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We investigated the effects of a 5°C soil + air experimental heating on root and microbial respiration in a boreal black spruce (Picea mariana (Mill.) B.S.P.) forest in northern Manitoba, Canada, that was warmed between 2004 and 2007. In 2007, the [sup.14]C/[sup.12]C signatures of soil C[O.sub.2] efflux and root and soil microbial respiration were used in a two-pool mixing model to estimate their proportional contributions to soil C[O.sub.2] efflux and to examine how each changed in response to the warming treatments. In laboratory incubations, we examined whether warming had altered microbial respiration rates or microbial temperature sensitivity. The [sup.14]C/[sup.12]C signature of soil C[O.sub.2] efflux and microbial respiration in the heating treatments were both significantly (p < 0.05) enriched relative to the control treatment, suggesting that C deposited nearer the atmospheric bomb peak in 1963 contributed more to microbial respiration in heated than control treatments. Soil C[O.sub.2] efflux was significantly greater in the heated than control treatments, suggesting the acclimation to temperature of either root or microbial respiration was not occurring in 2007. Microbial respiration in laboratory incubations was similar in heated and control soils. This study shows that microbial respiration rates still responded to temperature even after 4 years of warming, highlighting that ecosystem warming can cause a prolonged release of soil organic matter from these soils. Key words: boreal, soil carbon, root respiration, warming, black spruce. Nous avons etudie les effets d'un rechauffement experimental de 5°C de la temperature de l'air et du sol sur la respiration racinaire et microbienne dans une foret boreale d'epinette noire (Picea mariana (Mill.) Britton, Sterns, Poggenb.) du nord du Manitoba, au Canada, de 2004 a 2007. En 2007, la signature isotopique ([sup.14]C/[sup.12]C) du C[O.sub.2] emanant du sol et provenant de la respiration racinaire et microbienne du sol a ete utilisee dans un modele de melange de deux fluides pour estimer leur contribution proportionnelle au C[O.sub.2] emanant du sol et la facon dont chacun a varie en reaction au rechauffement de la temperature. En utilisant des incubations en laboratoire, nous avons etudie si le rechauffement a modifie le taux de respiration microbienne ou la sensibilite microbienne a la temperature. La signature isotopique ([sup.14]C/[sup.12]C) du C[O.sub.2] emanant du sol et de la respiration microbienne dans les traitements ayant fait l'objet d'un rechauffement etaient toutes deux significativement (p < 0,05) enrichies comparativement au traitement temoin, ce qui indique que le C depose plus pres du point culminant des essais nucleaires atmospheriques en 1963 a davantage contribue a la respiration microbienne dans le traitement ayant fait l'objet d'un rechauffement que dans le traitement temoin. Le C[O.sub.2] emanant du sol etait significativement plus important a la suite du rechauffement que dans le traitement temoin, ce qui indique que les racines ou la respiration microbienne ne s'etaient pas acclimatees a la temperature en 2007. Dans les incubations en laboratoire la respiration microbienne etait similaire dans le sol ayant subi un rechauffement et dans le sol temoin. Cette etude demontre que le taux de respiration microbienne reagit encore a la temperature meme apres quatre annees de rechauffement, faisant ressortir le fait que le rechauffement de l'ecosysteme peut engendrer une perte prolongee de matiere organique dans ces sols. [Traduit par la Redaction] Mots-cles : boreal, carbone du sol, respiration racinaire, rechauffement, epinette noire., Introduction During the next century, air temperatures in some regions of the boreal forest may warm by as much as 8°C ( Intergovernmental Panel on Climate Change 2007). Warmer air [...]

Arctic tundra soils store large amounts of carbon (C) in organic soil layers hundreds to thousands of years old that insulate, and in some cases maintain, permafrost soils (1,2). Fire has been largely absent from most of this biome since the early Holocene epoch (3), but its frequency and extent are increasing, probably in response to climate warming (4). The effect of fires on the C balance of tundra landscapes, however, remains largely unknown. The Anaktuvuk River fire in 2007 burned 1,039 square kilometres of Alaska's Arctic slope, making it the largest fire on record for the tundra biome and doubling the cumulative area burned since 1950 (ref. 5). Here we report that tundra ecosystems lost 2,016 ± 435 g C [m.sup.-2] in the fire, an amount two orders of magnitude larger than annual net C exchange in undisturbed tundra (6). Sixty per cent of this C loss was from soil organic matter, and radiocarbon dating of residual soil layers revealed that the maximum age of soil C lost was 50 years. Scaled to the entire burned area, the fire released approximately 2.1 teragrams of C to the atmosphere, an amount similar in magnitude to the annual net C sink for the entire Arctic tundra biome averaged over the last quarter of the twentieth century (7). The magnitude of ecosystem C lost by fire, relative to both ecosystem and biome-scale fluxes, demonstrates that a climate-driven increase in tundra fire disturbance may represent a positive feedback, potentially offsetting Arctic greening (8) and influencing the net C balance of the tundra biome., The Arctic terrestrial C cycle has the potential to influence global climate through feedbacks to recent warming, but the net sign, magnitude and pace of these feedbacks remains uncertain (7,9,10). [...]

The resilience and vulnerability of permafrost to climate change depends on complex interactions among topography, water, soil, vegetation, and snow, which allow permafrost to persist at mean annual air temperatures (MAATs) as high as +2 °C and degrade at MAATs as low as -20 °C. To assess these interactions, we compiled existing data and tested effects of varying conditions on mean annual surface temperatures (MASTs) and 2 m deep temperatures (MADTs) through modeling. Surface water had the largest effect, with water sediment temperatures being ~10 °C above MAAT. A 50% reduction in snow depth reduces MADT by 2 °C. Elevation changes between 200 and 800 m increases MAAT by up to 2.3 °C and snow depths by ~40%. Aspect caused only a ~1 °C difference in MAST. Covarying vegetation structure, organic matter thickness, soil moisture, and snow depth of terrestrial ecosystems, ranging from barren silt to white spruce (Picea glauca (Moench) Voss) forest to tussock shrub, affect MASTs by ~6 °C and MADTs by ~7 °C. Groundwater at 2-7 °C greatly affects lateral and internal permafrost thawing. Analyses show that vegetation succession provides strong negative feedbacks that make permafrost resilient to even large increases in air temperatures. Surface water, which is affected by topography and ground ice, provides even stronger negative feedbacks that make permafrost vulnerable to thawing even under cold temperatures. Resume: La resilience et la vulnerabilite: La resilience et la vulnerabilite du pergelisol face aux changements climatiques dependent d'interactions complexes entre la topographie, l'eau, le sol, la vegetation et la neige qui permettent au pergelisol de se maintenir a des temperatures moyennes annuelles de l'air (TMAA) aussi elevees que +2 °C et de se degrader a des TMAA aussi basses que -20 °C. Pour evaluer ces interactions, nous avons compile des donnees existantes et teste les effets de diverses conditions de temperature moyenne annuelle en surface (TMAS) et a une profondeur de 2 m (TMAP) par l'entremise de la modelisation. L'eau de surface avait l'effet le plus prononce alors que la temperature a l'interface entre l'eau et les sediments etait ~10 °C plus elevee que la TMAA. Une diminution de l'epaisseur de la couche de neige de 50% reduit la TMAP de 2 °C. Un changement d'altitude entre 200 et 800 m augmente la TMAA de 2,3 °C et l'epaisseur de la couche de neige de ~40 %. L'orientation cause une difference de seulement ~1 °C de la TMAS. La structure de la vegetation, l'epaisseur de la matiere organique, l'humidite du sol et l'epaisseur de la couche de neige qui covarient dans les ecosystemes terrestres, allant de la toundra limoneuse a la foret d'epinette blanche (Picea glauca (Moench) Voss) et a la zone de transition entre les buttes de gazon et les arbustes, affectent la TMAS de ~6 °C et la TMAP de ~7 °C. A une temperature de 2-7 °C, l'eau souterraine influence grandement la fonte laterale et interne du pergelisol. Les analyses montrent que la succession de la vegetation engendre de fortes retroactions negatives qui rendent le pergelisol resistant a une augmentation meme importante de la temperature de l'air. L'eau de surface qui est influencee par la topographie et la glace au sol engendre des retroactions negatives encore plus prononcees qui rendent le pergelisol vulnerable a la fonte meme si la temperature est froide. [Traduit par la Redaction], Introduction Perennially frozen ground (permafrost) is a unique characteristic of polar regions and high mountains and is fundamental to geomorphic processes and ecological development in tundra and boreal forests. Because [...]

Permafrost soils in boreal and Arctic ecosystems store almost twice as much carbon (1,2) as is currently present in the atmosphere (3). Permafrost thaw and the microbial decomposition of previously frozen organic carbon is considered one of the most likely positive climate feedbacks from terrestrial ecosystems to the atmosphere in a warmer world (1,2,4-7). The rate of carbon release from permafrost soils is highly uncertain, but it is crucial for predicting the strength and timing of this carbon-cycle feedback effect, and thus how important permafrost thaw will be for climate change this century and beyond (1,2,4-7). Sustained transfers of carbon to the atmosphere that could cause a significant positive feedback to climate change must come from old carbon, which forms the bulk of the permafrost carbon pool that accumulated over thousands of years (8-11). Here we measure net ecosystem carbon exchange and the radiocarbon age of ecosystem respiration in a tundra landscape undergoing permafrost thaw (12) to determine the influence of old carbon loss on ecosystem carbon balance. We find that areas that thawed over the past 15 years had 40 per cent more annual losses of old carbon than minimally thawed areas, but had overall net ecosystem carbon uptake as increased plant growth offset these losses. In contrast, areas that thawed decades earlier lost even more old carbon, a 78 per cent increase over minimally thawed areas; this old carbon loss contributed to overall net ecosystem carbon release despite increased plant growth. Our data document significant losses of soil carbon with permafrost thaw that, over decadal timescales, overwhelms increased plant carbon uptake (13-15) at rates that could make permafrost a large biospheric carbon source in a warmer world., We measured ecosystem carbon (C) dynamics at a tundra site in Alaska where permafrost thaw has been documented since 1990, and was occurring in the area before that time, probably [...]

Thawing permafrost and the resulting microbial decomposition of previously frozen organic carbon (C) is one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing [...]

The response of tropical forests to climate change will depend on individual plant species' nutritional strategies, which have not been defined in the case of the nitrogen nutrition that is critical to sustaining plant growth and photosynthesis. We used isotope natural abundances to show that a group of tropical plant species with diverse growth strategies (trees and ferns, canopy, and subcanopy) relied on a common pool of inorganic nitrogen, rather than specializing on different nitrogen pools. Moreover, the tropical species we examined changed their dominant nitrogen source abruptly, and in unison, in response to precipitation change. This threshold response indicates a coherent strategy among species to exploit the most available form of nitrogen in soils. The apparent community-wide flexibility in nitrogen uptake suggests that diverse species within tropical forests can physiologically track changes in nitrogen cycling caused by climate change. global change | isotope | community ecology

A number of operationally defined methods exist for pretreating plant tissues in order to measure C, N, and O isotopes. Because these isotope measurements are used to infer information about environmental conditions that existed at the time of tissue growth, it is important that these pretreatments remove compounds that may have exchanged isotopes or have been synthesized after the original formation of these tissues. In stable isotope studies, many pretreatment methods focus on isolating 'cellulose' from the bulk tissue sample because cellulose does not exchange C and O isotopes after original synthesis. We investigated the efficacy of three commonly applied pretreatment methods, the Brendel method and two variants of the Brendel method, the Jayme--Wise method and successive acid/base/acid washes, for use on three tissue types (wood, leaves, roots). We then compared the effect of each method on C and O isotope composition ([sup.13]C, [sup.14]C, [sup.18]O), C and N content, and chemical composition of the residue produced (using [sup.13]C nuclear magnetic resonance (NMR)). Our results raised concerns over use of the Brendel method as published, as it both added C and N to the sample and left a residue that contains remnant lipids and waxes. Furthermore, this method resulted in [sup.18]O values that are enriched relative to the other methods. Modifying the Brendel method by adding a NaOH step (wash) solved many of these problems. We also found that processed residues vary by tissue type. For wood and root tissues, the [sup.13]C NMR spectra and the [sup.18]O and [sup.13]C data showed only small differences between residues for the Jayme--Wise and modified Brendel methods. However, for leaf tissue, [sup.13]C NMR data showed that Jayme--Wise pretreatments produced residues that are more chemically similar to cellulose than the other methods. The acid/base/acid washing method generated [sup.13]C NMR spectra with incomplete removal of lignin for all tissues tested and both isotopic, and [sup.13]C NMR results confirmed that this method should not be used if purified cellulose is desired.

The response of tropical forest carbon balance to global change is highly dependent on the factors limiting net primary productivity (NPP) in this biome. Current empirical global NPP-climate relationships predict that the response of NPP to climate diminishes at higher levels of mean annual precipitation (MAP) and mean annual temperature (MAT), but data have been relatively scarce in warm and wet tropical ecosystems. By integrating data from a new comprehensive global survey of NPP from tropical forests and a climate gradient from Maui, Hawaii, along with data previously used to develop NPP-climate relationships, I show that there is a strong negative relationship between MAP and NPP in humid ecosystems. The relationships derived here clearly demonstrate that NPP in wet tropical forests is sensitive to climate, and that future forest growth may be limited by increased precipitation forecast by global climate models for the wet tropics. Key words: carbon; global climate; mean annual precipitation; mean annual temperature; net primary productivity; tropical forest; tropics.

Global surveys have shown that increased precipitation in mesic to wet ecosystems increases soil carbon storage, but it is not clear how changes in water availability act to affect soil carbon since water is generally in excess of plant demand within these humid ecosystems. Differences in precipitation can affect soil carbon storage either by altering the biotic processes involved in carbon cycling or by altering abiotic processes such as carbon adsorption on soil minerals. We compared the relative effect of water on net primary productivity and decomposition, and on soil weathering and mineralogy in controlling soil carbon storage in forests. Patterns of ecosystem carbon cycling and soil carbon storage were measured in Hawaiian montane forest sites similar in temperature regime, parent material, ecosystem age, vegetation, and topographical relief, while mean annual precipitation alone varied from 2200 to >5000 mm/yr. Soil carbon storage ranged from 30.9 to 62.5 kg/[m.sup.2] and increased by a factor of 1.7 with increased precipitation across the gradient. Neither changes in total secondary minerals nor noncrystalline mineral content could explain increased soil carbon as both decreased with increasing precipitation. Aboveground net primary productivity, soil carbon dioxide flux, and plant litter decomposition rates also decreased with increased precipitation. These data suggest that increased soil carbon storage was not stimulated by an increase in net primary productivity, but instead by a decrease in decomposition rates. Decreased decomposition rates corresponded with low soil reduction-oxidation potentials, suggesting that soil oxygen availability was limiting to microbes. While soil oxygen did not appear to affect plant growth directly, the decline in net primary productivity corresponded with decreased nitrogen availability, a consequence of slower decomposition and nutrient release. Thus, in humid environments the most important effect of variation in water on carbon cycling appears to be its control on the diffusion of oxygen into the soil. Key words: anaerobiosis; decomposition; gradient; Hawaii; net primary productivity; nutrients; precipitation; reduction-oxidation potential; soil carbon; soil minerals; soil oxygen; tropical forest.

1 We chose seven sites across the Hawaiian archipelago differing only in substrate age (400 years to 4.1 x [10.sup.6] years). All sites were at 1200m elevation, and mean annual rainfall was [greater than or equal to] 4000 mm. This chronosequence reflects long-term ecosystem development from basaltic lava parent material under a humid climatic regime. 2 Live above-ground biomass of woody species [greater than or equal to] 1 m tall changed unimodally along the chronosequence from 217 tons [ha.sup.-1] at the youngest site to a peak of 406 tons [ha.sup.-1] at the 5000-year site, before declining to 75 tons [ha.sup.-1] at the oldest site. 3 The size of the soil organic C pool above sub-surface lava or hardpan initially followed the pattern of above-ground biomass, increasing from the youngest site to the 5000-year site, and declining at the 9000-year site. However, it then steadily increased to the oldest site. The proportion of above-ground biomass C to the total C (above-ground biomass + soil) decreased linearly against logarithmic age from 74% at the youngest site to 8% at the oldest site. 4 Net soil N mineralization rate increased from the youngest site to the 5000-year site, and then declined with age to a nearly constant value except for an outstandingly high value at the oldest site. Exchangeable Ca and available P in topsoil increased from the youngest to the 5000-year site, before declining at older sites. 5 Soil redox potential (Eh7) was invariably high ([greater than or equal to] c. 500 mv) at the sites [less than or equal to] 9000 years, but declined at two old sites (410 000 years and 4100 000 years). 6 Live fine-root biomass in the topsoil increased steadily with substrate age. The distribution of fine roots in the soil profile was positively correlated with redox values. 7 High precipitation rates appear to lead to the development of iron hardpan during pedogenesis. This in turn initiates a positive feedback that promotes waterlogging and anaerobiosis, resulting in reduced organic matter mineralization and increased soil C accumulation. Reduction of biomass with age can be explained by increasingly restricted root penetration, as well as by the reduction in available soil P, N and Ca as a result of geochemical immobilization, leaching and/or reduced mineralization. Keywords: above-ground biomass, anaerobiosis, nutrient availability, redox potential, soil organic carbon