods of 3 to 12 mo. Nitrogen deposition measure- landscapes. As a result, total N deposition estimates to ments in bulk throughfall collected under pine (Pinus sp.) canopies ecosystems are generally highly uncertain or nonexistent. and in forest clearings were compared between co-located conven- Where N deposition data are available, a variety of ap- tional throughfall solution collectors and IER throughfall collectors proaches,suchastheinferentialmethod,simulationmod- using mixed bed IER columns. Deposition data were collected for eling, and throughfall (Lovett, 1994) have been used to 1 yr at a high deposition site (Camp Paivika, CP) and a relatively low

Microenvironmental data was collected long an elevational gradient, from Ash Mountain, Crystal Cave, Lower Kaweah, and Wolverton from mid April until mid November in 1999. At each site, soil temperature (-50 cm, -10 cm), air temperature, relative humidity, windspeed (all at 1.5 m), and photosynthetically active radiation were monitored. At all sites except Crystal Cave, ambient ozone was actively monitored every 5 min and averaged hourly. At Lower Kaweah only, an active NOx analyzer was also actively monitored every 5 min and averaged hourly. Calibrations were independently audited by Californian Air Resources Board. Passive ozone monitors were installed at all sites and accumulated exposure over a two week basis. Passive monitors for nitrous and nitric acid, ammonia, and SO2 was measured at all sites on a bimonthly basis for 48 h. Throughfall was collected from mid November, 1998 through July 14, 1999. Subsequent precipitation occurred only in trace amounts and insufficient solution volumes were available for chemical analysis. ticulate nitrate (NO3 - ), ammonium (NH4+), and sulfate (SO4 -2 ); and (3) passive samplers for O3, HNO3 and NO2. Elevated concentrations of O3 (seasonal means 41-71 ppb), HNO3 (seasonal means 0.4-2.9 µg/m 3 ), NH3 (seasonal means 1.6-4.5 µg/m 3 ), NO3 - (1.1-2.0 µg/m 3 ) and NH4 + (1.0-1.9 µg/m 3 ) were determined.