Soil-Atmosphere CO Exchanges and Microbial Biogeochemistry of CO Transformations in a Brazilian Agricultural Ecosystem

Although anthropogenic land use has major impacts on the exchange of soil and atmosphere gas in general, relatively little is known about its impacts on carbon monoxide. We compared soil-atmosphere CO exchanges as a function of land use, crop type, and tillage treatment on an experimental farm in Parãna, Brazil, that is representative of regionally important agricultural ecosystems. Our results showed that cultivated soils consumed CO at rates between 3 and 6 mg of CO m⁻² day⁻¹, with no statistically significant effect of tillage method or crop. However, CO exchange for a pasture soil was near zero, and an unmanaged woodlot emitted CO at a rate of 9 mg of CO m⁻² day⁻¹. Neither nitrite, aluminum sulfate, nor methyl fluoride additions affected CO consumption by tilled or untilled soils from soybean plots, indicating that CO oxidation did not depend on ammonia oxidizers and that CO oxidation patterns differed in part from patterns reported for forest soils. The apparent K_m for CO uptake, 5 to 11 ppm, was similar to values reported for temperate forest soils; V_max values, approximately 1 μg of CO g (dry weight)⁻¹ h⁻¹, were comparable for woodlot and cultivated soils in spite of the fact that the latter consumed CO under ambient conditions. Short-term (24-h) exposure to elevated levels of CO (10% CO) partially inhibited uptake at lower concentrations (i.e., 100 ppm), suggesting that the sensitivity to CO of microbial populations that are active in situ differs from that of known carboxydotrophs. Soil-free soybean and corn roots consumed CO when they were incubated with 100-ppm concentrations and produced CO when they were incubated with ambient concentrations. These results document for the first time a role for cultivated plant roots in the dynamics of CO in an agricultural ecosystem.     

Enrichment of high-affinity CO oxidizers in Maine forest soil

Carboxydotrophic activity in forest soils was enriched by incubation in a flowthrough system with elevated concentrations of headspace CO (40 to 400 ppm). CO uptake increased substantially over time, while the apparent K(m) ((app)K(m)) for uptake remained similar to that of unenriched soils (<10 to 20 ppm). Carboxydotrophic activity was transferred to and further enriched in sterile sand and forest soil. The (app)K(m)s for secondary and tertiary enrichments remained similar to values for unenriched soils. CO uptake by enriched soil and freshly collected forest soil was inhibited at headspace CO concentrations greater than about 1%. A novel isolate, COX1, obtained from the enrichments was inhibited similarly. However, in contrast to extant carboxydotrophs, COX1 consumed CO with an (app)K(m) of about 15 ppm, a value comparable to that of fresh soils. Phylogenetic analysis based on approximately 1,200 bp of its 16S rRNA gene sequence suggested that the isolate is an alpha-proteobacterium most closely related to the genera Pseudaminobacter, Aminobacter, and Chelatobacter (98.1 to 98.3% sequence identity).     

Microbial carbon monoxide consumption in salt marsh sediments

We have examined sediments from a fringing salt marsh in Maine to further understand marine CO metabolism, about which relatively little is known. Intact cores from the marsh emitted CO during dark oxic incubations, but emission rates were significantly higher during anoxic incubations, which provided evidence for simultaneous production and aerobic consumption in surface sediments. CO emission rates were also elevated when cores were exposed to light, which indicated that photochemical reactions play a role in CO production. A kinetic analysis of marsh surface sediments yielded an apparent K(m) of about 82 ppm, which exceeded values reported for well-aerated soils that consume atmospheric CO (65nM). Surface (0-0.2 cm depth interval) sediment slurries incubated under oxic conditions rapidly consumed CO, and methyl fluoride did not inhibit uptake, which indicated that neither ammonia nor methane oxidizers contributed to the observed activity. In contrast, aerobic CO uptake was inhibited by additions of readily available organic substrates (pyruvate, glucose and glycine), but not by cellulose. CO was also consumed by surface and sub-surface sediment slurries incubated under anaerobic conditions, but rates were less than during aerobic incubations. Molybdate and nitrate or nitrite, but not 2-bromoethanesulfonic acid, partially inhibited anaerobic uptake. These results suggest that sulfidogens and acetogens, but not dissimilatory nitrate reducers or methanogens, actively consume CO. Sediment-free plant roots also oxidized CO aerobically; rates for Spartina patens and Limonium carolinianum roots were significantly higher than rates for Spartina alterniflora roots. Thus plants may also impact CO cycling in estuarine environments.     

Enrichment of High-Affinity CO Oxidizers in Maine Forest Soil

Carboxydotrophic activity in forest soils was enriched by incubation in a flowthrough system with elevated concentrations of headspace CO (40 to 400 ppm). CO uptake increased substantially over time, while the apparent K_m (_appK_m) for uptake remained similar to that of unenriched soils (<10 to 20 ppm). Carboxydotrophic activity was transferred to and further enriched in sterile sand and forest soil. The _appK_ms for secondary and tertiary enrichments remained similar to values for unenriched soils. CO uptake by enriched soil and freshly collected forest soil was inhibited at headspace CO concentrations greater than about 1%. A novel isolate, COX1, obtained from the enrichments was inhibited similarly. However, in contrast to extant carboxydotrophs, COX1 consumed CO with an _appK_m of about 15 ppm, a value comparable to that of fresh soils. Phylogenetic analysis based on approximately 1,200 bp of its 16S rRNA gene sequence suggested that the isolate is an α-proteobacterium most closely related to the genera Pseudaminobacter, Aminobacter, and Chelatobacter (98.1 to 98.3% sequence identity).     

Volume Contents

Distribution of root systems through soils and recolonization of root channels by successive crops are fundamental, though difficult to study, processes of soil ecology. This article reports a minirhizotron (MR) study of corn and alfalfa root systems throughout the soil profile of Kalamazoo loam (fine-loamy, mixed, mesic Typic Hapludalf) monolith lysimeters for a three-year succession of corn, alfalfa and corn. Multiple-date comparisons within and between years were conducted to estimate total root densities in each soil horizon. Root recolonization was assessed by comparing every video frame of paired minirhizotrons, from recordings conducted one growing season apart. Distributions of corn root systems were modified by tillage practices. In 1994, root populations of corn in the Bt1 horizon peaked 75–90 days after planting (DAP). Numbers of corn roots per m2 in the Bt1 horizon were consistently higher for no-tillage (NT) than for conventional tillage (CT) lysimeters, in 1994 and 1996. Distribution of alfalfa roots within the soil profile was not significantly modified by tillage. However, alfalfa root decomposition rates responded to conventional and no-tillage practices and were specific for each soil horizon. Corn root systems growing in soils previously cropped with alfalfa presented similar patterns of root distribution by horizons as that of the previous alfalfa crop. Successive corn root systems did not display similar distribution patterns throughout the soil profile from one growing season to the next. Proportions of roots of the current crop recolonizing root induced macropores (RIMs) of the previous crop averaged 18% for corn after corn, 22% for alfalfa after corn and 41% for corn after alfalfa, across Bt horizons and tillage treatments. In conclusion, distribution of corn root systems appeared to be modified by tillage practices and root recolonization of RIMs was controlled by the preceding crop.     

开放式空气CO2浓度增高条件下旱地土壤气体CO2浓度廓线测定

设计了一套适合于FACE(free airCO2 enrichment)平台的旱地土壤气体CO2 浓度廓线测定方法 ,并将其应用于田间实验 .在江苏省无锡市郊区具有太湖地区典型水稻土的稻麦轮作农田 ,对FACE和对照麦田以及裸土 0～ 30cm土层的土壤气体CO2 浓度廓线进行了观测研究 .结果表明 ,所采用的方法满足进行旱地农田土壤气体CO2 浓度廓线研究的要求 ;在 0～ 30cm土层中 ,上层土壤气体中的CO2 向上垂直扩散要比下层土壤快 ;在作物旺盛生长期 ,大气CO2 浓度升高 2 0 0± 4 0 μmol·mol-1使 0～ 30cm土层的土壤气体CO2 浓度显著提高 14 %± 5 % (t 检验P <0 .0 0 1) .     

不同耕作措施下冬小麦生长季农田二氧化碳排放通量及其与土壤温度的关系

利用长期定位试验研究了华北平原不同耕作措施下冬小麦生育期农田CO2排放通量,并对CO2排放通量和土壤温度进行了回归模拟.结果表明冬小麦生育期CO2排放速率表现为翻耕＞旋耕＞免耕,平均分别为343.69、337.54和190.47 mg·m-2·h-1.各处理冬小麦生育期CO2排放通量与土壤温度呈显著正相关;翻耕地CO2排放通量与10 cm地温相关性最高,旋耕地和免耕地则与10 cm和20 cm地温相关性较高.CO2排放通量和土壤温度呈指数函数关系(P＜0.01),利用10 cm地温对冬小麦生育期农田CO2排放通量进行估算表明,翻耕、旋耕和免耕地CO2排放分别为1.88、1.89和1.03 kg·m-2.     

Determination of carbon monoxide (CO) in rodent tissue: effect of heme administration and environmental CO exposure

Carbon monoxide (CO), produced endogenously during heme degradation, is considered a messenger molecule in vascular and neurologic tissues. To study this role, it is important to determine CO concentration in target tissues pre- and post-perturbations. Here, we describe a sensitive and reproducible method, which is linear and accurate, and provide some examples of its application for quantitation of CO concentrations in tissues pre- and post-perturbations. Tissues from adult rats and mice were sonicated (20% w/w), and volumes representing 0.04-8 mg fresh weight (FW) were incubated at 0 degrees C for 30 min with sulfosalicylic acid. CO liberated into the headspace was quantitated by gas chromatography. Tissue CO concentrations (mean+/-SD, pmol CO/mg FW) were as follows: blood (47+/-10, 45+/-5), muscle (4+/-4, 10+/-1), kidney (5+/-2, 7+/-2), heart (6+/-3, 6+/-1), spleen (11+/-3, 6+/-1), liver (4+/-1, 5+/-1), intestine (2+/-1, 4+/-2), lung (2+/-1, 3+/-1), testes (1+/-1, 2+/-1), and brain (2+/-1, 2+/-0) in untreated rat (n=3) and mouse (n=5), respectively. Between the rat and the mouse, only CO concentrations in the muscle and spleen were significantly different (p0.05). Endogenous CO generation, after administration of heme arginate to mice (n=3), increased CO concentrations by 0-43 pmol/mg FW. Exposure of mice (n=3) to 500 ppm CO for 30 min yielded significantly elevated CO concentrations by 4-2603 pmol/mg FW in all tissues over the native state. While blood had the highest CO concentration for all conditions, muscle, kidney, heart, spleen, and liver, all rich in hemoglobin and/or other CO-binding hemoproteins, also contained substantial CO concentrations. Intestine, lung, testes, and brain contained the lowest CO concentrations.     

Soil organic matter dynamics under soybean exposed to elevated [CO2]

It is unclear how changing atmospheric composition will influence the plant–soil interactions that determine soil organic matter (SOM) levels in fertile agricultural soils. Positive effects of CO₂ fertilization on plant productivity and residue returns should increase SOM stocks unless mineralization or biomass removal rates increase in proportion to offset gains. Our objectives were to quantify changes in SOM stocks and labile fractions in prime farmland supporting a conventionally managed corn–soybean system and the seasonal dynamics of labile C and N in soybean in plots exposed to elevated [CO₂] (550 ppm) under free-air concentration enrichment (FACE) conditions. Changes in SOM stocks including reduced C/N ratios and labile N stocks suggest that SOM declined slightly and became more decomposed in all plots after 3 years. Plant available N (>273 mg N kg⁻¹) and other nutrients (Bray P, 22–50 ppm; extractable K, 157–237 ppm; Ca, 2,378–2,730 ppm; Mg, 245–317 ppm) were in the high to medium range. Exposure to elevated [CO₂] failed to increase particulate organic matter C (POM-C) and increased POM-N concentrations slightly in the surface depth despite known increases (≈30%) in root biomass. This, and elevated CO₂ efflux rates indicate accelerated decay rates in fumigated plots (2001: elevated [CO₂]: 10.5 ± 1.2 μmol CO₂ m⁻² s⁻¹ vs. ambient: 8.9 ± 1.0 μmol CO₂ m⁻² s⁻¹). There were no treatment-based differences in the within-season dynamics of SOM. Soil POM-C and POM-N contents were slightly greater in the surface depth of elevated than ambient plots. Most studies attribute limited ability of fumigated soils to accumulate SOM to N limitation and/or limited plant response to CO₂ fertilization. In this study, SOM turnover appears to be accelerated under elevated [CO₂] even though soil moisture and nutrients are non-limiting and plant productivity is consistently increased. Accelerated SOM turnover rates may have long-term implications for soil’s productive potential and calls for deeper investigation into C and N dynamics in highly-productive row crop systems.     

Mineralization of soil organic P induced by drying and rewetting as a source of plant-available P in limed and unlimed samples of an acid soil

This study assessed the effects of different farming systems, namely woodlot (WL), alley farming (AL), conventional tillage (CT) and natural fallow (NF) on the variability of organic carbon (OC) content and mean weight diameter (MWD) of a degraded Ferric Acrisol in the sub-humid zone of Ghana. The soils under woodlot accumulated the highest amount of organic carbon (18.6 g kg^–1) with the least spatial variability apparently due to the greater additions of litter and minimum tillage. The conventionally tilled soil had the least OC content (13.1 g kg^–1). Similar to the OC content, the woodlot soils also had the highest aggregate stability (MWD = 1.78 mm) and the least spatial variability. The stability of soil aggregates under the farming systems was greatly influenced by OC content; there was a good correlation between OC and MWD (r > 0.62^**). Correlograms showed that OC and MWD are space dependent. The correlation length for OC under the different farming systems followed the order WL > NF > AL > CT, indicating that WL ensured a greater uniform distribution soil organic matter. The spatial distribution in MWD followed the same trend observed for OC. The MWD in the other farming systems was poorly related from point to point with shorter k-values, suggesting lack of uniformity due to low accumulation of OC. Generally, the woodlot system appeared to be a better, low-input restorer of soil productivity.     

Attributes of atmospheric carbon monoxide oxidation by Maine forest soils

CO, one of the most important trace gases, regulates tropospheric methane, hydroxyl radical, and ozone contents. Ten to 25% of the estimated global CO flux may be consumed by soils annually. Depth profiles for (14)CO oxidation and CO concentration indicated that CO oxidation occurred primarily in surface soils and that photooxidation of soil organic matter did not necessarily contribute significantly to CO fluxes. Kinetic analyses revealed that the apparent K(m) was about 18 nM (17 ppm) and the V(max) was 6.9 micromol g (fresh weight)(-1) h(-1); the apparent K(m) was similar to the apparent K(m) for atmospheric methane consumption, but the V(max) was more than 100 times higher. Atmospheric CO oxidation responded sensitively to soil water regimes; decreases in water content in initially saturated soils resulted in increased uptake, and optimum uptake occurred at water contents of 30 to 60%. However, extended drying led to decreased uptake and net CO production. Rewetting could restore CO uptake, albeit with a pronounced hysteresis. The responses to changing temperatures indicated that the optimum temperature for net uptake was between 20 and 25 degrees C and that there was a transition to net production at temperatures above 30 degrees C. The responses to methyl fluoride and acetylene indicated that populations other than ammonia oxidizers and methanotrophs must be involved in forest soils. The response to acetylene was notable, since the strong initial inhibition was reversed after 12 h of incubation; in contrast, methyl fluoride did not have an inhibitory effect. Ammonium did not inhibit CO uptake; the level of nitrite inhibition was initially substantial, but nitrite inhibition was reversible over time. Nitrite inhibition appeared to occur through indirect effects based on abiological formation of NO.     

Maximal aerobic capacity at several ambient concentrations of CO at several altitudes

To assess the nature of the combined effect of the hypoxias of altitude (ALT) and CO exposure, 11 men and 12 women nonsmokers served as subjects in a double-blind experiment. The exposure conditions were four ambient CO levels (0, 50, 100, and 150 ppm) at each of four ALT (55, 1,524, 2,134, and 3,048 m). Each subject, after attaining the required ALT and ambient CO level, performed a maximal aerobic capacity test (VO2max). Blood samples were obtained before, at 50-W, 100-W, 150-W, and maximum work loads and at the 5th min of recovery. Blood were analyzed for hemoglobin, hematocrit, plasma proteins, lactates, and carboxyhemoglobin (HbCO). VO2max was similar at 55 and 1,524 m and decreased by 4 and 8% from the 55-m value at 2,134 and 3,048 m, respectively. On the basis of all statistical analyses, we concluded that VO2max values measured in men were only slightly diminished due to increased ambient CO. HbCO attained at maximum was highest at 55 m and lowest at 3,048 m. Women's HbCO concentrations were lower than men's. At maximal work loads CO shifted into extravascular spaces and returned to the vascular space within 5 min after exercise stopped. The independence of altitude and CO hypoxias on parameters of the maximum aerobic capacity test and a decrease in the CO to HbCO uptake with increasing altitude were demonstrated and attributed in part to the decrease in driving pressure of CO at altitude.     

The effect of CO2 availability on the growth, iron oxidation and CO2-fixation rates of pure cultures of Leptospirillum ferriphilum and Acidithiobacillus ferrooxidans

Understanding how bioleaching systems respond to the availability of CO(2) is essential to developing operating conditions that select for optimum microbial performance. Therefore, the effect of inlet gas and associated dissolved CO(2) concentration on the growth, iron oxidation and CO(2) -fixation rates of pure cultures of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum was investigated in a batch stirred tank system. The minimum inlet CO(2) concentrations required to promote the growth of At. ferrooxidans and L. ferriphilum were 25 and 70 ppm, respectively, and corresponded to dissolved CO(2) concentrations of 0.71 and 1.57 μM (at 30°C and 37°C, respectively). An actively growing culture of L. ferriphilum was able to maintain growth at inlet CO(2) concentrations less than 30 ppm (0.31-0.45 μM in solution). The highest total new cell production and maximum specific growth rates from the stationary phase inocula were observed with CO(2) inlet concentrations less than that of air. In contrast, the amount of CO(2) fixed per new cell produced increased with increasing inlet CO(2) concentrations above 100 ppm. Where inlet gas CO(2) concentrations were increased above that of air the additional CO(2) was consumed by the organisms but did not lead to increased cell production or significantly increase performance in terms of iron oxidation. It is proposed that At. ferrooxidans has two CO(2) uptake mechanisms, a high affinity system operating at low available CO(2) concentrations, which is subject to substrate inhibition and a low affinity system operating at higher available CO(2) concentrations. L. ferriphilum has a single uptake system characterised by a moderate CO(2) affinity. At. ferrooxidans performed better than L. ferriphilum at lower CO(2) availabilities, and was less affected by CO(2) starvation. Finally, the results demonstrate the limitations of using CO(2) uptake or ferrous iron oxidation data as indirect measures of cell growth and performance across varying physiological conditions.     

Methanogenic archaea and CO2-dependent methanogenesis on washed rice roots

Washed excised roots of rice (Oryza sativa) immediately started to produce CH4 when they were incubated in phosphate buffer under anoxic conditions (N2 atmosphere), with initial rates varying between 2 and 70nmolh(-1)g(-1) dry weight of root material (mean +/- SE: 20.3 +/- 5.9 nmol h(-1) g(-1) dry weight; n = 18). Production of CH4 continued for at least 500 h, with rates usually decreasing slowly. CH4 production was not significantly affected by methyl fluoride, an inhibitor of acetoclastic methanogenesis. Less than 0.5% of added [2-14C]-acetate was converted to 14CH4, and conversion of 14CO2 to 14CH4 indicated that CH4 was almost exclusively produced from CO2. Occasionally, however, especially when the roots were incubated without additional buffer, CH4 production started to accelerate after about 200h reaching rates of > 100 nmol h(-1) g(-1) dry weight. Methyl fluoride inhibited methanogenesis by more than 20% only in these cases, and the conversion of 14CO2 to 14CH4 decreased. These results indicate that CO2-dependent rather than acetoclastic methanogenesis was primarily responsible for CH4 production in anoxically incubated rice roots. Determination of most probable numbers of methanogens on washed roots showed highest numbers (10(6)g(-1) dry roots) on H2 and ethanol, i.e. substrates that support CH4 production from CO2. Numbers on acetate (10(5) g(-1) dry roots) and methanol (10(4)g(-1) dry roots) were lower. Methanogenic consortia enriched on H2 and ethanol were characterized phylogenetically by comparative sequence analysis of archaeal small-subunit (SSU) ribosomal RNA-encoding genes (rDNA). These sequences showed a high similarity to SSU rDNA clones that had been obtained previously by direct extraction of total DNA from washed rice roots. The SSU rDNA sequences recovered from the H2/CO2-using consortium either belonged to a novel lineage of methanogens that grouped within the phylogenetic radiation of the Methanosarcinales and Methanomicrobiales or were affiliated with Methanobacterium bryantii. SSU rDNA sequences retrieved from the ethanol-using consortium either grouped within the genus Methanosarcina or belonged to another novel lineage within the phylogenetic radiation of the Methanosarcinales and Methanomicrobiales. Cultured organisms belonging to either of the two novel lineages have not been reported yet.     

The response of soil microorganisms and roots to elevated CO2 and temperature in a terrestrial model ecosystem

We investigate the response of soil microorganisms to atmospheric CO2 and temperature change within model terrestrial ecosystems in the Ecotron. The model communities consisted of four plant species (Cardamine hirsuta, Poa annua, Senecio vulgaris, Spergula arvensis), four herbivorous insect species (two aphids, a leaf-miner, and a whitefly) and their parasitoids, snails, earthworms, woodlice, soil-dwelling Collembola (springtails), nematodes and soil microorganisms (bacteria, fungi, mycorrhizae and Protista). In two successive experiments, the effects of elevated temperature (ambient plus 2 °C) at both ambient and elevated CO2 conditions (ambient plus 200 ppm) were investigated. A 40:60 sand:Surrey loam mixture with relatively low nutrient levels was used. Each experiment ran for 9 months and soil microbial biomass (Cmic and Nmic), soil microbial community (fungal and bacterial phospholipid fatty acids), basal respiration, and enzymes involved in the carbon cycling (xylanase, trehalase) were measured at depths of 0–2, 0–10 and 10–20 cm. In addition, root biomass and tissue C:N ratio were determined to provide information on the amount and quality of substrates for microbial growth.Elevated temperature under both ambient and elevated CO2 did not show consistent treatment effects. Elevation of air temperature at ambient CO2 induced an increase in Cmic of the 0–10 cm layer, while at elevated CO2 total phospholipid fatty acids (PLFA) increased after the third generation. The metabolic quotient qCO2 decreased at elevated temperature in the ambient CO2 run. Xylanase and trehalase showed no changes in both runs. Root biomass and C:N ratio were not influenced by elevated temperature in ambient CO2. In elevated CO2, however, elevated temperature reduced root biomass in the 0–10 cm and 30–40 cm layers and increased N content of roots in the deeper layers. The different response of root biomass and C:N ratio to elevated temperature may be caused by differences in the dynamics of root decomposition and/or in allocation patterns to coarse or fine roots (i.e. storage vs. resource capture functions). Overall, our data suggests that in soils of low nutrient availability, the effects of climate change on the soil microbial community and processes are likely to be minimal and largely unpredicatable.     

Impacts of plant roots on soil CO cycling and soil–atmosphere CO exchange

Carbon monoxide (CO) plays a major role in tropospheric chemical dynamics. Accordingly, global CO budgets have been reasonably well documented. Atmospheric CO consumption by soils contributes significantly to these budgets, with the magnitude of the sink generally considered to reflect a balance between microbial uptake and abiological production. However, assays of live fine roots showed that diverse intact plants produced carbon monoxide at net rates ranging from 2 to 3000 mµg gdw⁻⁻¹ d⁻⁻¹. CO production was greater for legumes than nonlegumes, and primarily associated with nodules. Excised roots from woody and herbaceous plants produced CO at comparable rates. CO production rates were similar for roots of intact plants and roots excised from those plants. The magnitude of net CO fluxes from roots was determined in part by the balance between simultaneous production and consumption. Surface sterilization of roots indicated that CO consumption was due, in part, rhizoplane CO-oxidizing bacteria, but maximum CO consumption rates were typically only a small fraction of net production rates. Assays in a Brazilian agroecosystem indicated that root CO production affects soil–atmosphere CO exchange. Estimates of global CO production rates indicated that roots contribute about 170–260 Tg CO to the soil atmosphere annually, an amount comparable to current estimates of atmospheric CO uptake by soils, and much larger than estimates of net abiological soil CO production.     

Altering young tomato plant growth by nitrate and CO2 preserves the proportionate relation linking long-term organic-nitrogen accumulation to intercepted radiation

* A previously published model of crop nitrogen (N) status based on intercepted photosynthetically active radiation (R(i), mol per plant) suggested that plant organic N accumulation is related to R(i) by a constant ratio, defined hereafter as the radiation use efficiency for N (NRUE). The aim of this paper was to compare the effects of N nutrition and CO2 enrichment on NRUE and RUE (radiation use efficiency for biomass accumulation). * In three unrelated glasshouse experiments, tomato plants (Solanum lycopersicum) grown in hydroponics were fed for 28 d (exponential growth) with full solutions containing constant NO3(-) concentrations ([NO3(-)]) ranging from 0.05 to 15 mol m(-3), both under ambient or CO2-enriched (1000 microl l(-1)) air. * Each experiment comprised five harvests. Low [NO3(-)] (< 0.3 mol m(-3)) limited growth via leaf area (LA) restriction and decreased light interception. CO2 enrichment enhanced dry weight and LA. RUE was not affected by [NO3(-)], but increased under CO2-enriched air. By contrast, NRUE was not affected by [NO3(-)] or CO2 enrichment. * It is suggested that the radiation efficiency for organic N acquisition (NRUE) did not depend on C or N nutrition for young plants grown under unstressed conditions.     

Nitrate-dependent anaerobic carbon monoxide oxidation by aerobic CO-oxidizing bacteria

Two dissimilatory nitrate-reducing (Burkholderia xenovorans LB400 and Xanthobacter sp. str. COX) and two denitrifying isolates (Stappia aggregata IAM 12614 and Bradyrhizobium sp. str. CPP), previously characterized as aerobic CO oxidizers, consumed CO at ecologically relevant levels (<100 ppm) under anaerobic conditions in the presence, but not absence, of nitrate. None of the isolates were able to grow anaerobically with CO as a carbon or energy source, however, and nitrate-dependent anaerobic CO oxidation was inhibited by headspace concentrations >100-1000 ppm. Surface soils collected from temperate, subtropical and tropical forests also oxidized CO under anaerobic conditions with no lag. The observed activity was 25-60% less than aerobic CO oxidation rates, and did not appear to depend on nitrate. Chloroform inhibited anaerobic but not aerobic activity, which suggested that acetogenic bacteria may have played a significant role in forest soil anaerobic CO uptake.     

Leaf and canopy photosynthetic characteristics of cotton (Gossypium hirsutum) under elevated CO2 concentration and UV-B radiation

Increases in both atmospheric CO2 concentration ([CO2]) and ultraviolet-B (UV-B) radiation on the Earth's surface are features of current climate change patterns. An experiment was conducted in sunlit, controlled environment chambers known as Soil-Plant-Atmosphere-Research (SPAR) units to determine interactive effects of elevated [CO2] and UV-B radiation on leaf and canopy photosynthetic characteristics of cotton. Six treatments were comprised of two CO2 levels of 360 (ambient) and 720 (elevated) microL L(-1) and three levels of 0 (control), 8, and 16 kJ m(-2) d(-1) biologically effective UV-B radiation. Treatments were imposed for 66 days from crop emergence through three weeks after the first flower stage. Plants grown in elevated [CO2] had significantly greater leaf area, higher leaf and canopy net photosynthetic rates (PN), lower dark respiration rate (Rd), and lower light compensation point (LCP) than plants grown in ambient [CO2]. There was no difference in CO2 compensation point (gamma), maximum rate of Rubisco activity (Vcmax), or light-saturated rate of electron transport (Jmax) between ambient and elevated CO2 treatments. When plants were grown in 8 kJ m(-2) d(-1) UV-B radiation, most of the measured photosynthetic parameters did not differ from control plants. High UV-B (16 kJ) radiation, however, caused 47-50% smaller leaf area, 38-44% lower leaf PN, 72-74% lower Vcmax, and 61-66% lower Jmax compared to the control. There were no interactive effects of [CO2] and UV-B radiation on most of the photosynthetic parameters measured. From the results, it is concluded that decreased canopy photosynthesis due to enhanced UV-B radiation in cotton is associated with both smaller leaf area and lower leaf PN, and loss of Rubisco activity and electron transport are two major factors in UV-B inhibition of leaf PN.