Exposure to ozone (O3) has been shown to decrease the
allocation of carbon to tree roots. Decreased allocation of
carbon to roots might disrupt root metabolism and rhizosphere organisms.
The effects of soil type and shoot O3
exposure on below-ground respiration and soil microbial populations were
investigated using container-grown
ponderosa pine (Pinus ponderosa Laws.) growing in a low-nutrient
soil, or a fertilizer-amended organic potting
media, and exposed to one of three levels of O3 for two growing
seasons in open-top exposure chambers. A closed
system, designed to measure below-ground respiratory activity (CO2
production, O2 consumption and RQ-Respiration Quotient;
(CO2[ratio ]O2) of plants growing in pots, was used
monthly to monitor below-ground respiration of 3-yr-old ponderosa pine.
Although seasonal differences were detected, CO2 production
(μmol h−1 g−1
total root d. wt), O2 consumption
(μmol h−1 g−1
total root d. wt) and RQ (CO2[ratio ]O2) increased
with
increasing O3 exposure level. Seasonal patterns
showed increased respiration rates during periods of rapid root growth
in
spring and early fall. Respiration
quotient tended to decrease during known periods of active root growth
in
control seedlings, but a similar response
was not observed in O3-treated seedlings. Responses to O3
were greatest in the soil-grown plants, which had a
lower fertility level than media-grown plants. Although root d. wt was
decreased,
root[ratio ]shoot ratios did not change
in response to O3. Soil-grown plants had higher root-shoot ratios
than media-grown plants, reflecting the lower fertility of the soil.
Plant exposure to O3 was found to affect both active and
total
populations of soil organisms. In both organic
potting media and in soil, biomass of active soil fungi, and the ratio
of
active-fungal to active-bacterial biomass
increased with increasing plant exposure to O3. The effect of
O3 on total fungal and bacterial biomass was not
linear: at low O3 levels, total fungal and bacterial biomass
increased;
at the high O3 level, total fungal and bacterial
biomass decreased compared with those of controls.
Our results show that O3 exposure to shoots significantly
disrupts CO2 production and O2 consumption of soil
and roots of ponderosa pine seedlings. Below-ground respiratory differences
were thought to be a result of changes
in respiratory substrates, carbon refixation within the plant and soil
microbial activity. Ozone also changes below-ground RQ, suggesting that
O3 substantially disrupts root metabolism and interactions with
rhizosphere organisms. Ozone exposure of ponderosa pine grown in different
soil types can disrupt below-ground respiration
and influence populations of soil organisms without alteration of biomass
partitioning between above- and below-ground plant components. Collectively,
the effect of O3 on the below-ground system is of concern since
it is likely that these changes are accompanied by a change in the ability
of root systems to acquire nutrient and water
resources and possibly to synthesize amino acids and proteins necessary
for normal plant function.