Roger M. Gifford

The effects of elevated [CO2] on the C:N and C:P mass ratios of plant tissues

The influence of elevated CO2 concentration ([CO2]) during plant growth on the carbon:nutrient ratios of tissues depends in part on the time and space scales considered. Most evidence relates to individual plants examined over weeks to just a few years. The C:N ratio of live tissues is found to increase, decrease or remain the same under elevated [CO2]. On average it increases by about 15% under a doubled [CO2]. A testable hypothesis is proposed to explain why it increases in some situations and decreases in others. It includes the notion that only in the intermediate range of N-availability will C:N of live tissues increase under elevated [CO2]. Five hypotheses to explain the mechanism of such increase in C:N are discussed; none of these options explains all the published results. Where elevated [CO2] did increase the C:N of green leaves, that response was not necessarily expressed as a higher C:N of senesced leaves. An hypothesis is explored to explain the observed range in the degree of propogation of a CO2 effect on live tissues through to the litter derived from them. Data on C:P ratios under elevated [CO2] are sparse and also variable. They do not yet suggest a generalising-hypothesis of responses. Although, unlike for C:N, there is no theoretical expectation that C:P of plants would increase under elevated [CO2], the average trend in the data is of such an increase. The processes determining the C:P response to elevated [CO2] seem to be largely independent of those for C:N. Research to advance the topic should be structured to examine the components of the hypotheses to explain effects on C:N. This involves experiments in which plants are grown over the full range of N and of P availability from extreme limitation to beyond saturation. Measurements need to: distinguish structural from non-structural dry matter; organic from inorganic forms of the nutrient in the tissues; involve all parts of the plant to evaluate nutrient and C allocation changes with treatments; determine resorption factors during tissue senescence; and be made with cognisance of the temporal and spatial aspects of the phenomena involved.

A comment on the quantitative significance of aerobic methane release by plants

A recent study by Keppler et al. (2006; Nature 439, 187-191) demonstrated CH4 emission from living and dead plant tissues under aerobic conditions. This work included some calculations to extrapolate the findings from the laboratory to the global scale and led various commentators to question the value of planting trees as a greenhouse mitigation option. The experimental work of Keppler et al. (2006) appears to be largely sound, although some concerns remain about the quantification of emission rates. However, whilst accepting their basic findings, we are critical of the method used for extrapolating results to a global scale. Using the same basic information, we present alternative calculations to estimate global aerobic plant CH4 emissions as 10-60 Mt CH4 year-1. This estimate is much smaller than the 62-236 Mt CH4 year-1 reported in the original study and can be more readily reconciled within the uncertainties in the established sources and sinks in the global CH4 budget. We also assessed their findings in terms of their possible relevance for planting trees as a greenhouse mitigation option. We conclude that consideration of aerobic CH4 emissions from plants would reduce the benefit of planting trees by between 0 and 4.4%. Hence, any offset from CH4 emission is small in comparison to the significant benefit from carbon sequestration. However, much critical information is still lacking about aerobic CH4 emission from plants. For example, we do not yet know the underlying mechanism for aerobic CH4 emission, how CH4 emissions change with light, temperature and the physiological state of leaves, whether emissions change over time under constant conditions, whether they are related to photosynthesis and how they relate to the chemical composition of biomass. Therefore, the present calculations must be seen as a preliminary attempt to assess the global significance from a basis of limited information and are likely to be revised as further information becomes available.

A problem for biodiversity-productivity studies: how to compare the productivity of multispecific plant mixtures to that of monocultures?

Abstract The study of the relationship between species richness of a plant community and its productivity has received much attention, recently renewed by the concern on the loss of biological diversity at a global scale. Here, we briefly review some indices widely used in agronomic and competition experiments to compare monocultures and mixtures, and compare them to other, more recently designed ones. These various indices are then calculated for two experiments. In the first experiment, two grass and two legume species were grown at six levels of nitrogen availability, either in monocultures or in mixtures of the four species in a substitutive design; in the second experiment, five grass species were grown at 16 levels of total nutrient availability, either in monocultures or in mixtures of the five species in an additive design. These data clearly show that the conclusions drawn from the experiments depend on the index used to compare the experimental communities. We argue that a clear test of whether the productivity of communities increases with species richness requires that: (1) all species present in the multispecies assemblages also be grown in monocultures under the same environmental conditions, and (2) the productivity of these assemblages be compared to the most productive monoculture. We conclude that there are as yet very few cases where superior productivity of multispecies assemblages as compared to monocultures has been clearly shown.

Impact of elevated CO2 on the metabolic diversity of microbial communities in N-limited grass swards

The impact of elevated atmospheric CO2 on qualitative and qua ntitative changes in rhizosphere carbon flow will have important consequences fo r nutrient cycling and storage in soil, through the effect on the activity, biom ass size and composition of soil microbial communities. We hypothesized that mic robial communities from the rhizosphere of Danthonia richardsonii, a n ative C3 Australian grass, growing at ambient and twice ambient CO2 a nd varying rates of low N application (20, 60, 180 kg N ha-1) will be different as a consequence of qualitative and quantitative change in rhizosphere carbon flow. We used the BiologTM system to construct sole carbon source utilisation profiles of these communities from the rhizosphere of D. richardsonii. BiologTM GN and MT plates, the latter to which more ecologically relevant root exudate carbon sources were added, were used to characterise the communities. Microbial communities from the rhizosphere of D. richardsonii grown for four years at twice ambient CO2 had significantly greater utilisation of all carbon sources except those with a low C:N ratio (neutral and acidic amino acids, amides, N-heterocycles, long chain aliphatic acids) than communities from plants grown at ambient CO2. This indicates a change in microbial community composition suggesting that under elevated CO2 compounds with a higher C:N ratio were exuded. Enumeration of microorganisms, using plate counts, indicated that there was a preferential stimulation of fungal growth at elevated CO2 and confirmed that bacterial metabolic activity (C utilisation rates), not population size (counts), were stimulated by additional C flow at elevated CO2. Nitrogen was an additional rate-limiting factor for microbial growth in soil and had a significant impact on the microbial response to elevated CO2. Microbial populations were higher in the rhizosphere of plants receiving the highest N application, but the communities receiving the lowest N application were most active. These results have important implications for carbon turnover and storage in soils where changes in soil microbial community structure and stimulation of the activity of microorganisms which prefer to grow on rhizodeposits may lead to a decrease in the composition of organic matter and result in an accumulation of soil carbon.

Vegetation thickening in an ecological perspective: significance to national greenhouse gas inventories

Abstract ‘Vegetation thickening’ often refers to an increasing shrub and tree density on many grazed rangelands, woodlands and forests that may or may not have supported such woody plant populations in the past. It is one of several ecosystem changes, including post-clearing re-growth, afforestation and reforestation, which are variants of the same biological phenomenon — the recovery phase of disturbance/recovery cycles that all vegetation undergoes continuously. There are various levels of human influence over both phases. It is important as part of the global carbon cycle and potentially for its implications for implementation of the Kyoto Protocol. Vegetation thickening poses some inventory and carbon accounting challenges in this regard because of difficulties with quantification and attribution. The attribution of carbon sinks to natural, indirect or direct human influence is difficult because of the complex interactions of factors in determining woodland dynamics. The lack of clear scientific distinction of causation requires decisions to be made on how this is calculated in inventories of greenhouse gas emissions. Advantages, disadvantages, workability and dilemmas of five possible accounting approaches to dealing with these human-influenced biological components are discussed. These approaches range from accounting solely for emissions from clearing ignoring complementary re-growth sinks, to full emissions accounting including methane, nitrous oxide and CO 2 emissions from the managed animals and land.

Global atmospheric change effects on terrestrial carbon sequestration: Exploration with a global C- and N-cycle model (CQUESTN)

A model of the interacting global carbon and nitrogen cycles (CQUESTN) is developed to explore the possible history of C-sequestration into the terrestrial biosphere in response to the global increases (past and possible future) in atmospheric CO2 concentration, temperature and N-deposition. The model is based on published estimates of pre-industrial C and N pools and fluxes into vegetation, litter and soil compartments. It was found necessary to assign low estimates of N pools and fluxes to be compatible with the more firmly established C-cycle data. Net primary production was made responsive to phytomass N level, and to CO2 and temperature deviation from preindustrial values with sensitivities covering the ranges in the literature. Biological N-fixation could be made either unresponsive to soil C:N ratio, or could act to tend to restore the preindustrial C:N of humus with different N-fixation intensities. As for all such simulation models, uncertainties in both data and functional relationships render it more useful for qualitative evaluation than for quantitative prediction.With the N-fixation response turned off, the historic CO2 increase led to standard-model sequestration into terrestrial ecosystems in 1995AD of 1.8 Gt C yr−1. With N-fixation restoring humus C:N strongly, C sequestration was 3 Gt yr−1 in 1995. In both cases C:N of phytomass and litter increased with time and these increases were plausible when compared with experimental data on CO2 effects. The temperature increase also caused net C sequestration in the model biosphere because decrease in soil organic matter was more than offset by the increase in phytomass deriving from the extra N mineralised. For temperature increase to reduce system C pool size, the biosphere “leakiness” to N would have to increase substantially with temperature. Assuming a constant N-loss coefficient, the historic temperature increase alone caused standard-model net C sequestration to be about 0.6 Gt C in 1995. Given the disparity of plant and microbial C:N, the modelled impact of anthropogenic N-deposition on C-sequestration depends substantially on whether the deposited N is initially taken up by plants or by soil microorganisms. Assuming the latter, standard-model net sequestration in 1995 was 0.2 Gt C in 1995 from the N-deposition effect alone. Combining the effects of the historic courses of CO2, temperature and N-deposition, the standard-model gave C-sequestration of 3.5 Gt in 1995. This involved an assumed weak response of biological N-fixation to the increased carbon status of the ecosystem. For N-fixation to track ecosystem C-fixation in the long term however, more phosphorus must enter the biological cycle. New experimental evidence shows that plants in elevated CO2 have the capacity to mobilize more phosphorus from so-called “unavailable” sources using mechanisms involving exudation of organic acids and phosphatases.

A critical overview of model estimates of net primary productivity for the Australian continent

Net primary production links the biosphere and the climate system through the global cycling of carbon, water and nutrients. Accurate quantification of net primary productivity (NPP) is therefore critical in understanding the response of the worlds ecosystems to global climate change, and how changes in ecosystems might themselves feed back to the climate system.

Elevated atmospheric CO2 concentrations increase wheat root phosphatase activity when growth is limited by phosphorus

Wheat seedlings were grown in solution culture under adequate and limited phosphorus treatments at current ambient and elevated (approximately 2× ambient) CO2 concentrations. Acid phosphomonoesterase (‘phosphatase’) activity of root segments was measured using p-nitrophenyl phosphate as substrate. When plant growth was P-limited, elevated CO2 concentrations increased phosphatase activity more than at ambient CO2. This result (1) was evident when expressed on a unit root dry weight or root length basis, indicating that increased root enzyme activity was unlikely to be associated with CO2-induced changes in root morphology; (2) occurred when plants were grown aseptically, indicating that the increase in phosphatase activity originated from root cells rather than root- associated microorganisms; (3) was associated with shoot P concentrations below 0.18%; (4) occurred only when wheat roots were grown under P deficiency but not when a transient P deficiency was imposed; and (5) suggest that a previously reported increase in phosphatase activity at elevated CO2 by an Australian native pasture grass (Gifford, Lutze and Barrett 1996; Plant and Soil 187, 369–387) was also a root mediated response. The observed increase in phosphatase activity by plant roots at elevated CO2, if confirmed for a wide range of field pasture and crop species, is one factor which may increase mineralisation of soil organic P as the anthropogenic increase of atmospheric CO2 concentrations continues. But, whether a concomitant increase in plant uptake of P occurs will depend on the relative influence of root and microbial phosphatases, and soil geochemistry in determining the rate of mineralisation of soil organic P for any given soil.

Fine root production and litter input : Its effects on soil carbon

Carbon storage by forests has potential for contributing to ‘Kyoto Protocol’ greenhouse gas emission reduction targets, but evidence about C-storage and loss below ground is conflicting. The study addresses why soil carbon stocks are increased by land use change from forest to pasture, but are reduced by planting conifer trees, though not broadleaf trees, onto prior pasture. Can species differences in fine root production and litter input play a role? Addressing that hypothesis, a 1-year pot experiment was established as a model system in a glasshouse. Two tree species, pine (Pinus radiata) and Blue Gum (Eucalyptus globulus), and two grass species, Kangaroo Grass (Themeda triandra) and Wallaby Grass (Austrodanthonia racemosa), were sown in pots of soil taken from a native pasture with Kangaroo Grass dominant or from an adjacent pine plantation forest. After 3 months, half of the grass pots were defoliated monthly to 7 cm above ground to test for any cutting effect. Fine root production and turnover was monitored via minirhizotrons, with a destructive harvest after 1 year. Fine root oven dry mass at the end of the year varied between species: Kangaroo Grass (17 g/pot), pine (13 g/pot), Blue Gum (8 g/pot), Wallaby Grass (4 g/pot). Cutting significantly reduced fine root production in Kangaroo Grass but not in Wallaby Grass. From minirhizotron monitoring, 70% of the fine root length produced by pine during the year had disappeared, presumably by decomposition, before the final harvest. The equivalent loss for Wallaby Grass was only 26%, for Kangaroo Grass 36%, and for Blue Gum 45%. Despite the faster fine root disappearance under pines than under Kangaroo Grass, soil C declined under pines but increased under Kangaroo Grass as found in the field. Thus the experiment did not support the idea that a lower dead fine root production was the source of decline in soil C under pine. There was only a weak correlation between soil C change and the net amount of live fine root mass produced. However, the soil carbon changes in this study were positively correlated with live fine root length density in the soil. Kangaroo Grass maintained large lengths of very fine roots while pines produced a small length of thick fine roots. Accordingly, it is hypothesized that the increase of soil carbon under Kangaroo Grass compared with pine may be caused more by the activity of live fine roots than by decomposition of fine root mass to humus. This hypothesis, formulated from the model experimental system, needs to be evaluated for field sites where pines have been planted onto pastures.

Carbon Storage and Productivity of a Carbon Dioxide Enriched Nitrogen Limited Grass Sward After One Year's Growth

Determining the response of nitrogen restricted ecosystems to carbon dioxide enrichment is important in evaluating the role of the terrestrial biosphere in the unidentified sink in global carbon cycle models. Swards of the C3 grass Danthonia richardsonii (Cashmore) were established in large pots filled with a soil of low C and N content. The swards were continuously supplied with N at rates of 2, 6 and 18 g m-2 yr -1, and exposed to atmospheric CO2 concentrations of either 357 or 712 ,uL L1. After 1 years growth the high CO2 treatments gained 19, 53 and 43% more C than at low CO2 concentrations for the low, medium and high N treatments, respectively. This extra C gain was found in all plant and soil pools at the medium N level. At the low N level no extra C was found in the roots. At the high N level no extra carbon was found in the soil. Leaf area index was not affected by growth at high CO2. The extra C was gained with the same total N investment in green leaf in the two lowest N treatments, and with 30% less N in green leaf at the highest N level. Growth at the high CO2 concentration resulted in all C pools having a higher C:N ratio. Total water use was decreased and water use efficiency increased by growth at the high CO2 concentration. It was noted that if these results were transferable to the field, and if the higher C:N ratios do not reduce longer term productivity by reducing N-mineralization rates, grasslands could form a substantial part of the unidentified C sink. The potential feedback of decreased N availability in the longer term is being investigated in the final 3 years of the experiment.