The use of fire in land management has come under increasing scrutiny with regard to its potential effects on sustainability and climate change. Moorlands in the United Kingdom have traditionally used rotational burning of the heather (Calluna vulgaris) to improve the grazing and habitat, especially for grouse (Lagopus lagopus scoticus). However, these ecosystems overlie carbon-rich soils and concerns have been raised about the merits of this practice. In order to assess the impact of rotational burning on carbon balance, an investigation was undertaken on a grouse moor in the Yorkshire Dales, UK. This showed that the quantity of carbon stored above ground in heather biomass ranged from 600 to 1325 g C/m2 (typical for UK upland heaths). However, the national UK carbon inventory assumes 200 g C/m2, thereby appearing to underestimate considerably the importance of these habitats for carbon storage above ground. Analysis of 2 burns in subsequent years showed that 16 ± 4% and 24 ± 5% (± s.e.) of the above-ground material was consumed in the fires, resulting in the direct release of 103 ± 22 and 201 ± 62 g/m2 of carbon, respectively. Indirect carbon losses, which other studies have shown to be primarily due to erosion, were estimated to release another 5–21 g C/m2.year. The significance of other major greenhouse gas fluxes was assessed for the whole system using published parameters and models. We show that, over the burning cycle of 15–20 years, losses of carbon from burning are <10% of the total losses of carbon from the system, implying that careful burning management at this site does not have a major detrimental effect on the carbon budget, which for this moor lies within the range of an annual net loss of 34 g C/m2.year to a net uptake of 146 g C/m2.year.

Historically, agriculturally induced CO2 release from soils has contributed to rising levels in the atmosphere. However, by using appropriate management, soils can be turned into carbon sinks. Many of the dryland regions of the world are characterised by degraded soils, a high incidence of poverty and a low capacity to invest in agriculture. Two well-proven soil organic matter models (CENTURY 4.0 and RothC-26 3) were used two explore the effects of modifying agricultural practices to increase soil carbon stocks. The changes to land management were chosen to avoid any significant increase in energy input whilst using technologies that would be available without radically altering the current agricultural methodology. Case studies were selected from dryland farming systems in Nigeria, Sudan and Argentina. Modelling showed that it would be possible to make alterations within the structure of the current farming systems to convert these soils from carbon sources to net sinks. Annual rates of carbon sequestration in the range 0.08–0.17Mgha−1year−1 averaged over the next 50 years could be obtained. The most effective practices were those that maximised the input of organic matter, particularly farmyard manure (up to 0.09Mgha−1year−1), maintaining trees (up to 0.15Mgha−1year−1) and adopting zero tillage (up to 0.04Mgha−1year−1). Verification of these predictions will require experimental data collected from field studies.

Two C4 plants, Miscanthus x giganteus and Cyperus longus L., were grown at suboptimal growth temperatures and the relationships between the quantum efficiencies of photosynthetic electron transport through photosystem II (PSII) (PSII operating efficiency; Fq'/Fm') and CO2 assimilation (phiCO2) in leaves were examined. When M. x giganteus was grown at 10 degrees C, the ratio of the PSII operating efficiency to phiCO2 increased relative to that found in leaves grown at 14 and 25 degrees C. Similar increases in the Fq'/Fm': phiCO2 occurred in the leaves of two C. longus ecotypes when the plants were grown at 17 degrees C, compared to 25 degrees C. These elevations of Fq'/Fm': phiCO2 at low growth temperatures were not attributable to the development of anthocyanins, as has been suggested for maize, and were indicative of the operation of an alternative sink to CO2 assimilation for photosynthetic reducing equivalents, possibly oxygen reduction via a Mehler reaction, which would act as a mechanism for protection of PSII from photoinactivation and damage. Furthermore, in M. x giganteus grown at 10 degrees C, further protection of PSII was effected by a 20-fold increase in zeaxanthin content in dark-adapted leaves, which was associated with much higher levels of non-photochemical quenching of excitation energy, compared to that observed in leaves grown at 14 and 25 degrees C. These differences may explain the long growing season and remarkable productivity of this C4 plant in cool climates, even in comparison to other C4 species such as C. longus, which occur naturally in such climates.

Previous studies have shown that short exposure of plants to high doses of ozone decreases subsequent photosynthesis; initially by reducing carboxylation capacity. This study tests the hypothesis that this is also the primary cause of loss of photosynthetic capacity in leaves affected by development under a low level of ozone. Triticum aestivum and Pisum sativum plants were exposed from germination to ozone in air (80 nmol mol-1 for 7 hours per day, for 18 days. Leaves that had completed lamina expansion at this time were free of visible injury and light absorptance was unaffected. However, some significant changes in photosynthetic gas exchange were evident. Photosynthetic CO2 uptake at light saturation was decreased significantly by 35% in T. aestivum but was unchanged in P. sativum. The reduction in photosynthesis of T. aestivum was accompanied by a 31% decline in the maximum velocity of carboxylation measured in vivo. Decreased stomatal conductance did not contribute to this reduction of photosynthesis because there was no significant change in the stomatal limitation to CO2. Processes directly dependent upon photochemical reactions; that is, the quantum yield of CO2 uptake and capacity for regeneration of ribulose 1,5-bisphosphate were not affected by O3 fumigation in either species. This suggests that for wheat, the quantitative cause of decreased photosynthetic rate in vivo is a decrease in the quantity of active ribulose-1,5- bisphosphate carboxylase-oxygenase.

The process of carbon sequestration, or flux of carbon, into soils forms part of the global carbon cycle. Movement of carbon between the soil and the above ground environment is bidirectional and consequently carbon storage in soils reflects the balance between the opposing processes of accumulation and loss. This reservoir of soil carbon is truly dynamic, not only is carbon continually entering and leaving the soil, the soil carbon itself is partitioned between several pools, the residence times of which, span several orders of magnitude. Neither is soil carbon an inert reservoir, the organic matter with which it is associated is vital for maintaining soil fertility and it plays a part in such varied phenomena as nutrient cycling and gaseous emissions. A detailed description and analysis of soil carbon and organic matter can be found elsewhere (Schnitzer, 1991; FAO, 2001).

The prediction of complex interactive effects of rising concentrations of ozone and CO2 on vegetation will require robust models based on mechanistic understanding of how these two gases affect photosynthesis. This paper describes the development of a model of acute ozone exposure effects on wheat leaf photosynthesis, based on the mechanism of reactive oxygen scavenging processes. Based on experimental data, the dose of ozone to the leaf above a threshold flux, here termed the effective ozone dose, was found to be linearly related to the decline in the in vivo maximum rate of carboxylation. The proposed mechanism is that ozone damage to the photosynthetic apparatus will only occur above a critical rate of ozone delivery.By combining the model of the response of ribulose-1,5-bisphosphate-saturated and limited photosynthesis to ozone exposure with both a mechanistic biochemical model of leaf photosynthesis and a phenomenological model of stomatal conductance, it was possible to investigate the degree of dependency of ozone-induced stomatal closure on changes in the mesophyll. The stomatal conductance of the model simulation compared well with the magnitude of measured stomatal closure. The results indicate that the stomatal changes caused by acute ozone exposure can be predicted from changes in the mesophyll rather than directly on the stomata.The findings that the effects of ozone on photosynthesis can be predicted by an effective ozone dose to the leaf, and that the resulting reduction in CO2 assimilation rate can, in turn, predict stomatal closure, greatly simplifies modelling the effects of elevated concentrations of ozone and CO2 on wheat photosynthesis. Future work should determine whether the model can be adapted to predict chronic ozone exposure effects on photosynthesis, and whether it can be applied to other species by adjusting the values of threshold flux, related to the maximum scavenging capacity within the leaf, and the ozone slope coefficient, representing the inherent sensitivity of the photosynthetic apparatus to ozone.

Long-term exposure of plants to elevated partial pressures of CO2 (pCO2) often depresses photosynthetic capacity. The mechanistic basis for this photosynthetic acclimation may involve accumulation of carbohydrate and may be promoted by nutrient limitation. However, our current knowledge is inadequate for making reliable predictions concerning the onset and extent of acclimation. Many studies have sought to investigate the effects of N supply but the methodologies used generally do not allow separation of the direct effects of limited N availability from those caused by a N dilution effect due to accelerated growth at elevated pCO2. To dissociate these interactions, wheat (Triticum aestivum L.) was grown hydroponically and N was added in direct proportion to plant growth. Photosynthesis did not acclimate to elevated pCO2 even when growth was restricted by a low-N relative addition rate. Ribulose-1, 5-bisphosphate carboxylase/oxygenase activity and quantity were maintained, there was no evidence for triose phosphate limitation of photosynthesis, and tissue N content remained within the range recorded for healthy wheat plants. In contrast, wheat grown in sand culture with N supplied at a fixed concentration suffered photosynthetic acclimation at elevated pCO2 in a low-N treatment. This was accompanied by a significant reduction in the quantity of active ribulose-1, 5-bisphosphate carboxylase/oxygenase and leaf N content.

ABSTRACTApex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m−2 of UV‐B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5‐bis‐phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady‐state photosynthesis, and the light and CO2‐saturated rate of linear electron transport. Ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV‐B treatment resulted in decreases in the light‐saturated rate of CO2 assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV‐B were observed on end‐product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO2 assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO2 assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO2 assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV‐B inhibition of CO2 assimilation.

summary Quercus robur saplings were exposed to elevated O3 (80 nmol mol‐1) in hemispherical glasshouses for one growing season in order to assess the effects of this pollutant on photosynthesis. Measurements of chlorophyll fluorescence induction kinetics in mid‐summer showed that plants exposed to both control O3 (20 nmol mol−1) and elevated O3 treatments experienced significant photoinhibition of photosynthesis. Elevated O3 increased photoinhibition. At the end of the summer, photosynthetic CO2 uptake, measured at either light saturation or under light‐limiting conditions, was not significantly different between plants from the two O3 treatments. Although the maximum rate of carboxylation velocity, as estimated from the response of CO2 uptake to intercellular CO2 concentration, the maximum quantum efficiency and the maximum rate of electron transport measured in vivo were decreased by O3 exposure, the responses differed between plants and, overall, the decreases were not statistically significant. The results suggest that O3 levels that are at present reached for a few days during photochemical episodes in the summer are not sufficient to cause measurable lasting damage to photosynthetic CO2 uptake in oak saplings assessed over one growing season.

The principles and limitations of leaf gas exchange measurements in portable gas exchange systems are described. Attention is given to the design and developments in infrared gas analysers used in portable systems, and the basic structure of single and dual beam instruments is presented. The significance of flow measurement in these systems and the principles of thermal mass flow measurement are illustrated, Considerations of leaf area measurement, chamber design and choice of materials are outlined, Two specific developments in field gas exchange systems are described and their significance in field measurements is illustrated with examples. (1) An integrating sphere leaf chamber for the determination of the quantum yield of photosynthesis, on the basis of absorbed light, is explained and equations for its use are developed. The significance of this approach is illustrated by a comparison of data for contrasting leaves plotted on an absorbed and incident light basis. This measurement of light-limited photosynthesis is also critical in understanding the contribution of shaded leaves to canopy photosynthesis. (2) A system for the measurement of canopy photosynthesis from arable crops and low stature natural vegetation is described, Results from a season-long study of wheat CO2 exchange are shown to illustrate its application.

This study investigated the interacting effects of carbon dioxide and ozone on photosynthetic physiology in the flag leaves of spring wheat (Triticum aestivum L. cv. Wembley), at three stages of development. Plants were exposed throughout their development to reciprocal combinations of two carbon dioxide and two ozone treatments: [CO2] at 350 or 700 μmol mol−1, [O3] at < 5 or 60 nmol mol−1. Gas exchange analysis, coupled spectrophotometric assay for RuBisCO activity, and SDS-PAGE, were used to examine the relative importance of pollutant effects on i) stomatal conductance, ii) quantum yield, and iii) RuBisCO activity, activation, and concentration. Independently, both elevated [CO2] and elevated [O3] caused a loss of RuBisCO protein and Vcmax. In combination, elevated [CO2] partially protected against the deleterious effects of ozone. It did this partly by reducing stomatal conductance, and thereby reducing the effective ozone dose. Elevated [O3] caused stomatal closure largely via its effect on photoassimilation.

The depressions of photosynthetic CO2 uptake following O3 exposures of 200 and 400 nmol mol(-1) for between 4 and 16 h were compared between Pisum sativum, Quercus robur and Triticum aestivum, and the potential causes of change identified in vivo. Photosynthetic change was examined by analysis of CO2, O2, O3 and water vapour exchanges together with chlorophyll fluorescence in controlled environments. Under identical fumigation conditions, each species showed very similar rates of O3 consumption. The light-saturated rate of CO2 uptake showed a statistically significant decrease in each species with increasing O3 dose. Although stomatal conductance declined in parallel with CO2 uptake this did not account for the observed decrease in photosynthesis. The decrease in mesophyll conductance resulted primarily from a decrease in the apparent carboxylation capacity, implying in decreased activity of ribulose 1,5-bisphosphate carboxylase/oxygenase. The maximum capacity of carboxylation was consequently reduced by over 30% and 50% after 16 h fumigation with 200 and 400 nmol mol(-1) O3 respectively. Additionally, in Q. robur, a statistically significant inhibition of the CO2 saturated rate of photosynthesis occurred after 16 h with 400 nmol mol(-1) O3, suggesting that the ability to regenerate ribulose 1,5-bisphosphate was also impaired. None of the species showed any significant decrease in the efficiency of light-limited photosynthesis following fumigation at 200 nmol mol(-1) O3, but effects were apparent at 400 nmol mol(-1) O3. The common feature in all three species was a decline in carboxylation capacity which preceded any other change in the photosynthetic apparatus.

Thebasis ofinhibition ofphotosynthesis bysingle acute03 exposures wasinvestigated invivo using analyses basedonleaf gasexchange measurements. Thefully expanded second leaves ofwheatplants (Triticum aestivum L.cvAvalon) werefumigated witheither 200or400nanomoles permole03forbetween 4and 16hours. Thisreduced significantly thelight-saturated rateof CO2uptake andwasaccompanied bya parallel decrease in stomatal conductance. However, thestomatal limitation, estimatedfromtherelationship between CO2uptake andtheintemal CO2concentration, onlyincreased significantly during thefirst 8 hours ofexposure to400nanomoles permole03;nosignificant increase occurred foranyoftheother treatments. Analysis ofthe response ofCO2uptake totheinternal CO2concentration implied that thepredominant factor responsible forthereduction inlightsaturated CO2uptake wasadecrease intheefficiency ofcarboxylation. Thiswas58and21%ofthecontrol value after 16hours at200and400nanomoles permole03,respectively. Atsaturating concentrations ofC02,photosynthesis wasinhibited bynomore than22%after 16hours, indicating thatthecapacity forregeneration ofribulose bisphosphate waslesssusceptible to03. Ozonefumigations alsohadalesspronounced effect onlightlimited photosynthesis. Themaximumquantum yield ofCO2uptakeandthequantumyield ofoxygenevolution showedno significant decline after 16hourswith200nanomoles permole 03,requiring 8hoursat400nanomoles permole03before a significant reduction occurred. Thephotochemical efficiency of photosystem 11estimated fromtheratio ofvariable tomaximum chlorophyll fluorescence andtheatrazine-binding capacity of isolated thylakoids demonstrated thatphotochemical reactions werenotresponsible fortheinitial inhibition ofCO2uptake. The results suggest thattheapparent carboxylation efficiency appears tobetheinitial causeofdecline inphotosynthesis invivo following acute03fumigation.