The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 308

P.Martin-Olmèdoet al. Interactions Between Elevated CO 2 and N in Soils

4.12

Interactions Between
Elevated CO 2 and N in Soils:
Influence on N 2 O Fluxes and
Rhizosphere Denitrifier Activity

P. M ARTIN -O LMÈDO 1 , J. G RACE 2 and R.M. R EES 3

1
EscuelaAndaluzadeSaludPublica, CampusUniversitariode
Cartuja, Ap.Correos2070, 18080 Granada, Spain; 2 IERM,
University of Edinburgh, King's Buildings, Edinburgh EH9 3JG;
and 3 Environment Division, Scottish Agricultural College,
Edinburgh EH9 3JG, UK

Introduction

Increasing atmospheric CO 2 concentrations are expected to alter both the
quality and quantity of products fixed by photosynthesis, and hence global
C and N cycles (Ross et al ., 1996). Plants influence soil microbial processes
in a variety of ways, largely through modifications of the soil physical and
chemical environment. The soil microbial communities likely to be influ-
enced most strongly by such perturbations are the rhizosphere communities
(Grayston et al ., 1996). One possible effect of an increased rate of CO 2 fixa-
tion under elevated CO 2 is the release of more C as exudates and detritus
transferred into the rhizosphere soil (Canadell et al ., 1996). If some of this
C is used by heterotrophic microbes, an effect on soil N transformations,
including those leading to N 2 O emissions, would be anticipated (Davidson,
1991). Several consequences are possible. Higher net primary productivity
under CO 2 enrichment may result in a greater net removal of water from
the rhizosphere and thus create better conditions for O 2 supply promoting
the establishment of nitrifying bacteria (Klemedtsson et al ., 1987).
Alternatively, greater plant water use efficiency, higher root biomass and
large microbial activity under elevated CO 2 may lead to a reduction in O 2
availability which, after wetting of soil and surplus of root exudates,
subsequently may favour the synthesis of denitrifying enzymes and thus
of denitrifier activity and growth (Klemedtsson et al ., 1987; Højberg et al .,
1996). Root N uptake and enhanced N immobilization by growing
soil microbial populations may also diminish the availability of mineral