ELEVATED CO2 ALTERS BELOW-GROUND EXOENZYME ACTIVITIES IN TUSSOCK TUNDRA

A three-year exposure to a CO2 concentration of 680 mu mol mol(-1) alt ered the enzymic characteristics of root surfaces, associated ectomyco rrhizae, and in soils surrounding roots in a tussock tundra ecosystem of north Alaska, USA. At elevated CO2, phosphatase activity was higher on Eriophorum vaginatum root surfaces, ectomycorrhizal rhizomorphs an d mantles associated with Betula nana roots, and in Oe and Oi soil hor izons associated with plant roots. Also, endocellulase and exocellulas e activities at elevated CO2 were higher in ectomycorrhizal rhizomorph s and lower in Oe and Oi soil horizons associated with roots. These re sults suggest that arctic plants respond to raised CO2 by increasing a ctivities associated with nutrient acquisition, e.g. higher phosphatas e activities on surfaces of roots and ectomycorrhizae, and greater cel lulase activity in ectomycorrhizae. Changes in enzyme activities of su rrounding soils are consistent with an increase in carbon exudation fr om plant roots, which would be expected to inhibit cellulase activitie s and stimulate phosphatase activities of soil microflora. These data were used to modify existing simulation models describing tussock phos phatase activities and litter decay. Model projections suggest that ob served increases in phosphatase activities at 680 mu mol mol(-1) CO2, could augment total annual phosphorus release within tussocks by more than 40%, at present levels of root and ectomycorrhizae biomass. This includes a nearly three-fold increase in potential phosphatase activit y of E. vaginatum roots, per unit of surface area. Observed reductions in cellulase activities could diminish cellulose turnover by 45% in s oils within rooting zones, which could substantially increase mineral nitrogen availability in soils due to lowered microbial immobilization .