Date of Award


Degree Name

Master of Science


Geological Sciences


Valeria I. Molina


Drylands, characterized by low and sporadic precipitation, require irrigation for crop growth. However, irrigation practices can lead to salt accumulation in soil due to high evaporation rates and reduced leaching. In addition to loading salts to soil, irrigation promotes the accumulation of secondary calcite. In natural systems, the formation of pedogenic carbonate (secondary calcite, CaCO3) is critical, impacting the soil properties hydrologically and biogeochemically, and modifying the global carbon cycle over geological time, albeit at a lower rate. In agricultural sites, irrigation water supplies HCO3- and Ca2+, accelerating the rates of CaCO3 formation and releasing abiotic CO2. This study investigated the abiotic and biotic processes that have produced soil CO2 in dryland soils at an irrigated pecan orchard in Tornillo, Texas, and a natural site within Jornada Experimental Range, New Mexico. Two sites within the pecan orchard, Pecan_Coarse, and Pecan_Fine, have contrasting soil textures resulting in different soil salinity, pedogenic carbonate accumulation rates, and tree sizes. A range of methods was employed including CO2, O2, moisture sensors, and soil gas samples for pCO2 and δ13CCO2 measurements, as well as irrigation water collection and analyses at the orchard, for pH, alkalinity, and δ13CDIC. The overall objective of this study is to quantify the release of abiotic CO2 during the precipitation of irrigation-induced calcite, as a function of spatial variability due to soil texture, and as a function of growing season and irrigation events at a high temporal scale. Three CO2 end-members in the agricultural system (the atmospheric, biotic, and abiotic/calcite derived) have their distinctive C isotope signatures. Soil gas samples at Pecan_Fine (> 30 cm), without the influence of atmospheric CO2, are plotted closer to abiotic CO2 endmember than those at Pecan_Coarse, due to finer soil texture, more salt buildup, faster calcite accumulation, and thus more abiotic CO2. In contrast at the Pecan_Coarse, more soil CO2 is generated biotically, with larger tree sizes, and thus more soil respiration than Pecan_Fine. Two-component mixing model results show that the abiotic process has contributed up to 72% of total soil CO2 at Pecan_Fine and only up to 47% at Pecan_Coarse. These contributions are much higher than those reported by Ortiz et al. (2022) because local groundwater of much higher total dissolved solids was utilized for irrigation in this study producing more calcite-derived abiotic CO2 than river water in the previous study. When irrigation sources are switched from local groundwater to river water in the summer, the δ13CDIC of irrigation water shifts, leading to a corresponding shift in the δ13C signature of abiotic CO2. The temporal variability of soil pCO2 and pO2 is driven by irrigation at both Pecan_Fine and Pecan_Coarse soils. After irrigation, water filtration pushes the O2 out, and when soil pores open up with evaporation, O2 diffuses in from the atmosphere until the next irrigation. Soil CO2 behaves differently: irrigation water, at equilibrium with atmospheric CO2, dissolves soil CO2 and lowers its concentrations after irrigation. With continuous evaporation, soil water concentrations increase leading to the precipitation of calcite and the release of abiotic CO2, leading to higher pCO2. Similarly, supported by higher pO2, soil respiration rate also is elevated, releasing biotic CO2 and leading to higher pCO2. Higher pCO2 also leads to diffusion loss of CO2 from soil to the atmosphere. When soil CO2 production is lower than the overall CO2 loss, the soil pCO2 reaches a maximum and begins to drop. Although the temporal trends are similar between two sites of different soil textures, the magnitude of variations in pCO2 and pO2 within each irrigation is different, dictated by texture-controlled water and gas transport. The Jornada Experimental Range provides an opportunity to study the natural dryland ecosystem, wherein rainfall patterns and seasonal variability are the primary factors controlling soil pCO2 and pO2. Through this project, we have deepened our understanding of the soil C budget in both natural and managed drylands by investigating the biotic and abiotic processes that contribute to soil CO2. Our research has demonstrated that soil CO2 production in drylands is strongly influenced by soil texture, water inputs, and its corresponding chemical composition. Specifically, the combination of fine soil texture and continuous evaporation can lead to chemical saturation of evaporate minerals, resulting in salt buildup and the formation of pedogenic carbonates, which in turn produce abiotic CO2. It is critical to comprehend these processes in agricultural soil systems in drylands, as the accumulation of pedogenic carbonates can impact soil health, crop yields, and the global C budget.




Recieved from ProQuest

File Size


File Format


Rights Holder

Valeria Isabel Molina