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Wang X.,CAS Institute of Botany | Wang X.,University of Chinese Academy of Sciences | Liu L.,CAS Institute of Botany | Piao S.,Peking University | And 8 more authors.
Global Change Biology | Year: 2014

Despite decades of research, how climate warming alters the global flux of soil respiration is still poorly characterized. Here, we use meta-analysis to synthesize 202 soil respiration datasets from 50 ecosystem warming experiments across multiple terrestrial ecosystems. We found that, on average, warming by 2 °C increased soil respiration by 12% during the early warming years, but warming-induced drought partially offset this effect. More significantly, the two components of soil respiration, heterotrophic respiration and autotrophic respiration showed distinct responses. The warming effect on autotrophic respiration was not statistically detectable during the early warming years, but nonetheless decreased with treatment duration. In contrast, warming by 2 °C increased heterotrophic respiration by an average of 21%, and this stimulation remained stable over the warming duration. This result challenged the assumption that microbial activity would acclimate to the rising temperature. Together, our findings demonstrate that distinguishing heterotrophic respiration and autotrophic respiration would allow us better understand and predict the long-term response of soil respiration to warming. The dependence of soil respiration on soil moisture condition also underscores the importance of incorporating warming-induced soil hydrological changes when modeling soil respiration under climate change. © 2014 John Wiley & Sons Ltd.

Tomasky-Holmes G.,The Ecosystem Center | Valiela I.,The Ecosystem Center | Charette M.A.,Woods Hole Oceanographic Institution
Marine Chemistry | Year: 2013

Despite a relatively short residence time of water in many shallow, semi-enclosed estuaries, phytoplankton blooms in nutrient enriched systems are a common phenomenon. This poses the question how is it possible to have phytoplankton populations bloom in response to local conditions of shallow estuaries, if the water residence times are similar to cell division times? To address this paradox we used the radium quartet as a tool to measure water mass age in coastal systems (Waquoit Bay, MA, USA) subject to different degrees of land-derived nitrogen load and hence differences in phytoplankton biomass. Recently, the radium quartet has been used as geochemical tracers to determine age of water masses. Based on a number of samples collected over the course of one year, the average radium-derived age (±. stdev) of water in three sub-estuaries of Waquoit Bay (Childs River, Quashnet River, and Sage Lot Pond) was ~. 7 (±. 4.7), 11 (±. 6.2), and 17 (±. 7.5). days, respectively. These values are significantly longer than previous estimates based on more traditional hydrodynamic methods. Furthermore, peak chlorophyll concentrations were associated with older water masses in the heavily freshwater-influenced sub-estuaries (Childs and Quashnet). Our results suggest that water age, temperature, and nutrients all play a role in controlling phytoplankton biomass however, water age was more important at the time of the year when temperature limits phytoplankton growth (late spring, early summer). We conclude that radium-derived age models, which are similar to artificial tracer-based approaches, may be the most appropriate method for studying the role of hydrodynamics on estuarine ecology. © 2013 Elsevier B.V.

PubMed | Max Planck Institute for Biogeochemistry, Copenhagen University, Purdue University, CAS Beijing Institute of Geographic Sciences and Nature Resources Research and 10 more.
Type: Journal Article | Journal: Ecology letters | Year: 2016

Nitrogen (N) deposition is impacting the services that ecosystems provide to humanity. However, the mechanisms determining impacts on the N cycle are not fully understood. To explore the mechanistic underpinnings of N impacts on N cycle processes, we reviewed and synthesised recent progress in ecosystem N research through empirical studies, conceptual analysis and model simulations. Experimental and observational studies have revealed that the stimulation of plant N uptake and soil retention generally diminishes as N loading increases, while dissolved and gaseous losses of N occur at low N availability but increase exponentially and become the dominant fate of N at high loading rates. The original N saturation hypothesis emphasises sequential N saturation from plant uptake to soil retention before N losses occur. However, biogeochemical models that simulate simultaneous competition for soil N substrates by multiple processes match the observed patterns of N losses better than models based on sequential competition. To enable better prediction of terrestrial N cycle responses to N loading, we recommend that future research identifies the response functions of different N processes to substrate availability using manipulative experiments, and incorporates the measured N saturation response functions into conceptual, theoretical and quantitative analyses.

van de Weg M.J.,University of Edinburgh | van de Weg M.J.,The Ecosystem Center | Meir P.,University of Edinburgh | Grace J.,University of Edinburgh | Ramos G.D.,National University San Antonio Abad del Cusco
Oecologia | Year: 2012

Few data are available describing the photosynthetic parameters of the leaves of tropical montane cloud forests (TMCF). Here, we present a study of photosynthetic leaf traits (V cmax and J max), foliar dark respiration (R d), foliar nitrogen (N) and phosphorus (P), and leaf mass per area (LMA) throughout the canopy for five different TMCF species at 3025 m a. s. l. in Andean Peru. All leaf traits showed a significant relationship with canopy height when expressed on an area basis, and V cmax-area and J max-area almost halved when descending through the TMCF canopy. When corrected to a common temperature, average V cmax and J max on a leaf area basis were similar to lowland tropical values, but lower when expressed on a mass basis, because of the higher TMCF LMA values. By contrast, R d on an area basis was higher than found in tropical lowland forests at a common temperature, and similar to lowland forests on a mass basis. The TMCF J max-V cmax relationship was steeper than in other tropical biomes, and we propose that this can be explained by either the light conditions or the relatively low VPD in the studied TMCF. Furthermore, V cmax had a significant-though relatively weak and shallow-relationship with N on an area basis, but not with P, which is consistent with the general hypothesis that TMCFs are N rather than P limited. Finally, the observed V cmax-N relationship (i.e., maximum photosynthetic nitrogen use efficiency) was distinctly different from those in tropical and temperate regions, probably because the TMCF leaves compensate for reduced Rubisco activity in cool environments. © 2011 Springer-Verlag.

Schadel C.,University of Florida | Schuur E.A.G.,University of Florida | Bracho R.,University of Florida | Elberling B.,Copenhagen University | And 6 more authors.
Global Change Biology | Year: 2014

High-latitude ecosystems store approximately 1700 Pg of soil carbon (C), which is twice as much C as is currently contained in the atmosphere. Permafrost thaw and subsequent microbial decomposition of permafrost organic matter could add large amounts of C to the atmosphere, thereby influencing the global C cycle. The rates at which C is being released from the permafrost zone at different soil depths and across different physiographic regions are poorly understood but crucial in understanding future changes in permafrost C storage with climate change. We assessed the inherent decomposability of C from the permafrost zone by assembling a database of long-term (>1 year) aerobic soil incubations from 121 individual samples from 23 high-latitude ecosystems located across the northern circumpolar permafrost zone. Using a three-pool (i.e., fast, slow and passive) decomposition model, we estimated pool sizes for C fractions with different turnover times and their inherent decomposition rates using a reference temperature of 5 °C. Fast cycling C accounted for less than 5% of all C in both organic and mineral soils whereas the pool size of slow cycling C increased with C : N. Turnover time at 5 °C of fast cycling C typically was below 1 year, between 5 and 15 years for slow turning over C, and more than 500 years for passive C. We project that between 20 and 90% of the organic C could potentially be mineralized to CO2 within 50 incubation years at a constant temperature of 5 °C, with vulnerability to loss increasing in soils with higher C : N. These results demonstrate the variation in the vulnerability of C stored in permafrost soils based on inherent differences in organic matter decomposability, and point toward C : N as an index of decomposability that has the potential to be used to scale permafrost C loss across landscapes. © 2013 John Wiley & Sons Ltd.

van de Weg M.J.,VU University Amsterdam | van de Weg M.J.,The Ecosystem Center | Shaver G.R.,The Ecosystem Center | Salmon V.G.,University of Florida
Plant Ecology | Year: 2013

We examined the effects of short (<1-4 years) and long-term (22 years) nitrogen (N) and/or phosphorus (P) addition on the foliar CO2 exchange parameters of the Arctic species Betula nana and Eriophorum vaginatum in northern Alaska. Measured variables included: the carboxylation efficiency of Rubisco (Vcmax), electron transport capacity (Jmax), dark respiration (Rd), chlorophyll a and b content (Chl), and total foliar N (N). For both B. nana and E. vaginatum, foliar N increased by 20-50 % as a consequence of 1-22 years of fertilisation, respectively, and for B. nana foliar N increase was consistent throughout the whole canopy. However, despite this large increase in foliar N, no significant changes in Vcmax and Jmax were observed. In contrast, Rd was significantly higher (>25 %) in both species after 22 years of N addition, but not in the shorter-term treatments. Surprisingly, Chl only increased in both species the first year of fertilisation (i.e. the first season of nutrients applied), but not in the longer-term treatments. These results imply that: (1) under current (low) N availability, these Arctic species either already optimize their photosynthetic capacity per leaf area, or are limited by other nutrients; (2) observed increases in Arctic NEE and GPP with increased nutrient availability are caused by structural changes like increased leaf area index, rather than increased foliar photosynthetic capacity and (3) short-term effects (1-4 years) of nutrient addition cannot always be extrapolated to a larger time scale, which emphasizes the importance of long-term ecological experiments. © 2013 Springer Science+Business Media Dordrecht.

Xu H.,University of Minnesota | Twine T.E.,University of Minnesota | Yang X.,Brown University | Yang X.,The Ecosystem Center
Remote Sensing | Year: 2014

Vegetation phenology plays an important role in regulating processes of terrestrial ecosystems. Dynamic ecosystem models (DEMs) require representation of phenology to simulate the exchange of matter and energy between the land and atmosphere. Location-specific parameterization with phenological observations can potentially improve the performance of phenological models embedded in DEMs. As ground-based phenological observations are limited, phenology derived from remote sensing can be used as an alternative to parameterize phenological models. It is important to evaluate to what extent remotely sensed phenological metrics are capturing the phenology observed on the ground. We evaluated six methods based on two vegetation indices (VIs) (i.e., Normalized Difference Vegetation Index and Enhanced Vegetation Index) for retrieving the phenology of temperate forest in the Agro-IBIS model. First, we compared the remotely sensed phenological metrics with observations at Harvard Forest and found that most of the methods have large biases regardless of the VI used. Only two methods for the leaf onset and one method for the leaf offset showed a moderate performance. When remotely sensed phenological metrics were used to parameterize phenological models, the bias is maintained, and errors propagate to predictions of gross primary productivity and net ecosystem production. Our results show that Agro-IBIS has different sensitivities to leaf onset and offset in terms of carbon assimilation, suggesting it might be better to examine the respective impact of leaf onset and offset rather than the overall impact of the growing season length. © 2014 by the authors; licensee MDPI, Basel, Switzerland.

Cai W.-J.,University of Georgia | Luther III G.W.,University of Delaware | Cornwell J.C.,University of Cambridge | Giblin A.E.,The Ecosystem Center
Aquatic Geochemistry | Year: 2010

We used fine-scale porewater profiles and rate measurements together with a multiple component transport-reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O2, Mn2+, Fe2+, HS-, pH and pCO2 at mm resolution by microelectrodes, and profiles of NO3-, SO42-, NH4+, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment-water fluxes of oxygen, DIC, nitrate, ammonium and N2 were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35. 2%), denitrification (10. 4%), MnO2 reduction (3. 6%), FeOOH reduction (9. 6%), sulfate reduction (14. 9%), and methanogenesis (26. 4%). In the lake sediments, about half of the benthic O2 flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O2 usage and Mn2+ oxidation. MnO2 is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH4+ to NO3-. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO2 are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions. © Springer Science+Business Media B.V. 2010.

Flombaum P.,University of Buenos Aires | Sala O.E.,Arizona State University | Rastetter E.B.,The Ecosystem Center
Oecologia | Year: 2014

Resource partitioning, facilitation, and sampling effect are the three mechanisms behind the biodiversity effect, which is depicted usually as the effect of plant-species richness on aboveground net primary production. These mechanisms operate simultaneously but their relative importance and interactions are difficult to unravel experimentally. Thus, niche differentiation and facilitation have been lumped together and separated from the sampling effect. Here, we propose three hypotheses about interactions among the three mechanisms and test them using a simulation model. The model simulated water movement through soil and vegetation, and net primary production mimicking the Patagonian steppe. Using the model, we created grass and shrub monocultures and mixtures, controlled root overlap and grass water-use efficiency (WUE) to simulate gradients of biodiversity, resource partitioning and facilitation. The presence of shrubs facilitated grass growth by increasing its WUE and in turn increased the sampling effect, whereas root overlap (resource partitioning) had, on average, no effect on sampling effect. Interestingly, resource partitioning and facilitation interacted so the effect of facilitation on sampling effect decreased as resource partitioning increased. Sampling effect was enhanced by the difference between the two functional groups in their efficiency in using resources. Morphological and physiological differences make one group outperform the other; once these differences were established further differences did not enhance the sampling effect. In addition, grass WUE and root overlap positively influence the biodiversity effect but showed no interactions. © 2013 Springer-Verlag Berlin Heidelberg.

Souther S.,University of Wisconsin - Madison | Fetcher N.,Wilkes University | Fowler Z.,West Virginia University | Shaver G.R.,The Ecosystem Center | McGraw J.B.,West Virginia University
Botany | Year: 2015

Ecotypic differentiation reduces climatic niche breadth at the population level relative to a species’ spatial distribution. For species that form climatic ecotypes, if future climate exceeds local population tolerance, climate change will precipitate the decline of extant populations range-wide. Here, we examine the variation in physiological and morphological traits of Eriophorum vaginatum L. collected from a 30-year-old reciprocal transplant experiment, in which six populations of E. vaginatum were transplanted along a latitudinal gradient from Eagle Creek to Prudhoe Bay, Alaska. We tested for ecotypic differentiation of photosynthesis, respiration, chlorophyll fluorescence, and biomass per tiller, which is a metric correlated with population growth in E. vaginatum. The light-saturated photosynthetic rate (Amax) showed homesite advantage in that tussocks in their homesites had significantly higher values of Amax relative to nonlocal populations. This pattern of homesite advantage was also observed for biomass per tiller, but not for fluorescence and respiration. Photosynthetic rate was positively correlated with biomass per tiller and survival, suggesting that adaptations related to photosynthesis may optimize performance of local populations to homesite conditions. Taken together, these findings indicate that a rapidly changing climate may elicit population decline of E. vaginatum, rendering this species at a competitive disadvantage to shrubs and boreal forest species, which are expanding northward as the climate changes. Transition from tussock-sedge tundra to boreal forest and shrubland alters features, such as albedo, soil temperature, and water-table depth, in ways that may accelerate climate change. © 2015, National Research Council of Canada. All rights reserved.

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