Roser Matamala

EFFECTS OF FREE-AIR CO2 ENRICHMENT (FACE) ON BELOWGROUND PROCESSES IN A PINUS TAEDA FOREST

Terrestrial vegetation and soils may act as important carbon sinks if rising atmospheric CO2 stimulates plant production. We used free-air CO2 enrichment (FACE) technology to expose three 30 m diameter plots of a loblolly pine (Pinus taeda) forest to elevated CO2 at 200 FLL/L above ambient levels, while three control plots were outfitted with FACE apparatus but were fumigated with ambient air. We quantified litterfall mass and chemistry, fine root biomass increment and turnover, CO2 efflux from soils, 83C in soil C02, soil CO2, soil microbial biomass C and N, and potential net N mineralization. After two growing seasons, elevated CO2 caused significant increases in loblolly pine litterfall mass and fine root increment. Within the first year of FACE treatment, the con- centration of CO2 in soil had increased, and soil surface CO2 efflux was generally higher at elevated C02, but this difference was not statistically significant. Loblolly pine litter C:N ratio, fine root turnover, microbial biomass C and N, and potential net N mineralization were not significantly affected by elevated CO2. Our results suggest that elevated atmo- spheric CO2 may accelerate inputs of organic matter to soil C pools in loblolly pine forests, but it may also accelerate losses of C from belowground by stimulating soil respiration.

Temperature effects on the diversity of soil heterotrophs and the δ13C of soil-respired CO2.

We measured the respiration rates, δ13C of respired CO2, and microbial community composition in root-free bulk soils incubated at 4, 22 and 40°C. The soils were obtained from the Duke Forest Free-Air CO2 Enrichment (FACE) experiment where organic carbon in soils sampled from the elevated CO2 plots contained a unique 13C label that was derived from FACE fumigation. The CO2 produced by soil heterotrophs at 4°C was 2.2 to 3.5‰ enriched in 13C relative to CO2 respired at 22 and 40°C and was similarly enriched relative to bulk soil carbon. There was no isotopic difference between CO2 produced at 22 and 40°C. Respiration rates increased exponentially with temperature from 0.25 mg CO2 g soil−1 d−1 at 4°C to 0.73 mg CO2 g soil−1 d−1 at 40°C. Microbial community composition, as measured by the differences in populations of morphology types, differed across the temperature range. Only eight of 67 microbial morphology types were common to all three incubation temperatures, while six types were unique to 4°C soil, 17 to 22°C soil and 18 to 40°C soil. Species richness, approximated from morphology type, was significantly lower at 4°C than at 22 and 40°C. This change in microbial community structure from 4 to 22 and 40°C caused a shift in mineralizable carbon pools, resulting in a shift in the isotopic composition of CO2 respired at the low temperature.

TEMPORAL CHANGES IN C AND N STOCKS OF RESTORED PRAIRIE: IMPLICATIONS FOR C SEQUESTRATION STRATEGIES

The recovery of ecosystem C and N dynamics after disturbance can be a slow process. Chronosequence approaches offer unique opportunities to use space-for-time substitution to quantify the recovery of ecosystem C and N stocks and estimate the potential of restoration practices for C sequestration. We studied the distribution of C and N stocks in two chronosequences that included long-term cultivated lands, 3- to 26-year-old prairie restorations, and remnant prairie on two related soil series. Results from the two chronosequences did not vary significantly and were combined. Based on modeling predictions, the recovery rates of different ecosystem components varied greatly. Overall, C stocks recovered faster than N stocks, but both C and N stocks recovered more rapidly for aboveground vegetation than for any other ecosystem component. Aboveground C and N reached 95% of remnant levels in only 13 years and 21 years, respectively, after planting to native vegetation. Belowground plant C and N recovered several decades later, while microbial biomass C, soil organic C (SOC), and total soil N recovered on a century timescale. In the cultivated fields, SOC concentrations were depleted within the surface 25 cm, coinciding with the depth of plowing, but cultivation apparently led to redistribution of soil C, increasing SOC stocks deeper in the soil profile. The restoration of prairie vegetation was effective at rebuilding soil organic matter (SOM) in the surface soil. Accrual rates were maintained at 43 g C x m(-2) x yr(-1) and 3 g N x m(-2) x yr(-1) in the surface 0.16 Mg/m2 soil mass during the first 26 years of restoration and were predicted to reach 50% of their storage potential (3500 g C/m2) in the first 100 years. We conclude that restoration of tallgrass prairie vegetation can restore SOM lost through cultivation and has the potential to sequester relatively large amounts of SOC over a sustained period of time. Whether restored prairies can retain the C apparently transferred to the subsoil by cultivation practices remains to be seen.

ELEVATED CO2 DIFFERENTIATES ECOSYSTEM CARBON PROCESSES: DECONVOLUTION ANALYSIS OF DUKE FOREST FACE DATA

Quantification of the flux of carbon (C) through different pathways is critical to predict the impact of global change on terrestrial ecosystems. Past research has en- countered considerable difficulty in separating root exudation, root turnover rate, and other belowground C fluxes as affected by elevated CO2. In this study we adopted a deconvolution analysis to differentiate C flux pathways in forest soils and to quantify the flux through those pathways. We first conducted forward analysis using a terrestrial-C sequestration (TCS) model to generate four alternative patterns of convolved responses of soil surface respiration to a step increase in atmospheric CC)2. The model was then validated against measured soil respiration at ambient CO2 before it was used to deconvolve the CO2 stim- ulation of soil respiration. Deconvolved data from the Duke Forest free-air CO2 enrichment (FACE) experiment suggest that fast C transfer processes, e.g., root exudation, are of minor importance in the ecosystem C cycling in the Duke Forest and were not affected by elevated CO2. The analysis indicates that the fine-root turnover is a major process adding C to the rhizosphere. This C has a residence time of several months to -2 yr and increases signif- icantly with increased CO2. In addition, the observed phase shift in soil respiration caused by elevated CO2 can be only reproduced by incorporation of a partial time delay function in C fluxes into the model. This paper also provides a detailed explanation of deconvolution analysis, since it is a relatively new research technique in ecology.

Global estimation of evapotranspiration using a leaf area index-based surface energy and water balance model

Studies of global hydrologic cycles, carbon cycles and climate change are greatly facilitated when. global estimates of evapotranspiration (E) are available. We have developed an air-relative-humidity-based two-source (ARTS) E model that simulates the surface energy balance, soil water balance, and environmental constraints on E. It uses remotely sensed leaf area index (L-ai) and surface meteorological data to estimate E by: 1) introducing a simple biophysical model for canopy conductance (G(c)), defined as a constant maximum stomatal conductance g(smax) of 12.2 mm s(-1) multiplied by air relative humidity (R-h) and L-ai (G(c) = g(srnax) x R-h X L-ai); 2) calculating canopy transpiration with the G(c)-based Penman-Monteith (PM) E model; 3) calculating soil evaporation from an air-relative-humidity-based model of evapotranspiration (Yan & Shugart, 2010); 4) calculating total E (E-0) as the sum of the canopy transpiration and soil evaporation, assuming the absence of soil water stress; and 5) correcting E-0 for soil water stress using a soil water balance model. This physiological ARTS E model requires no calibration. Evaluation against eddy covariance measurements at 19 flux sites, representing a wide variety of climate and vegetation types, indicates that daily estimated E had a root mean square error = 0.77 mm d(-1). bias = -0.14 mm d(-1), and coefficient of determination, R-2 = 0.69. Global, monthly, 0.5 degrees-gridded ARTS E simulations from 1984 to 1998, which were forced using Advanced Very High Resolution Radiometer Lai data, Climate Research Unit climate data, and surface radiation budget data, predicted a mean annual land E of 58.4 x 10(3) km(3). This falls within the range (58 x 10(3)-85 x 10(3) km(3)) estimated by the Second Global Soil Wetness Project (GSWP-2: Dirmeyer et al., 2006). The ARTS E spatial pattern agrees well with that of the global E estimated by GSWP-2. The global annual ARTS E increased by 15.5 mm per decade from 1984 to 1998, comparable to an increase of 9.9 mm per decade from the model tree ensemble approach (Jung et al., 2010). These comparisons confirm the effectivity of the ARTS E model to simulate the spatial. pattern and climate response of global E. This model is the first of its kind among remote-sensing-based PM E models to provide global land E estimation with consideration of the soil water balance

Tracing Changes in Ecosystem Function under Elevated Carbon Dioxide Conditions

Abstract Responses of ecosystems to elevated levels of atmospheric carbon dioxide (CO2) remain a critical uncertainty in global change research. Two key unknown factors are the fate of carbon newly incorporated by photosynthesis into various pools within the ecosystem and the extent to which elevated CO2 is transferred to and sequestered in pools with long turnover times. The CO2 used for enrichment in many experiments incorporates a dual isotopic tracer, in the sense that ratios of both the stable carbon-13 (13C) and the radioactive carbon-14 (14C) isotopes with respect to carbon-12 are different from the corresponding ratios in atmospheric CO2. Here we review techniques for using 13C and 14C abundances to follow the fate of newly fixed carbon and to further our understanding of the turnover times of ecosystem carbon pools. We also discuss the application of nitrogen, oxygen, and hydrogen isotope analyses for tracing changes in the linkages between carbon, nitrogen, and water cycles under conditions of elevated CO2.

Stored carbon partly fuels fine‐root respiration but is not used for production of new fine roots

The relative use of new photosynthate compared to stored carbon (C) for the production and maintenance of fine roots, and the rate of C turnover in heterogeneous fine-root populations, are poorly understood. We followed the relaxation of a (13)C tracer in fine roots in a Liquidambar styraciflua plantation at the conclusion of a free-air CO(2) enrichment experiment. Goals included quantifying the relative fractions of new photosynthate vs stored C used in root growth and root respiration, as well as the turnover rate of fine-root C fixed during [CO(2)] fumigation. New fine-root growth was largely from recent photosynthate, while nearly one-quarter of respired C was from a storage pool. Changes in the isotopic composition of the fine-root population over two full growing seasons indicated heterogeneous C pools; < 10% of root C had a residence time < 3 months, while a majority of root C had a residence time > 2 yr. Compared to a one-pool model, a two-pool model for C turnover in fine roots (with 5 and 0.37 yr(-1) turnover times) doubles the fine-root contribution to forest NPP (9-13%) and supports the 50% root-to-soil transfer rate often used in models.

Empirical estimates to reduce modeling uncertainties of soil organic carbon in permafrost regions: a review of recent progress and remaining challenges

The vast amount of organic carbon (OC) stored in soils of the northern circumpolar permafrost region is a potentially vulnerable component of the global carbon cycle. However, estimates of the quan ...

The Duke Forest FACE Experiment: CO2 Enrichment of a Loblolly Pine Forest

Free-air CO2 enrichment (FACE) in the Duke Forest provides a whole-ecosystem arena in which to examine the response of a temperate coniferous forest to high, future levels of atmospheric CO2. At the end of 8 years of the experiment, we conclude: Photosynthetic rates by canopy foliage have increased up to 50 % over controls. Basal area increment has been stimulated 13–27 % versus that in control plots, with interannual variation due to variations in temperature and moisture during the growing season. Biomass increment has increased by 108 g C m-2 year-1 (27 %) over that in control plots. Growth and respiration of roots are higher in CO2 fumigated plots. Litterfall is greater in high CO2 plots and forest floor accumulation has increased. There has been little or no change in the total amount of soil organic matter as a result of CO2 fumigations. While the stimulation of growth by high CO2 persists after 8 years of fumigation, there is evidence of nitrogen limitation in the fumigated plots.

Observational needs for estimating Alaskan soil carbon stocks under current and future climate

Representing land surface spatial heterogeneity when designing observation networks is a critical scientific challenge. Here, we present a geospatial approach that utilizes the multivariate spatial heterogeneity of soil-forming factors — namely climate, topography, land cover types, and surficial geology — to identify observation sites to improve soil organic carbon (SOC) stock estimates across the State of Alaska, USA. Standard deviations in existing SOC samples indicated that 657, 870, and 906 randomly distributed pedons would be required to quantify the average SOC stocks for 0-1 m, 0-2 m, and whole-profile depths, respectively, at a confidence interval of 5 kg m-2. Using the spatial correlation range of existing SOC samples, we identified that 309, 446, and 484 new observation sites are needed to estimate current SOC stocks to 1-m, 2-m, and whole-profile depths, respectively. We also investigated whether the identified sites might change under future climate by using eight decadal (2020–2099) projections of precipitation, temperature, and length of growing season for three representative concentration pathway (RCP 4.5, 6.0, and 8.5) scenarios of the Intergovernmental Panel on Climate Change. These analyses determined that 12 to 41 additional sites (906 + 12 to 41; depending upon the emission scenarios) would be needed to capture the impact of future climate on Alaskan whole-profile SOC stocks by 2100. The identified observation sites represent spatially distributed locations across Alaska that captures the multivariate heterogeneity of soil-forming factors under current and future climatic conditions. This information is needed for designing monitoring networks and benchmarking of Earth System Model results.