Three perennial grass species, Lolium perenne L.,
Agrostis capillaris L. and Festuca ovina L., were homogeneously
labelled in phytotrons with 14CO2 at two CO2
concentrations (350 and 700 μl l−1). Plants were grown
under two
nitrogen regimes: one with a minor addition of 8 kg N ha−1,
the other with an addition of 278 kg N ha−1. Carbon
allocation over the different compartments of the plant/soil systems
was measured: shoots, roots, rhizosphere soil
(soil solution, microbial biomass and soil residue), and bulk soil. Elevated
CO2 increased total net 14C recovery in
all species by 14%, and significantly enhanced the below-ground
14C allocation by 26%, this enhancement was
24%, 39% and 21% for root, rhizosphere soil and bulk soil, respectively.
Within
the rhizosphere soil, the 14C
amounts in the soil solution (+69%) and soil residue (+49%) increased
significantly. Total microbial biomass-C in the rhizosphere soil was also
increased (15%) by the elevated CO2 treatment, but only in proportion
to the increased root mass. No interactions were observed between the elevated
CO2 and N treatments. The N treatment increased total net
14C recovery by more than 300% and
14C was preferentially allocated to the shoots, leading to
a significant increase in shoot-to-root ratio. However, N fertilization
also
increased (+111%) the absolute amount of
14C in soil. The three species behaved differently, but no
interactions were observed between CO2 treatment
and plant species. These results show that elevated CO2 induces
an
increased C input into soil for all three grass
species at both N levels. However, the highest absolute amounts were found
in
the soils of the fastest growing species and at the highest N level.