Field studies of atmospheric CO 2 effects on ecosystems usually include few levels of CO 2 and a single soil type, making it difficult to ascertain the shape of responses to increasing CO 2 or to generalize across soil types. The Lysimeter CO 2 Gradient (LYCOG) chambers were constructed to maintain a linear gradient of atmospheric CO 2 ($250 to 500 ll l -1 ) on grassland vegetation established on intact soil monoliths from three soil series. The chambers maintained a linear daytime CO 2 gradient from 263 ll l -1 at the subambient end of the gradient to 502 ll l -1 at the superambient end, as well as a linear nighttime CO 2 gradient. Temperature variation within the chambers affected aboveground biomass and evapotranspiration, but the effects of temperature were small compared to the expected effects of CO 2 . Aboveground biomass on Austin soils was 40% less than on Bastrop and Houston soils. Biomass differences between soils resulted from variation in biomass of Sorghastrum nutans, Bouteloua curtipendula, Schizachyrium scoparium (C 4 grasses), and Solidago canadensis (C 3 forb), suggesting the CO 2 sensitivity of these species may differ among soils. Evapotranspiration did not differ among the soils, but the CO 2 sensitivity of leaf-level photosynthesis and water use efficiency in S. canadensis was greater on Houston and Bastrop than on Austin soils, whereas the CO 2 sensitivity of soil CO 2 efflux was greater on Bastrop soils than on Austin or Houston soils. The effects of soil type on CO 2 sensitivity may be smaller for some processes that are tightly coupled to microclimate. LYCOG is useful for discerning the effects of soil type on the CO 2 sensitivity of ecosystem function in grasslands.

Soils sequester and release substantial atmospheric carbon, but the contribution of fungal communities to soil carbon balance under rising CO 2 is not well understood. Soil properties likely mediate these fungal responses but are rarely explored in CO 2 experiments. We studied soil fungal communities in a grassland ecosystem exposed to a preindustrial-to-future CO 2 gradient (250 to 500 ppm) in a black clay soil and a sandy loam soil. Sanger sequencing and pyrosequencing of the rRNA gene cluster revealed that fungal community composition and its response to CO 2 differed significantly between soils. Fungal species richness and relative abundance of Chytridiomycota (chytrids) increased linearly with CO 2 in the black clay (P < 0.04, R 2 > 0.7), whereas the relative abundance of Glomeromycota (arbuscular mycorrhizal fungi) increased linearly with elevated CO 2 in the sandy loam (P ‫؍‬ 0.02, R 2 ‫؍‬ 0.63). Across both soils, decomposition rate was positively correlated with chytrid relative abundance (r ‫؍‬ 0.57) and, in the black clay soil, fungal species richness. Decomposition rate was more strongly correlated with microbial biomass (r ‫؍‬ 0.88) than with fungal variables. Increased labile carbon availability with elevated CO 2 may explain the greater fungal species richness and Chytridiomycota abundance in the black clay soil, whereas increased phosphorus limitation may explain the increase in Glomeromycota at elevated CO 2 in the sandy loam. Our results demonstrate that soil type plays a key role in soil fungal responses to rising atmospheric CO 2 .

Changes in soil carbon storage could affect and be affected by rising atmospheric CO 2 . However, it is unlikely that soils will respond uniformly, as some soils are more sensitive to changes in the amount and chemistry of plant tissue inputs whereas others are less sensitive because of mineralogical, textural, or microbial processes. We studied soil carbon and microbial responses to a preindustrial-to-future CO 2 gradient (250-500 ppm) in a grassland ecosystem in the field. The ecosystem contains three soil types with clay fractions of 15% to 55%: a sandy loam Alfisol, a silty clay Mollisol, and a black clay Vertisol. Soil and microbial responses to atmospheric CO 2 are plant-mediated; and aboveground plant productivity in this ecosystem increased linearly with CO 2 in the sandy loam and silty clay. Although total soil organic carbon (SOC) did not change with CO 2 treatment after four growing seasons, fast-cycling SOC pools increased with CO 2 in the two clay soils. Microbial biomass increased 18% and microbial activity increased 30% across the CO 2 gradient in the black clay (55% clay), but neither factor changed with CO 2 in the sandy loam (15% clay). Similarly, size fractionation of SOC showed that coarse POM-C, the youngest and most labile fraction, increased four-fold across the CO 2 gradient in the black clay, but increased by only 50% across the gradient in the sandy loam. Interestingly, mineral-associated C, the oldest and most recalcitrant fraction, declined 23% across the gradient in the third soil type, a silty clay (45% clay). Our results provide evidence for priming in this soil type, as labile C availability and decomposition rate (measured as soil respiration and soil C mineralization) also increased across the CO 2 gradient in the silty clay soil.In summary, CO 2 enrichment in this grassland increased the fast-cycling SOC pool as in other CO 2 studies, but only in the two high-clay soils. Priming in the silty clay could limit SOC accumulation after prolonged CO 2 exposure. Because soil texture varies geographically, including data on soil types could enhance predictions of soil carbon and microbial responses to future CO 2 levels.

SummaryMilitary bases resemble small cities and face similar sustainability challenges. As pilot studies in the U.S. Army Net Zero program, 17 locations are moving to 100% renewable energy, zero depletion of water resources, and/or zero waste to landfill by 2020. Some bases target net zero in a single area, such as water, whereas two bases, including Fort Carson, Colorado, target net zero in all three areas. We investigated sustainability strategies that appear when multiple areas (energy, water, and waste) are integrated. A system dynamics model is used to simulate urban metabolism through Fort Carson's energy, water, and waste systems. Integrated scenarios reduce environmental impact up to 46% from the 2010 baseline, whereas single-dimension scenarios (energy-only, water-only, and wasteonly) reduce impact, at most, 20%. Energy conserving technologies offer mutual gains, reducing annual energy use 18% and water use 15%. Renewable energy sources present trade-offs: Concentrating solar power could supply 11% of energy demand, but increase water demand 2%. Waste to energy could supply 40% of energy demand and reduce waste to landfill >80%, but increase water demand between 1% and 22% depending on cooling system and waste tonnage. Outcomes depend on how the Fort Carson system is defined, because some components represent multiple net zero areas (food represents waste and energy), and some actions require embodied resources (energy generation potentially requires water and off-base feedstock). We suggest that integrating multiple net zero goals can lead to lower environmental impact for military bases. Keywords:industrial ecology military renewable energy system dynamics urban metabolism water recycling Supporting information is available on the JIE Web site

Like many urban areas around the world, Durham and Orange counties in North Carolina, USA are experiencing population growth and sprawl that is putting stress on the transportation system. Light rail and denser transit-oriented development are being considered as possible solutions. However, local agencies and stakeholders are concerned the light rail may worsen housing affordability and have questioned whether investment in both light rail and dense redevelopment are necessary to achieve community goals. We developed an integrated system dynamics model to quantitatively explore the outcomes of these land use and transportation options across multiple societal dimensions. The model incorporates feedbacks among the land, transportation, economic, equity, and energy sectors. This paper uses the results of four model scenarios, run between 2000 and 2040, to address two main questions: (1) what role does redevelopment play in capturing the socioeconomic benefits of transit infrastructure investment? And (2) how do redevelopment and light-rail transit interact to affect housing and transportation affordability? We find that transit investment and dense redevelopment combine synergistically to better achieve the goals of the light-rail line, including economic development, mobility, and compact growth. However, housing affordability does worsen in the combined scenario, as transportation-cost savings are not sufficient to offset the rise in housing costs. We emphasize that model users may input their own assumptions to explore the dynamics of alternative scenarios. We demonstrate how spatially-aggregated systems models can complement traditional land use and transportation models in the regional planning process.