ABSTRACT Forest biomes are major reserves for terrestrial carbon, and major components of global primary productivity. The carbon balance of forests is determined by a number of component processes of carbon acquisition and carbon loss, and a small shift in the magnitude of these processes would have a large impact on the global carbon cycle. In this paper, we discuss the climatic influences on the carbon dynamics of boreal, temperate and tropical forests by presenting a new synthesis of micrometeorological, ecophysiological and forestry data, concentrating on three case-study sites. Historical changes in the carbon balance of each biome are also reviewed, and the evidence for a carbon sink in each forest biome and its likely behaviour under future global change are discussed. We conclude that there have been significant advances in determining the carbon balance of forests, but there are still critical uncertainties remaining, particularly in the behaviour of soil carbon stocks. Key-words: biosphere -atmosphere interactions; carbon dioxide; eddy covariance; forests; global carbon cycle; global change. INTRODUCTION The increasing concentration of carbon dioxide in the atmosphere since the industrial revolution is among the most significant of human influences on the global environment. The source of this carbon dioxide has been convincingly ascribed to the use of fossil fuels, cement manufacture and deforestation, but considerable mystery remains because only a fraction of the estimated emissions of CO 2 remains in the atmosphere. Between 1980, it is estimated (Schimel 1995) that 5·5 ± 0·5 Gt C year -1 were released through fossil fuel combustion and cement production, and 1·6 ± 1·0 Gt C year -1 through tropical deforestation, resulting in total anthropogenic emissions of 7·1 ± 1·1 Gt C year -1 . Of this total, only 3·3 ± 0·2 Gt C year -1 (46%) remained in the atmosphere, and a combination of modelling and measurements of carbon isotopes and atmospheric O 2 /N 2 ratios suggest that 2·0 ± 0·8 Gt C year -1 were transferred into the oceans. This leaves 1·8 ± 1·6 Gt C year -1 that are ascribed to a terrestrial 'missing sink', a term that encompasses land-use change processes such as forest regrowth resulting from abandonment of agricultural land in mid-latitudes, as well as ecophysiological processes such as enhanced forest growth attributable to CO 2 fertilization, nitrogen deposition and response to climatic anomalies. It should be emphasized that the above figures for the global carbon cycle, although frequently quoted, are the mean for the 1980s, and global emissions have already moved significantly above this level. Recent (1996) global emissions resulting from the use of fossil fuels and cement production are estimated to be 6·5 Gt C year -1 (World Energy Council, Nature 390, p. 215) and increasing at a rate of about 0·1 Gt year -1 . There is evidently considerable uncertainty about the magnitude of the terrestrial missing sink, and even larger uncertainty about its location. With a few significant exceptions, such as tropical grasslands The net carbon budget of a forest is a fine balance between processes of carbon acquisition (photosynthesis, tree growth, forest ageing, carbon accumulation in soils), and processes of carbon release (respiration of living biomass, tree mortality, microbial decomposition of litter, oxidation of soil carbon, degradation and disturbance). These processes operate on a variety of time scales from diurnal to seasonal, interannual, interdecadal and beyond, and are influenced by a number of climatic and environmental variables, such as temperature, moisture availability and frequency of disturbance. Moreover, there are large differences between different forest types such as the humid tropical forests of Amazonia and the cold boreal forests of Siberia, so the major forest biomes need to be treated separately. In this article we first review the historical natural and anthropogenic processes that have affected the areal extent and total carbon balance of forest biomes. We then examine the evidence for a present-day carbon sink in the major biomes from forestry, micrometeorological and atmospheric studies. Utilizing three sites for which detailed data are available, we also present a comparison of carbon cycling in the tropical, temperate and boreal forest biomes (using a combination of micrometeorological, ecophysiological and forest mensuration measurements), and a discussion of the differing response of each forest type to environmental Plant, Cell and Environment (1999) variables. Finally, we discuss the future behaviour of this sink under possible climatic change. THE HISTORICAL AND CURRENT STATE OF FORESTS The most extensive forest biomes are the boreal, temperate and tropical forests. The boreal forest (or taiga) occupies a circumpolar belt in high northern latitudes, between circumpolar tundra and temperate forests and grasslands Carbon sequestration since the last glacial maximum The climate in all regions of the earth was very different at the last glacial maximum (LGM; 18 000 years before present), and all forest biomes have undergone major changes in distribution in recent prehistory. Low temperatures were associated with the presence of permanent ice sheets over much of northern North America and Eurasia, and air bubbles in ice cores reveal atmospheric CO 2 concentrations of approximately 200 µmol mol -1 The change in carbon storage since the LGM is a matter of controversy. Using paleogeographical surveys and carbon density estimates for each biome, Whatever the exact figures, it is likely that all forest biomes have sequestered significant amounts of carbon since the LGM, and have acted as an indirect negative feedback on the rise in CO 2 concentration since deglaciation. If this sequestered carbon were resident in the atmosphere, atmospheric CO 2 concentrations would be 130-640 µmol mol -1 higher than at present (1 Gt C ~0·47 µmol mol -1 CO 2 ), although in practice the oceans would have buffered much of this fluctuation in the long term. Much of this sequestration probably occurred rapidly at the time of deglaciation (Crowley 1995), but it is possible that the forest biomes have still not equilibrated after this sudden transition. A recent analysis of an Antarctic ice core Historical anthropogenic change Since the discovery of fire management, all human societies have relied on modifications of forest landscapes with consequent changes in the carbon storage densities of forests Expansion of agriculture since 1850 Over the period 1850 Logging since 1850 In contrast to forest clearance, most logging over the past century has been in the temperate and boreal zones. Rates of logging of boreal forests have steadily increased from about 1850 and 1990, an area that was 77% larger than the area of forest converted to agriculture. Once logged, however, forests are frequently left to recover and sequester carbon; hence the overall impact of logging on CO 2 emissions is less than that of land clearance. Forest degradation and fragmentation Fragmentation of remaining areas of forest is a common side-effect of logging and clearance The amount of carbon released through forest conversion can be estimated by use of a land-use model which combines historical data of clearance and abandonment with subsequent response curves of carbon stocks in live vegetation, soils, slash and wood products. Using such an approach, Houghton (1996a) estimated that, between 1850 and 1980, a net 83 Gt C were released by forest clearance and logging, and a further 14 Gt C by clearance of desert scrub and temperate grasslands. The annual net release increased from 0·3 Gt C year -1 in 1850 to 1·6 Gt C year -1 in 1980-90. According to Houghton's estimates, the expansion of croplands has been responsible for the largest net carbon release (63 Gt C), followed by logging and regrowth of forests (23 Gt C) and conversion of forests to pasture (10 Gt C). These are net values, and thus take into account regrowth after logging and abandonment of agriculture. Current anthropogenic change Over the period THE CARBON CYCLE OF INTACT FORESTS As summarized above, forest clearance and degradation continue to be a major source of atmospheric CO 2 . Less clear, however, is the extent to which intact or low disturbance forests are currently sinks for carbon dioxide. In recent years considerable progress has been made in understanding the processes which determine forest carbon balance, through a combination of physiological, micrometeorological and mensurational studies. In this section we discuss the carbon dynamics of various intact forest biomes, and the interaction between climate and the carbon cycle. All forest biomes contain a significant amount of spatial heterogeneity, but at the same time have fundamental properties (related to climate and plant phenotype) that are characteristic of that biome. Rather than attempt to describe each biome in all its complexity, we shall here illustrate and contrast the properties of different biomes by concentrating on three intensively studied sites: a Canadian boreal forest Characteristics of each site Climate at these three sites differs markedly, particularly with respect to the degree of seasonality and length of growing season. The main features of the prevailing climates are given in The boreal forest site exhibits extreme seasonality in day length and air temperature. Winter conditions are severe, and soils remain frozen until late spring. The frost-free season lasts between 50 and 100 d, the growing season about 100 d and the photosynthetic season about 170 d. Annual precipitation is low, but in spite of this the site is not in general short of water, because of low evaporation rates and impeded drainage . The site is situated in the southern, mixed forest zone of the Canadian boreal forest, in a flat landscape composed of a patchwork of pure and mixed stands of black and white spruce, jack pine, aspen, fen and post-glacial lakes. The study site was dominated (≈ 90%) by evergreen black spruce (Picea mariana), which can take advantage of mild days in spring and autumn when broadleaf species would not be in full leaf. About 10% of the stand is composed of deciduous tamarack (Larix laricina), with occasional jack pine (Pinus banksiana) and balsam poplar (Populus balsamifera). The stand was fairly uniform in age (≈ 115 years), probably because it developed after fire, which plays an important role in the natural dynamics of the boreal forest landscape (Payette 1992). The forest floor had a hummock-hollow microtopography dominated by peat moss (Sphagnum spp.), with some mixed feather mosses in drier areas. The living moss overlay a 2-10 cm thick layer of dead moss, which in turn overlay dense acidic peat. Away from hummocks the soil was generally anaerobic and waterlogged. There was no permafrost at this site. As is the case for many boreal forests At the other extreme, the forest in central Amazonia experiences year-round warm temperatures, and comparatively small ranges of diurnal and seasonal temperature and day length. Annual precipitation is high, but shows significant interannual variability and, despite popular impression, distinct seasonality. The dry season usually lasts from mid-June to late October The climate of the temperate region spans a wide range of intermediate conditions that influence productivity, including both low winter temperatures and low summer water availability. At the site in Tennessee the climate is mild and annual precipitation is high. Rainfall is fairly evenly dis- The total stocks of carbon at each site are given in The total stocks of carbon at the boreal and tropical sites are remarkably similar in amount Stand dynamics The carbon balance of a forest ecosystem as a whole is the resultant of the dynamics of the component processes that comprise the system. The NEE, or carbon balance, between a forest and the atmosphere can be measured using micrometeorological techniques such as eddy covariance (see section entitled Eddy covariance below). The NEE can be considered in terms of its two principal components, acting in opposite directions: the influx of CO 2 in canopy photosynthesis, and the combined effluxes of CO 2 resulting from autotrophic and heterotrophic respiration. These components can be quantified using two different strategies: a 'bottom-up' strategy of integrating small-scale measurements of photosynthesis, litterfall, etc. (Rayment & Jarvis 1999c), and a 'top-down' approach of decomposing the NEE into bulk photosynthetic and respiratory fluxes (as in Baldocchi 1997; The dynamics of these fluxes can be better understood by breaking them down into their subcomponents and making a carbon balance above and below ground. Accounting for net root biomass increment (8) leaves a combined estimate of root respiration (7) and combined detritus production (9). The input of detritus from roots (9) and from above-ground (4) comprises the flux of carbon from autotrophs to soil organic carbon (SOC). If the SOC is neither increasing nor decreasing in amount on interannual timescales, this influx would equal the efflux of heterotrophically respired CO 2 from the soil (11). Since the soil CO 2 efflux can be independently measured (or obtained by difference between the net ecosystem flux and the net influx of CO 2 in canopy photosynthesis plus the aboveground autotrophic respiration), it is therefore possible to determine whether the SOC may be increasing or decreasing in amount (13), something very difficult to determine by stock taking. This approach is dependent only on quantification of the changes in root biomass (8), and not on accurate partitioning of below-ground carbon fluxes between autotrophic (12) and heterotrophic (11) respiration. If the changes in root biomass have not been accurately estimated, the approach can still provide estimates of changes in total below-ground carbon (i.e. roots plus SOC). Simplified annual carbon balance diagrams (see The main difference between the three C budgets is that G p , and hence N p and the general magnitude of the carbon cycle, show a decreasing trend from the tropical site via the temperate site to the boreal site. Consequently, there is a much larger translocation of C to below the ground at the tropical site, and much higher rates of root and heterotrophic respiration. The calculation suggests large rates of increase in soil carbon at the tropical and temperate sites, and a small The mean residence times for carbon for each component of the ecosystem can be estimated by dividing the total carbon stocks As argued above, the measurements of NEE on an annual basis should provide a major constraint that closes the forest carbon balance. However, there are likely to be significant differences between estimates of annual NEE derived from eddy flux measurements and estimates derived from integration of the measurements of component processes. This is because there are uncertainties with both measurement methodologies: first, measurements of each of the component processes have large error terms associated with them (Rayment 1998), and second there are still some uncertainties attached to the post-collection analysis procedures of the eddy covariance data (discussed in 4·1). Influence of climate on carbon dynamics Many of the properties and much of the temporal variation of carbon dynamics at each site can be related to three predominant climatic variables: light availability, temperature and soil moisture availability. Light Incident light (or photosynthetic photon flux density, PPFD) is the most immediate environmental control on photosynthesis. Whilst the light response curve of an individual leaf can be approximated by a non-rectangular hyperbola Availability of light is strongly affected by cloudiness and low sun-angle conditions, but the effect of this on canopy photosynthesis is partially counteracted by the increased proportion of diffuse solar radiation, which is more effective at penetrating deeper layers of the forest canopy and illuminating usually shaded leaves At mid-and high latitudes, the availability of light controls potential photosynthesis by determining the maximum length of the growing season in spring and autumn. Away from the tropics, variation in day length and noon solar zenith angle are the predominant constraints that define potential photosynthesis of a forest. Cloudiness can affect light availability on daily or weekly timescales, but in midlatitude climates there is rarely a strong seasonal cycle in cloudiness Temperature At the extremely low temperatures that occur in winter in the boreal region, chloroplast organization breaks down and Photosystem 2 becomes inactivated In middle and high latitudes, an additional effect of temperature on photosynthesis is its potential to inflict frost damage on buds and leaves. As the annual cycle of air temperature lags some weeks behind that of insolation, the main effect is to constrict the length of the photosynthetic season in the spring, when frosts can curtail daytime photosynthesis below expectation Temperature is thus a major determinant on seasonal processes which regulate both carbon gain and carbon loss in temperate and boreal forests. In the tropics, seasonality in temperature is generally minimal; for the central Amazonian forest, for example, the annual range in monthly mean temperature is 2 to 4°C (Malhi et al. 1998). On interannual timescales, however, temperature may influence the carbon balance of all biomes. Soil moisture In high latitudes the start of the photosynthetically active season is determined by the availability of soil moisture. Frozen soil prevents water uptake by roots so that leaf turgor and stomatal opening are dependent on the limited supply of stored water within the trees (Whitehead & Jarvis 1981). As air temperature rises and the snow cover begins to thaw, melt water at close to 0°C percolates down through the soil, replacing the ice and enabling water uptake. In high and mid-latitudes, soil moisture reserves are largely replenished by autumn and winter precipitation Seasonal cycles of carbon uptake and loss The seasonal dynamics of each of the forest sites in are illustrated in Daytime uptake The tropical forest site showed the least seasonality in both daytime uptake (mean value 5·2 g C m -2 d -1 ) and night-time release (mean value 3·5 g C m -2 d -1 ), although there was significant seasonality in canopy photosynthetic capacity: In the boreal evergreen, black spruce forest, the length of the active season is strongly influenced by the length of the frost-free period. The growing season begins and ends with abrupt temperature transitions. As the thaw begins in the spring and snow starts to melt, the temperature rises rapidly towards zero and as it passes through about -1·5°C (in mid-April in 1996), CO 2 exchange switches from carbon loss to carbon gain (Massheder 1998). When the thaw occurs late, substantial amounts of solar radiation are effectively wasted and the annual carbon gain is correspondingly small. Carbon uptake rises to a peak of 3 g C m -2 d -1 in late June, and then fluctuates at around 2 g C m -2 d -1 until mid-September. Both uptake and loss rates are much smaller at all times in the boreal black spruce forest. Night-time CO 2 efflux For these forests, soil respiration rates are primarily determined by temperature, and to a lesser extent by soil water content and the quantity and quality of the soil organic matter. In tropical forests, more than 70% of respiratory CO 2 originates from autotrophic and heterotrophic processes within the soil biomass . At the Amazonian site, soil temperature is the main cause of diurnal variability of soil respiration, both through its mean diurnal cycle (23·0-24·5°C at 5 cm depth; Night-time carbon efflux rates in the temperate oakhickory and boreal spruce forest sites follow the seasonal changes in soil temperature. At the oak-hickory site in 1997, rates rose from a constant 1 g C m -2 d -1 to a steady 4 g C m -2 d -1 in the June-August period (slightly higher than the tropical rain forest), as the average daily soil temperature rose to approximately 22°C, before falling away again in September. In a study of soil respiration in this region, At the boreal site in winter-time there is a small but persistent efflux of about 0·2 g C m -2 d -1 , when insulation by permanent snow cover maintains surface soil temperature at close to zero, whereas air temperatures are -15 to -35°C To conclude, the broad photosynthetic responses of temperate and tropical broadleaf forests to light and temperature are very similar. The differences between tropical and temperate deciduous forests in the annual course of CO 2 fluxes are driven by the seasonality of day length, timing of bud burst and senescence, variation in cloud cover, and soil moisture availability. The light sensitivity of the boreal spruce forest seems less pronounced than that of the broadleaf forests (but is consistent with that measured at other coniferous forests, e.g. Diurnal cycles Interannual variability The data shown in As discussed above, the primary meteorological determinants of the carbon balance of intact stands are: (1) the length of the growing season determined by the radiation inputs and modified by soil and air temperatures in spring and autumn; (2) the length of the period of snow cover; (3) the amount of cloud cover in the growing season; and (4) the occurrence of drought in the late summer or dry season. Cloudiness and drought can directly limit photosynthetic uptake in the growing season, whereas the cold period controls the overall length of the growing season through its impact on phenology. Snow cover can be important in insulating the soil surface from extremely low temperatures, thus permitting a moderate amount of soil respiration throughout the winter In mid-and high latitudes in the Americas the correlation is weaker, but El Niño is associated with mild, wet winters, with a reduced period of snow cover While the description of climatic controls outlined above describes the impact of climatic variability on intact forest stands, catastrophic events (such as drought, flood, fire and wind-throw) may have a spatially variable impact, but a strong influence on the overall carbon balance of the biome at regional scale. For example, drought may reduce forest photosynthesis and increase tree mortality, shifting the carbon balance towards a net source, but a more significant factor may be associated fires that turn patches of the forest into strong, albeit temporary, carbon sources. Wind-throw may create similar patches. Since such events are not infrequent, the interannual variability of the spatially integrated net biome production (NBP; MEASURING THE CURRENT CARBON BALANCE OF FORESTS In recent years there has been an accumulating body of evidence for a net sink of carbon in the terrestrial biosphere. In this section, we discuss the evidence for such a sink in the major forest biomes. The three principal approaches to estimating the carbon balance of forest biomes are micrometeorological measurements of surface fluxes, biomass inventories and the inversion of atmospheric gas concentrations. All three approaches still have methodological problems, but are pointing towards a combined net terrestrial carbon sink (excluding deforestation) of 1 to 3 Gt C year -1 . However, the exact partitioning of carbon sequestration amongst the tropical, temperate and boreal forest biomes remains unclear. Eddy covariance This micrometeorological technique relies on directly measuring the turbulent transport of CO 2 above a forest canopy Although eddy covariance has already been demonstrated to provide reliable estimates of the bulk photosynthesis and respiration of a forest site Much of the uncertainty has focused on the night-time fluxes, an area that was ignored in much of traditional micrometeorology because of its irrelevance to energy and water budgets. On calm nights, respired CO 2 tends to accumulate within the forest canopy, and is transferred to the atmosphere through intermittent events. Several studies The annual cycle of net ecosystem exchange of each of the case-study sites is shown in The tropical forest appears to be a net sink of CO 2 throughout the year, whereas the higher latitude forests are sinks throughout most of the growing season and sources throughout the winter. All three forests, however, can be carbon sources on warm, cloudy days in the summer when photosynthetic rates are low and soil respiration rates are high. NEE at the tropical forest site peaks in the early wet season (December to March), and then declines steadily to minimum values in the late dry season (October to November). This seasonality is the resultant of two opposing factors: variation in soil moisture status and amount of cloud cover. The observed minimum in the dry season indicates that soil moisture availability is a more important constraint than available sunshine at this site. NEE at the temperate site is positive throughout the winter, and rises to a maximum in early March when warming temperatures result in a peak of respiration of accumulated autumn and

Abstract. The rate and factors controlling denitrification in marine sediments have been investigated using a prognostic diagenetic model. The model is forced with observed carbon fluxes, bioturbation and sedimentation rates, and bottom water conditions. It can reproduce rates of aerobic mineralization, denitrification, and fluxes of oxygen, nitrate, and ammonium. The globally integrated rate o f denitrification is estimated by this model to be about 230-285 Tg N yr'1, with about 100 Tg N yr"1 occurring in shelf sediments. This estimate is significantly higher than literature estimates (12-89 Tg N yr'1), mainly because o f a proposed upward revision of denitrification rates in slope and deep-sea sediments. Higher sedimentary denitrification estimates require a revision of the marine nitrogen budget and lowering o f the oceanic residence time o f nitrogen down to about 2 xlO 3 years and are consistent with reported low N/P remineralization ratios between 1000 and 3000 m. Rates o f benthic denitrification are most sensitive to the flux o f labile organic carbon arriving at the sediment-water interface and bottom water concentrations o f nitrate and oxygen. Denitrification always increases when bottom water nitrate increases but may increase or decrease if oxygen in the bottom water increases. Nitrification is by far the most important source o f nitrate for denitrification, except for organic-rich sediments underlying oxygen-poor and nitrate-rich water. 1.

Over timescales of months to years, the export of organic nitrogen from the oceanic euphotic zone (principally as sinking particulate nitrogen, PN) is believed to balance the input of exogenous combined inorganic nitrogen (i.e., export production balances new production). In the oligotrophic North Pacific subtropical gyre, there are two significant sources of new nitrogen: the upward flux of nitrate from deep water and the biological fixation of dissolved N2 in near-surface waters. Because these sources have distinct stable isotopic signatures, we were able to use the total PN and d15N measurements of sinking particles and an isotopic mass balance model to deconvolute the relative and absolute contributions of the nitrate flux and nitrogen fixation to the gravitational export of PN. The sinking flux of PN and its isotopic composition both varied widely between 1989 and 2001. Seasonally, N2 fixation correlated inversely with mixed-layer depth, reaching a maximum in June–August, whereas nitrate-supported export correlated inversely with sea surface temperature, reaching a maximum in February–March. These patterns were consistent with summertime increases in diazotroph biomass and water column N2 fixation rates, as indicated by phycoerythrin pigment concentrations and 15N2 tracer studies. Annually, the relative contribution of N2 fixation to N export varied from 36 to 69 % (mean 5 48%) and showed a significant increasing trend over the period of observation. Although total PN export correlated with the Southern Oscillation Index, the nitrate- and nitrogen

coastal eutrophication

Abstract not found

[1] We report hourly in-situ observations of C1-C8 speciated volatile organic compounds

[1] The export of nitrate by the Mississippi River to the Gulf of Mexico has tripled since the 1950s primarily due to an increase in agricultural fertilizer application and hydrological changes. Here we have adapted two physically based models, the Integrated Biosphere Simulator (IBIS) terrestrial ecosystem model and the Hydrological Routing Algorithm (HYDRA) hydrological transport model, to simulate the nitrate export in the Mississippi River system and isolate the role of hydrological processes in the observed increase and interannual variability in nitrate export. Using an empirical nitrate input algorithm based on constant land cover and variability in runoff, the modeling system is able to represent much of the spatial and interannual variability in aquatic nitrate export. The results indicate that about a quarter of the sharp increase in nitrate export from 1966 to 1994 was due to an increase in runoff across the basin. This illustrates the pivotal role of hydrology and climate in the balance between storage of nitrate in the terrestrial system

Abstract not found

[1] Stable nitrogen isotope ratios ( 15 N / 14 N; d 15 N) were determined in sediments, suspended matter, and water at selected sites in the Baltic Sea area in order to set up a source budget and trace the fate of anthropogenic N sources. Sediments of the shallow near-coastal area of the southern and eastern Baltic Sea have an average d 15 Nof 7.3 ± 2.1%, interpreted as a characteristic trace of residual anthropogenic nitrogen delivered by rivers and diffuse runoff. In contrast, d 15 N values in sediments deposited in the basins of the central Baltic Sea are depleted (average d 15 N of 3.5 ± 0.6%), indicating a significant contribution by diazotrophic cyanobacteria. Statistical analysis of long-term nutrient data (1969–2001) indicates no increase in nitrate concentrations in the central Baltic Proper, where only phosphate concentrations have increased. The physical circulation pattern and the enthalpy, as derived from a circulation model, show a closed circulation cell in the Baltic Proper with limited transport of riverine material into the basins and elevated temperatures in the Baltic Proper. Together, the isotope and nutrient data suggest that eutrophication by riverine nitrogen is pronounced in the coastal rim of the Baltic Sea, and that coastal sediments appear to be very efficient in removing riverborne nitrogen by denitrification. A nitrogen isotope mass balance model suggests that N loss by sediment denitrification and N input by N fixation can be as high as 855 ktons N yr 1.

Preliminary results of studies on nutrient fluxes in forests of the eastern Sierra Nevada were compared to those from more humid and polluted ecosystems. Snowmelt, soil solution, soil, and streamwater were collected from Jeffrey and lodgepole pine (Pinus jeffreyii [Grev. and Balf.] and Pinus contorta Dougl.) stands in Little Valley, Nevada, and from California red fir (Abies magnifica A. murr.) and Jeffrey pine/white fir (Pinus jeffreyii/Abies concolor [Gord and Glend.] Lindl.) stands at Sagehen, California. Snowmelt, soil solutions, and streamwaters from both sites were circumneutral and dominated by base cations and bicarbonate. The red fir stand at Sagehen had high NO 3 concentrations (approximately 30-100 µmol /L) in both snowmelt and soil solution during the relatively dry 1993-4 water year. The Little Valley sites had substantially lower NO 3 concentrations in both snowmelt (5-20 µmol /L) and soil solution (0.5-3 µmol /L) in both wet and dry years. At both sites, a pulse of streamwater NO 3 (from 0.5 to 20-40 µmol /L) was detected during dry years but not wet years. The Andic soils at the Sagehen site have trace levels of available P in soils, whereas the Entisols and Inceptisols in Little Valley have 10 to 100 times greater levels. The results suggest a hypothesis that the greater mobility of NO 3 in the Sagehen red fir site was caused by the amount and timing