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Contenuto archiviato il 2022-12-23

EFFECTS OF ATMOSPHERIC CO2 CO- INCREASE ON CARBON FLUXES IN GRASSLAND ECOSYSTEMS

Obiettivo

A.GENERAL BACKGROUND

1.Current state of research in the proposed field of research

1.1.Increase in atmospheric CO2-concentration and individual plant growth

For plants, CO2 is the carbon source for the photosynthetic production of carbohydrates and their subsequent conversion into biomass. Assuming no extraneous growth-limiting factors, an increased atmospheric CO2-concentration causes primarily:

(a)a higher rate of photosynthesis, notably in C3-plants, which represent the main part of the vegetation in temperate and cool climates. The increase in photosynthesis is more pronounced at higher temperatures;

(b)an increase in the optimal growth-temperature of C3-plants;

(c)a decrease in transpiration due to a CO2-induced stomata-closure. This effect results in a higher growth rate under water-limiting conditions.

These primary effects may generally cause secondary responses. The following ones are important: enhanced root, leaf and shoot growth; modified morphological characteristics such as tillering or branching, and modified development; altered assimilate partitioning; improved nutrient use efficiency (the available nutrients allow of a larger biomass production); improved water use efficiency (less water is needed for similar productivity); enhanced biological nitrogen fixation (BNF) by the symbiotic rhizobial bacteria in the root nodules of legumes; increased biological activity of soil organisms, which in turn affects chemical and physical soil parameters; increased C/N-ratios of plants. The latter influences the quality of plant material and the decomposition of plant litter.

These effects have been tested with many plant species, among them many important crops such as wheat, barley, rice and soybean. In these tests a doubling of the atmospheric CO2-concentration usually resulted in a 25 to 100% increased yield (Kimball, 1983). However, most of these experiments were done on single plants in growth chambers and greenhouses.

1.2.Response of annual arable crops

The above yield increase was confirmed by a limited number of field studies of annual crops (reviewed by Lawlor and Mitchell, 1991). The extent of the CO2-effects is species-specific, but it depends also on temperature, nutrient and water availability. A moderate temperature rise of about 2?C and a CO2-increase to 600 ppm would also enhance the growth-stimulating CO2-effect. However, little is known about the possible responses to CO2 of more complex ecosystems.

1.3.Responses of ecosystems

In complex ecosystems we have to expect not only species-related differences in biomass production but also changes in the interactions between the various plant species since the CO2-effects mentioned above are specific for single plant species or even for genotypes. Thus, it is possible that significant changes in floristic composition or even in species number may occur within an ecosystem.

The competitiveness of a plant species is influenced by many plant and environmentally based growth factors. Under elevated atmospheric CO2-concentrations, many of these factors are affected directly or indirectly. Therefore, it is very difficult to predict the response of a single plant species in a complex ecosystem. The interpretations of the CO2-related responses of plants in complex ecosystems requires close observation of the response of the whole ecosystem, combined with sound information about the key growth-processes in the shoot and root zone.

Moreover, the CO2-related responses of species depend on the management of the ecosystem. Nutrient availability, especially that of nitrogen, strongly affects not only growth and biomass but also floristic composition. The presence or absence of legumes in a plant community, e.g. has such effects on nitrogen availability, too. The harvesting frequency influences not only the amount of harvested biomass and nutrients but canopy structure as well and, thus, above ground competition.

Considering the great number of species in complex ecosystems, it is important to characterize "functional types", i.e. groups of species or genotypes characterized by similar responses in CO2 increase under distinct management practices. This requires a screening of many species of different "functional types" and their genotypes.

Changes in morphological characteristics and in the development of leaf area and canopy structure may affect the competitiveness of the species. Changes in the rooting depth and in root morphology will affect the plants' competitive ability with respect to the uptake of water and nutrients.

The increase of atmospheric CO2-concentration causes a reduction of transpiration and therefore of water use. The increased leaf-area (particularly during the early growth stage), the increased leaf-temperature of CO2-fertilized plants, and the increased transpiration caused by an elevated air temperature (in climate change) could neutralize the reduced water use to a certain extent. The extent of these effects has to be tested experimentally. Changes in the water use of a canopy will also modify the water status of the soil, such as the drain-water and the amount of water available to the plant. This is especially important for plant growth during periods of drought. Thus, a changed water use or rooting depth of a single species might alter its competitive ability.

Soil organisms are crucial to the decomposition of organic matter and thus to providing plant nutrients. They also affect many physical and chemical parameters in the soil. It can be expected that biological activity in the soil will be changed by a CO2-induced increase in root exudation and by an increased and qualitatively different litter availability. The changed biological activity in

the soil could lead to substantial changes in physical and chemical soil parameters. Apart from the above activities, it is also possible that the populations of soil organisms will be affected. This could either increase or decrease the populations of certain soil organisms. Thus, a new balance between pathogenic (e.g. nematodes) and beneficial (e.g. mycorrhiza or rhizobia, see below) organisms in the soil would result. This could potentially change soil quality very dramatically. These changes might also affect plant growth in a species specific way.

How all these potential changes will influence ecosystems is completely uncertain. It is not possible to study a single phenomenon or process in the controlled environment and then to extrapolate the data to field conditions because natural conditions are far more complex as compared to laboratory conditions. Moreover, due to the many interactions among the different single components, the final response of a natural ecosystem is not predictable at all. However, the impact of all these factors on the relative competitiveness of all the plant species is unquestionable and has to be investigated in field experiments.

1.3.1.Grassland ecosystems

Grasses and legumes are important components for the yield of grassland; in Europe white clover is the most important legume in temperate grassland and its importance is increasing due to economic and ecological reasons. The proportion of white clover in mixed canopies is mainly determined by the light competitiveness of the associated plants. White clover can symbiotically fix nitrogen (N2) in root nodules in association with the soil bacterium Rhizobium leguminosarum biovar. trifolii. This particularly interesting feature makes white clover a powerful competitor in grassland, if managed appropriately. The symbiotically fixed nitrogen is the most important nitrogen source in natural ecosystems. All the plants and soil organisms associated with white clover will ultimately benefit from biological

nitrogen fixation. The expected increase in biological nitrogen fixation due to an increase of plant growth may cause substantial changes in the nitrogen economy of grassland. This would lead to changes in the botanical composition because not all the plants can use the increased amount of nitrogen equally. These kinds of effects could probably be counteracted by appropriate management in intensively or moderately used grassland. Only in extensively exploited ecosystems could it lead to substantial changes in botanical composition.

In classifying the "functional types" of grassland species the following must be considered: morphological characters, growth and propagation strategy (stolons, rhizomes, tillering), nutrient requirements and management practices. The variability of the CO2 responses of ecotypes of grassland species may be an important feature for the survival of the species in a CO2 rich world.

1.3.2.C-fluxes in the soil

Only a few studies have been done so far to determine the effect of increased CO2 on the C-fluxes of agricultural ecosystems (Lekkerkerk et. al. 1990, Wolf and Janssen 1991, both in the Netherlands), based on laboratory experiments. Most of the data used in global and regional C balance studies use estimated values for the C-fluxes (Bouwman 1990). The decomposition of the soil organic matter pool and the C input into it

depend on soil properties, vegetation type, and climate (Bouwman 1990). Therefore, the C-fluxes and pools have to be studied at many different sites to allow for appropriate estimates of a regional or global C balance. The methods suited to measuring gas flux are described by Mosier (1990). For small plots, the chamber methods are recommended. Root exudates are difficult to determine in the field. To study C-pools in the soil, isotope methods are increasingly used. The 13C isotope technique, described by Insam (1991), has been successfully used by several groups.

B.OBJECTIVES OF THE ACTION

The main objectives of our co-operative effort are:

-to advance research on the effects of increased C02 concentration on the C-fluxes of differently managed and fertilized grassland ecosystems, including the processes that affect plant growth and the accumulation and decomposition of soil organic matter;

-to improve or develop models that describe these C-fluxes or their components at different scales (from leaf to globe).

Secondary objectives:

-to compare and evaluate experimental approaches that are appropriate for the field by improving the exchange of concepts and knowledge between participating institutes;

-to stimulate co-operation between the participants and the relevant national, EC and global programmes like the IGBP (International Geosphere Biosphere Programme).

C.SCIENTIFIC CONTENT (= PROGRAMME) OF THE ACTION

Projects aiming at the determination of C-fluxes in grassland ecosystems at present and at increased CO2-concentrations must examine:

-The growth of the plants and especially root growth. Plant roots have a limited lifetime. After death they are decomposed by soil organisms. One part of this root-biomass is decomposed quickly and the C is released as CO2. The rest is decomposed slowly; it contributes to the soil organic matter. The increased root growth will presumably increase this C-pool in the soil. The decomposition rate of dead roots will presumably be affected by the C/N ratio. Root growth and C/N ratio must therefore be measured.

-Higher CO2 levels will presumably modify the competitive ability of the species and therefore effect changes in the floristic composition of the ecosystem. This above-ground character of grassland affects root growth, too. Thus, detailed studies of the growth processes and their effects on the root and shoot competition of different functional types have to be included. Changes of a species' competitive ability might also result, if the amount or quality of seeds were affected. Thus, studies of seed production and quality and their effects on seedling development must be included. It can be expected that the results about the changes in botanical composition will be of great interest to scientists working on biodiversity.

-The amount and quality (C/N ratio) of plant litter. An increased leaf-growth at a higher CO2-concentration might increase the amount of plant litter. The decomposition of plant litter by soil organisms is affected by its quality. A slower decomposition, e.g. would increase the proportion of plant litter added to soil organic matter. Thus, a changed proportion of aboveground biomass might be transferred into soil organic matter.

-The amount and quality of root exudates. An increase of root exudates is hypothesized which might stimulate root growth and the activity of soil organisms. This in turn may affect the release of nutrients and therefore plant growth. A greater availability of root exudates might cause soil organisms to feed preferentially from this easily digestible material instead of the soil organic matter or plant litter. This would affect the decomposition of these C-pools. Qualitative changes of root exudates, as, e.g. the C/N ratio, might affect the soil organisms, too. The lack of appropriate methods to measure root exudates in the field is an important drawback.

-The activity of soil organisms and their effects on the decomposition of soil organic matter. This includes the determination of the CO2 released from the fresh and old components of this C-pool. The activity of soil organisms is also responsible for the mobilization of nutrients that, in turn, affects plant growth.

-Soil water content and physical and chemical soil parameters. They affect plant growth and the activity of soil organisms.

These components of grassland ecosystems will be studied by the participants by different experimental approaches and at different levels of complexity. The use of models at different scales will help to integrate the many approaches. The COST project will create a network to exchange information and methods by organizing workshops. It will stimulate the exchange of scientists for visits and collaboration.

D.TIMETABLE

The objectives of the COST Action are so extensive that a duration of five years is considered a minimum.

The exact timetable will be decided by the Management Committee.

E.ORGANIZATION MANAGEMENT AND RESPONSIBILITIES

At the preparatory meeting on 9 September 1993 in Brussels, it was decided to form three working groups for:

1)plant processes
2)soil processes
3)modelling and system analysis.

The co-ordinators of these working groups are members of the Management Committee. Each working group can invite experts to their workshops.

Furthermore, it was decided to organize, when possible, a workshop to discuss experimental approaches to have a basis for the comparison of the results.

A preparatory meeting should be organized to discuss the modelling aspects of this project. The system analytical approach should provide a better outline of the requirements to the experimental protocols and to the design of new experiments.

Frequency of meetings:

1.The frequency of the meetings of the working groups will be decided by the management committee.

2.All working groups have a common annual meeting to improve exchange of information between the groups.

F.ECONOMIC DIMENSION OF THE ACTION

So far, more than twenty groups from fourteen countries have sent a COST schema 2. Accordingly, the man-years carried out in the countries that will presumably participate may be estimated approximately at 85. The personnel costs are estimated to ECU 17 million within the five years duration of this project. Adding the overhead costs of 15%, the total cost is estimated at ECU 20 million.

Current status
The Action 619 started in 1994 and 15 countries have signed the MoU. The first Management Committee meeting was held on 4 July 1994 and Mr J. Nösberger (CH) and Mr S. van de Geijn (NL) were elected Chairman and Vice-chairman respectively.
Owing to an early preparatory project meeting in Wageningen (NL), the Management Committee was able at the second MC meeting, held in Zurich (CH), 26/27 Oct 1994, to organise the Action into 4 Working Groups and appoint Co-ordinators. At the second meeting in Wageningen (1995), one of the Working Groups was divided and there are now 5 Working Groups (listed below) :
Working Group 1 "Photosynthesis and Gas Exchange" (Dr S. LONG)
Working Group 2 "Partitioning and Growth" (Dr J. FARRAR)
Working Group 3 "C-Fluxes in Soils" (Dr P. KUIKMAN)
Working Group 4 "Species" (Dr J.F. SOUSSANA)
Working Group 5 "Integration" (Dr H. BLUM)

Several of the scientists has participated in Short Term Scientific Missions.
Work planned
The Action will maintain and develop the good relations it has with GCTE (Global Change & Terrestrial Ecosystems).

Argomento(i)

Invito a presentare proposte

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Meccanismo di finanziamento

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Coordinatore

CEC
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Indirizzo
Rue de la Loi 200
1049 Brussels
Belgio

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