The quantifiable objectives are formulated in terms of scientific hypotheses to be tested by work-packages (WPs,).

Objective 1 To test four hypotheses about the effects of heavy metals and time on the C:N:P stoichiometry in field conditions.

WP1 Change in time (field plot scale long-term feed-back loop); PE10_9 and LS8_2 priorities. Leads to Deliverable 2 (D2), target journal for publication Scientific Reports.

• H1a The ratios of C:P and N:P in soil (0-20 cm) contaminated with heavy metals (HMs) increased more at the time scale of 8 years than at the time scale of 5 years.
  • Rationale: ecological succession after the end of disturbance by pollution would lead to accumulation of C and N in the top soil.
• H1b The increase of C:P and N:P ratios in soil (0-20 cm) contaminated with HMs is positively correlated with the aggregated effect traits at the scale of grassland plots and negatively correlated with the bare soil relative cover.
  • Rationale: vegetation recovery after disturbance by pollution is associated with response traits facilitating re-colonization, and presumably with a decrease in the bare soil surface

WP2 Runoff stoichiometry (field plot and mini-plot scale); PE10_9 priority. Leads to D3, target journal for publication Biogeochemistry

• H2a The concentration of HMs in soil is positively correlated with the slope of the positive relationship between the N:P ratios in water and the amount of precipitation produced for erosion plots with 100% plant cover.
• H2b The concentration of HMs in soil is positively correlated with the slope of the negative relationship between the N:P ratios in water and the amount of precipitation produced for erosion plots with small plant cover (large area relative cover of bare soil).
  • Rationales: as in Sardans and Penuelas (2014, figure 3) developed with the extra effect on plants development due to HMs.

Objective 2 To test six hypotheses about the effects of HMs and temperature on the C:N:P stoichiometry at microcosm and mezocosm scales ex-situ.

WP3 Plant uptake and leaching (pot scale); PE10_9 priority. D4, target journal New Phytologist.

• H3a The increase of temperature leads to higher C:N and C:P ratios in roots and aboveground parts of plant grown in soils contaminate with HMs.
• H3b The relative increase of C:N and C:P ratios due to temperature is positively correlated with the concentrations of HMs concentrations in plant parts.
  • Rationales: as in Sardans and Penuelas (2012) with the extra stress due to HMs.
• H3c The increase of temperature leads to a larger amount of N and P exported by leaching after flooding with a fixed amount of water. The relative increase due to temperature in the exported amount of N and P  is positively correlated with the HMs concentrations in plants.
  • Rationale: as in Sardans and Penuelas (2014, figure 3) extrapolated from runoff from plots covered by vegetation to leaching from pots and with the extra effect of HMs.
• H3d The effects hypothesized in H2a-H2c will be smaller in the case of multispecies pots than in the case of monospecies pots.
  • Rationale: The aggregated functional traits at pot scale will be more favorable to nutrients retention in the case of multiple plant species by mechanisms of plant interactions.

WP4 Leaching (lysimeters); PE10_9 priority. D5, target journal Advances in Water Resources

• H4a The increase in the concentration of metals in soil decrease the retention time of the percolating water and increase the N:P ratio in the seepage water.
  • Rationales: as in Sardans and Penuelas (2014, figure 3) extrapolated from runoff from plots covered by vegetation to leaching from such plots, and developed with the extra effect on plants development due to HMs.
• H4b The effects hypothesized in H4a will be smaller when the simulated rain occurs on multispecies lysimeters than in the case of monospecies lysimeters.
  • Rationale: it is expected that the multispecies vegetation will resist better to the metal stress

Objective 3 To test three hypotheses about the effects of HMs and temperature on the structure of traits space and to disentangle the significance of the results about the effects of complex stress on the production of ecosystem services.

WP 5 Complex stress effects on the overall traits space (cross-cutting WP); LS8_2 and SH2_7.

• Extra activity: to produce the final form of the theoretical framework, D1, target journal Biogeochemistry.
• H5a The increase of temperature and HMs in soil is inversely correlated with the variability of the physiologically relevant stoichiometric traits in a plant species and with the hypervolume occupied by their nutrients elementome.
• H5b The increase of temperature and HMs in soil leads to a stronger discrimination (segregation) between the hypervolumes occupied by the nutrients elementome (the biogeochemical niches) of each plant species
  • Rationale: as in He et al. (2019) and Penuelas et al. (2019) tested for the stress due only to HMs, or in combination with temperature.
• H5c The plant samples scores on the factors extracted from the functional traits space of a plant species are significantly correlated with their stoichiometric traits, with the scores of the factors extracted from their nutrients elementome, and with the scores extracted from their toxic elements elementome.
  • Rationale: general multivariate physiological causation (with biogeochemical consequences by the functional traits indirectly controlling the hydrological fluxes and the associated elemental fluxes). D6, target journal for H5a-c Science Advances.
• Extra activity: to disentangle the potential effects of the results of all hypotheses testing on the production of ecosystem services by grassland plants. D7, journal Ecosystem Services.