Kristina J. Anderson-Teixeira

Changes in soil organic carbon under biofuel crops

One potentially significant impact of growing biofuel crops will be the sequestration or release of carbon (C) in soil. Soil organic carbon (SOC) represents an important C sink in the lifecycle C balances of biofuels and strongly influences soil quality. We assembled and analyzed published estimates of SOC change following conversion of natural or agricultural land to biofuel crops of corn with residue harvest, sugarcane, Miscanthus x giganteus, switchgrass, or restored prairie. We estimated SOC losses associated with land conversion and rates of change in SOC over time by regressing net change in SOC relative to a control against age since establishment year. Conversion of uncultivated land to biofuel agriculture resulted in significant SOC losses – an effect that was most pronounced when native land was converted to sugarcane agriculture. Corn residue harvest (at 25–100% removal) consistently resulted in SOC losses averaging 3–8 Mg ha−1 in the top 30 cm, whereas SOC accumulated under all four perennial grasses, with SOC accumulation rates averaging <1 Mg ha−1 yr−1 in the top 30 cm. More intensive harvests led to decreased C gains or increased C losses – an effect that was particularly clear for residue harvest in corn. Direct or indirect conversion of previously uncultivated land for biofuel agriculture will result in SOC losses that counteract the benefits of fossil fuel displacement. Additionally, SOC losses under corn residue harvest imply that its potential to offset C emissions may be overestimated, whereas SOC sequestration under perennial grasses represents an additional benefit that has rarely been accounted for in life cycle analyses of biofuels.

Life-cycle analysis and the ecology of biofuels.

Biofuels have been proposed as an ecologically benign alternative to fossil fuels. There is, however, considerable uncertainty in the scientific literature about their ecological benefit. Here, we review studies that apply life-cycle analysis (LCA), a computational tool for assessing the efficiency and greenhouse gas (GHG) impact of energy systems, to biofuel feedstocks. Published values for energy efficiency and GHG differ significantly even for an individual species, and we identify three major sources of variation in these LCA results. By providing new information on biogeochemistry and plant physiology, ecologists and plant scientists can increase the accuracy of LCA for biofuel production systems.

Larger trees suffer most during drought in forests worldwide

The frequency of severe droughts is increasing in many regions around the world as a result of climate change1–3. Droughts alter the structure and function of forests4,5. Site- and region-specific studies suggest that large trees, which play keystone roles in forests6 and can be disproportionately important to ecosystem carbon storage7 and hydrology8, exhibit greater sensitivity to drought than small trees4,5,9,10. Here, we synthesize data on tree growth and mortality collected during 40 drought events in forests worldwide to see whether this size-dependent sensitivity to drought holds more widely. We find that droughts consistently had a more detrimental impact on the growth and mortality rates of larger trees. Moreover, drought-related mortality increased with tree size in 65% of the droughts examined, especially when community-wide mortality was high or when bark beetles were present. The more pronounced drought sensitivity of larger trees could be underpinned by greater inherent vulnerability to hydraulic stress11–14, the higher radiation and evaporative demand experienced by exposed crowns4,15, and the tendency for bark beetles to preferentially attack larger trees16. We suggest that future droughts will have a more detrimental impact on the growth and mortality of larger trees, potentially exacerbating feedbacks to climate change.

Reduced Nitrogen Losses after Conversion of Row Crop Agriculture to Perennial Biofuel Crops

Current biofuel feedstock crops such as corn lead to large environmental losses of N through nitrate leaching and NO emissions; second-generation cellulosic crops have the potential to reduce these N losses. We measured N losses and cycling in establishing miscanthus (), switchgrass ( L. fertilized with 56 kg N ha yr), and mixed prairie, along with a corn ( L.)-corn-soybean [ (L.) Merr.] rotation (corn fertilized at 168-202 kg N ha). Nitrous oxide emissions, soil N mineralization, mid-profile nitrate leaching, and tile flow and nitrate concentrations were measured. Perennial crops quickly reduced nitrate leaching at a 50-cm soil depth as well as concentrations and loads from the tile systems (year 1 tile nitrate concentrations of 10-15 mg N L declined significantly by year 4 in all perennial crops to <0.6 mg N L, with losses of <0.8 kg N ha yr). Nitrous oxide emissions were 2.2 to 7.7 kg N ha yr in the corn-corn-soybean rotation but were <1.0 kg N ha yr by year 4 in the perennial crops. Overall N balances (atmospheric deposition + fertilization + soybean N fixation - harvest, leaching losses, and NO emissions) were positive for corn and soybean (22 kg N ha yr) as well as switchgrass (9.7 kg N ha yr) but were -18 and -29 kg N ha yr for prairie and miscanthus, respectively. Our results demonstrate rapid tightening of the N cycle as perennial biofuel crops established on a rich Mollisol soil.

Altered dynamics of forest recovery under a changing climate

Forest regeneration following disturbance is a key ecological process, influencing forest structure and function, species assemblages, and ecosystem-climate interactions. Climate change may alter forest recovery dynamics or even prevent recovery, triggering feedbacks to the climate system, altering regional biodiversity, and affecting the ecosystem services provided by forests. Multiple lines of evidence - including global-scale patterns in forest recovery dynamics; forest responses to experimental manipulation of CO2 , temperature, and precipitation; forest responses to the climate change that has already occurred; ecological theory; and ecosystem and earth system models - all indicate that the dynamics of forest recovery are sensitive to climate. However, synthetic understanding of how atmospheric CO2 and climate shape trajectories of forest recovery is lacking. Here, we review these separate lines of evidence, which together demonstrate that the dynamics of forest recovery are being impacted by increasing atmospheric CO2 and changing climate. Rates of forest recovery generally increase with CO2 , temperature, and water availability. Drought reduces growth and live biomass in forests of all ages, having a particularly strong effect on seedling recruitment and survival. Responses of individual trees and whole-forest ecosystems to CO2 and climate manipulations often vary by age, implying that forests of different ages will respond differently to climate change. Furthermore, species within a community typically exhibit differential responses to CO2 and climate, and altered community dynamics can have important consequences for ecosystem function. Age- and species-dependent responses provide a mechanism by which climate change may push some forests past critical thresholds such that they fail to recover to their previous state following disturbance. Altered dynamics of forest recovery will result in positive and negative feedbacks to climate change. Future research on this topic and corresponding improvements to earth system models will be a key to understanding the future of forests and their feedbacks to the climate system.

Altered Belowground Carbon Cycling Following Land-Use Change to Perennial Bioenergy Crops

Belowground carbon (C) dynamics of terrestrial ecosystems play an important role in the global C cycle and thereby in climate regulation. Globally, land-use change is a major driver of changes in belowground C storage. The emerging bioenergy industry is likely to drive widespread land-use changes, including the replacement of annually tilled croplands with perennial bioenergy crops, and thereby to impact the climate system through alteration of belowground C dynamics. Mechanistic understanding of how land-use changes impact belowground C storage requires elucidation of changes in belowground C flows; however, altered belowground C dynamics following land-use change have yet to be thoroughly quantified through field measurements. Here, we show that belowground C cycling pathways of establishing perennial bioenergy crops (0- to 3.5-year-old miscanthus, switchgrass, and a native prairie mix) were substantially altered relative to row crop agriculture (corn-soy rotation); specifically, there were substantial increases in belowground C allocation (>400%), belowground biomass (400–750%), root-associated respiration (up to 2,500%), moderate reductions in litter inputs (20–40%), and respiration in root-free soil (up to 50%). This more active root-associated C cycling of perennial vegetation provides a mechanism for observed net C sequestration by these perennial ecosystems, as well as commonly observed increases in soil C under perennial bioenergy crops throughout the world. The more active root-associated belowground C cycle of perennial vegetation implies a climate benefit of grassland maintenance or restoration, even if biomass is harvested annually for bioenergy production.

Ethanol from sugarcane in Brazil: a ‘midway’ strategy for increasing ethanol production while maximizing environmental benefits

This article reviews the history and current state of ethanol production from sugarcane in Brazil and presents a strategy for improving ecosystem services and production. We propose that it is possible to produce ethanol from sugarcane while maintaining or even recovering some of Brazils unique neotropical biodiversity and ecosystem climate services. This approach to the future of sustainable and responsible ethanol production is termed the ‘midway’ strategy. The ‘midway’ strategy involves producing the necessary biotechnology to increase productivity while synergistically protecting and regenerating rainforest. Three main areas of scientific and technological advance that are key to realizing the ‘midway’ strategy are: (i) improving the quality of scientific data on sugarcane biology as pertains to its use as a bioenergy crop; (ii) developing technologies for the use of bagasse for cellulosic ethanol; and (iii) developing policies to improve the ecosystem services associated with sugarcane landscapes. This article discusses these three issues in the general context of biofuels production and highlights examples of scientific achievements that are already leading towards the ‘midway’ strategy.

Amplified temperature dependence in ecosystems developing on the lava flows of Mauna Loa, Hawai'i

Through its effect on individual metabolism, temperature drives biologically controlled fluxes and transformations of energy and materials in ecological systems. Because primary succession involves feedbacks among multiple biological and abiotic processes, we expected it to exhibit complex dynamics and unusual temperature dependence. We present a model based on first principles of chemical kinetics to explain how biologically mediated temperature dependence of “reactant” concentrations can inflate the effective temperature dependence of such processes. We then apply this model to test the hypothesis that the temperature dependence of early primary succession is amplified due to more rapid accumulation of reactants at higher temperatures. Using previously published data from the lava flows of Mauna Loa, HI, we show that rates of vegetation and soil accumulation as well as rates of community compositional change all display amplified temperature dependence (Q10 values of ≈7–50, compared with typical Q10 values of 1.5–3 for the constituent biological processes). Additionally, in young ecosystems, resource concentrations increase with temperature, resulting in inflated temperature responses of biogeochemical fluxes. Mauna Loas developing ecosystems exemplify how temperature-driven, biologically mediated gradients in resource availability can alter the effective temperature dependence of ecological processes. This mechanistic theory should contribute to understanding the complex effects of temperature on the structure and dynamics of ecological systems in a world where regional and global temperatures are changing rapidly.

Carbon dynamics of mature and regrowth tropical forests derived from a pantropical database (TropForC-db)

Tropical forests play a critical role in the global carbon (C) cycle, storing ~45% of terrestrial C and constituting the largest component of the terrestrial C sink. Despite their central importance to the global C cycle, their ecosystem-level C cycles are not as well-characterized as those of extra-tropical forests, and knowledge gaps hamper efforts to quantify C budgets across the tropics and to model tropical forest-climate interactions. To advance understanding of C dynamics of pantropical forests, we compiled a new database, the Tropical Forest C database (TropForC-db), which contains data on ground-based measurements of ecosystem-level C stocks and annual fluxes along with disturbance history. This database currently contains 3568 records from 845 plots in 178 geographically distinct areas, making it the largest and most comprehensive database of its type. Using TropForC-db, we characterized C stocks and fluxes for young, intermediate-aged, and mature forests. Relative to existing C budgets of extra-tropical forests, mature tropical broadleaf evergreen forests had substantially higher gross primary productivity (GPP) and ecosystem respiration (Reco), their autotropic respiration (Ra) consumed a larger proportion (~67%) of GPP, and their woody stem growth (ANPPstem) represented a smaller proportion of net primary productivity (NPP, ~32%) or GPP (~9%). In regrowth stands, aboveground biomass increased rapidly during the first 20 years following stand-clearing disturbance, with slower accumulation following agriculture and in deciduous forests, and continued to accumulate at a slower pace in forests aged 20-100 years. Most other C stocks likewise increased with stand age, while potential to describe age trends in C fluxes was generally data-limited. We expect that TropForC-db will prove useful for model evaluation and for quantifying the contribution of forests to the global C cycle. The database version associated with this publication is archived in Dryad (DOI: 10.5061/dryad.t516f) and a dynamic version is maintained at https://github.com/forc-db.

Predicting Greenhouse Gas Emissions and Soil Carbon from Changing Pasture to an Energy Crop

Bioenergy related land use change would likely alter biogeochemical cycles and global greenhouse gas budgets. Energy cane (Saccharum officinarum L.) is a sugarcane variety and an emerging biofuel feedstock for cellulosic bio-ethanol production. It has potential for high yields and can be grown on marginal land, which minimizes competition with grain and vegetable production. The DayCent biogeochemical model was parameterized to infer potential yields of energy cane and how changing land from grazed pasture to energy cane would affect greenhouse gas (CO2, CH4 and N2O) fluxes and soil C pools. The model was used to simulate energy cane production on two soil types in central Florida, nutrient poor Spodosols and organic Histosols. Energy cane was productive on both soil types (yielding 46–76 Mg dry mass⋅ha−1). Yields were maintained through three annual cropping cycles on Histosols but declined with each harvest on Spodosols. Overall, converting pasture to energy cane created a sink for GHGs on Spodosols and reduced the size of the GHG source on Histosols. This change was driven on both soil types by eliminating CH4 emissions from cattle and by the large increase in C uptake by greater biomass production in energy cane relative to pasture. However, the change from pasture to energy cane caused Histosols to lose 4493 g CO2 eq⋅m−2 over 15 years of energy cane production. Cultivation of energy cane on former pasture on Spodosol soils in the southeast US has the potential for high biomass yield and the mitigation of GHG emissions.