Plant and soil microbial biomasses in Agrostis capillaris and Lathyrus pratensis monocultures exposed to elevated O3 and CO2 for three growing seasons

Elevated CO2 usually promotes plant growth, whereas elevated O3 often has a negative effect, especially on root biomass. Thus both these gases can indirectly affect the soil microbial community. We exposed Agrostis capillaris and Lathyrus pratensis to realistic levels of O3 (40–50 ppb) and CO2 (ambient air + 100 ppm) in open-top chambers during 2002–2004. The experiment shows negative effects of both O3 and CO2, especially on the bulk soil of L. pratensis, in terms of the decreased biomasses of total (25% and 31%), actinobacterial (29% and 31%), bacterial (26% and 33%) and mycorrhizal (AM fungal) (31% and 35%) indicator subgroups, analysed by the PLFA (phospholipid fatty acid) method. The fungal:bacterial PLFA biomass ratio decreased in the bulk soil of A. capillaris, especially with elevated CO2 alone (38%). These longer-term changes are considered to arise mainly from differences between the plant functional types (i.e. grass cf. N2-fixing legume) in litter quality and soil C:N ratio. The results also point to interactions and multi-trophic feedbacks between elevated O3, plant, parasitic rust fungi and soil readily available P, accompanied by a shift in N balance in favour of plants rather than soil microorganisms.