Effects of elevated CO2 and O3 on soil respiration under ponderosa pine

Soil respiration represents the integrated response of plant roots and soil organisms to environmental conditions and the availability of C in the soil. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing a reconstructed ponderosa pine/soil–litter system. The study used a 2×2 factorial design with two levels of CO2 and two levels of O3 and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO2 and O3, singly and in combination, on soil respiration, fine root growth and soil organisms. Fine root growth and soil organisms were included in the study as indicators of the autotrophic and heterotrophic components of soil respiration. The study evaluated three hypotheses: (1) elevated CO2 will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O3 will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO2 and O3 have opposing effects on C assimilation and allocation, elevated CO2 will eliminate or reduce the negative effects of elevated O3 on soil respiration. A mixed-model covariance analysis was used to remove the influences of soil temperature, soil moisture and days from planting when testing for the effects of CO2 and O3 on soil respiration. The covariance analysis showed that elevated CO2 significantly reduced the soil respiration while elevated O3 had no significant effect. Despite the lack of a direct CO2 stimulation of soil respiration, there were significant interactions between CO2 and soil temperature, soil moisture and days from planting indicating that elevated CO2 altered soil respiration indirectly. In elevated CO2, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO2. Soil respiration increased more with days from planting in elevated than in ambient CO2. Elevated CO2 had no effect on fine root biomass but increased abundance of culturable bacteria and fungi suggesting that these increases were associated with increased C allocation belowground. Elevated CO2 had no significant effect on microarthropod and nematode abundance. Elevated O3 had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature.