Workpackage Ecosystem CO2 flux components
and their interaction with the nitrogen cycle


Workpackage manager: Dr. Michael BAHN & Dr. Rolf SIEGWOLF


Objectives:
Aim of WP 2 is i) to quantify the contribution of the major ecosystem components to the net ecosystem CO2 exchange (NEE, cf. WP 1), ii) to analyse and interpret their response to nitrogen contents and environmental driving forces, iii) to assess of effects of land-use changes on plant light and nitrogen use efficiency of carbon fluxes along an altitudinal and a latitudinal transect, and iv) to provide estimates of off-site carbon losses.

Methodology / work description:
Ecosystem component CO2 fluxes and their response to nitrogen contents and environmental driving forces will be measured at various levels of organisation:

  • The response of phytoelement (leaf, stem, litter) CO2 exchange rates to photosynthetically active radiation, temperature, CO2 concentration, air water vapour deficit, soil water availability and tissue nitrogen contents will be measured both in the field and laboratory by means of CO2/H2O-porometers (CIRAS-1, PP-Systems, UK; Li-6400, Li-Cor, USA) for selected key species within four functional groups (graminoids, forbs, legumes, dwarf shrubs). Leaf structure versus function relationships will be established to simplify the estimation of foliage photosynthetic parameters and leaf optical properties. Seasonal effects (phenology, water and cold stress) on physiology will be considered.
  • Effects of land-use changes on plant light and nitrogen use efficiency of carbon fluxes will be studied along an altitudinal and a latitudinal transect.
  • Bulk soil respiration will be measured on a continuous basis in the field with an open respiration chamber
    system.
  • Root and microbial contributions to bulk soil respiration will be quantified by means of the substrate induced respiration method (SIR). The dependency of the root and microbial soil respiration components on temperature, water and nitrogen content will be measured with oxygen electrodes and infra-red gas analysers.
  • Phytoelement and soil nitrogen contents will be measured by means of an elemental analyser (CHNS-932,
    LECO Instruments, USA).
  • Net flux partitioning into the canopy and soil component will be accomplished by the com-bination of flux measurements (WP 1 and this work package) and the analysis of the stable C and O isotopes. Two mass spectrometers (Delta-S and a Delta-plus-XL, Finnigan, Germany) are available and equipped with the periphery necessary for the analysis of organic, water and gaseous samples.
  • Off-site carbon losses via grazing and harvesting will be quantified combining harvesting (cf. WP 3) and literature data and will permit an assessment of net biome production.
  • Nitrogen inputs into the ecosystem will be determined by assessing atmospheric nitrogen deposition, inorganic and manure inputs.


Deliverables:

  • Response curves of phytoelement (leaf, stem, litter) CO2 exchange rates to environmental driving forces and tissue nitrogen contents for selected key species/functional groups
  • Database of foliage optical properties of dominant species
  • Leaf structure versus function relationships to simplify the estimation of foliage photosynthetic parameters
  • Continuous data sets for bulk soil respiration
  • Partitioning of bulk soil respiration into root and microbial contributions
  • Response curves of root and soil microbial respiration to temperature, water and nitrogen content
  • Partitioning of the net ecosystem fluxes into its main components for several episodic field campaigns during the season
  • Isotopic characterisation of key species
  • Complete data sets for the isotopic signature of trace gases at selected days (spring, summer fall) during the vegetation period
  • Amounts of off-site carbon losses
  • Amounts of nitrogen input into the ecosystem


Milestones:

  • Quantification of the contribution of the major ecosystem components to NEE
  • Response of CO2 exchange rates of the major ecosystem components to biotic (in particular nitrogen availability) and abiotic driving forces
  • Quantification of net biome production (NBP)