Responses of net ecosystem CO2 exchange in managed grassland to long-term CO2 enrichment, N fertilization and plant species

The effects of elevated pCO₂ on net ecosystem CO₂ exchange were investigated in managed Lolium perenne (perennial ryegrass) and Trifolium repens (white clover) monocultures that had been exposed continuously to elevated pCO₂ (60 Pa) for nine growing seasons using Free Air CO₂ Enrichment (FACE) technology. Two levels of nitrogen (N) fertilization were applied. Midday net ecosystem CO₂ exchange (mNEE) and night-time ecosystem respiration (NER) were measured in three growing seasons using an open-flow chamber system. The annual net ecosystem carbon (C) input resulting from the net CO₂ fluxes was estimated for one growing season. In both monocultures and at both levels of N supply, elevated pCO₂ stimulated mNEE by up to 32%, the exact amount depending on intercepted PAR. The response of mNEE to elevated pCO₂ was larger than that of harvestable biomass. Elevated pCO₂ increased NER by up to 39% in both species at both levels of N supply. NER, which was affected by mNEE of the preceding day, was higher in T. repens than in L. perenne. High N increased NER compared to low N supply. According to treatment, the annual net ecosystem C input ranged between 210 and 631 g C m⁻² year⁻¹ and was not significantly affected by the level of pCO₂. Low N supply led to a higher net C input than high N supply. We demonstrated that at the ecosystem level, there was a long-term stimulation in the net C assimilation during daytime by elevated pCO₂. However, because NER was also stimulated, net ecosystem C input was not significantly increased at elevated pCO₂. The annual net ecosystem C input was primarily affected by the amount of N supplied.     

Species of plants and associated arbuscular mycorrhizal fungi mediate mycorrhizal responses to CO2 enrichment

It has been suggested that enrichment of atmospheric CO₂ should alter mycorrhizal function by simultaneously increasing nutrient‐uptake benefits and decreasing net C costs for host plants. However, this hypothesis has not been sufficiently tested. We conducted three experiments to examine the impacts of CO₂ enrichment on the function of different combinations of plants and arbuscular mycorrhizal (AM) fungi grown under high and low soil nutrient availability. Across the three experiments, AM function was measured in 14 plant species, including forbs, C₃ and C₄ grasses, and plant species that are typically nonmycorrhizal. Five different AM fungal communities were used for inoculum, including mixtures of Glomus spp. and mixtures of Gigasporaceae (i.e. Gigaspora and Scutellospora spp.). Our results do not support the hypothesis that CO₂ enrichment should consistently increase plant growth benefits from AM fungi, but rather, we found CO₂ enrichment frequently reduced AM benefits. Furthermore, we did not find consistent evidence that enrichment of soil nutrients increases plant growth responses to CO₂ enrichment and decreases plant growth responses to AM fungi. Our results show that the strength of AM mutualisms vary significantly among fungal and plant taxa, and that CO₂ levels further mediate AM function. In general, when CO₂ enrichment interacted with AM fungal taxa to affect host plant dry weight, it increased the beneficial effects of Gigasporaceae and reduced the benefits of Glomus spp. Future studies are necessary to assess the importance of temperature, irradiance, and ambient soil fertility in this response. We conclude that the affects of CO₂ enrichment on AM function varies with plant and fungal taxa, and when making predictions about mycorrhizal function, it is unwise to generalize findings based on a narrow range of plant hosts, AM fungi, and environmental conditions.     

Above- and below-ground responses of C3–C4 species mixtures to elevated CO2 and soil water availability

We evaluated the influences of CO₂[Control, ～ 370 µ mol mol ^{? 1}; 200 µ mol mol ^{? 1} above ambient applied by free‐air CO₂ enrichment (FACE)] and soil water (Wet, Dry) on above‐ and below‐ground responses of C₃ (cotton, Gossypium hirsutum) and C₄ (sorghum, Sorghum bicolor) plants in monocultures and two density mixtures. In monocultures, CO₂ enrichment increased height, leaf area, above‐ground biomass and reproductive output of cotton, but not sorghum, and was independent of soil water treatment. In mixtures, cotton, but not sorghum, above‐ground biomass and height were generally reduced compared to monocultures, across both CO₂ and soil water treatments. Density did not affect individual plant responses of either cotton or sorghum across the other treatments. Total (cotton + sorghum) leaf area and above‐ground biomass in low‐density mixtures were similar between CO₂ treatments, but increased by 17–21% with FACE in high‐density mixtures, due to a 121% enhancement of cotton leaf area and a 276% increase in biomass under the FACE treatment. Total root biomass in the upper 1.2 m of the soil was not influenced by CO₂ or by soil water in monoculture or mixtures; however, under dry conditions we observed significantly more roots at lower soil depths ( > 45 cm). Sorghum roots comprised 81–85% of the total roots in the low‐density mixture and 58–73% in the high‐density mixture. CO₂‐enrichment partly offset negative effects of interspecific competition on cotton in both low‐ and high‐density mixtures by increasing above‐ground biomass, with a greater relative increase in the high‐density mixture. As a consequence, CO₂‐enrichment increased total above‐ground yield of the mixture at high density. Individual plant responses to CO₂ enrichment in global change models that evaluate mixed plant communities should be adjusted to incorporate feedbacks for interspecific competition. Future field studies in natural ecosystems should address the role that a CO₂‐mediated increase in C₃ growth may have on subsequent vegetation change.     

The effects of elevated CO2 concentrations on the root growth of Lolium perenne and Trifolium repens grown in a FACE* system

Lolium perenne and Trifolium repens were grown in a Free Air CO₂ Enrichment (FACE) system at elevated (600 μimol mol-¹) and ambient (340 μmol mol-¹) carbon dioxide concentrations during a whole growing season. Using a root ingrowth bag technique the extent to which CO₂ enrichment influenced the growth of L, perenne and T. repens roots under two contrasting nutrient regimes was examined. Root ingrowth bags were inserted for a fixed time into the soil in order to trap roots. It was also possible to follow the mortality of roots in bags inserted for different time intervals. Root ingrowth of both L. perenne and T. repens increased under elevated CO₂ conditions. In L. perenne, root ingrowth decreased with increasing nutrient fertilizer level, but for T. repens the root ingrowth was not affected by the nutrient application rate. Besides biomass measurements, root length estimates were made for T, repens. These showed an increase under elevated CO₂ concentrations. Root decomposition appeared to decrease under elevated CO₂ concentrations. A possible explanation for this effect is the observed changes in tissue composition, such as the increase in the carbon: nitrogen ratio in roots of L. perenne at elevated CO₂ concentrations.     

Saprobic microfungi under Lolium perenne and Trifolium repens at different fertilization intensities and elevated atmospheric CO2 concentration

Anthropogenic increases in atmospheric CO₂ concentration and the connected deposition of organic matter into the soil influence the occurrence of decomposers who regulate carbon release back into the atmosphere. The effects of increased concentration of atmospheric carbon dioxide, plant species cover quality and nitrogen (N) fertilization on the coenosis composition of soil saprobic microfungi were studied under field conditions (Swiss Free Air Carbon Dioxide Enrichment experiment). In total, 42 species of microfungi were detected in examined soil. The most significant response of soil mycoflora was induced by the species identity of plant cover. Higher N fertilization significantly suppressed the abundance of soil microfungi at ambient CO₂. The effect of increased CO₂ on colony‐forming units was not significant when taken as an independent treatment; however, this factor interacted significantly with N availability. Some species, e.g. the Clonostachys rosea, were proven associated with the plant cover components, in this particular case with Trifolium repens. Therefore, we suggest the identity of plant species constituting plant cover as the most important factors affecting soil microfungi in agroecosystems.     

P uptake by arbuscular mycorrhizal hyphae: effect of soil temperature and atmospheric CO2 enrichment

Mycorrhizas are ubiquitous symbioses that may have an important role in the movement of C from air to soil. Studies on the effects of climate change factors on mycorrhizas have been concentrated on the effects of atmospheric [CO₂] whereas temperature effects have been neglected. Based on previous results showing no effect of varying atmospheric [CO₂] on the development and P uptake of the arbuscular mycorrhizal fungi (AMF) colonizing plants growing in controlled conditions, we hypothesized that soil temperature would have a higher impact on AMF development and nutrient uptake than the effects of [CO₂] on the host plant. Pea plants were grown in association with either a single isolate of Glomus caledonium or AMF from field soil in factorial combination with the corresponding current (10 °C) or elevated (15 °C) soil temperatures at current (350 p.p.m) or elevated (700 p.p.m) atmospheric [CO₂]. ³³P uptake by extraradical AMF hyphae was measured independently from root P uptake in a root exclusion compartment. Intraradical colonization developed well at both soil temperatures and almost duplicated from 10 to 15 °C. Extraradical mycelium developed only at 15 °C in the root exclusion compartment and hyphal P uptake could therefore be studied at 15 °C only. Hyphal P uptake differed markedly between inoculum types, but was not altered by growing the host plants at two atmospheric [CO₂] levels. No significant [CO₂] × soil temperature interactions were observed. The results suggested that, in the system tested, AMF development and function is likely more influenced by the temperature component of climate change than by its [CO₂] component. We suggest that much more attention should be paid to temperature effects in future studies.     

Ten years of free-air CO2 enrichment altered the mobilization of N from soil in Lolium perenne L. swards

Effects of free‐air carbon dioxide enrichment (FACE, 60 Pa pCO₂) on plant growth as compared with ambient pCO₂ (36 Pa) were studied in swards of Lolium perenne L. (perennial ryegrass) at two levels of N fertilization (14 and 56 g m^?2 a^?1) from 1993 to 2002. The objectives were to determine how plant growth responded to the availability of C and N in the long term and how the supply of N to the plant from the two sources of N in the soil, soil organic matter (SOM) and mineral fertilizer, varied over time. In three field experiments, ¹⁵N‐labelled fertilizer was used to distinguish the sources of available N. In 1993, harvestable biomass under elevated pCO₂ was 7% higher than under ambient pCO₂. This relative pCO₂ response increased to 32% in 2002 at high N, but remained low at low N. Between 1993 and 2002, the proportions and amounts of N in harvestable biomass derived from SOM (excluding remobilized fertilizer) were, at high N, increasingly higher at elevated pCO₂ than at ambient pCO₂. Two factorial experiments confirmed that at high N, but not at low N, a higher proportion of N in harvestable biomass was derived from soil (including remobilized fertilizer) following 7 and 9 years of elevated pCO₂, when compared with ambient pCO₂. It is suggested that N availability in the soil initially limited the pCO₂ response of harvestable biomass. At high N, the limitation of plant growth decreased over time as a result of the stimulated mobilization of N from soil, especially from SOM. Consequently, harvestable biomass increasingly responded to elevated pCO₂. The underlying mechanisms which contributed to the increased mobilization of N from SOM under elevated pCO₂ are discussed. This study demonstrated that there are feedback mechanisms in the soil which are only revealed during long‐term field experiments. Such investigations are thus, a prerequisite for understanding the responses of ecosystems to elevated pCO₂ and N supply.     

Arbuscular mycorrhizal colonization on carbon economy in perennial ryegrass: quantification by 13CO2/12CO2 steady-state labelling and gas exchange

Effects of the arbuscular mycorrhizal fungus (AMF) Glomus hoi on the carbon economy of perennial ryegrass (Lolium perenne) were investigated by comparing nonmycorrhizal and mycorrhizal plants of the same size, morphology and phosphorus status. Plants were grown in the presence of CO2 sources with different C isotope composition (delta13C -1 or -44). Relative respiration and gross photosynthesis rates, and belowground allocation of C assimilated during one light period ('new C'), as well as its contribution to respiration, were quantified by the concerted use of 13CO2/12CO2 steady-state labelling and 13CO2/12CO2 gas-exchange techniques. AMF (G. hoi) enhanced the relative respiration rate of the root + soil system by 16%, inducing an extra C flow amounting to 3% of daily gross photosynthesis. Total C flow into AMF growth and respiration was estimated at < 8% of daily gross photosynthesis. This was associated with a greater amount of new C allocated belowground and respired in mycorrhizal plants. AMF colonization affected the sources supplying belowground respiration, indicating a greater importance of plant C stores in supplying respiration and/or the participation of storage pools within fungal tissues. When ontogenetic and nutritional effects were accounted for, AMF increased belowground C costs, which were not compensated by increased photosynthesis rates. Therefore the instantaneous relative growth rate was lower in mycorrhizal plants.     

Nitrogen-regulated effects of free-air CO2 enrichment on methane emissions from paddy rice fields

Using the free‐air CO₂ enrichment (FACE) techniques, we carried out a 3‐year mono‐factorial experiment in temperate paddy rice fields of Japan (1998–2000) and a 3‐year multifactorial experiment in subtropical paddy rice fields in the Yangtze River delta in China (2001–2003), to investigate the methane (CH₄) emissions in response to an elevated atmospheric CO₂ concentration (200±40 mmol mol^?1 higher than that in the ambient atmosphere). No significant effect of the elevated CO₂ upon seasonal accumulative CH₄ emissions was observed in the first rice season, but significant stimulatory effects (CH₄ increase ranging from 38% to 188%, with a mean of 88%) were observed in the second and third rice seasons in the fields with or without organic matter addition. The stimulatory effects of the elevated CO₂ upon seasonal accumulative CH₄ emissions were negatively correlated with the addition rates of decomposable organic carbon (P<0.05), but positively with the rates of nitrogen fertilizers applied in either the current rice season (P<0.05) or the whole year (P<0.01). Six mechanisms were proposed to explain collectively the observations. Soil nitrogen availability was identified as an important regulator. The effect of soil nitrogen availability on the observed relation between elevated CO₂ and CH₄ emission can be explained by (a) modifying the C/N ratio of the plant residues formed in the previous growing season(s); (b) changing the inhibitory effect of high C/N ratio on plant residue decomposition in the current growing season; and (c) altering the stimulatory effects of CO₂ enrichment upon plant growth, as well as nitrogen uptake in the current growing season. This study implies that the concurrent enrichment of reactive nitrogen in the global ecosystems may accelerate the increase of atmospheric methane by initiating a stimulatory effect of the ongoing dramatic atmospheric CO₂ enrichment upon methane emissions from nitrogen‐poor paddy rice ecosystems and further amplifying the existing stimulatory effect in nitrogen‐rich paddy rice ecosystems.     

Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem

We measured the short‐term direct and long‐term indirect effects of elevated CO₂ on leaf dark respiration of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) in an intact forest ecosystem. Trees were exposed to ambient or ambient + 200 µmol mol^?1 atmospheric CO₂ using free‐air carbon dioxide enrichment (FACE) technology. After correcting for measurement artefacts, a short‐term 200 µmol mol^?1 increase in CO₂ reduced leaf respiration by 7–14% for sweetgum and had essentially no effect on loblolly pine. This direct suppression of respiration was independent of the CO₂ concentration under which the trees were grown. Growth under elevated CO₂ did not appear to have any long‐term indirect effects on leaf maintenance respiration rates or the response of respiration to changes in temperature (Q₁₀, R₀). Also, we found no relationship between mass‐based respiration rates and leaf total nitrogen concentrations. Leaf construction costs were unaffected by growth CO₂ concentration, although leaf construction respiration decreased at elevated CO₂ in both species for leaves at the top of the canopy. We conclude that elevated CO₂ has little effect on leaf tissue respiration, and that the influence of elevated CO₂ on plant respiratory carbon flux is primarily through increased biomass.