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.Humified soil organic matter storage in boreal forests is large, and its responses to warming over relatively long timescales is critical for predicting soil feedbacks to climate change. To derive information relevant across decades to centuries from manipulative short-term experiments, we conducted incubations of soils from two forested sites along the Newfoundland-Labrador Boreal Ecosystem Latitude Transect in eastern Canada and assessed linkages between incubation data and these sites' profile characteristics. The sites differ in mean annual temperature by 3.4°C, but vegetation and soil types are similar. Organic soils (Oe + Oa) were incubated for 120 days at 15°C and 20°C, with and without a replaced Oi subhorizon possessing a distinct δ13C signature. Laboratory warming induced significantly greater mineralization and leaching of humified SOM relative to replaced Oi, congruent with greater warming-induced increases in phenol oxidase activity relative to enzymes associated with labile C acquisition (percent increases of 101% versus 50%, respectively). These data suggest that warming can influence microbial communities and their enzymatic dynamics such that relative losses of humified SOM are disproportionately enhanced. This is consistent with stable isotopic, C:N, and radiocarbon profile differences between the two sites, which suggest a greater degree of microbial processing and greater relative losses of older SOC over the preceding decades at the warmer site, given our knowledge of organic inputs in these soils. This study is a first step toward linking the divergent timescales represented by soil profiles and laboratory manipulations, an important goal for biogeochemists assessing climate change impacts on SOM dynamics

Tolerance of a standard intact protein formula versus a partially hydrolyzed formula in healthy, term infants

<p>Abstract</p> <p>Background</p> <p>Parents who perceive common infant behaviors as formula intolerance-related often switch formulas without consulting a health professional. Up to one-half of formula-fed infants experience a formula change during the first six months of life.</p> <p>Methods</p> <p>The objective of this study was to assess discontinuance due to study physician-assessed formula intolerance in healthy, term infants. Infants (335) were randomized to receive either a standard intact cow milk protein formula (INTACT) or a partially hydrolyzed cow milk protein formula (PH) in a 60 day non-inferiority trial. Discontinuance due to study physician-assessed formula intolerance was the primary outcome. Secondary outcomes included number of infants who discontinued for any reason, including parent-assessed.</p> <p>Results</p> <p>Formula intolerance between groups (INTACT, 12.3% vs. PH, 13.7%) was similar for infants who completed the study or discontinued due to study physician-assessed formula intolerance. Overall study discontinuance based on parent- vs. study physician-assessed intolerance for all infants (14.4 vs.11.1%) was significantly different (P = 0.001).</p> <p>Conclusion</p> <p>This study demonstrated no difference in infant tolerance of intact vs. partially hydrolyzed cow milk protein formulas for healthy, term infants over a 60-day feeding trial, suggesting nonstandard partially hydrolyzed formulas are not necessary as a first-choice for healthy infants. Parents frequently perceived infant behavior as formula intolerance, paralleling previous reports of unnecessary formula changes.</p> <p>Trial Registration</p> <p>clinicaltrials.gov: NCT00666120</p

Short- and long-term temperature responses of soil denitrifier net N2O efflux rates, inter-profile N2O dynamics, and microbial genetic potentials

Production and reduction of nitrous oxide (N2O) by soil denitrifiers influence atmospheric concentrations of this potent greenhouse gas. Accurate projections of the net N2O flux have three key uncertainties: (1) short- vs. long-term responses to warming, (2) interactions among soil horizons, and (3) temperature responses of different steps in the denitrification pathway. We addressed these uncertainties by sampling soil from a boreal forest climate transect encompassing a 5.2 ∘C difference in the mean annual temperature and incubating the soil horizons in isolation and together at three ecologically relevant temperatures in conditions that promote denitrification. Both short-term exposure to warmer temperatures and long-term exposure to a warmer climate increased N2O emissions from organic and mineral soils; an isotopic tracer suggested that an increase in N2O production was more important than a decline in N2O reduction. Short-term warming promoted the reduction of organic horizon-derived N2O by mineral soil when these horizons were incubated together. The abundance of nirS (a precursor gene for N2O production) was not sensitive to temperature, whereas that of nosZ clade I (a gene for N2O reduction) decreased with short-term warming in both horizons and was higher from a warmer climate. These results suggest a decoupling of gene abundance and process rates in these soils that differs across horizons and timescales. In spite of these variations, our results suggest a consistent, positive response of denitrifier-mediated net N2O efflux rates to temperature across timescales in these boreal forests. Our work also highlights the importance of understanding cross-horizon N2O fluxes for developing a predictive understanding of net N2O efflux from soils

Short- and long-term temperature responses of soil denitrifier net N2O efflux rates, inter2 profile N2O dynamics, and microbial genetic potentials

Production and reduction of nitrous oxide (N2O) by soil denitrifiers influence atmospheric concentrations of this potent greenhouse gas. Accurate projections of the net N2O flux have three key uncertainties: (1) short- vs. long-term responses to warming, (2) interactions among soil horizons, and (3) temperature responses of different steps in the denitrification pathway. We addressed these uncertainties by sampling soil from a boreal forest climate transect encompassing a 5.2 ∘C difference in the mean annual temperature and incubating the soil horizons in isolation and together at three ecologically relevant temperatures in conditions that promote denitrification. Both short-term exposure to warmer temperatures and long-term exposure to a warmer climate increased N2O emissions from organic and mineral soils; an isotopic tracer suggested that an increase in N2O production was more important than a decline in N2O reduction. Short-term warming promoted the reduction of organic horizon-derived N2O by mineral soil when these horizons were incubated together. The abundance of nirS (a precursor gene for N2O production) was not sensitive to temperature, whereas that of nosZ clade I (a gene for N2O reduction) decreased with short-term warming in both horizons and was higher from a warmer climate. These results suggest a decoupling of gene abundance and process rates in these soils that differs across horizons and timescales. In spite of these variations, our results suggest a consistent, positive response of denitrifier-mediated net N2O efflux rates to temperature across timescales in these boreal forests. Our work also highlights the importance of understanding cross-horizon N2O fluxes for developing a predictive understanding of net N2O efflux from soils

Is the Earth Crying Wolf? Exploring Knowledge Source and Certainty in High School Students\u27 Analysis of Global Warming News

The marked contrast between the scientific consensus on global warming and public beliefs indicates a need to research how high schoolers, as future citizens, engage with and make meaning from news articles on such topics. In the case of socioscientific issues (SSIs) such as global warming, students’ acquisition of knowledge from the news is mediated by their epistemic understandings of the nature of science (NOS) and use of informal reasoning in evaluating claims, evidence, and sources. This exploratory qualitative study examined twelve U.S. high school students’ understandings, opinions, and epistemic beliefs concerning global warming knowledge. Researchers examined microgenetic changes as students discussed global warming during semi-structured interviews and a close reading of global warming news texts. Although results showed that most students could articulate a working concept of global warming, in follow-up questions, a subset offered personal opinions that differed from or contradicted their previously stated understandings. Meanwhile, students who offered opinions consistent with the scientific consensus often argued that the dangers of global warming were exaggerated by politicians and scientists who wished to profit from the issue. This study suggests a need for more explicit focus on NOS and scientific news literacy in curricula, as well as further research into the interplay between epistemic beliefs and the informal reasoning students use to negotiate diverse sources of SSI knowledge—from the classroom to the news media and public life

Distinct Contributions of Eroding and Depositional Profiles to Land-Atmosphere CO2 Exchange in Two Contrasting Forests

This work is licensed under a Creative Commons Attribution 4.0 International License.Lateral movements of soil organic C (SOC) influence Earth's C budgets by transporting organic C across landscapes and by modifying soil-profile fluxes of CO2. We extended a previously presented model (Soil Organic C Erosion Replacement and Oxidation, SOrCERO) and present SOrCERODe, a model with which we can project how erosion and subsequent deposition of eroded material can modify biosphere-atmosphere CO2 fluxes in watersheds. The model permits the user to quantify the degree to which eroding and depositional profiles experience a change in SOC oxidation and production as formerly deep horizons become increasingly shallow, and as depositional profiles are buried. To investigate the relative importance of erosion rate, evolving SOC depth distributions, and mineralization reactivity on modeled soil C fluxes, we examine two forests exhibiting distinct depth distributions of SOC content and reactivity, hydrologic regimes and land use. Model projections suggest that, at decadal to centennial timescales: (1) the quantity of SOC moving across a landscape depends on erosion rate and the degree to which SOC production and oxidation at the eroding profile are modified as deeper horizons become shallower, and determines the degree to which depositional profile SOC fluxes are modified; (2) erosional setting C sink strength increases with erosion rate, with some sink effects reaching more than 40% of original profile SOC content after 100 y of a relatively high erosion rate (i.e., 1 mm y−1); (3) even large amounts of deposited SOC may not promote a large depositional profile C sink even with large gains in autochthonous SOC post-deposition if oxidation of buried SOC is not limited; and (4) when modeled depositional settings receive a disproportionately large amount of SOC, simulations of strong C sink scenarios mimic observations of modest preservation of buried SOC and large SOC gains in surficial horizons, suggesting that C sink scenarios have merit in these forests. Our analyses illuminate the importance of cross-landscape linkages between upland and depositional environments for watershed-scale biosphere-atmosphere C fluxes, and emphasize the need for accurate representations and observations of time-varying depth distributions of SOC reactivity across evolving watersheds if we seek accurate projections of ecosystem C balances

Climate Warming Can Accelerate Carbon Fluxes without Changing Soil Carbon Stocks

Climate warming enhances multiple ecosystem C fluxes, but the net impact of changing C fluxes on soil organic carbon (SOC) stocks over decadal to centennial time scales remains unclear. We investigated the effects of climate on C fluxes and soil C stocks using space-for-time substitution along a boreal forest climate gradient encompassing spatially replicated sites at each of three latitudes. All regions had similar SOC concentrations and stocks (5.6 to 6.7 kg C m−2). The three lowest latitude forests exhibited the highest productivity across the transect, with tree biomass:age ratios and litterfall rates 300 and 125% higher than those in the highest latitude forests, respectively. Likewise, higher soil respiration rates (~55%) and dissolved organic C fluxes (~300%) were observed in the lowest latitude forests compared to those in the highest latitude forests. The mid-latitude forests exhibited intermediate values for these indices and fluxes. The mean radiocarbon content (Δ14C) of mineral-associated SOC (+9.6‰) was highest in the lowest latitude forests, indicating a more rapid turnover of soil C compared to the mid- and highest latitude soils (Δ14C of −35 and −30‰, respectively). Indicators of the extent of soil organic matter decomposition, including C:N, δ13C, and amino acid and alkyl-C:O-alkyl-C indices, revealed highly decomposed material across all regions. These data indicate that the lowest latitude forests experience accelerated C fluxes that maintain relatively young but highly decomposed SOC. Collectively, these observations of within-biome soil C responses to climate demonstrate that the enhanced rates of SOC loss that typically occur with warming can be balanced on decadal to centennial time scales by enhanced rates of C inputs

Distinct Contributions of Eroding and Depositional Profiles to Land-Atmosphere CO2 Exchange in Two Contrasting Forests

Lateral movements of soil organic C (SOC) influence Earth's C budgets by transporting organic C across landscapes and by modifying soil-profile fluxes of CO2. We extended a previously presented model (Soil Organic C Erosion Replacement and Oxidation, SOrCERO) and present SOrCERODe, a model with which we can project how erosion and subsequent deposition of eroded material can modify biosphere-atmosphere CO2 fluxes in watersheds. The model permits the user to quantify the degree to which eroding and depositional profiles experience a change in SOC oxidation and production as formerly deep horizons become increasingly shallow, and as depositional profiles are buried. To investigate the relative importance of erosion rate, evolving SOC depth distributions, and mineralization reactivity on modeled soil C fluxes, we examine two forests exhibiting distinct depth distributions of SOC content and reactivity, hydrologic regimes and land use. Model projections suggest that, at decadal to centennial timescales: (1) the quantity of SOC moving across a landscape depends on erosion rate and the degree to which SOC production and oxidation at the eroding profile are modified as deeper horizons become shallower, and determines the degree to which depositional profile SOC fluxes are modified; (2) erosional setting C sink strength increases with erosion rate, with some sink effects reaching more than 40% of original profile SOC content after 100 y of a relatively high erosion rate (i.e., 1 mm y−1); (3) even large amounts of deposited SOC may not promote a large depositional profile C sink even with large gains in autochthonous SOC post-deposition if oxidation of buried SOC is not limited; and (4) when modeled depositional settings receive a disproportionately large amount of SOC, simulations of strong C sink scenarios mimic observations of modest preservation of buried SOC and large SOC gains in surficial horizons, suggesting that C sink scenarios have merit in these forests. Our analyses illuminate the importance of cross-landscape linkages between upland and depositional environments for watershed-scale biosphere-atmosphere C fluxes, and emphasize the need for accurate representations and observations of time-varying depth distributions of SOC reactivity across evolving watersheds if we seek accurate projections of ecosystem C balances