Impact of root and mycorrhizal exudation on soil carbon fluxes : influence of elevated CO2 and metals

Sammanfattning: The thesis concerns the behavior of root and ectomycorrhizal (ECM) exudates. In particular, the dynamics of soluble low molecular weight (LMW) organic compounds such as organic acids (LMWOAs), amino acids, monosaccharides, and dissolved organic carbon (DOC) have been studied. Our knowledge of exudation rates for tree roots and especially associated ECM is limited, and also factors influencing exudation rates. Two environmental factors, metal stress and elevated atmospheric CO2 level, have been investigated. Both are of great environmental concern, but function in different ways (detoxification and C allocation) and may be highly important for the C flux caused by root/ECM exudation. The project has been carried out with mycorrhizal and nonmycorrhizal (NM) Scots pine seedlings, or saprotrophic fungi, under both sterile and non-sterile (soil) conditions. Analytical determination of exudates and calculation of exudation rates have been major tools for assessment. Assessing the possibility of using naturally occurring chelating agents (e.g. citrate and oxalate) for bioremediation of metals contaminated soils and development and validation of analytical techniques have been additional foci. The results show that from soil-living fungi and ectomycorrhizal roots exudation rates of especially LMWOAs increase significantly at Cd and Pb stress (1-100 μM), while As (as arsenate) and mixtures of metals with As have little effect. The impact of ECM fungi is large and much higher exudation rates are found when the symbionts are present both for controls and metal treatments compared to NM plants. In soil systems there was a significant mobilization of metals from soils under presence of saprotrophic fungi. Both N as well as elevated CO2 (700 ppm) causes sizable increases in exudation rates, independent of biomass, and is a finding that suggests that the availability of easily degradable carbon in soil raises, which may be highly important for the carbon flux in soil. Mycorrhizal seedlings (10 months old) increased total soil respiration ~50% compared to controls without plants in non-sterile soil systems. Key words: amino acids, 13C, carbon cycle, ectomycorrhiza, elevated CO2, exudation, DOC, LMWOA, metal stress, monosaccharides, oxalate, Pinus sylvestris, saprotrophic fungi, soil respiration