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Andersen, C.P. 2001. Ozone stress and changes below ground: linking root and soil processes. Phyton 40:7-12. WED-00-026
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 CO2 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 CO2 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.