THE EFFECTS OF ELEVATED CO2 ON SYMBIOTIC N-2 FIXATION - A LINK BETWEEN THE CARBON AND NITROGEN CYCLES IN GRASSLAND ECOSYSTEMS

The response of plants to elevated CO2 is dependent on the availabilit y of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO2 concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilizati on which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO2 stimulated increase in plant growth (thus requiri ng more nitrogen) and an increased N demand for the decomposition of s oil residues with a large C:N will result under elevated CO2 in a larg er N-sink of the whole grassland ecosystem. One way to maintain the ba lance between the C and N cycles in elevated CO2 would be to increase N-import to the grassland ecosystem through symbiotic N-2 fixation. Wh ether this might happen in the context of temperate ecosystems is disc ussed, by assessing the following hypothesis: i) symbiotic N-2 fixatio n in legumes will be enhanced under elevated CO2, ii) this enhancement of N-2 fixation will result in a larger N-input to the grassland ecos ystem, and iii) a larger N input will allow the sequestration of addit ional carbon, either above or below-ground, into the ecosystem. Data f rom long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, gr own under elevated CO2 under free-air or field-like conditions, suppor ts the first two hypothesis, since: i) both the percentage and the amo unt of fixed N increases in white clover grown under elevated CO2 ii) the contribution of fixed N to the nitrogen nutrition of the mixed gra ss also increases in elevated CO2. Concerning the third hypothesis? an increased nitrogen input to the grassland ecosystem from N-2 fixation usually promotes shoot growth (above-ground C storage) in elevated CO 2. However, the consequences of this larger N input under elevated CO2 on the belowground carbon fluxes are not fully understood. On one han d, the positive effect of elevated CO2 on the quantity of plant residu es might be overwhelming and lead to an increased long-term below-grou nd C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over an d, hence, limit the storage of below-ground carbon.