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.