We present two case studies to illustrate the active roles critical zones play in the global carbon cycle under natural and cultivated systems. We evaluate the potential of gray shale to (1) take up atmospheric CO2 during mineral weathering and (2) release CO2 during degradation of its ancient organic matter by investigating the evolution of dissolved inorganic carbon (DIC) concentrations and its carbon isotopic ratio (δ13CDIC) along water flow paths in the well-characterized Susquehanna Shale Hills Critical Zone Observatory. Here, soil porewaters are characterized by low DIC, in chemical and isotopic equilibrium with soil CO2. The degradation of ancient organic matter is negligible in contributing to soil CO2. The chemistry of groundwater varies along different flowpaths as soil porewater recharges to the water table and then dissolves ankerite or secondary calcite. Weathering of carbonate leads to much higher concentrations of DIC in groundwaters than soil waters. The δ13CDIC values of these groundwaters document mixing between the carbonate phases and soil CO2. The major element concentrations, DIC, and δ13CDIC data show the first-order stream in the valley of the catchment is derived from groundwater and soil waters in proportions that vary both spatially and temporally. The CO2 has degassed slightly from the headwaters to the mouth of the stream but there is little evidence of C isotopic equilibration with the atmosphere, due to its short length of the stream and short contact time with air. Drylands, covering more than 40% of the land surface, have been intensively cultivated worldwide. To date, however, few studies have quantified the production and emission of CO2 during the development of pedogenic carbonate in dryland agricultural settings. The challenge is to separate the contributions of biogenic CO2 (from soil respiration) and abiotic CO2 (from calcite precipitation). We studied such soil-water-gas interaction in a flood-irrigated pecan field in El Paso, Texas, and characterized the organic matter and carbonates in the soil profiles, DIC in the irrigation and soil waters, CO2 in soil gases, and CO2 efflux from the land to the atmophsere. Using stable carbon isotopes and carbon mass balance, we demonstrated that supplies of Ca and DIC by irrigation waters accelerated accumulation of pedogenic carbonates and led to a measurable flux of CO2 from the soil to the atmosphere that may be important in land-carbon-climate feedback.

Understanding inorganic carbon dynamics in natural and human-impacted critical zones

253rd National Meeting (2017)

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