TRANSIENT EFFECT OF CALCITE DISSOLUTION AND STEADY STATE EFFECT OF SOIL MOISTURE ON THE APPARENT RESPIRATORY QUOTIENT IN PORE SPACE GAS OF EXPERIMENTAL SOILS

The cycling of carbon in soils impacts climate and soil health. Soil respiration is the primary flux of carbon from terrestrial ecosystems to the atmosphere. Aerobic respiration consumes O₂ and produces CO₂. Soil respiration is usually studied by measuring the CO₂ emitted from soils. The respiratory quotient (The number of moles of CO₂ produced per mole of O₂ consumed) provides additional information about respiration in soils. However, other soil processes including mineral-water-gas reactions also affect CO₂ and O₂ in soils. In this study, carbon dioxide and oxygen concentrations from soil gas were monitored daily with an automated system at depths of 15, 30, and 60 cm from nine plots planted with Bouteloua curtipendula (Sideoats grama). 12 total plots were created by excavating 1 cubic meter of soil and backfilling with homogenized soil. For 6 of the plots, 10% by weight powered calcite was mixed with the homogenized material when backfilling from 10-20 cm. Nine of these plots were ultimately monitored, with high precision data recorded over a 7-month span from September to April. The measured CO₂ and O₂ concentrations were used to calculate values of the apparent respiratory quotients (ARQ), which ranged from 0.34 to 1.64. Large, transient decreases in ARQ occur after rainstorms, with values falling by 0.3 to 0.5 Larger decreases occur in carbonate treatments than controls, which we attribute to calcite dissolution. We filtered the transient responses to rainfall and compared the remaining, steady state ARQ values against soil moisture, soil temperature, and air temperature. A strong linear relationship was found between soil moisture and ARQ where an increase in soil moisture resulted in a decrease in ARQ. This could be explained by changes in the type of root exudates, changes in microbial and/or plant biosynthesis, and/or changes in the relative proportion of heterotrophic versus autotrophic respiration as a response to moisture.