Andersen, C.P. 2000. Ozone stress and changes below-ground: Linking root and soil process. Phyton, (Austria), special issue: Root-soil interactions. 40:7-12.
Physiological changes in roots in response to tropospheric ozone can lead to altered below-ground processes, and responses are not always predictable due to the complexity of the belowground ecosystem. Previous experiments have shown that ozone reduces carbon allocation to roots and mycorrhizal hyphae, and reduces root starch in ponderosa pine. Ozone was found to increase C02 flux from soils containing ponderosa pine seedlings in controlled exposures, a response seemingly inconsistent with results showing decreased allocation to roots. We hypothesize this apparent paradox is due to a combination of factors including altered root metabolism and increased soil microbial respiration. Increased microbial respiration is hypothesized to result from increased root exudation, which was observed in trials with wheat, and possibly increased root mortality and turnover. Increased microbial respiration was supported by findings of increased bacterial and fungal populations in soil of plants exposed to ozone. Responses are expected to be short term, since chronic ozone exposure would lead to lower standing root biomass over time and decreased C02 flux from soil. Changes in carbon allocation to roots and mycorrhizae, reduced root growth, and altered carbon release to soil are all important factors that affect carbon fluxes into and out of forested ecosystems. The dependence of the soil organisms on carbon substrates from plants illustrates the potentially important role that ozone may play in altering ecosystem carbon fluxes. Ozone may alter nutrient availability, soil moisture holding capacity, and ultimately plant productivity through indirect effects on soil chemical and physical characteristics. Studies are currently underway in naturally regenerated stands of ponderosa pine to better understand how natural and anthropogenic stresses such as ozone affect roots and soil processes, and how changes occurring below-ground affect ecosystem productivity and sustainability.