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Sistla S.A.,University of California at Santa Barbara | Rastetter E.B.,Ecosystems Center | Schimel J.P.,University of California at Santa Barbara
Ecological Monographs | Year: 2014

Soils, plants, and microbial communities respond to global change perturbations through coupled, nonlinear interactions. Dynamic ecological responses complicate projecting how global change disturbances will influence ecosystem processes, such as carbon (C) storage. We developed an ecosystem-scale model (Stoichiometrically Coupled, Acclimating MicrobePlantSoil model, SCAMPS) that simulates the dynamic feedbacks between aboveground and belowground communities that affect their shared soil environment. The belowground component of the model includes three classes of soil organic matter (SOM), three microbially synthesized extracellular enzyme classes specific to these SOM pools, and a microbial biomass pool with a variable C-to-N ratio (C:N). The plant biomass, which contributes to the SOM pools, flexibly allocates growth toward wood, root, and leaf biomass, based on nitrogen (N) uptake and shoot-to-root ratio. Unlike traditional ecosystem models, the microbial community can acclimate to changing soil resources by shifting its C:N between a lower C:N, faster turnover (bacteria-like) community, and a higher C:N, slower turnover (fungal-like) community. This stoichiometric flexibility allows for the microbial C and N use efficiency to vary, feeding back into system decomposition and productivity dynamics. These feedbacks regulate changes in extracellular enzyme synthesis, soil pool turnover rates, plant growth, and ecosystem C storage. We used SCAMPS to test the interactive effects of winter, summer, and year-round soil warming, in combination with microbial acclimation ability, on decomposition dynamics and plant growth in a tundra system. Over 50-year simulations, both the seasonality of warming and the ability of the microbial community to acclimate had strong effects on ecosystem C dynamics. Across all scenarios, warming increased plant biomass (and therefore litter inputs to the SOM), while the ability of the microbial community to acclimate increased soil C loss. Winter warming drove the largest ecosystem C losses when the microbial community could acclimate, and the largest ecosystem C gains when it could not acclimate. Similar to empirical studies of tundra warming, modeled summer warming had relatively negligible effects on soil C loss, regardless of acclimation ability. In contrast, winter and year-round warming drove marked soil C loss when decomposers could acclimate, despite also increasing plant biomass. These results suggest that incorporating dynamically interacting microbial and plant communities into ecosystem models might increase the ability to link ongoing global change field observations with macroscale projections of ecosystem biogeochemical cycling in systems under change. © 2014 by the Ecological Society of America.


Sistla S.A.,University of California at Santa Barbara | Moore J.C.,Colorado State University | Simpson R.T.,Colorado State University | Gough L.,University of Texas at Arlington | And 2 more authors.
Nature | Year: 2013

High latitudes contain nearly half of global soil carbon, prompting interest in understanding how the Arctic terrestrial carbon balance will respond to rising temperatures. Low temperatures suppress the activity of soil biota, retarding decomposition and nitrogen release, which limits plant and microbial growth. Warming initially accelerates decomposition, increasing nitrogen availability, productivity and woody-plant dominance. However, these responses may be transitory, because coupled abiotic-biotic feedback loops that alter soil-temperature dynamics and change the structure and activity of soil communities, can develop. Here we report the results of a two-decade summer warming experiment in an Alaskan tundra ecosystem. Warming increased plant biomass and woody dominance, indirectly increased winter soil temperature, homogenized the soil trophic structure across horizons and suppressed surface-soil-decomposer activity, but did not change total soil carbon or nitrogen stocks, thereby increasing net ecosystem carbon storage. Notably, the strongest effects were in the mineral horizon, where warming increased decomposer activity and carbon stock: a 'biotic awakening' at depth. © 2013 Macmillan Publishers Limited. All rights reserved.


Ducklow H.W.,Ecosystems Center | Doney S.C.,Woods Hole Oceanographic Institution
Annual Review of Marine Science | Year: 2013

For more than a decade there has been controversy in oceanography regarding the metabolic state of the oligotrophic subtropical gyres of the open ocean. Here we review the background of this controversy, commenting on several issues to set the context for a moderated debate between two groups of scientists. In one of the two companion articles, Williams et al. (2013) take the view that these gyres exhibit a state of net autotrophy-that is, their gross primary production (GPP) exceeds community respiration (R) when averaged over some suitably extensive region and over a long duration. In the other companion article, Duarte et al. (2013) take the opposite view, that these gyres are net heterotrophic, with R exceeding the GPP. This idea-that large, remote areas of the upper ocean could be net heterotrophic-raises a host of fundamental scientific questions about the metabolic processes of photosynthesis and respiration that underlie ocean ecology and global biogeochemistry. The question remains unresolved in part because the net state is finely balanced between large opposing fluxes and most current measurements have large uncertainties. This challenging question must be studied against the background of large, anthropogenically driven changes in ocean ecology and biogeochemistry. Current trends of anthropogenic change make it an urgent problem to solve and also greatly complicate finding that solution. © 2013 by Annual Reviews. All rights reserved.


Schofield O.,Rutgers University | Ducklow H.W.,Ecosystems Center | Martinson D.G.,Columbia University | Meredith M.P.,British Antarctic Survey | And 2 more authors.
Science | Year: 2010

Climate change will alter marine ecosystems; however, the complexity of the food webs, combined with chronic undersampling, constrains efforts to predict their future and to optimally manage and protect marine resources. Sustained observations at the West Antarctic Peninsula show that in this region, rapid environmental change has coincided with shifts in the food web, from its base up to apex predators. New strategies will be required to gain further insight into how the marine climate system has influenced such changes and how it will do so in the future. Robotic networks, satellites, ships, and instruments mounted on animals and ice will collect data needed to improve numerical models that can then be used to study the future of polar ecosystems as climate change progresses. Copyright Science 2010 by the American Association for the Advancement of Science; all rights reserved.


Johnson D.S.,Ecosystems Center
Marine Ecology Progress Series | Year: 2011

In salt marshes, high-marsh habitats are infrequently flooded (typically only during spring tides). Organisms in these habitats, however, may still be susceptible to the effects of increased nutrients delivered by tidal water (i.e. eutrophication). In a Massachusetts salt marsh, I examined the responses of the epibenthic invertebrates in the Spartina patens-dominated high marsh to long-term (7 yr) and landscape-level (4?5 ha) nutrient enrichment. In this ecosystemlevel experiment, nutrients (N and P; ∼15× reference conditions) were added to the flooding waters of tidal creeks-which flooded the high marsh during spring tides-to mimic cultural eutrophication. Three detritivores: Melampus bidentatus (gastropod), Philoscia vittata (isopod), and Orchestia grillus (amphipod) numerically dominated the benthic invertebrate community (97% by abundance). These species had higher densities (47 to 199% increase) in enriched versus reference creeks. Melampus size structure shifted to larger individuals with enrichment. End-of-season aboveground biomass and detritus stocks of S. patens did not differ between treatments; thus, increased litter quality and/or alternative food-source increases (e.g. microbes) led to increased detritivore density/biomass. Predator densities (spiders and Tabanus larvae) increased 125 to 160% with enrichment, likely due to increased prey densities (including Orchestia and Philoscia). Analysis of similarities (ANOSIM) revealed that communities were dissimilar between treatments; differences were driven primarily by changes in detritivore abundance. These results suggest that despite being infrequently flooded and thus infrequently exposed to elevated nutrients, high-marsh invertebrates are susceptible to eutrophication. Hence, the high marsh should be integrated into our understanding of how eutrophication impacts saltmarsh functioning. © Inter-Research 2011.

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