Growth response of Trifolium repens L. and Lolium perenne L. as monocultures and bi-species mixture to free air CO2 enrichment and management

Trifolium repens L. and Lolium perenne L. were grown in monocultures and bi-species mixture in a Free Air Carbon Dioxide Enrichment (FACE) experiment at elevated (60 Pa) and ambient (35 Pa) CO₂ partial pressure (pCO₂) for three years. The effects of defoliation frequencies (4 and 7 cuts in 1993; 4 and 8 cuts in 1994/95) and nitrogen fertilization (10 and 42 g m^–2 y^–1 N in 1993; 14 and 56 g m^–2 y^–1 N in 1994/95) on the growth response to pCO₂ were investigated. There were significant interspecific differences in the CO₂ responses during the first two years, while in the third growing season, these interspecific differences disappeared. Yield of T. repens in monocultures increased in the first two years by 20% when grown at elevated pCO₂. This CO₂ response was independent of defoliation frequency and nitrogen fertilization. In the third year, the CO₂ response of T. repens declined to 11%. In contrast, yield of L. perenne monocultures increased by only 7% on average over three years at elevated pCO₂. The yield response of L. perenne to CO₂ changed according to defoliation frequency and nitrogen fertilization, mainly in the second and third year. The ratio of root/yield of L. perenne increased under elevated pCO₂, low N fertilizer rate, and frequent defoliation, but it remained unchanged in T. repens. We suggest that the more abundant root growth of L. perenne was related to increased N limitation under elevated pCO₂. The consequence of these interspecific differences in the CO₂ response was a higher proportion of T. repens in the mixed swards at elevated pCO₂. This was evident in all combinations of defoliation and nitrogen treatments. However, the proportion of the species was more strongly affected by N fertilization and defoliation frequency than by elevated pCO₂. Based on these results, we conclude that the species proportion in managed grassland may change as the CO₂ concentration increases. However, an adapted management could, at least partially, counteract such CO₂ induced changes in the proportion of the species. Since the availability of mineral N in the soil may be important for the species’ responses to elevated pCO₂, more long-term studies, particularly of processes in the soil, are required to predict the entire ecosystem response.     

Soil 13C–15N dynamics in an N2-fixing clover system under long-term exposure to elevated atmospheric CO2

Reduced soil N availability under elevated CO₂ may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N₂‐fixing system. We studied the effects of Trifolium repens (an N₂‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO₂ under FACE conditions. ¹⁵N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha^?1 yr^?1 and the CO₂ concentration was increased to 60 Pa pCO₂ using ¹³C‐depleted CO₂. The total soil C content was unaffected by elevated CO₂, species and rate of ¹⁵N fertilization. However, under elevated CO₂, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (f_N) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO₂, had a significant effect on f_N values of the total soil N pool. The fractions of newly sequestered C (f_C) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO₂, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more ¹⁵N fertilizer than T. repens: 179 vs. 101 kg N ha^?1 for the low rate of N fertilization and 393 vs. 319 kg N ha^?1 for the high N‐fertilization rate. As the loss of fertilizer‐¹⁵N contributed to the ¹⁵N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N₂ fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N₂‐fixing L. perenne system. This suggests that N₂ fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.     

Total soil C and N sequestration in a grassland following 10 years of free air CO2 enrichment

Soil C sequestration may mitigate rising levels of atmospheric CO_2. However, it has yet to be determined whether net soil C sequestration occurs in N‐rich grasslands exposed to long‐term elevated CO₂. This study examined whether N‐fertilized grasslands exposed to elevated CO₂ sequestered additional C. For 10 years, Lolium perenne, Trifolium repens, and the mixture of L. perenne/T. repens grasslands were exposed to ambient and elevated CO₂ concentrations (35 and 60 Pa pCO₂). The applied CO₂ was depleted in δ¹³C and the grasslands received low (140 kg ha^?1) and high (560 kg ha^?1) rates of ¹⁵N‐labeled fertilizer. Annually collected soil samples from the top 10 cm of the grassland soils allowed us to follow the sequestration of new C in the surface soil layer. For the first time, we were able to collect dual‐labeled soil samples to a depth of 75 cm after 10 years of elevated CO₂ and determine the total amount of new soil C and N sequestered in the whole soil profile. Elevated CO₂, N‐fertilization rate, and species had no significant effect on total soil C. On average 9.4 Mg new C ha^?1 was sequestered, which corresponds to 26.5% of the total C. The mean residence time of the C present in the 0–10 cm soil depth was calculated at 4.6±1.5 and 3.1±1.1 years for L. perenne and T. repens soil, respectively. After 10 years, total soil N and C in the 0–75 cm soil depth was unaffected by CO₂ concentration, N‐fertilization rate and plant species. The total amount of ¹⁵N‐fertilizer sequestered in the 0–75 cm soil depth was also unaffected by CO₂ concentration, but significantly more ¹⁵N was sequestered in the L. perenne compared with the T. repens swards: 620 vs. 452 kg ha^?1 at the high rate and 234 vs. 133 kg ha^?1 at the low rate of N fertilization. Intermediate values of ¹⁵N recovery were found in the mixture. The fertilizer derived N amounted to 2.8% of total N for the low rate and increased to 8.6% for the high rate of N application. On average, 13.9% of the applied ¹⁵N‐fertilizer was recovered in the 0–75 cm soil depth in soil organic matter in the L. perenne sward, whereas 8.8% was recovered under the T. repens swards, indicating that the N₂‐fixing T. repens system was less effective in sequestering applied N than the non‐N₂‐fixing L. perenne system. Prolonged elevated CO₂ did not lead to an increase in whole soil profile C and N in these fertilized pastures. The potential use of fertilized and regular cut pastures as a net soil C sink under long‐term elevated CO₂ appears to be limited and will likely not significantly contribute to the mitigation of anthropogenic C emissions.     

Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO2: impact on C cycling and modeling

Elevated atmospheric CO₂ may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO₂ decomposes differently from plant material produced under ambient CO₂. Moreover, a long‐term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO₂ made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO₂ at two levels of N fertilizer (140 vs. 560 kg ha^?1 yr^?1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO₂ under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO₂ and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO₂ did not affect decomposition rates of the plant material. If under prolonged elevated CO₂ changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low‐N fertilization treatment was enhanced after exposure to elevated CO₂. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the ¹³C‐CO₂ signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO₂, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.     

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.     

Net soil carbon input under ambient and elevated CO2 concentrations: isotopic evidence after 4 years

Elevation of atmospheric CO₂ concentration is predicted to increase net primary production, which could lead to additional C sequestration in terrestrial ecosystems. Soil C input was determined under ambient and Free Atmospheric Carbon dioxide Enrichment (FACE) conditions for Lolium perenne L. and Trifolium repens L. grown for four years in a sandy‐loam soil. The ¹³C content of the soil organic matter C had been increased by 5‰ compared to the native soil by prior cropping to corn (Zea mays) for > 20 years. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above‐ground biomass produced by L. perenne during the 4‐year period was strongly dependent on the amount of N fertilizer applied but did not respond to increased CO₂. In contrast, the total accumulated above‐ground biomass of T. repens doubled under elevated CO₂ but remained independent of N fertilizer rate. The C:N ratio of above‐ground biomass for both species increased under elevated CO₂ whereas only the C:N ratio of L. perenne roots increased under elevated CO₂. Root biomass of L. perenne doubled under elevated CO₂ and again under high N fertilization. Total soil C was unaffected by CO₂ treatment but dependent on species. After 4 years and for both crops, the fraction of new C (F‐value) under ambient conditions was higher (P= 0.076) than under FACE conditions: 0.43 vs. 0.38. Soil under L. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO₂ had no effect on total soil organic matter content for both systems. The net amount of C sequestered in 4 years was unaffected by the CO₂ concentration (overall average of 8.5 g C kg^?1 soil). There was a significant species effect and more new C was sequestered under highly fertilized L. perenne. The amount of new C sequestered in the soil was primarily dependent on plant species and the response of root biomass to CO₂ and N fertilization. Therefore, in this FACE study net soil C sequestration was largely depended on how the species responded to N rather than to elevated CO₂.     

Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes?

The extent of the response of plant growth to atmospheric CO₂ enrichment depends on the availability of resources other than CO₂. An important growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO₂ response of plants. The effect of elevated CO₂ (60 Pa) partial pressure (pCO₂) on the N nutrition of field-grown Lolium perenne swards, cultivated alone or in association with Trifolium repens, was investigated using free air carbon dioxide enrichment (FACE) technology over 3 years. The established grassland ecosystems were treated with two N fertilization levels and were defoliated at two frequencies. Under elevated pCO₂, the above-ground plant material of the L. perenne monoculture showed a consistent and significant decline in N concentration which, in general, led to a lower total annual N yield. Despite the decline in the critical N concentration (minimum N concentration required for non-N-limited biomass production) under elevated pCO₂, the index of N nutrition (ratio of actual N concentration and critical N concentration) was lower under elevated pCO₂ than under ambient pCO₂ in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO₂ was evoked indirectly by a reduction of plant-available N. For L. perenne grown in association with T. repens and exposed to elevated pCO₂, there was an increase in the contribution of symbiotically fixed N to the total N yield of the grass. This can be explained by an increased apparent transfer of N from the associated N₂-fixing legume species to the non-fixing grass. The total annual N yield of the mixed grass/legume swards increased under elevated pCO₂. All the additional N yielded was due to symbiotically fixed N. Through the presence of an N₂-fixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevated pCO₂. Received: 11 November 1996 / Accepted: 20 May 1997     

Carbon‐13 input and turn‐over in a pasture soil exposed to long‐term elevated atmospheric CO2

The impact of elevated CO₂ and N‐fertilization on soil C‐cycling in Lolium perenne and Trifolium repens pastures were investigated under Free Air Carbon dioxide Enrichment (FACE) conditions. For six years, swards were exposed to ambient or elevated CO₂ (35 and 60 Pa pCO₂) and received a low and high rate of N fertilizer. The CO₂ added in the FACE plots was depleted in ¹³C compared to ambient (Δ? 40‰) thus the C inputs could be quantified. On average, 57% of the C associated with the sand fraction of the soil was ‘new’ C. Smaller proportions of the C associated with the silt (18%) and clay fractions (14%) were derived from FACE. Only a small fraction of the total C pool below 10 cm depth was sequestered during the FACE experiment. The annual net input of C in the FACE soil (0–10 cm) was estimated at 4.6 ± 2.2 and 6.3 ± 3.6 (95% confidence interval) Mg ha^{? 1} for T. repens and L. perenne, respectively. The maximum amount of labile C in the T. repens sward was estimated at 8.3 ± 1.6 Mg ha^{? 1} and 7.1 ± 1.0 Mg ha^{? 1} in the L. perenne sward. Mean residence time (MRT) for newly sequestered soil C was estimated at 1.8 years in the T. repens plots and 1.1 years for L. perenne. An average of 18% of total soil C in the 0–10 cm depth in the T. repens sward and 24% in the L. perenne sward was derived from FACE after 6 years exposure. The majority of the change in soil δ¹³C occurred in the first three years of the experiment. No treatment effects on total soil C were detected. The fraction of FACE‐derived C in the L. perenne sward was larger than in the T. repens sward. This suggests a priming effect in the L. perenne sward which led to increased losses of the old C. Although the rate of C cycling was affected by species and elevated CO₂, the soil in this intensively managed grassland ecosystem did not become a sink for additional new C.     

Arbuscular mycorrhizal fungi benefit from 7 years of free air CO2 enrichment in well-fertilized grass and legume monocultures

Rising atmospheric carbon dioxide partial pressure (pCO₂) and nitrogen (N) deposition are important components of global environmental change. In the Swiss free air carbon dioxide enrichment (FACE) experiment, the effect of altered atmospheric pCO₂ (35 vs. 60 Pa) and the influence of two different N‐fertilization regimes (14 vs. 56 g N m^?2 a^?1) on root colonization by arbuscular mycorrhizal fungi (AMF) and other fungi (non‐AMF) of Lolium perenne and Trifolium repens were studied. Plants were grown in permanent monoculture plots, and fumigated during the growth period for 7 years. At elevated pCO₂ AMF and non‐AMF root colonization was generally increased in both plant species, with significant effects on colonization intensity and on hyphal and non‐AMF colonization. The CO₂ effect on arbuscules was marginally significant (P=0.076). Moreover, the number of small AMF spores (≤100 μm) in the soils of monocultures (at low‐N fertilization) of both plant species was significantly increased, whereas that of large spores (>100 μm) was increased only in L. perenne plots. N fertilization resulted in a significant decrease of root colonization in L. perenne, including the AMF parameters, hyphae, arbuscules, vesicles and intensity, but not in T. repens. This phenomenon was probably caused by different C‐sink limitations of grass and legume. Lacking effects of CO₂ fumigation on intraradical AMF structures (under high‐N fertilization) and no response to N fertilization of arbuscules, vesicles and colonization intensity suggest that the function of AMF in T. repens was non‐nutritional. In L. perenne, however, AM symbiosis may have amended N nutrition, because all root colonization parameters were significantly increased under low‐N fertilization, whereas under high‐N fertilization only vesicle colonization was increased. Commonly observed P‐nutritional benefits from AMF appeared to be absent under the phosphorus‐rich soil conditions of our field experiment. We hypothesize that in well‐fertilized agricultural ecosystems, grasses benefit from improved N nutrition and legumes benefit from increased protection against pathogens and/or herbivores. This is different from what is expected in nutritionally limited plant communities.     

Greater seed production in elevated CO2 is not accompanied by reduced seed quality in Pinus taeda L.

For herbaceous species, elevated CO₂ often increases seed production but usually leads to decreased seed quality. However, the effects of increased atmospheric CO₂ on tree fecundity remain uncertain, despite the importance of reproduction to the composition of future forests. We determined how seed quantity and quality differed for pine trees grown for 12 years in ambient and elevated (ambient+200 μL L^?1) CO₂, at the Duke Forest free‐air CO₂ enrichment (FACE) site. We also compared annual reproductive effort with yearly measurements of aboveground net primary productivity (ANPP), precipitation (P), potential evapotranspiration (PET) and water availability [precipitation minus potential evapotranspiration (P?PET)] to investigate factors that may drive interannual variation in seed production. The number of mature, viable seeds doubled per unit basal area in high‐CO₂ plots from 1997 to 2008 (P<0.001), but there was no CO₂ effect on mean seed mass, viability, or nutrient content. Interannual variation in seed production was positively related to ANPP, with a similar percentage of ANPP diverted to reproduction across years. Seed production was negatively related to PET (P<0.005) and positively correlated with water availability (P<0.05), but showed no relationship with precipitation (P=0.88). This study adds to the few findings that, unlike herbaceous crops, woody plants may benefit from future atmospheric CO₂ by producing larger numbers of seeds without suffering degraded seed quality. Differential reproductive responses between functional groups and species could facilitate woody invasions or lead to changes in forest community composition as CO₂ rises.     

Long-term effects of an increased CO2 concentration on terrestrial plants in model ecosystems. Morphology and reproduction ofTrifolium repens L.and Lolium perenne L.

Mixtures (ratio 11, 45 seeds·dm^–2) of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) were enclosed in environmentally controlled acrylic mini-glasshouses and supplied with additional CO₂ from September 1981 until September 1982 (547–600 ppm, control: 340 ppm CO₂). At growth optimum the single leaf area of white clover was 30% greater and the specific leaf weight 19% higher with additional CO₂; with the perennial ryegrass it was 18% resp. 29%. The length growth of the white clover petioles was enhanced by the additional CO₂: in the beginning 67% longer leafstalks, at growth optimum no effect and at the end of the experiment 100%. At growth optimum the specific petiole weight was approximately 52% higher. When the growing was terminated the blades of perennial ryegrass were 19% longer at high CO₂. The average volume of the flowerheads was about 53% greater and the adult ears 13% longer. The number of flowers per flower head and the number of earlets per ear did not increase significantly. On an average the white clover seeds were 28% heavier and the caryopses were 4% lighter.Supported by the Umweltbundesamt, Fed. Rep. of Germany, Contract BSM 104 02 620Presented at the 10th International Congress of Biometeorology in Tokyo, 1984.     

Impact of elevated CO2 on soil organic matter dynamics as related to changes in aggregate turnover and residue quality

Increasing global atmospheric CO₂ concentration can potentially affect C cycling in terrestrial ecosystems. This study was conducted to assess the impact of elevated CO₂ concentration on soil organic matter and aggregate dynamics in Lolium perenne and Trifolium repens pastures. Soil samples from a 6 year old `free air CO₂ enrichment' (FACE) experiment were separated in four aggregate size classes (<53, 53–250, 250–2000, and > 2000 m). Free light fraction (i.e. particulate organic matter (POM) outside of aggregates; free LF) and intra-aggregate-POM (i.e. POM occluded within the aggregate structure; iPOM) were isolated. The distinct ¹³C-signature of the CO₂ used to raise the ambient CO₂ concentration in FACE allowed us to calculate proportions of recently incorporated C (< 6 yr) in the physically defined soil fractions. The proportion of new C increased with increasing aggregate size class, except the two largest aggregate size classes had a similar proportion of new C; this indicates a faster turnover of macroaggregates compared to microaggregates. In addition, higher proportions of new C in macroaggregates under T. repens compared to L. perenne indicate a faster macroaggregate turnover under T. repens. This faster macroaggregate turnover is hypothesized to be a result of the higher residue quality (C:N ratio) of T. repens compared to L. perenne and reduces the potential of sequestering C under elevated CO₂. In the L. perenne soil, elevated CO₂ did not significantly increase total C, but led to: (1) a 54% increase in aggregation and (2) a 40% increase in total iPOM-C. It is hypothesized that the sequestration of iPOM-C induced by elevated CO₂ in the low residue quality, L. perenne treatment, resulted from an increase in the proportion of large macroaggregates with a slow turnover.     

The impact of long-term elevated CO2 on C and N retention in stable SOM pools

Elevated atmospheric CO₂ frequently increases plant production and concomitant soil C inputs, which may cause additional soil C sequestration. However, whether the increase in plant production and additional soil C sequestration under elevated CO₂ can be sustained in the long-term is unclear. One approach to study C–N interactions under elevated CO₂ is provided by a theoretical framework that centers on the concept of progressive nitrogen limitation (PNL). The PNL concept hinges on the idea that N becomes less available with time under elevated CO₂. One possible mechanism underlying this reduction in N availability is that N is retained in long-lived soil organic matter (SOM), thereby limiting plant production and the potential for soil C sequestration. The long-term nature of the PNL concept necessitates the testing of mechanisms in field experiments exposed to elevated CO₂ over long periods of time. The impact of elevated CO₂ and ¹⁵N fertilization on L. perenne and T. repens monocultures has been studied in the Swiss FACE experiment for ten consecutive years. We applied a biological fractionation technique using long-term incubations with repetitive leaching to determine how elevated CO₂ affects the accumulation of N and C into more stable SOM pools. Elevated CO₂ significantly stimulated retention of fertilizer-N in the stable pools of the soils covered with L. perenne receiving low and high N fertilization rates by 18 and 22%, respectively, and by 45% in the soils covered by T. repens receiving the low N fertilization rate. However, elevated CO₂ did not significantly increase stable soil C formation. The increase in N retention under elevated CO₂ provides direct evidence that elevated CO₂ increases stable N formation as proposed by the PNL concept. In the Swiss FACE experiment, however, plant production increased under elevated CO₂, indicating that the additional N supply through fertilization prohibited PNL for plant production at this site. Therefore, it remains unresolved why elevated CO₂ did not increase labile and stable C accumulation in these systems.     

Growth, senescence and water use efficiency of spring oilseed rape (Brassica napus L. cv. Mozart) grown in a factorial combination of nitrogen supply and elevated CO2

Atmospheric CO₂ enrichment is expected to affect the resource use efficiency of C3 plants with respect to water, nutrients and light in an interactive manner. The responses of oilseed rape (OSR) to elevated CO₂ have not much been addressed. Since the crop has low nitrogen use efficiency, the interactive effects of CO₂ enrichment and nitrogen supply deserve particular attention.Spring OSR was grown in climate chambers simulating the seasonal increments of day length and temperature in South-Western Germany. Three levels of N fertilisation representing 75, 150 and 225 kg ha⁻¹ and two CO₂ concentrations (380 and 550 μmol mol⁻¹) were used to investigate changes in source-sink relationships, plant development and senescence, water use efficiency of the dry matter production (WUE_prod.), allocation patterns to different fractions, growth, yield and seed oil contents. Seven harvests were performed between 72 and 142 days after sowing (DAS).Overall, plant performance in the chambers was comparable to the development under field conditions. While CO₂ responses were small in the plants receiving lowest N-levels, several significant N × CO₂ interactions were observed in the other treatments. Increasing the N availability resulted in longer flowering windows, which were furthermore extended at elevated CO₂ concentrations. Nevertheless, significantly less biomass was allocated to reproductive structures under elevated CO₂, while the vegetative C-storing organs continued to grow. At the final harvest shoot mass of the CO₂ exposed plants had increased by 9, 8 and 15% in the low, medium and high N treatments. Root growth was increased even more by 17, 43 and 33%, respectively and WUE_prod. increased by 23, 42 and 35%. At the same time, seed oil contents were significantly reduced by CO₂ enrichment in the treatments with ample N supply.Obviously, under high N-supply, the CO₂ fertilisation induced exaggerated growth of vegetative tissues at the expense of reproductive structures. The interruption of source-sink relationships stimulated the formation of side shoots and flowers (branching out). While direct effects of elevated CO₂ on flowering can be excluded, we assume that the increased growth under high N and CO₂ supply created nutrient imbalances which hence affected flowering and seed set.Nevertheless, the final seed macronutrient concentrations were slightly increased by elevated CO₂, indicating that remobilisation of nutrients from the sources (leaves) to the sinks (seeds) remained effective. These findings were supported by the lower nitrogen concentrations in senescing leaves and probably increased N remobilisation to other plant parts under elevated concentrations of CO₂. All the same, CO₂ enrichment caused a decline in seed oil contents, which may translate into a reduced crop quality.     

Effects of CO2 enrichment and intraspecific competition on biomass partitioning, nitrogen content and microbial biomass carbon in soil of perennial ryegrass and white clover

Seedlings of perennial ryegrass (Lolium perenne L. cv. Parcour)and white clover (Trifolium repens L. cv. Karina) grown at fivedifferent plant densities were exposed to ambient (390 ppm)and elevated (690 ppm) CO₂ concentrations. After 43 d the effectsof CO₂ enrichment and plant density on growth of shoot and root,nitrogen concentration of tissue, and microbial biomass carbon(C_mic) in soil were determined. CO₂ enrichment of Lolium perenneincreased shoot growth on average by 17% independent of plantdensity, while effects on root biomass ranged between -4% and+ 107% due to an interaction with plant density. Since tilernumber per plant was unaffected by elevated CO₂, the small responseof shoot growth to CO₂ enrichment was atributed to low sinkstrength. A significant correlation between nitrogen concentrationof total plant biomass and root fraction of total plant drymatter, which was not changed by CO₂ enrichment, indicates thatnitrogen status of the plant controls biomass partitioning andthe effect of CO₂ enrichment on root growth. Effects of elevatedCO₂ and plant density on shoot and root growth of Trifoliumrepens were not significantly interacting and mean CO₂-relatedincrease amounted to 29% and 66%, respectively. However, growthenhancement due to elevated CO₂ was strongest when leaf areaindex was lowest. Total amounts of nitrogen in shoots and rootswere bigger at 690 ppm than at 390 ppm CO₂. There was a significantincrease in C_mic in experiments with both species whereas plantdensity had no substantial effect. Key words: CO₂ enrichment, intraspecific competition, biomass partitioning, Lolium perenne, Trifolium repens, grassland     

Due to symbiotic N2 fixation,five years of elevated atmospheric pCO2 had no effect on the N concentration of plant litter in fertile,mixed grassland

Experimental findings indicate that, in terrestrial ecosystems, nitrogen cycling changes under elevated partial pressure of atmospheric CO₂ (pCO₂). It was suggested that the concentration of N in plant litter as well as the amount of litter are responsible for these changes. However, for grassland ecosystems, there have been no relevant data available to support this hypothesis. Data from five years of the Swiss FACE experiment show that, under fertile soil conditions in a binary plant community consisting of Lolium perenne L. and Trifolium repens L., the concentration of litter N does not change under elevated atmospheric pCO₂; this applies to harvest losses, stubble, stolons and roots as the sources of litter. This is in strong contrast to the CO₂ response of L. perenne swards without associated legumes; in this case the above-ground concentration of biomass N decreased substantially. Increased symbiotic N₂ fixation in T. repens nodules and a greater proportion of the N-rich T. repens in the community are regarded as the main mechanisms that buffer the increased C introduction into the ecosystem under elevated atmospheric pCO₂. Our data also suggest that elevated atmospheric pCO₂ results in greater amounts of litter, mainly due to increased root biomass production. This study indicates that, in a fertile grassland ecosystem with legumes, the concentration of N in plant litter is not affected by elevated atmospheric pCO₂ and, thus, cannot explain CO₂-induced changes in the cycling of N. This revised version was published online in June 2006 with corrections to the Cover Date.     

Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment

Legumes, with the ability to fix atmospheric nitrogen (N), may help alleviate the N limitations thought to constrain plant community response to elevated concentrations of atmospheric carbon dioxide (CO₂). To address this issue we assessed: (1) the effects of the presence of the perennial grassland N₂ fixer, Lupinus perennis, on biomass accumulation and plant N concentrations of nine-species plots of differing plant composition; (2) leaf-level physiology of co-occurring non-fixing species (Achillea millefolium, Agropyron repens, Koeleria cristata) in these assemblages with and without Lupinus; (3) the effects of elevated CO₂ on Lupinus growth and symbiotic N₂ fixation in both monoculture and the nine-species assemblages; and (4) whether assemblages containing Lupinus exhibit larger physiological and growth responses to elevated CO₂ than those without. This study was part of a long-term grassland field experiment (BioCON) that controls atmospheric CO₂ at current ambient and elevated (560 µmol mol^–1) concentrations using free-air CO₂ enrichment. Nine-species plots with Lupinus had 32% higher whole plot plant N concentrations and 26% higher total plant N pools than those without Lupinus, based on both above and belowground measurements. Co-occurring non-fixer leaf N concentrations increased 22% and mass-based net photosynthetic rates increased 41% in plots containing Lupinus compared to those without. With CO₂ enrichment, Lupinus monocultures accumulated 32% more biomass and increased the proportion of N derived from fixation from 44% to 57%. In nine-species assemblages, Lupinus N derived from fixation increased similarly from 43% to 54%. Although Lupinus presence enhanced photosynthetic rates and leaf N concentrations of co-occurring non-fixers, and increased overall plant N pools, Lupinus presence did not facilitate stronger photosynthetic responses of non-fixing species or larger growth responses of overall plant communities to elevated CO₂. Non-fixer leaf N concentrations declined similarly in response to elevated CO₂ with and without Lupinus present and the relationship between net photosynthesis and leaf N was not affected by Lupinus presence. Regardless of the presence or absence of Lupinus, CO₂ enrichment resulted in reduced leaf N concentrations and rates of net photosynthesis.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

Changes in microbial activity and composition in a pasture ecosystem exposed to elevated atmospheric carbon dioxide

Elevated atmospheric CO₂ increases aboveground plant growth and productivity. However, carbon dioxide-induced alterations in plant growth are also likely to affect belowground processes, including the composition of soil biota. We investigated the influence of increased atmospheric CO₂on bacterial numbers and activity, and on soil microbial community composition in a pasture ecosystem under Free-Air Carbon Dioxide Enrichment (FACE). Composition of the soil microbial communities, in rhizosphere and bulk soil, under two atmospheric CO₂ levels was evaluated by using phospholipid fatty acid analysis (PLFA), and total and respiring bacteria counts were determined by epifluorescence microscopy. While populations increased with elevated atmospheric CO₂ in bulk soil of white clover (Trifolium repens L.), a higher atmospheric CO₂ concentration did not affect total or metabolically active bacteria in bulk soil of perennial ryegrass (Lolium perenne L.). There was no effect of atmospheric CO₂ on total bacteria populations per gram of rhizosphere soil. The combined effect of elevated CO₂ on total root length of each species and the bacterial population in these rhizospheres, however, resulted in an 85% increase in total rhizosphere bacteria and a 170% increase in respiring rhizosphere bacteria for the two plant species, when assessed on a per unit land area basis. Differences in microbial community composition between rhizosphere and bulk soil were evident in samples from white clover, and these communities changed in response to CO₂ enrichment. Results of this study indicate that changes in soil microbial activity, numbers, and community composition are likely to occur under elevated atmospheric CO₂, but the extent of those changes depend on plant species and the distance that microbes are from the immediate vicinity of the plant root surface.     

Nitrous oxide emissions from grass swards during the eighth year of elevated atmospheric pCO2 (Swiss FACE)

Emissions of N₂O were measured during the growth season over a year from grass swards under ambient (360 μL L^?1) and elevated (600 μL L^?1) CO₂ partial pressures at the Free Air Carbon dioxide Enrichment (FACE) experiment, Eschikon, Switzerland. Measurements were made following high (56 g N m^?2 yr^?1) and low (14 g N m^?2 yr^?1) rates of fertilizer application, split over 5 re‐growth periods, to Lolium perenne, Trifolium repens and mixed Lolium/Trifolium swards. Elevated pCO₂ increased annual emissions of N₂O from the high fertilized Lolium and mixed Lolium/Trifolium swards resulting in increases in GWP (N₂O emissions) of 179 and 111 g CO₂ equivalents m^?2, respectively, compared with the GWP of ambient pCO₂ swards, but had no significant effect on annual emissions from Trifolium monoculture swards. The greater emissions from the high fertilized elevated pCO₂Lolium swards were attributed to greater below‐ground C allocation under elevated pCO₂ providing the energy for denitrification in the presence of excess mineral N. An annual emission of 959 mg N₂O‐N m^?2 yr^?1 (1.7% of fertilizer N applied) was measured from the high fertilized Lolium sward under elevated pCO₂. The magnitude of emissions varied throughout the year with 84% of the total emission from the elevated pCO₂Lolium swards measured during the first two re‐growths (April–June 2001). This was associated with higher rainfall and soil water contents at this time of year. Trends in emissions varied between the first two re‐growths (April–June 2001) and the third, fourth and fifth re‐growths (late June–October 2000), with available soil NO₃^? and rainfall explaining 70%, and soil water content explaining 72% of the variability in N₂O in these periods, respectively. Caution is therefore required when extrapolating from short‐term measurements to predict long‐term responses to global climate change. Our findings are of global significance as increases in atmospheric concentrations of CO₂ may, depending on sward composition and fertilizer management, increase greenhouse gas emissions of N₂O, thereby exacerbating the forcing effect of elevated CO₂ on global climate. Our results suggest that when applying high rates of N fertilizer to grassland systems, Trifolium repens swards, or a greater component of Trifolium in mixed swards, may minimize the negative effect of continued increasing atmospheric CO₂ concentrations on global warming.