Influences of Rising Atmospheric Carbon Dioxide and Ozone Concentrations On Soil Respiration, Soil Microbial Biomass, Nutrient Availability and Soil C Dynamics in a Soybean-Wheat No-till System.

Agricultural soils under no-till management are thought to act as a carbon sink in a changing global climate because rising atmospheric carbon dioxide often stimulates agricultural productivity and enhances plant-derived carbon belowground, though elevated tropospheric ozone concentration likely offsets the carbon dioxide-fertilization effect. However, limited information is available on long-term belowground responses to elevated carbon dioxide and ozone.  The objective of this four-year experiment was to determine the separate and combined effects of elevated carbon dioxide and ozone on soil respiration, microbial biomass, nutrient availability and soil C levels.  Plants were treated with either ambient or elevated carbon dioxide (550 ppm) in combination with charcoal-filtered (CF) air or CF air plus ozone (1.4 x ambient ozone) using open-top field chambers.  Soil respiration during three growing seasons was stimulated by 28% with elevated carbon dioxide and suppressed by ozone during soybean reproductive growth, possibly due to early root senescence.  Microbial soil respiration and biomass were also higher with elevated carbon dioxide at later stages of the study but similar to the control in the added ozone treatment.  Availability of soil cations (Ca and Mg) was increased with elevated carbon dioxide while ammonia availability was lower in the added ozone treatment.  Delta ¹³C values of the coarse organic matter fraction decreased with elevated carbon dioxide while effects on percent C were mixed.  In combination, elevated ozone attenuated the effect of elevated carbon dioxide on soil respiration, but other effects were not significant.  Changes in soil biology, C and nutrient availability were primarily driven by effects of elevated atmospheric carbon dioxide concentrations.