Adrian S. Walcroft

Thermal acclimation of respiration but not photosynthesis in Pinus radiata

Pinus radiata L. were grown in climate-controlled cabinets under three night/day temperature treatments, and transferred between treatments to mimic changes in growth temperature. The objective was to determine the extent to which dark respiration and photosynthesis in pre-existing and new needles acclimate to changes in growth temperatures. We also assessed whether needle nitrogen influenced the potential for photosynthetic and respiratory acclimation, and further assessed if short-term (instantaneous, measured over a few hours) respiratory responses are accurate predictors of long-term (acclimated, achieved in days-weeks) responses of respiration to changing temperature. Results show that respiration displayed considerable potential for acclimation. Cold and warm transfers resulted in some acclimation of respiration in pre-existing needles, but full acclimation was displayed only in new needles formed at the new growth temperature. Short-term respiratory responses were poor predictors of the long-term response of respiration due to acclimation. There was no evidence that photosynthesis in pre-existing or new needles acclimated to changes in growth temperature. N status of leaves had little impact on the extent of acclimation. Collectively, our results indicate that there is little likelihood that respiration would be significantly stimulated in this species as night temperatures increase over the range of 10-20°C, but that inclusion of temperature acclimation of respiration would in fact lead to a shift in the balance between photosynthesis and respiration in favour of carbon uptake.

Understanding the Stable Isotope Composition of Biosphere‐Atmosphere CO2 Exchange

Stable isotopes of atmospheric carbon dioxide (CO2) contain a wealth of information regarding biosphere-atmosphere interactions. The carbon isotope ratio of CO2 (δ13C) reflects the terrestrial carbon cycle including processes of photosynthesis, respiration, and decomposition. The oxygen isotope ratio (δ18O) reflects terrestrial carbon and water coupling due to CO2-H2O oxygen exchange. Isotopic CO2 measurements, in combination with ecosystem-isotopic exchange models, allow for the quantification of patterns and mechanisms regulating terrestrial carbon and water cycles, as well as for hypothesis development, data interpretation, and forecasting. Isotopic measurements and models have evolved significantly over the past two decades, resulting in organizations that promote model-measurement networks, e.g., the U.S. National Science Foundations Biosphere-Atmosphere Stable Isotope Network, the European Stable Isotopes in Biosphere-Atmosphere Exchange Network, and the U.S. National Environmental Observatory Network.

In vitro methane removal by volcanic pumice soil biofilter columns over one year.

Soil methane (CH(4)) biofilters, containing CH(4)-oxidizing bacteria (methanotrophs), are a promising technology for mitigating greenhouse gas emissions. However, little is known about long-term biofilter performance. In this study, volcanic pumice topsoils (0-10 cm) and subsoils (10-50 cm) were tested for their ability to oxidize a range of CH(4) fluxes over 1 yr. The soils were sampled from an 8-yr-old and a 2-yr-old grassed landfill cover and from a nearby undisturbed pasture away from the influence of CH(4) generated by the decomposing refuse. Methane was passed through the soils in laboratory chambers with fluxes ranging from 0.5 g to 24 g CH(4) m(-3) h(-1). All topsoils efficiently oxidized CH(4). The undisturbed pasture topsoil exhibited the highest removal efficiency (24 g CH(4) m(-3) h(-1)), indicating rapid activation of the methanotroph population to the high CH(4) fluxes. The subsoils were less efficient at oxidizing CH(4) than the topsoils, achieving a maximum rate oxidation rate of 7 g CH(4) m(-3) h(-1). The topsoils exhibited higher porosities; moisture contents; surface areas; and total C, N, and available-P concentrations than the subsoils, suggesting that these characteristics strongly influence growth and activity of the CH(4)-oxidizing bacteria. Soil pH values and available-P levels gradually declined during the trial, indicating a need to monitor chemical parameters closely so that adjustments can be made when necessary. However, other key soil physicochemical parameters (moisture, total C, total N) increased over the course of the trial. This study showed that the selected topsoils were capable of continually sustaining high CH(4) removal rates over 1 yr, which is encouraging for the development of biofilters as a low-maintenance greenhouse gas mitigation technology.

CH4/CO2 ratios indicate highly efficient methane oxidation by a pumice landfill cover-soil.

Landfills that generate too little biogas for economic energy recovery can potentially offset methane (CH(4)) emissions through biological oxidation by methanotrophic bacteria in cover soils. This study reports on the CH(4) oxidation efficiency of a 10-year old landfill cap comprising a volcanic pumice soil. Surface CH(4) and CO(2) fluxes were measured using field chambers during three sampling intervals over winter and summer. Methane fluxes were temporally and spatially variable (-0.36 to 3044 mgCH(4)m(-2)h(-1)); but were at least 15 times lower than typical literature CH(4) fluxes reported for older landfills in 45 of the 46 chambers tested. Exposure of soil from this landfill cover to variable CH(4) fluxes in laboratory microcosms revealed a very strong correlation between CH(4) oxidation efficiency and CH(4)/CO(2) ratios, confirming the utility of this relationship for approximating CH(4) oxidation efficiency. CH(4)/CO(2) ratios were applied to gas concentrations from the surface flux chambers and indicated a mean CH(4) oxidation efficiency of 72%. To examine CH(4) oxidation with soil depth, we collected 10 soil depth profiles at random locations across the landfill. Seven profiles exhibited CH(4) removal rates of 70-100% at depths <60 cm, supporting the high oxidation rates observed in the chambers. Based on a conservative 70% CH(4) oxidation efficiency occurring at the site, this cover soil is clearly offsetting far greater CH(4) quantities than the 10% default value currently adopted by the IPCC.