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.

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.

Thermal acclimation of leaf respiration but not photosynthesis in Populus deltoides x nigra.

Dark respiration and photosynthesis were measured in leaves of poplar Populus deltoides x nigra ('Veronese') saplings to investigate the extent of respiratory and photosynthetic acclimation in pre-existing and newly emerged leaves to abrupt changes in air temperature. The saplings were grown at three temperature regimes and at high and low nitrogen availabilities. Rates of photosynthesis and dark respiration (R(d)) were measured at the initial temperature and the saplings were then transferred to a different temperature regime, where the plants remained for a second and third round of measurements on pre-existing and newly emerged leaves. Acclimation of photosynthesis was limited following transfer to warmer or cooler growing conditions. There was strong evidence of cold and warm acclimation of R(d) to growth temperature, but this was limited in pre-existing leaves. Full acclimation of R(d )was restricted to newly emerged leaves grown at the new growth temperature. These findings indicate that the extent of thermal acclimation differs significantly between photosynthesis and respiration. Importantly, pre-existing leaves in poplar were capable of some respiratory acclimation, but full acclimation was observed only in newly emerged leaves. The R(d)/A(max) ratio declined at higher growth temperatures, and nitrogen status of leaves had little impact on the degree of acclimation.

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.

Spatial and temporal scaling of intercellular CO2 concentration in a temperate rain forest dominated by Dacrydium cupressinum in New Zealand.

Seven methods, including measurements of photosynthesis (A) and stomatal conductance (g(s)), carbon isotope discrimination, ecosystem CO2 and water vapour exchange using eddy covariance and the use of a multilayer canopy model and ecosystem Keeling plots, were employed to derive estimates of intercellular CO2 concentration (Ci) across a range of spatial and temporal scales in a low productivity rain forest ecosystem dominated by the conifer Dacrydium cupressinum Lamb. in New Zealand. Estimates of shoot and canopy Ci across temporal scales ranging from minutes to years were remarkably similar (range of 274-294 micromol mol(-1)). The gradual increase in shoot Ci with depth in the canopy was more likely attributable to decreases in A resulting from lower irradiance (Q) than to increases in g, due to changes in air saturation deficit (D). The lack of marked vertical gradients in A and g(s) at saturating Q through the canopy and the low seasonal variability in environmental conditions contributed to the efficacy of scaling Ci. However, the canopy Ci estimate calculated from the carbon isotope composition of respired ecosystem CO2 (delta13CR; 236 micromol mol(-1)) was much lower than other estimates of canopy Ci. Partitioning delta13CR into four components (soil, roots, litter and foliage) indicated root respiration as the dominant (> 50%) contributor to delta13CR. Variable time lags and differences in isotopic composition during photosynthesis and respiration make the direct estimation of canopy Ci from delta 13CR problematic.

Characteristics of photosynthesis and stomatal conductance in the shrubland species manuka (Leptospermum scoparium) and kanuka (Kunzea ericoides) for the estimation of annual canopy carbon uptake.

Responses of photosynthesis to carbon dioxide (CO2) partial pressure and irradiance were measured on leaves of 39-year-old trees of manuka (Leptospermum scoparium J. R. Forst. & G. Forst.) and kanuka (Kunzea ericoides var. ericoides (A. Rich.) J. Thompson) at a field site, and on leaves of young trees grown at three nitrogen supply rates in a nursery, to determine values for parameters in a model to estimate annual net carbon uptake. These secondary successional species belong to the same family and commonly co-occur. Mean (+/- standard error) values of the maximum rate of carboxylation (hemi-surface area basis) (Vcmax) and the maximum rate of electron transport (Jmax) at the field site were 47.3 +/- 1.9 micromol m(-2) s(-1) and 94.2 +/- 3.7 micromol m(-2) s(-1), respectively, with no significant differences between species. Both Vcmax and Jmax were positively related to leaf nitrogen concentration on a unit leaf area basis, and the slopes of these relationships did not differ significantly between species or between the trees in the field and young trees grown in the nursery. Mean values of Jmax/Vcmax measured at 20 degrees C were significantly lower (P < 0.01) for trees in the field (2.00 +/- 0.05) than for young trees in the nursery with similar leaf nitrogen concentrations (2.32 +/- 0.08). Stomatal conductance decreased sharply with increasing air saturation deficit, but the sensitivity of the response did not differ between species. These data were used to derive parameters for a coupled photosynthesis-stomatal conductance model to scale estimates of photosynthesis from leaves to the canopy, incorporating leaf respiration at night, site energy and water balances, to estimate net canopy carbon uptake. Over the course of a year, 76% of incident irradiance (400-700 nm) was absorbed by the canopy, annual net photosynthesis per unit ground area was 164.5 mol m(-2) (equivalent to 1.97 kg C m(-2)) and respiration loss from leaves at night was 37.5 mol m(-2) (equivalent to 0.45 kg m(-2)), or 23% of net carbon uptake. When modeled annual net carbon uptake for the trees was combined with annual respiration from the soil surface, estimated net primary productivity for the ecosystem (0.30 kg C m(-2)) was reasonably close to the annual estimate obtained from independent mensurational and biomass measurements made at the site (0.22 +/- 0.03 kg C m(-2)). The mean annual value for light-use efficiency calculated from the ratio of net carbon uptake (net photosynthesis minus respiration of leaves at night) and absorbed irradiance was 13.0 mmol C mol(-1) (equivalent to 0.72 kg C GJ(-1)). This is low compared with values reported for other temperate forests, but is consistent with limitations to photosynthesis in the canopy attributable mainly to low nitrogen availability and associated low leaf area index.

Analysis of the growth of rimu (Dacrydium cupressinum) in South Westland, New Zealand, using process-based simulation models.

Two process-based models were used to identify the environmental variables limiting productivity in a pristine, mature forest dominated by rimu (Dacrydium cupressinum Sol. ex Lamb.) trees in South Westland, New Zealand. A model of canopy net carbon uptake, incorporating routines for radiation interception, photosynthesis and water balance was used to determine a value for quantum efficiency when climate variables were not limiting. The annual net carbon uptake by the canopy was estimated to be 1.1 kg C m(-2) and the quantum efficiency 22.6 mmol mol quanta(-1). This value of quantum efficiency, combined with other parameters obtainable from the literature, was then used in a model of forest productivity (3-PG), to simulate changes in net productivity and the allocation of carbon to tree components. The model was adjusted to match a measured stem increment of 10.6 Mg ha(-1) over a period of 13 years. To achieve this while maintaining a low, but stable value for leaf area index, it was necessary to set the site fertility rating very low and select high values for the parameters describing the proportional allocation of total carbon to roots. This approach highlighted nutrient availability as the principal constraint on productivity for the ecosystem and identified critical measurements that will be necessary for using the model to predict the effects of climate change on carbon sequestration. The low rates of carbon uptake and productivity are consistent with the low nutrient supply available from the highly leached, acid soils, most likely attributable to frequent saturation and a very shallow aerobic zone.

Effects of branch length on carbon isotope discrimination in Pinus radiata.

Gas exchange was measured on a pruned Pinus radiata D. Don hedge and on a long-branch P. radiata tree near Hamilton, New Zealand, in spring 1993 when soil water content was close to field capacity. Foliage at the end of long branches (9.0 m) showed a marked drop in net photosynthetic rate and stomatal conductance as the saturation deficit increased, whereas foliage on short branches (0.5 m) showed little change. Mean foliage delta(13)C was -30.1 per thousand for short branches and -26.3 per thousand for long branches. Foliage delta(13)C was correlated with branch length in two genetically improved P. radiata seedlots at four stocking densities. The multinodal seedlot had shorter branches and more (13)C-depleted foliage compared with branches and foliage from the long internode seedlot. There was a strong effect of stocking density on carbon isotope composition in both seedlots. We conclude that branch morphology affects foliage gas exchange properties and foliage carbon isotope composition.