Effects Of Free-air CO2 Enrichment Research Articles

Differential Effect of Free-Air CO2 Enrichment (FACE) in Different Organs and Growth Stages of Two Cultivars of Durum Wheat.

Wheat is a target crop within the food security context. The responses of wheat plants under elevated concentrations of CO2 (e[CO2]) have been previously studied; however, few of these studies have evaluated several organs at different phenological stages simultaneously under free-air CO2 enrichment (FACE) conditions. The main objective of this study was to evaluate the effect of e[CO2] in two cultivars of wheat (Triumph and Norin), analyzed at three phenological stages (elongation, anthesis, and maturation) and in different organs at each stage, under FACE conditions. Agronomic, biomass, physiological, and carbon (C) and nitrogen (N) dynamics were examined in both ambient CO2 (a[CO2]) fixed at 415 µmol mol-1 CO2 and e[CO2] at 550 µmol mol-1 CO2. We found minimal effect of e[CO2] compared to a[CO2] on agronomic and biomass parameters. Also, while exposure to 550 µmol mol-1 CO2 increased the photosynthetic rate of CO2 assimilation (An), the current study showed a diminishment in the maximum carboxylation (Vc,max) and maximum electron transport (Jmax) under e[CO2] conditions compared to a[CO2] at physiological level in both cultivars. However, even if no significant differences were detected between cultivars on photosynthetic machinery, differential responses between cultivars were detected in C and N dynamics at e[CO2]. Triumph showed starch accumulation in most organs during anthesis and maturation, but a decline in N content was observed. Contrastingly, in Norin, a decrease in starch content during the three stages and an increase in N content was observed. The amino acid content decreased in grain and shells at maturation in both cultivars, which might indicate a minimal translocation from source to sink organs. These results suggest a greater acclimation to e[CO2] enrichment in Triumph than Norin, because both the elongation stage and e[CO2] modified the source-sink relationship. According to the differences between cultivars, future studies should be performed to test genetic variation under FACE technology and explore the potential of cultivars to cope with projected climate scenarios.

Effects of free-air CO2 enrichment (FACE) and nitrogen (N) supply on N uptake and utilization of indica and japonica cultivars (Oryza sativa L.)

BackgroundIncreasing atmospheric [CO2] can increase photosynthesis and promote plant growth, consequently influencing nitrogen (N) cycling. Yet, there is no systematic information on the N response among different organs of japonica and indica rice genotypes to elevated [CO2] as affected by N application. To investigate the impacts of elevated [CO2] and N fertilization on N uptake and utilization of different genotypic rice (Oryza sativa L.) during grain filling, a free-air CO2 enrichment (FACE) experiment with indica cv. Liangyou 084 (IIY084) and japonica cv. Wuyunjing 23 (WYJ23) was conducted in Eastern China. Crops were exposed to ambient [CO2] and elevated [CO2] (200 μmol mol−1 above ambient) at two levels of N: control (0N) and 22.5 g N m−2 (normal N, NN), and they were sampled at 82 days after transplanting (DAT), 99 DAT, and maturity, respectively.ResultsUnder FACE, significant declines of N concentration in all tissues and the whole plants were observed with the greater decrease in leaves and stems at three stages. Positive responses of N use efficiency (NUE) to elevated [CO2] were recorded over the study period. The total N accumulation remained unchanged, while a large amount of N was partitioned to panicles at the expense of leaves and stems. As compared to WYJ23, greater N transportation from roots to aboveground, especially the panicles, was observed on IIY084 accompanied by higher panicle biomass (82 DAT and 99 DAT), N concentrations (maturity), and greater NUE for leaves through the study season. Across all [CO2] and cultivars, N fertilization increased N partitioning to leaves and stems while decreasing that to panicles. Additionally, N supply decreased NUE while stimulating N concentrations and N amounts of rice plants. Among all treatments, IIY084 had the highest N accumulation and allocation in panicles under elevated [CO2] in combination with N fertilizer at maturity.ConclusionData from this study were helpful for understanding the temporal N uptake and utilization of different rice genotypes as affected by N availability and suggest that IIY084 promises a considerable prospect for its grain yield and quality under future elevated atmospheric [CO2].

Effects of free-air CO2 enrichment on flower opening time in rice

ABSTRACTFlower opening time (FOT) is important for reproductive success in higher plants. Rice plants exhibit phenotypic plasticity in FOT in response to environmental factors such as temperature. However, the effect of the concentration of atmospheric CO2 ([CO2]) on FOT has not yet been reported. Elevated [CO2] (E-[CO2]) increases the temperature of panicles, which may in turn advance FOT. We investigated the effect of E-[CO2] on FOT in rice using a free-air CO2 enrichment facility where we increased [CO2] by about 200 μmol mol−1 above the ambient level (A-[CO2]). By photographing panicles at 10-min intervals, we determined 10%FOT and 50%FOT (the time of the day in Japan Standard Time when 10% and 50% of the flowers had opened, respectively). E-[CO2] advanced 10%FOT and 50%FOT by 4 and 5 min at the 10% and 5% levels of significance, respectively. Daily mean air temperature (Ta), solar radiation, and vapor-pressure deficit were negatively correlated with 50%FOT. Regression line slopes for Ta versus 10%FOT and 50%FOT were slightly steeper for A-[CO2] than those for E-[CO2]. Our results suggest that the most probable reason why E-[CO2] advanced FOT is an increase in panicle temperature arising from a reduction in leaf stomatal conductance.Abbreviations: 10%FOT: the time of the day in Japan Standard Time when 10% of the flowers had opened; 50%FOT: the time of the day in Japan Standard Time when 50% of the flowers had opened; A-[CO2]: ambient CO2; [CO2]: the concentration of atmospheric carbon dioxide; E-[CO2]: elevated [CO2]; FACE: free-air CO2 enrichment; FOT: flower opening time; JST: Japan Standard Time; RH: relative humidity; Rs: solar radiation; Ta: air temperature; Tp: panicle temperature; VPD: vapor-pressure deficit

Effects of free-air CO2 enrichment on heat-induced sterility and pollination in rice

ABSTRACTGlobal climate changes may cause heat-induced sterility in rice, threatening the global production of this important crop. Although little is currently known about the combined effects of the concentration of atmospheric CO2 ([CO2]) and temperature on heat-induced sterility, elevated [CO2] (E-[CO2]) will likely increase the panicle temperature and thereby exacerbate heat-induced sterility, but this was not tested in open fields. Therefore, we investigated the effect of E-[CO2] on heat-induced sterility and sterility-related traits in rice by increasing E-[CO2] by approximately 200 μmol mol−1 above ambient levels using a free-air CO2 enrichment (FACE) facility for six growing seasons with variable growing season temperatures. The percentage fertility was not significantly correlated with the air temperature (Ta) between 09:00 and 12:00 on each flowering day, but it did significantly vary among the years, with 2011 experiencing cool temperatures resulting in chilling-induced mild sterility. When data from 2011 were removed, there was a significant negative correlation between fertility and Ta between 09:00 and 12:00 on each flowering day under E-[CO2], whereas no such effect was seen under ambient [CO2]. E-[CO2] also significantly reduced the number of pollen grains deposited on the stigma by 10%, but it slightly increased the anther length by 1.3%, indicating that it had both negative and positive effects on heat-induced sterility. These findings suggest that E-[CO2] affects many traits related to heat-induced sterility and may sometimes exacerbate sterility by reducing pollen grain deposition.Abbreviations: A-[CO2]: ambient CO2; [CO2]: the concentration of carbon dioxide; E-[CO2]: elevated [CO2]; FACE: free-air CO2 enrichment; Ta: air temperature

Effects of free-air CO2 enrichment (FACE) on N, P and K uptake of soybean in northern China

The objective of this study was to investigate the effect of elevated (550±19μmolmol−1) [CO2] on uptake and utilization of nitrogen (N), phosphorus (P) and potassium (K) by soybean (Glycine max (L.) Merr) at the free-air carbon dioxide enrichment (FACE) experimental facility in northern China. The above-ground biomass and root biomass were significantly increased under elevated [CO2]. Elevated [CO2] significantly decreased the N concentration of the above-ground part at the beginning bloom (R1) stage, but had no effect at the beginning pod (R3), beginning seed (R5) or harvest stage. The concentration of ureide in the upper most fully-expanded leaf was not significantly affected by elevated [CO2] at any growth stage. Elevated [CO2] increased P concentration of the above-ground plant parts at the R1 and R5 stages, but did not affect P concentration at the R3 stage or at harvest. However, K concentration of the above-ground plant parts and root was not affected by elevated [CO2] at any growth stage. At harvest, elevated [CO2] significantly increased N, P and K uptake in soybean seed. Results indicate that more N, P and K fertilizers may be required to maintain the availability of these elements in the soil for soybean under future elevated [CO2] environments.

Effects of free-air CO2 enrichment (FACE) on the uptake and utilization of N, P and K in Vigna radiata

Elevated [CO2] stimulates plant growth, which in turn demands more nutrients to sustain it. The nutrient demand of N-fixing plants may differ from that of other plants under elevated [CO2]. We conducted an experiment to determine how elevated [CO2] affected N, P and K content, assimilation of nutrients and accumulation of metabolites in the legume mung bean [Vigna radiata L.]. We investigated the effect of 550±19μmolmol−1 [CO2] on N, P, K uptake and utilization by mung bean at the free-air carbon dioxide enrichment (FACE) experimental facility in north China. At maturity, N concentration in whole plants decreased by 4.4%, but P and K concentration was unchanged at elevated [CO2]. The weight of nodules per plant significantly increased at elevated [CO2] but N, P, K-use efficiency for seed and the ratio of seed yield to cumulative absorption of N, P and K was unaffected. These results indicate that under elevated [CO2] the mechanisms governing N absorption and metabolism in mung bean was different from that for P and K. The nutrient dynamics between different elements of overall plant biomass and the soil nutrients pool could, therefore, be changed in future by elevated [CO2].

Effects of free-air CO2 enrichment on adventitious root development of rice under low and normal soil nitrogen levels

Free air CO2 enrichment (FACE) and nitrogen (N) have marked effects on rice root growth, and numerical simulation can explain these effects. To further define the effects of FACE on root growth of rice, an experiment was performed, using the hybrid indica cultivar Xianyou 63. The effects of increasing atmospheric CO2 concentration [CO2], 200μmolmol−1 higher than ambient, on the growth of rice adventitious roots were evaluated, with two levels of N: low (LN, 125kgha−1) and normal (NN, 250kgha−1). The results showed a significant increase in both adventitious root number (ARN) and adventitious root length (ARL) under FACE treatment. The application of nitrogen also increased ARN and ARL, but these increases were smaller than that under FACE treatment. On the basis of the FACE experiment, numerical models for rice adventitious root number and length were constructed with time as the driving factor. The models illustrated the dynamic development of rice adventitious root number and length after transplanting, regulated either by atmospheric [CO2] or by N application. The simulation result was supported by statistical tests comparing experimental data from different years, and the model yields realistic predictions of root growth. These results suggest that the models have strong predictive potential under conditions of atmospheric [CO2] rises in the future.

The effects of free-air CO₂ enrichment (FACE) on carbon and nitrogen accumulation in grains of rice (Oryza sativa L.).

Rising atmospheric CO2 concentrations will probably increase rice (Oryza sativa L.) yield but decrease grain nitrogen (GN) concentration. Grains attached to different positions in the panicles differ greatly in weight and quality, but their responses to elevated CO2 (e[CO2]) are poorly understood, which limits our understanding of the mechanisms of yield enhancement and quality degradation. Thus a free-air CO2 enrichment experiment was conducted to examine the effects of e[CO2] on grain mass (GM), grain carbon (GC), and GN accumulation in the spikelets attached to the upper primary rachis branch (superior spikelets; SS) and those attached to the lower secondary rachis (inferior spikelets; IS). e[CO2] stimulated the rice yield by 13% but decreased the N concentration in the panicle by 7% when averaged over two levels of N fertilizations (P < 0.01). The responses of SS and IS to e[CO2] were different particularly under higher N supply. For SS, e[CO2] decreased GN by 24% (P < 0.01) but did not affect GM. For IS, e[CO2] increased GM by 13% (P < 0.05) but GN was not affected. The reduction of GN due to e[CO2] started to appear at the beginning of grain filling. These results suggest that future [CO2] levels probably stimulate the grain growth of IS, most of which are not marketable due to limited size, at the expense of GN reduction in SS. Translocation of N from SS to IS may be a possible mechanism for reduction in GN of SS. This may degrade the grain quality of marketable rice under e[CO2].

Interactive effects of free-air CO2 enrichment and drought stress on maize growth

Predicting future maize yields requires quantifying anticipated climate change impacts on maize growth and yield. In the present study, maize was grown over 2 years (2007 and 2008) under sufficient (WET) and reduced water supply (DRY) and under ambient (378μll−1, AMB) and elevated (550μll−1, FACE) atmospheric CO2 concentration ([CO2]) using free air CO2 enrichment (FACE). The objective of the present study was to test the hypothesis that maize growth does not respond to elevated [CO2] under WET but under DRY conditions due to an increase of water use efficiency (WUE) of biomass production realized through reduced transpiration. Moreover, in 2008 soil cover was varied to test whether mitigation of evaporation by straw mulch increases the CO2 effect on WUE. The DRY treatment received 12% and 48% less water than the WET treatment in 2007 and 2008, respectively, which was achieved with the aid of rainout shelters. In the first year, drought stress was insignificant and crop growth was similar among the two watering regimes. CO2 enrichment did not affect crop growth in 2007 and also in the WET treatment of 2008. In the second year, a pronounced drought stress decreased green leaf index, accumulated seasonal radiation absorption and radiation use efficiency (RUE) significantly. However, these effects were mitigated by CO2 enrichment and the decrease of RUE was higher under AMB (−18%) than under FACE (−2%) conditions. In the DRY treatment in 2008, CO2 enrichment significantly increased final biomass (+24%) and grain yield (+41%) as compared to the DRY AMB treatment. CO2 enrichment significantly increased soil water content under WET and DRY conditions but did not affect the soil water exploitation. There was a significant interaction of [CO2] and water supply on WUE with no (2007) or a small CO2-response (+10% in 2008) under WET and a strong effect under DRY conditions in 2008 (+25%). Soil cover did not intensify the CO2 effect on WUE. It is concluded that maize will benefit from the increase in [CO2] only under drought but not under sufficient water supply.

Four years of free-air CO2 enrichment enhance soil C concentrations in a Chinese wheat field

Elevated atmospheric CO2 can influence soil C dynamics in agroecosystems. The effects of free-air CO2 enrichment (FACE) and N fertilization on soil organic C (Corg), dissolved organic C (DOC), microbial biomass C (Cmic) and soil basal respiration (SBR) were investigated in a Chinese wheat field after expose to elevated CO2 for four full years. The results indicated that elevated CO2 has stimulative effects on soil C concentrations regardless of N fertilization. Following the elevated CO2, the concentrations of Corg and SBR were increased at wheat jointing stage, and those of DOC and Cmic were enhanced obviously across the wheat jointing stage and the fallow period after wheat harvest. On the other hand, N fertilization did not significantly affect the content of soil C. Significant correlations were found among DOC, Cmic, and SBR in this study.

Effect of elevated atmospheric CO2 concentration on soil and root respiration in winter wheat by using a respiration partitioning chamber

Soil respiration in a cropland is the sum of heterotrophic (mainly microorganisms) and autotrophic (root) respiration. The contribution of both these types to soil respiration needs to be understood to evaluate the effects of environmental change on soil carbon cycling and sequestration. In this paper, the effects of free-air CO2 enrichment (FACE) on hetero- and autotrophic respiration in a wheat field were differentiated and evaluated by a novel split-root growth and gas collection system. Elevated atmospheric pCO2 of approximately 200 μmol mol−1 above the ambient pCO2 significantly increased soil respiration by 15.1 and 14.8% at high nitrogen (HN) and low nitrogen (LN) application rates, respectively. The effect of elevated atmospheric pCO2 on root respiration was not consistent across the wheat growth stages. Elevated pCO2 significantly increased and decreased root respiration at the booting-heading stage (middle stage) and the late-filling stage (late stage), respectively, in HN and LN treatments; however, no significant effect was found at the jointing stage (early stage). Thus, the effect of increased pCO2 on cumulative root respiration for the entire wheat growing season was not significant. Cumulative root respiration accounted for approximately 25–30% of cumulative soil respiration in the entire wheat growing season. Consequently, cumulative microbial respiration (soil respiration minus root respiration) increased by 22.5 and 21.1% due to elevated pCO2 in HN and LN, respectively. High nitrogen application significantly increased root respiration at the late stage under both elevated pCO2 and ambient pCO2; however, no significant effects were found on cumulative soil respiration, root respiration, and microbial respiration. These findings suggest that heterotrophic respiration, which is influenced by increased substrate supplies from the plant to the soil, is the key process to determine C emission from agro-ecosystems with regard to future scenarios of enriched pCO2.

Effect of Free-Air CO2 Enrichment (FACE) on the Community Structure of Aquatic Organisms in Floodwater of a Japanese Paddy Field during the Flooding Period of Rice Cultivation

In the present study, the effect of free-air CO2 enrichment (FACE) on the diversity and the community structure of aquatic organisms in the floodwater of a paddy field in northern Japan was evaluated. The varieties and the number of aquatic organisms ranging from 30 μm to 2 cm were compared four times at intervals of 20 d between the FACE (599 ppm CO2) and ambient air (384 ppm CO2) plots during the flooding period of rice cultivation. Aquatic organisms were classified mainly at the order level. Twenty-eight taxonomical groups were detected, and the number of groups in the FACE plots was significantly lower than that in the ambient plots. Zygnematales and Chaetonotida were less frequently detected in the FACE plots. The population densities of Zygnematales, and Chaetonotida also tended to be lower in the FACE plots than in the ambient plots in the late flooding period (after 50 d of flooding). No groups showed a significantly higher population density and frequency of presence in the FACE plots than in the ambient plots. The seasonal variations in the number of taxonomical groups and the community structure of aquatic organisms were less conspicuous in the FACE plots than in the ambient plots. We concluded that the FACE conditions adversely affected the communities of aquatic organisms in paddy fields.

Effect of Free-Air CO2 Enrichment (FACE) on Methanogenic Archaeal Communities Inhabiting Rice Roots in a Japanese Rice Field

The effect of free-air CO2 enrichment (FACE) on the methanogenic archaeal communities inhabiting rice roots was studied in a Japanese rice field by separately collecting rice roots three times (at mid-tillering, panicle initiation, and heading stages) according to their nodal number, extracting DNA from the roots and subjecting it to polymerase-chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) and sequence analyses. Appearance of roots indicated that aging and senescence occurred faster under the FACE conditions than under the ambient conditions. The number of DGGE bands of methanogenic archaeal communities tended to increase with the growth stages. Cluster analysis showed that the succession of methanogenic archaeal communities in the ambient plot preceded that in the FACE plot, while the trend of the appearance of rice roots was opposite. All the closest relatives associated with the DGGE bands belonged to Methanomicrobiales and Rice cluster I, and FACE did not affect the phylogenetic position of the closest relatives associated with the characteristic DGGE bands. Faster succession of methanogenic archaeal communities in the ambient plot and similar phylogenetic members between the plots were observed in rice roots in years with both warmer (1999) and cooler (2003) weather during the rice cultivation period than in average years.

Elevated CO2 increases carbon allocation to the roots of Lolium perenne under free‐air CO2 enrichment but not in a controlled environment

• Grass swards often show a higher root–shoot ratio of dry matter (R : SDM) at elevated [CO2] than at ambient [CO2]. However, it is not known whether this is a result of a sustained increase in C allocation to the roots. • The effects of free-air CO2 enrichment (FACE) on carbon allocation to roots in established Lolium perenne (perennial ryegrass) swards during regrowth were investigated by means of 14C labelling and compared with data from a controlled environment experiment. • Elevated [CO2] in the field increased root dry matter (109%), R : SDM (44%) and, in accordance, root–shoot 14C ratios (R : S14C, 39%), as well as the intercept of the allometric line. By contrast, elevated [CO2] did not affect the allometry of the plants under controlled conditions. However, at low N concentration in the nutrient solution, the R : SDM and the intercept of the allometric line increased. • It is suggested that the remarkably different responses to elevated [CO2] under field and controlled conditions result from differences in both N availability for growth, and C sink strength of the shoots.

Effect of free-air CO2 enrichment (FACE) on microbial biomass in paddy field soil

To determine the effect of free-air CO2 enrichment (FACE)on microbial biomass in paddy soil, a field experiment was conducted in an Andosol field in Shizukuishi (Iwate Prefecture, Japan). Rice plants were grown under ambient CO2 or FACE (ambient +200 ppmv CO2 ) during the growing season in 1998 and 1999. Soil samples were collected at 5 different times from the upper (0–1 cm) and lower (1–10 cm) layers and analyzed for the amounts of microbial biomass C (B c) and N (B N) and mineralizable N (Min. N). The amount of chlorophylltype compounds (ChIs), an index of algal growth, in the upper soil layers was also measured. During the first growing season (1998), FACE treatment did not cause any significant difference in the above-measured parameters except for B C at harvest, compared to the ambient CO2 treatment. However in 1999, FACE caused a significant increase in the amounts of B N, Min. N, and ChIs in the upper soil layers at harvest, compared to ambient CO2. Furthermore, FACE significantly increased the amount of Be in both soil layers, from mid-plant growth until harvest. From early plant growth until harvest, the upper soil layers subjected to both FACE and ambient CO2 treatments exhibited significantly higher values for B C, B N and Min. N than those in the lower soil layers. The amount of ChIs in the FACE treatment increased compared to that in the ambient CO2 treatment, especially at harvest. These findings suggest that FACE exerted a positively significant effect on the amounts of B C and B N, Min. N, and ChIs in paddy soil, particularly in the surface soil layers.

Biological dinitrogen fixation and soil microbial biomass carbon as influenced by free-air carbon dioxide enrichment (FACE) at three levels of nitrogen fertilization in a paddy field

An experiment was conducted in an Andosol paddy field in Shizukuishi (Iwate Prefecture, Japan) to determine the effects of free-air CO2 enrichment (FACE) on biological N2-fixation activity and soil microbial biomass C at three levels of N application. Rice (Oryza sativa L. cv. Akitakomachi) plants were grown under ambient CO2 or FACE (ambient +200 µmol mol–1 CO2) conditions throughout the growing season with each treatment having four replicated plots. Three levels of N fertilizer (high, standard and low; 15, 9 and 4 g N m–2, respectively) were applied to examine the effect of different N availability under both CO2 conditions. Soil samples were collected at four different times from upper and lower soil layers (0–1-cm and 1–10-cm soil depths, respectively) and analysed for biological N2-fixation (BNF) activity and microbial biomass C (MBC) by the acetylene reduction and chloroform fumigation-extraction methods, respectively. The amounts of chlorophyll-type compounds (Chls), an index of algal growth, and soil available C were also determined. Compared to the ambient CO2 treatment, the FACE treatment had significantly higher BNF activity in both the upper and lower soil layers at ripening only in low-N soil and at harvest at all three levels of N fertilization rates. MBC was significantly increased by FACE in both the upper and lower soil layers from the middle to later period of the growing season compared to the ambient CO2 treatment. The FACE treatment increased the Chls in the upper soil layers at ripening only in low-N soil and at harvest at all three levels of N fertilization rates. The amount of soil available C was not significantly different between FACE and ambient CO2 treatments in both the upper and lower soil layers throughout the cropping season. From these results it can be concluded that the FACE treatment had a significantly positive influence on BNF activity, MBC and Chls at different levels of N fertilization rates in paddy field during the cropping season.

Effects of free-air CO2 enrichment on energy balance and evapotranspiration of cotton

The effects of free-air CO2 enrichment (FACE) at 550 μmol mol−1 on the energy balance and evapotranspiration, ET, of cotton (Gossypium hirsutum L.) were investigated. Latent heat flux, λET was calculated as the residual in an energy balance approach from determinations of net radiation, Rn minus surface soil heat flux, G0, minus sensible heat flux, H. Rn was directly measured. G0 was determined from measurements with soil heat flux plates at 10 mm depth, corrected for temperature changes in the soil above. H was determined from measurements of air temperature with aspirated psychrometers, of foliage temperature with IR thermometers, and of wind speed with cup anemometers.Under ambient CO2 (control) conditions (about 370 μmol mol−1), the λET from the energy balance approach agreed fairly well with values from several other methods, including the Bowen ratio method, lending credence to the technique. However, the results had an uncertainty of the order of 20% associated with the Rn measurements. Therefore, an apparent increase in ET of about 13% in the FACE plots was judged insignificant. The conclusion that any effects of CO2 enrichment to 550 μmol mol−1 on the ET of cotton were too small to be detected was consistent with the results of other investigators who determined ET in the same experiment using stem flow gauges and the soil water balance.

Effect of free-air CO2 enrichment on the chlorophyll content of cotton leaves

In vivo chlorophyll concentrations were estimated using a Minolta SPAD 502 meter on upper-canopy leaves of cotton plants exposed to air enriched to an atmospheric CO2 concentration of approximately 550 μmol mol−1 in a free-air CO2 enrichment (FACE) study. Measurements were made on 27 days during the final 90 days of the 1991 growing season. In both well-watered and moderately water-stressed plants, leaves in the FACE plots had greater chlorophyll a concentrations than leaves in the ambient air control plots (about 370 μmol CO2 mol−1): season-long chlorophyll a averages were 7.1% greater in the ‘wet’ treatment and 8.2% greater in the ‘dry’ treatment. This finding differs from what has been observed in a number of studies where experimental plants were grown in small pots. It is, however, typical of what has been observed in studies employing larger pots and open fields, and is a compelling rationale for conducting additional studies of this nature in FACE projects.

Effects of free-air CO2 enrichment on microbial populations in the rhizosphere and phyllosphere of cotton

Cotton (Gossypium hirsutum L.) plants were exposed to free-air CO2 enriched (FACE = 550 μmol mol−1) or ambient (CONTROL = 370 μmol mol−1) levels of atmospheric CO2 and to wet (100% of evapotranspiration replaced) or dry (67% of ET replaced) soil water content treatments. Foliar, soil and root samples were collected in June and August 1991 to determine the effects of elevated CO2 on selected groups of phyllosphere and rhizosphere microorganisms. Foliage and rhizosphere soil were analyzed for bacteria and/or fungi using dilution plating. Mycorrhizal colonization of cotton roots was assessed. Root-zone soil was analyzed for populations of nematodes, microarthropods and Rhizoctonia using various extraction methods. A dehydrogenase assay for total microbial respiration and a bioassay for cotton root infecting organisms were also conducted using root-zone soil. Populations of fungi on cotton leaves varied, by genera, in response to CO2 enrichment, but none was affected by soil water content treatments; populations of foliar bacteria were not affected by either CO2 or soil water content treatments. In August, higher total numbers of rhizosphere fungi were found under the wet compared with the dry soil water treatment, but differences related to CO2 were not detected. There was a trend for infestation by Rhizoctonia solani to be higher under FACE in the August sample, but the soil bioassay demonstrated no increase in damping-off potential. There was a significant interaction between CO2 concentration and soil water content for populations of saprophagous nematodes; populations were different between the CO2 levels in the dry soil treatment only, with higher numbers under FACE. Microarthropod numbers were low; however, there was a trend for Collembola populations to be higher under FACE in the August sample and more fungi were isolated from Collembola in June. Total microbial activity was higher under FACE at both sample dates. Effects of elevated atmospheric CO2 on plant microbe interactions could have profound influence on the productivity of agro-ecosystems, and deserve further research.

Effects of free-air CO2 enrichment on cotton root growth

The rise in atmospheric CO2 concentration is predicted to have a positive effect on agroecosystem productivity. However, an area which requires further investigation centers on responses of crop root systems to elevated atmospheric CO2 under field conditions. The advent of free-air CO2 enrichment (FACE) technology provides a new method of CO2 exposure with minimal alteration of plant microclimate. In 1990 and 1991, cotton (Gossypium hirsutum (L.) ‘Deltapine 77’) was grown under two atmospheric CO2 levels (370 and 550 γmol mol−1) and two water regimes (wet (100% of ET replaced) and dry (75% of ET replaced in 1990 and 67% in 1991)). Plant root samples were collected at early vegetative and mid-reproductive growth. Taproots of CO2-enriched plants displayed greater volume, dry weight, length, and tissue density. Water treatment effects were noted for length, volume and dry weight of roots at the second sampling in 1991. In general, whole soil profile root densities (both length and dry weight densities) and root weight per unit length at the initial sampling were increased under CO2 enrichment at each of three positions (0.00, 0.25, and 0.50 m) from row center to the middle of the inter-row space. At the second sampling, root length density and root dry weight density were generally unaffected by water stress, whereas root weight per unit length was somewhat higher. In addition, extra CO2 increased whole profile root length density only at the 0.50 m inter-row position, whereas whole profile root dry weight density and root weight per unit length were generally higher under elevated CO2 at all three positions. The results from this field experiment strongly indicated that increased atmospheric CO2 level would enhance plant root growth.