Effects of elevated CO 2 and O 3 on soil respiration under ponderosa pine

Abstract

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 CO 2 and two levels of O 3 and three replicates of each treatment. The objectives of our study were to assess the effects of long-term exposure to elevated CO 2 and O 3, 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 CO 2 will increase C assimilation and allocation belowground increasing soil respiration; (2) elevated O 3 will decrease C assimilation and allocation belowground decreasing soil respiration and (3) as elevated CO 2 and O 3 have opposing effects on C assimilation and allocation, elevated CO 2 will eliminate or reduce the negative effects of elevated O 3 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 CO 2 and O 3 on soil respiration. The covariance analysis showed that elevated CO 2 significantly reduced the soil respiration while elevated O 3 had no significant effect. Despite the lack of a direct CO 2 stimulation of soil respiration, there were significant interactions between CO 2 and soil temperature, soil moisture and days from planting indicating that elevated CO 2 altered soil respiration indirectly. In elevated CO 2, soil respiration was more sensitive to soil temperature changes and less sensitive to soil moisture changes than in ambient CO 2. Soil respiration increased more with days from planting in elevated than in ambient CO 2. Elevated CO 2 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 CO 2 had no significant effect on microarthropod and nematode abundance. Elevated O 3 had no significant effects on any parameter except it reduced the sensitivity of soil respiration to changes in temperature.