Caitlin E. Hicks Pries

Thawing permafrost increases old soil and autotrophic respiration in tundra: Partitioning ecosystem respiration using δ13C and ∆14C

Ecosystem respiration (Reco ) is one of the largest terrestrial carbon (C) fluxes. The effect of climate change on Reco depends on the responses of its autotrophic and heterotrophic components. How autotrophic and heterotrophic respiration sources respond to climate change is especially important in ecosystems underlain by permafrost. Permafrost ecosystems contain vast stores of soil C (1672 Pg) and are located in northern latitudes where climate change is accelerated. Warming will cause a positive feedback to climate change if heterotrophic respiration increases without corresponding increases in primary production. We quantified the response of autotrophic and heterotrophic respiration to permafrost thaw across the 2008 and 2009 growing seasons. We partitioned Reco using Δ(14) C and δ(13) C into four sources-two autotrophic (above - and belowground plant structures) and two heterotrophic (young and old soil). We sampled the Δ(14) C and δ(13) C of sources using incubations and the Δ(14) C and δ(13) C of Reco using field measurements. We then used a Bayesian mixing model to solve for the most likely contributions of each source to Reco . Autotrophic respiration ranged from 40 to 70% of Reco and was greatest at the height of the growing season. Old soil heterotrophic respiration ranged from 6 to 18% of Reco and was greatest where permafrost thaw was deepest. Overall, growing season fluxes of autotrophic and old soil heterotrophic respiration increased as permafrost thaw deepened. Areas with greater thaw also had the greatest primary production. Warming in permafrost ecosystems therefore leads to increased plant and old soil respiration that is initially compensated by increased net primary productivity. However, barring large shifts in plant community composition, future increases in old soil respiration will likely outpace productivity, resulting in a positive feedback to climate change.

Permafrost degradation stimulates carbon loss from experimentally warmed tundra

A large pool of organic carbon (C) has been accumulating in the Arctic for thousands of years because cold and waterlogged conditions have protected soil organic material from microbial decomposition. As the climate warms this vast and frozen C pool is at risk of being thawed, decomposed, and released to the atmosphere as greenhouse gasses. At the same time, some C losses may be offset by warming-mediated increases in plant productivity. Plant and microbial responses to warming ultimately determine net C exchange from ecosystems, but the timing and magnitude of these responses remain uncertain. Here we show that experimental warming and permafrost (ground that remains below 0 degrees C for two or more consecutive years) degradation led to a two-fold increase in net ecosystem C uptake during the growing season. However, warming also enhanced winter respiration, which entirely offset growing-season C gains. Winter C losses may be even higher in response to actual climate warming than to our experimental manipulations, and, in that scenario, could be expected to more than double overall net C losses from tundra to the atmosphere. Our results highlight the importance of winter processes in determining whether tundra acts as a C source or sink, and demonstrate the potential magnitude of C release from the permafrost zone that might be expected in a warmer climate.

The whole-soil carbon flux in response to warming

Warming the top meter increased soil CO2 production by 37%, with 40% of the response coming from soils more than 15 centimeters deep. Digging deeper into soils Soils contain about twice as much carbon as Earths atmosphere, so their response to warming is crucial to understanding carbon fluxes in a changing climate. Past studies have heated soil to a depth of 5 to 20 cm to examine such fluxes. Hicks Pries et al. heated the ground to a depth of 100 cm. Extending measurements to that depth revealed that 4°C of warming increased annual soil respiration by 34 to 37%—a considerable amount more than previously observed. Science, this issue p. 1420 Soil organic carbon harbors three times as much carbon as Earth’s atmosphere, and its decomposition is a potentially large climate change feedback and major source of uncertainty in climate projections. The response of whole-soil profiles to warming has not been tested in situ. In a deep warming experiment in mineral soil, we found that CO2 production from all soil depths increased with 4°C warming; annual soil respiration increased by 34 to 37%. All depths responded to warming with similar temperature sensitivities, driven by decomposition of decadal-aged carbon. Whole-soil warming reveals a larger soil respiration response than many in situ experiments (most of which only warm the surface soil) and models.

Beyond clay: towards an improved set of variables for predicting soil organic matter content

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.

Inquiring into Familiar Objects: An Inquiry-Based Approach to Introduce Scientific Vocabulary

ABSTRACT Learning science vocabulary is an often tedious but important component of many curricula. Frequently, students are expected to learn science vocabulary indirectly, but this method can hinder the success of lower-performing students (Carlisle, Fleming, and Gudbrandsen 2000). We have developed an inquiry-based vocabulary activity wherein students explore how new words apply to familiar objects, piquing their interest in the words’ meanings, prior to direct instruction. Groups of students were asked to apply new vocabulary words to familiar objects set up at various rotation stations. The students were encouraged to play with the objects, discuss answers with their group, and write down the vocabulary words they thought best described each object. Since the words were new to the students, there was no credit for correct answers, making the students comfortable exploring a new topic. After the rotation stations, we led students in a discussion of their answers and followed up with direct instruction on the words’ definitions. Here we give three examples of this activity used in our physical science classroom—one each for energy, simple machines, and force vocabulary—but this activity can easily be modified for biology and Earth science curricula.

Response to Comment on “The whole-soil carbon flux in response to warming”

Temperature records and model predictions demonstrate that deep soils warm at the same rate as surface soils, contrary to Xiao et al.’s assertions. In response to Xiao et al.’s critique of our Q10 analysis, we present the results with all data points included, which show Q10 values of >2 throughout the soil profile, indicating that all soil depths responded to warming.