CO2浓度倍增对冬小麦生长发育产量形成及发芽率的影响

利用OTC-1型开顶式气室进行了CO₂浓度倍增对冬小麦影响的诊断试验，结果表明，CO₂浓度倍增对冬小麦生长发育、叶面积变化、生物量及产量形成等影响显著，且均为正效应。     

The counteracting effects of elevated atmospheric CO2 concentrations and drought episodes: Studies of enchytraeid communities in a dry heathland

The potential impacts of interactions of multiple climate change factors in soil ecosystems have received little attention. Most studies have addressed effects of single factors such as increased temperature or atmospheric CO₂ but little is known about how such environmental factors will interact. In the present study we investigate the effects of in situ exposure to elevated atmospheric CO₂ concentration, increased temperatures and prolonged drought episodes on field communities of Enchytraeidae (Oligochaeta) in a dry heathland (Brandbjerg, Denmark). Increased CO₂ had a positive effect on enchytraeid biomass, whereas drought significantly reduced it. Elevated temperature did not result in any detectable effects. No interactions between the three factors were observed. Interestingly, the positive effect of increased CO₂ and the negative effect of drought were cancelled out when applied in combination. Thus, in the combined drought and CO₂ treatment, and when additionally combined with increased temperature, the total biomass of enchytraeids was similar to those in the ambient plots. The positive effect of increased CO₂ seemed to be influenced by season, as the response was most pronounced during summer, and less so in autumn. The impact of the drought was more consistent, with enchytraeids reduced at all sampling occasions. Moreover, the negative effect of drought seemed to depend on the inter-annual variability of precipitation. The year with a dry summer and autumn (2006) showed a stronger impact of drought on the enchytraeids, compared to the year with a wet summer and autumn (2007). Our study emphasises the importance of multi-factorial experimental design as a means to investigate effects of climatic changes.     

Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2

Elevated CO₂ may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO₂-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO₂. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to ¹³C labeling at ambient (392 μmol mol⁻¹) and elevated (792 μmol mol⁻¹) CO₂ concentrations, in soil containing ¹⁵N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-¹³C was enhanced by 50% under elevated compared to ambient CO₂. Concurrently, plant ¹⁵N uptake increased (+7%) under elevated CO₂ while ¹⁵N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO₂. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO₂ can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO₂.     

Long term CO2 enrichment stimulates N-mineralisation and enzyme activities in calcareous grassland

Elevated concentration of atmospheric carbon dioxide will affect carbon cycling in terrestrial ecosystems. Possible effects include increased carbon input into the soil through the rhizosphere, altered nutrient concentrations of plant litter and altered soil moisture. Consequently, the ongoing rise in atmospheric carbon dioxide might indirectly influence soil biota, decomposition and nutrient transformations.N-mineralisation and activities of the enzymes invertase, xylanase, urease, protease, arylsulfatase, and alkaline phosphatase were investigated in spring and summer in calcareous grassland, which had been exposed to ambient and elevated CO₂ concentrations (365 and 600 μl l⁻¹) for six growing seasons.In spring, N-mineralisation increased significantly by 30% at elevated CO₂, while there was no significant difference between treatments in summer (+3%). The response of soil enzymes to CO₂ enrichment was also more pronounced in spring, when alkaline phosphatase and urease activities were increased most strongly by 32 and 21%. In summer, differences of activities between CO₂ treatments were greatest in the case of urease and protease (+21 and +17% at elevated CO₂).The stimulation of N-mineralisation and enzyme activities at elevated CO₂ was probably caused by higher soil moisture and/or increased root biomass. We conclude that elevated CO₂ will enhance below-ground C- and N-cycling in grasslands.     

不同氮素水平下冬小麦叶片酚酸类物质代谢对FACE的响应

利用开放式空气CO2浓度升高(Free Air Carbon-dioxide Enrichment, FACE)平台, 研究了低氮(LN)和常氮(NN)水平下, 大气CO2浓度升高对冬小麦叶片酚酸类物质代谢的影响.结果表明, CO2浓度升高对小麦叶片水杨酸、对羟基苯甲酸、肉桂酸、阿魏酸和香草酸含量的影响随供氮水平的不同而有所差异.低氮下小麦通过提高叶片苯丙氨酸解氨酶(PAL)活性(30.1%)而使其含量均显著增加, 增幅分别达33.7%、119.6%、26.7%、39.9%和28.6%; 而常氮下PAL活性和酚酸类含量变化均未达显著水平.可见, 大气CO2浓度升高对冬小麦酚酸类物质代谢的影响受氮水平的调控, 在未来CO2浓度升高条件下, 选择适宜的施肥水平将显得更为重要.此外, 总酚含量与水杨酸、对羟基苯甲酸、肉桂酸、阿魏酸和香草酸等含量变化趋势基本一致, 且总酚含量变化的79.6%～151.4%是由这几种酚酸含量变化引起的, 说明CO2浓度升高使水杨酸、对羟基苯甲酸、肉桂酸、阿魏酸和香草酸等含量增加是总酚含量增加的直接原因.低氮条件下大气CO2浓度升高将通过改变酚酸类物质代谢而间接影响小麦与伴生杂草的关系.     

Repeated CO2 pulse-labelling reveals an additional net gain of soil carbon during growth of spring wheat under free air carbon dioxide enrichment (FACE)

Rising levels of atmospheric CO₂ have often been found to increase above and belowground biomass production of C3 plants. The additional translocation of organic matter into soils by increased root mass and exudates are supposed to possibly increase C pools in terrestrial ecosystems. Corresponding investigations were mostly conducted under more or less artificial indoor conditions with disturbed soils. To overcome these limitations, we conducted a ¹⁴CO₂ pulse-labelling experiment within the German FACE project to elucidate the role of an arable crop system in carbon sequestration under elevated CO₂. We cultivated spring wheat cv. “Minaret” with usual fertilisation and ample water supply in stainless steel cylinders forced into the soil of a control and a FACE plot. Between stem elongation and beginning of ripening the plants were repeatedly pulse-labelled with ¹⁴CO₂ in the field. Soil born total CO₂ and ¹⁴CO₂ was monitored daily till harvest. Thereafter, the distribution of ¹⁴C was analysed in all plant parts, soil, soil mineral fractions and soil microbial biomass. Due to the small number of grown wheat plants (40) in each ring and the inherent low statistical power, no significant above and belowground growth effect of elevated CO₂ was detected at harvest. But in comparison to ambient conditions, 28% more ¹⁴CO₂ and 12% more total CO₂ was evolved from soil under elevated CO₂ (550 μmol CO₂ mol⁻¹). In the root-free soil 27% more residual ¹⁴C was found in the FACE soil than in the soil from the ambient ring. In soil samples from both treatments about 80% of residual ¹⁴C was found in the clay fraction and 7% in the silt fraction. Very low ¹⁴C contents in the CFE extracts of microbial biomass in the soil from both CO₂ treatments did not allow assessing their influence on this parameter. Since the calculated specific radioactivity of soil born ¹⁴CO₂ gave no indication of an accelerated priming effect in the FACE soil, we conclude that wheat plants grown under elevated CO₂ can contribute to an additional net carbon gain in soils.     

Kinetics of the respiratory response of the soil and rhizosphere microbial communities in a field experiment with an elevated concentration of atmospheric CO2

The effect of an elevated concentration of atmospheric CO₂ and the application rate of nitrogen fertilizers on the microbial biomass and maximum specific growth rate of microorganisms in the soil and rhizosphere was studied in a long-term field experiment involving the growing of sugar beets and winter wheat. It was shown that the treatment of field plots with carbon dioxide at a concentration higher than that in the atmosphere (550 ppm) for three-four years resulted in the formation of a microbial community with a higher maximum specific growth rate and a larger share of R-strategy microorganisms as compared to the soil under the control plants. No reliable differences in the total microbial biomass in the soil under the winter wheat were revealed between the treatments with the ambient and elevated CO₂ concentrations; in the soil under the beet plants, a reliable increase in the total microbial biomass at the elevated CO₂ concentration was noted only in the close vicinity of the plant roots.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Effects of elevated CO2 and O3 on soil respiration under ponderosa pine

Soil respiration represents the integrated response of plant roots and soil organisms to environmental conditions and the availability of C in the soil. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing a reconstructed ponderosa pine/soil-litter system. The study used a 2×2 factorial design with two levels of CO₂ and two levels of O₃ and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO₂ and O₃, singly and in combination, on soil respiration, fine root growth and soil organisms. Fine root growth and soil organisms were included in the study as indicators of the autotrophic and heterotrophic components of soil respiration. The study evaluated three hypotheses: (1) elevated CO₂ will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O₃ will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO₂ and O₃ have opposing effects on C assimilation and allocation, elevated CO₂ will eliminate or reduce the negative effects of elevated O₃ on soil respiration. A mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting when testing for the effects of CO₂ and O₃ on soil respiration. The covariance analysis showed that elevated CO₂ significantly reduced the soil respiration while elevated O₃ had no significant effect. Despite the lack of a direct CO₂ stimulation of soil respiration, there were significant interactions between CO₂ and soil temperature, soil moisture and days from planting indicating that elevated CO₂ altered soil respiration indirectly. In elevated CO₂, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO₂. Soil respiration increased more with days from planting in elevated than in ambient CO₂. Elevated CO₂ had no effect on fine root biomass but increased abundance of culturable bacteria and fungi suggesting that these increases were associated with increased C allocation belowground. Elevated CO₂ had no significant effect on microarthropod and nematode abundance. Elevated O₃ had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature.     

Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands

Several recent studies have indicated that an enriched atmosphere of carbon dioxide (CO₂) could exacerbate the intensity of plant invasions within natural ecosystems, but little is known of how rising CO₂ impacts the belowground characteristics of these invaded systems. In this study, we examined the effects of elevated CO₂ and nitrogen (N) inputs on plant and soil microbial community characteristics of plant communities invaded by reed canary grass, Phalaris arundinacea L. We grew the invasive grass under two levels of invasion: the invader was either dominant (high invasion) at >90% plant cover or sub-dominant (low invasion) at <50% plant cover. Experimental wetland communities were grown for four months in greenhouses that received either 600 or 365 μl l⁻¹ (ambient) CO₂. Within each of three replicate rooms per CO₂ treatment, the plant communities were grown under high (30 mg l⁻¹) or low (5 mg l⁻¹) N. In contrast to what is often predicted under N limitation, we found that elevated CO₂ increased native graminoid biomass at low N, but not at high N. The aboveground biomass of reed canary grass did not respond to elevated CO₂, despite it being a fast-growing C3 species. Although elevated CO₂ had no impact on the plant biomass of heavily invaded communities, the relative abundance of several soil microbial indicators increased. In contrast, the moderately invaded plant communities displayed increased total root biomass under elevated CO_2, while little impact occurred on the relative abundance of soil microbial indicators. Principal components analysis indicated that overall soil microbial community structure was distinct by CO₂ level for the varying N and invasion treatments. This study demonstrates that even when elevated CO₂ does not have visible effects on aboveground plant biomass, it can have large impacts belowground.     

Switchgrass Influences on Soil Biogeochemical Processes in the Dryland Region of the Pacific Northwest

Switchgrass and other perennial grasses have been promoted as biomass crops for production of renewable fuels. The objective of this study was to evaluate the effect of biomass removal on soil biogeochemical processes. A 3-year field study consisting of three levels of net primary productivity (NPP; low, medium, and high growing season precipitation) and two biomass crops (winter wheat and switchgrass) was conducted near Pendleton, Oregon. Switchgrass increased soil carbon (C)–nitrogen (N) ratio, but the effect varied with net primary productivity (NPP) and soil depth. In situ soil respiration (carbon dioxide; CO₂) rate from switchgrass increased with NPP level but switchgrass had greater cumulative flux than wheat in medium and low NPP. Nitrogen mineralization and microbial biomass carbon were significantly greater under switchgrass than under wheat at high and medium NPP. Introduction of switchgrass initiates major changes in soil nutrient dynamics through organic-matter input.     

Effects of elevated CO2 concentration on rhizodeposition from Lolium perenne grown on soil exposed to 9 years of CO2 enrichment

The effects of enriched CO₂ atmosphere on partitioning of recently assimilated carbon were investigated in a plant-soil-microorganism system in which Lolium perenne seedlings were planted into cores inserted into the resident soil within a sward that had been treated with elevated CO₂ for 9 consecutive years, under two N fertilisation levels (Swiss FACE experiment). The planted cores were excavated from the ambient (35 Pa pCO₂) and enriched (60 Pa pCO₂) rings at two dates, in spring and autumn, during the growing season. The cores were brought back to the laboratory for ¹⁴C labelling of shoots in order to trace the transfer of recently assimilated C both within the plant and to the soil and microbial biomass. At the spring sampling, high N supply stimulated shoot and total dry matter production. Consistently, high N enhanced the allocation of recently fixed C to shoots, and reduced it to belowground compartments. Elevated CO₂ had no consequences for DM or the pattern of C allocation. At the autumn sampling, at high N plot, yield of L. perenne was stimulated by elevated CO₂. Consistently, ¹⁴C was preferentially allocated aboveground and, consequently belowground recent C allocation was depressed and rhizodeposition reduced. At both experimental periods, total soil C content was similar in all treatments, providing no evidence for soil carbon sequestration in the Swiss Free Air CO₂ Enrichment experiment (FACE) after 9 years of enrichment. Recently assimilated C and soil C were mineralised faster in soils from enriched rings, suggesting a CO₂-induced shift in the microbial biomass characteristics (structure, diversity, activity) and/or in the quality of the root-released organic compounds.     

增施CO2降低小白菜硝酸盐积累的机理研究

以低硝酸盐积累基因型（东妃）和高硝酸盐积累基因型（高雄甜脆）两种小白菜为材料,采用溶液培养法研究了增施CO2降低蔬菜硝酸盐积累的生理机制。结果表明,CO2浓度升高能显著提高2种基因型小白菜的生物量和硝酸还原酶活性,并降低根、茎叶各部位的硝酸盐含量。CO2浓度升高不仅促进了植株对硝态氮的吸收,而且植株吸收硝酸盐的累积量增幅均高于鲜重的增幅。由此可见,除了鲜重增加的稀释作用,处理后生理机制的变化也可能是CO2浓度升高引起硝酸盐含量降低的重要原因。研究还表明,增施CO2后“东妃”的硝酸盐含量降低百分率与硝酸还原酶活性的增加百分率呈极显著相关,而“高雄甜脆”的硝酸盐含量降低百分率则与鲜重的增加百分率的相关性达极显著水平。说明增施CO2后植株各部位硝酸还原酶活性提高及鲜重的增加均为引起硝酸盐含量降低的重要原因,但贡献率具有明显的基因型差异。     

Interactive effects of elevated CO2 and nitrogen fertilization levels on photosynthesized carbon allocation in a temperate spring wheat and soil system

Increasing atmospheric CO2 concentration impacts the terrestrial carbon(C) cycle by affecting plant photosynthesis, the flow of photosynthetically fixed C belowground, and soil C pool turnover. For managed agroecosystems, how and to what extent the interactions between elevated CO2 and N fertilization levels influence the accumulation of photosynthesized C in crops and the incorporation of photosynthesized C into arable soil are in urgent need of exploration.We conducted an experiment simulating elevated CO2 with spring wheat(Triticum aestivum L.) planted in growth chambers.13C-enriched CO2 with an identical 13C abundance was continuously supplied at ambient and elevated CO2 concentrations(350 and 600 μmol mol-1, respectively) until wheat harvest.Three levels of N fertilizer application(equivalent to 80, 120, and 180 kg N ha-1 soil) were supplied for wheat growth at both CO2 concentrations. During the continuous 62-d 13CO2 labeling period, elevated CO2 and increased N fertilizer application increased photosynthesized C accumulation in wheat by 14%–24% and 11%–20%, respectively, as indicated by increased biomass production, whereas the C/N ratio in the roots increased under elevated CO2 but declined with increasing N fertilizer application levels. Wheat root deposition induced 1%–2.5% renewal of soil C after 62 d of 13CO2 labeling. Compared to ambient CO2, elevated CO2 increased the amount of photosynthesized C incorporated into soil by 20%–44%. However, higher application rates of N fertilizer reduced the net input of root-derived C in soil by approximately 8% under elevated CO2. For the wheat-soil system, elevated CO2 and increased N fertilizer application levels synergistically increased the amount of photosynthesized C. The pivotal role of plants in photosynthesized C accumulation under elevated CO2 was thereby enhanced in the short term by the increased N application. Therefore, robust N management could mediate C cycling and sequestration by influencing the interactions between plants and soil in agroecosystems under elevated CO2.     

Elevated CO2 increases the effect of an arbuscular mycorrhizal fungus and a plant-growth-promoting rhizobacterium on structural stability of a semiarid agricultural soil under drought conditions

In arid and semiarid Mediterranean regions, an increase in the severity of drought events could be caused by rising atmospheric CO₂ concentrations. We studied the effects of the interaction of CO₂, water supply and inoculation with a plant-growth-promoting rhizobacterium (PGPR), Pseudomonas mendocina Palleroni, or inoculation with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith), on aggregate stabilisation of the rhizosphere soil of Lactuca sativa L. cv. Tafalla. The influence of such structural improvements on the growth of lettuce was evaluated. We hypothesised that elevated atmospheric CO₂ concentration would increase the beneficial effects of inoculation with a PGPR or an AM fungus on the aggregate stability of the rhizosphere soil of lettuce plants. Leaf hydration, shoot dry biomass and mycorrhizal colonisation were decreased significantly under water-stress conditions, but this decrease was more pronounced under ambient vs elevated CO₂. The root biomass decreased under elevated CO₂ but only in non-stressed plants. Under elevated CO₂, the microbial biomass C of the rhizosphere of the G. intraradices-colonised plants increased with water stress. Bacterial and mycorrhizal inoculation and CO₂ had no significant effect on the easily-extractable glomalin concentration. Plants grown under elevated CO₂ had a significantly higher percentage of stable aggregates under drought stress than under well-watered conditions, particularly the plants inoculated with either of the assayed microbial inocula (about 20% higher than the control soil). We conclude that the contribution of mycorrhizal fungi and PGPR to soil aggregate stability under elevated atmospheric CO₂ is largely enhanced by soil drying.     

Elevated CO2 stimulates N2O emissions in permanent grassland

To evaluate climate forcing under increasing atmospheric CO₂ concentrations, feedback effects on greenhouse gases such as nitrous oxide (N₂O) with a high global warming potential should be taken into account. This requires long-term N₂O flux measurements because responses to elevated CO₂ may vary throughout annual courses. Here, we present an almost 9 year long continuous N₂O flux data set from a free air carbon dioxide enrichment (FACE) study on an old, N-limited temperate grassland. Prior to the FACE start, N₂O emissions were not different between plots that were later under ambient (A) and elevated (E) CO₂ treatments, respectively. However, over the entire experimental period (May 1998–December 2006), N₂O emissions more than doubled under elevated CO₂ (0.90 vs. 2.07 kg N₂O-N ha⁻¹ y⁻¹ under A and E, respectively). The strongest stimulation occurred during vegetative growth periods in the summer when soil mineral N concentrations were low. This was surprising because based on literature we had expected the highest stimulation of N₂O emissions due to elevated CO₂ when mineral N concentrations were above background values (e.g. shortly after N application in spring). N₂O emissions under elevated CO₂ were moderately stimulated during late autumn–winter, including freeze–thaw cycles which occurred in the 8th winter of the experiment. Averaged over the entire experiment, the additional N₂O emissions caused by elevated CO₂ equaled 4738 kg CO₂-equivalents ha⁻¹, corresponding to more than half a ton (546 kg) of CO₂ ha⁻¹ which has to be sequestered annually to balance the CO₂-induced N₂O emissions. Without a concomitant increase in C sequestration under rising atmospheric CO₂ concentrations, temperate grasslands may be converted into greenhouse gas sources by a positive feedback on N₂O emissions. Our results underline the need to include continuous N₂O flux measurements in ecosystem-scale CO₂ enrichment experiments.     

Elevated carbon dioxide and nitrogen supply affect photosynthesis and nitrogen partitioning of two wheat varieties

The response of wheat to elevated carbon dioxide concentration (e[CO₂]) is likely to be dependent on nitrogen supply. To investigate the underlying mechanism of growth response to e[CO₂], two wheat cultivars were grown under different carbon dioxide concentration [CO₂] in a chamber experimental facility. The changes in leaf photosynthesis, C and N concentration, and biomass were investigated under different [CO₂] and N supply. The result showed an increase in photosynthesis under e[CO₂] at all N level except the one with the lowest N supply. Furthermore, a significant decrease in g_s and Tr for both the cultivars was also observed under e[CO₂] at all N levels. A considerable increase in WUEi was observed for both the cultivars under e[CO₂] at all N levels except for the lowest concentration one. Therefore, the study shows that a stimulation of plant growth under e[CO₂] to be marginal at higher N supply.     

Short and long-term effects of elevated CO2 on Lolium perenne rhizodeposition and its consequences on soil organic matter turnover and plant N yield

It is still unclear whether elevated CO₂ increases plant root exudation and consequently affects the soil microbial biomass. The effects of elevated CO₂ on the fate of the C and nitrogen (N) contained in old soil organic matter pools is also unclear. In this study the short and long-term effects of elevated CO₂ on C and N pools and fluxes were assessed by growing isolated plants of ryegrass (Lolium perenne) in glasshouses at elevated and ambient atmospheric CO₂ and using soil from the New Zealand FACE site that had >4 years exposure to CO₂ enrichment. Using ¹⁴CO₂ pulse labelling, the effects of elevated CO₂ on C allocation within the plant-soil system were studied. Under elevated CO₂ more root derived C was found in the soil and in the microbial biomass 48 h after labelling. The increased availability of substrate significantly stimulated soil microbial growth and acted as priming effect, enhancing native soil organic matter decomposition regardless of the mineral N supply. Despite indications of faster N cycling in soil under elevated CO₂, N availability to plants stayed unchanged. Soil previously exposed to elevated CO₂ exhibited a higher N cycling rate but again there was no effect on plant N uptake. With respect to the difficulties of extrapolating glasshouse experiment results to the field, we concluded that the accumulation of coarse organic matter observed in the field under elevated CO₂ was probably not created by an imbalance between C and N but was likely to be due to more complex phenomena involving soil mesofauna and/or other nutrients limitations.     

Soil carbon dioxide flux, carbon sequestration and crop productivity in a tropical dryland agroecosystem: Influence of organic inputs of varying resource quality

In view of the significance of agricultural soils in affecting global C balance, the impact of manipulation of the quality of exogenous inputs on soil CO₂–C flux was studied in rice–barley annual rotation tropical dryland agroecosystem. Chemical fertilizer, Sesbania shoot (high quality resources), wheat straw (low quality resource) and Sesbania + wheat straw (high + low quality), all carrying equivalent recommended dose of N, were added to soil. A distinct seasonal variation in CO₂–C flux was recorded in all treatments, flux being higher during rice period, and much reduced during barley and summer fallow periods. During rice period the mean CO₂–C flux was greater in wheat straw (161% increase over control) and Sesbania + wheat straw (+129%) treatments; however, during barley and summer fallow periods differences among treatments were small. CO₂–C flux was more influenced by seasonal variations in water-filled pore space compared to soil temperature. In contrast, the role of microbial biomass and live crop roots in regulating soil CO₂–C flux was highly limited. Wheat straw input showed smaller microbial biomass with a tendency of rapid turnover rate resulting in highest cumulative CO₂–C flux. The Sesbania input exhibited larger microbial biomass with slower turnover rate, leading to lower cumulative CO₂–C flux. Addition of Sesbania to wheat straw showed higher cumulative CO₂–C flux yet supported highest microbial biomass with lowest turnover rate indicating stabilization of microbial biomass. Although single application of wheat straw or Sesbania showed comparable net change in soil C (18% and 15% relative to control, respectively) and crop productivity (32% and 38%), yet they differed significantly in soil C balance (374 and −3 g C m⁻² y⁻¹ respectively), a response influenced by the recalcitrant and labile nature of the inputs. Combining the two inputs resulted in significant increment in net change in soil C (33% over control) and crop yield (49%) in addition to high C balance (152 g C m⁻² y⁻¹). It is suggested that appropriate mixing of high and low quality inputs may contribute to improved crop productivity and soil fertility in terms of soil C sequestration.     

Effect of elevated carbon dioxide and water stress on gas exchange and water use efficiency in corn

CO₂ has been predicted to increase in the future, and thus leading to possible changes in precipitation patterns. The objectives of this study were to investigate water use and canopy level photosynthesis of corn plants, and to quantify water use efficiency in corn plants under two different CO₂ levels combined with four different water stress levels. Corn plants were planted in sunlit plant growth chambers and a day/night temperature of (28/18 °C) was applied. From 21 days after emergence (DAE), the eight treatments including two levels of carbon dioxide concentrations (400 and 800 μmol mol⁻¹) and four levels of water stress (well-watered control, “mild”, “moderate”, and “severe” water stress) treatments at each CO₂ level were imposed. Height, number of leaves, leaf lengths, and growth stages of corn plants were monitored from nine plants twice a week. Corn plants were separately collected, dried, and analyzed for the biomass accumulation at 21 and 60 DAE. Soil water contents were monitored by a time domain reflectometry (TDR) system (15 probes per chamber). The “breaking points” (changes from high to low rates of soil water uptake) were observed in the bottom of soil depth for the water stressed conditions, and the “breaking points” under ambient CO₂ appeared 6-9 days earlier than under elevated CO₂. Although approximately 20-49% less water was applied for the elevated CO₂ treatments than for ambient CO₂ from 21 DAE, higher soil water contents were recorded under elevated CO₂ than under ambient CO₂. However, corn growth variables such as height, leaf area, and biomass accumulation were not significantly different in CO₂ or water stressed treatments. This result may be explained by considering that significant differences in canopy level gross photosynthesis among the water stress treatments was observed only toward the end of the experiment. The higher soil water contents observed under elevated CO₂ resulted mainly from less water use than under ambient CO_2. WUE (above ground biomass per water use since 21 DAE) at the final harvest was consistently higher and varied with a smaller range under elevated CO₂ than under ambient CO₂. This study suggests that less water will be required for corn under high-CO₂ environment in the future than at present.