Sharon A. Billings

Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO2

The earths future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO₂. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO₂ stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO₂ as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.

The ecology of algal biodiesel production

Sustainable energy production represents one of the most formidable problems of the 21st century, and plant-based biofuels offer significant promise. We summarize the potential advantages of using pond-grown microalgae as feedstocks relative to conventional terrestrial biofuel crop production. We show how pond-based algal biofuel production, which requires significantly less land area than agricultural crop-based biofuel systems, can offer additional ecological benefits by reducing anthropogenic pollutant releases to the environment and by requiring much lower water subsidies. We also demonstrate how key principles drawn from the science of ecology can be used to design efficient pond-based microalgal systems for the production of biodiesel fuels.

How interactions between microbial resource demands, soil organic matter stoichiometry, and substrate reactivity determine the direction and magnitude of soil respiratory responses to warming

Recent empirical and theoretical advances inform us about multiple drivers of soil organic matter (SOM) decomposition and microbial responses to warming. Absent from our conceptual framework of how soil respiration will respond to warming are adequate links between microbial resource demands, kinetic theory, and substrate stoichiometry. Here, we describe two important concepts either insufficiently explored in current investigations of SOM responses to temperature, or not yet addressed. First, we describe the complete range of responses for how warming may change microbial resource demands, physiology, community structure, and total biomass. Second, we describe how any relationship between SOM activation energy of decay and carbon (C) and nitrogen (N) stoichiometry can alter the relative availability of C and N as temperature changes. Changing availabilities of C and N liberated from their organic precursors can feedback to microbial resource demands, which in turn influence the aggregated respiratory response to temperature we observe. An unsuspecting biogeochemist focused primarily on temperature sensitivity of substrate decay thus cannot make accurate projections of heterotrophic CO2 losses from diverse organic matter reservoirs in a warming world. We establish the linkages between enzyme kinetics, SOM characteristics, and potential for microbial adaptation critical for making such projections. By examining how changing microbial needs interact with inherent SOM structure and composition, and thus reactivity, we demonstrate the means by which increasing temperature could result in increasing, unchanging, or even decreasing respiration rates observed in soils. We use this exercise to highlight ideas for future research that will develop our abilities to predict SOM feedbacks to climate.

Warming‐enhanced preferential microbial mineralization of humified boreal forest soil organic matter: Interpretation of soil profiles along a climate transect using laboratory incubations

Accepted for publication in Journal of Geophysical Research. Copyright 2012 American Geophysical Union. Further reproduction or electronic distribution is not permitted.

Stable-isotope Analysis of Diets of Short-tailed Fruit Bats (Chiroptera: Phyllostomidae: Carollia)

Abstract The coexistence of multiple species of short-tailed fruit bats (Phyllostomidae: Carollia) is common throughout the range of the genus. Previous studies of fecal and stomach contents have documented differences in dietary breadth and in habitat use as mechanisms by which these species may partition dietary niches. By comparing values of δ15N and δ13C across species of Carollia from 17 sites in Central and South America, we show that co-occurring Carollia frequently differ in dietary breadth, foraging habitats, and level of insectivory. Values of δ15N, which tended to be enriched in C. castanea, depleted in C. perspicillata, and intermediate in C. brevicauda and C. sowelli, indicate trophic stratification. Values of δ13C followed the opposite trend, tending to be enriched in C. perspicillata and depleted in C. castanea, suggesting interspecific differences in breadth of the foraging area. Isotopic comparisons among Carollia, other bat species, and potential food items at 5 of our sites illustrate that populations of Carollia tend to be trophically intermediate between strictly phytophagous and strictly insectivorous organisms, and, contrary to the paucity of insect remains found in fecal samples, indicate that the consumption of insects by Carollia is more common and potentially more important than previously was thought.

Biogeochemistry: nitrous oxide in flux.

In drought conditions, forest soils can serve as a small but surprisingly persistent sink for the greenhouse gas nitrous oxide. The effect highlights a research avenue necessary for predicting Earths climate.

Incorporation of Plant Residues into Soil Organic Matter Fractions With Grassland Management Practices in the North American Midwest

Disturbed grassland soils are often cited as having the potential to store large amounts of carbon (C). Fertilization of grasslands can promote soil C storage, but little is known about the generation of recalcitrant pools of soil organic matter (SOM) with management treatments, which is critical for long-term soil C storage. We used a combination of soil incubations, size fractionation and acid hydrolysis of SOM, [C], [N], and stable isotopic analyses, and biomass quality indices to examine how fertilization and haying can impact SOM dynamics in Kansan grassland soils. Fertilized soils possessed 113% of the C possessed by soils subjected to other treatments, an increase predominantly harbored in the largest size fraction (212–2,000 μm). This fraction is frequently associated with more labile material. Haying and fertilization/haying, treatments that more accurately mimic true management techniques, did not induce any increase in soil C. The difference in 15N-enrichment between size fractions was consistent with a decoupling of SOM processing between pools with fertilization, congruent with gains of SOM in the largest size fraction promoted by fertilization not moving readily into smaller fractions that frequently harbor more recalcitrant material. Litterfall and root biomass C inputs increased 104% with fertilization over control plots, and this material possessed lower C:N ratios. Models of incubation mineralization kinetics indicate that fertilized soils have larger pools of labile organic C. Model estimates of turnover rates of the labile and recalcitrant C pools did not differ between treatments (65.5 ± 7.2 and 2.9 ± 0.3 μg C d−1, respectively). Although fertilization may promote greater organic inputs into these soils, much of that material is transformed into relatively labile forms of soil C; these data highlight the challenges of managing grasslands for long-term soil C sequestration.

Seasonal variations in plant nitrogen relations and photosynthesis along a grassland to shrubland gradient in Owens Valley, California

Community composition in semi-arid ecosystems has largely been explained by water availability; however, nitrogen is a common limiting nutrient, and may be an important control on plant function and carbon uptake. We investigated nitrogen relations and photosynthesis of several dominant species at shallow groundwater sites in Owens Valley, California. We measured soil nitrogen (N) availability, leaf N and isotopes, water isotopes, and gas exchange of dominant shrub species Atriplex torreyi and Ericameria nauseosa and grass species Distichlis spicata throughout the summer season in three sites that had similar watertable depths, but that varied in community composition and N availability. Surface soil inorganic N was greatest at the grassland site and declined from June to September at all sites. Leaf N declined throughout the season in all species, and was correlated with soil inorganic N. Photosynthesis of A. torreyi remained relatively constant throughout the season. In contrast, D. spicata and E. nauseosa experienced seasonal declines in photosynthesis at sites with greater inorganic N availability. Leaf N was significantly correlated with photosynthesis in D. spicata across all sites and measurement periods. Controls on N cycling are likely to be an important determinant of photosynthesis of D. spicata in this region.

Topographic variability and the influence of soil erosion on the carbon cycle

Soil erosion, particularly that caused by agriculture, is closely linked to the global carbon (C) cycle. There is a wide range of contrasting global estimates of how erosion alters soil-atmosphere C exchange. This can be partly attributed to limited understanding of how geomorphology, topography, and management practices affect erosion and oxidation of soil organic C (SOC). This work presents a physically based approach that stresses the heterogeneity at fine spatial scales of SOC erosion, SOC burial, and associated soil-atmosphere C fluxes. The Holcombes Branch watershed, part of the Calhoun Critical Zone Observatory in South Carolina, USA, is the case study used. The site has experienced some of the most serious agricultural soil erosion in North America. We use SOC content measurements from contrasting soil profiles and estimates of SOC oxidation rates at multiple soil depths. The methodology was implemented in the tRIBS-ECO (Triangulated Irregular Network-based Real-time Integrated Basin Simulator-Erosion and Carbon Oxidation), a spatially and depth-explicit model of SOC dynamics built within an existing coupled physically based hydro-geomorphic model. According to observations from multiple soil profiles, about 32% of the original SOC content has been eroded in the study area. The results indicate that C erosion and its replacement exhibit significant topographic variation at relatively small scales (tens of meters). The episodic representation of SOC erosion reproduces the history of SOC erosion better than models that use an assumption of constant erosion in space and time. The net atmospheric C exchange at the study site is estimated to range from a maximum source of 14.5 g m−2 yr−1 to a maximum sink of −18.2 g m−2 yr−1. The small-scale complexity of C erosion and burial driven by topography exerts a strong control on the landscapes capacity to serve as a C source or a sink.

CARBON CONTROLS ON NITROUS OXIDE PRODUCTION WITH CHANGES IN SUBSTRATE AVAILABILITY IN A NORTH AMERICAN GRASSLAND

Fluxes of nitrous oxide (N2O) are governed by the availability of substrate nitrogen (N), soil moisture, and soil organic carbon (SOC) concentration. Grassland management techniques such as fertilization and haying can influence both SOC and soil N transformations. In mesic grasslands, where SOC can be high compared with drier regions, it is unclear to what extent heterotrophic denitrifiers will respond to such management practices. Biomass removal via haying can reduce SOC, whereas fertilization decreases C:N ratios of plant residues, which can decrease or increase heterotrophic microbial activity, respectively. We experimentally manipulated haying and fertilization regimens on a grassland in northeastern Kansas and measured resulting inorganic N availability, field fluxes of N2O and potential denitrification, and litter C:N ratios to assess how these management practices may influence soil N2O evolution. We observed N2O fluxes that were an order of magnitude larger than those reported in drier grassland systems with lower SOC. The largest N2O fluxes observed were in fertilized and fertilized/hayed plots, immediately after precipitation events. Haying periodically mitigated N2O fluxes. Denitrification enzyme activity was lower in hayed plots than unhayed plots, and greater with glucose-C additions, indicating that the C substrate in these soils is an important driver of denitrification. The N2O fluxes that represented at least 0.1% of available inorganic N exhibited a weak negative relationship with C:N ratios of litter in fertilized/hayed plots, suggesting that the lower C:N ratios associated the shift to C3 plants with fertilization may promote soil N2O production, in addition to SOC and inorganic N availability.