DO DRYLANDS SOILS ACT AS A NET SINK OF CO2 TO REDUCE ATMOSPHERIC CO2 IN THE GLOBAL CARBON CYCLE?

The present CO2 concentration in the atmosphere is ~416 ppm, which is the highest concentration in over the past 0.8 My, presenting a challenge to identify new and existing processes of C sequestration. Dryland soils extend over 4.9 billion hectares and are characterized by pedogenic carbonates. These pedogenic carbonates are naturally accumulating high concentrations of carbon (i.e., 700 to 940 Pg C) in the form of soil inorganic carbon (SIC), equivalent to the atmospheric carbon pool or ~two-thirds of the soil organic carbon pool. Moreover, agriculture plays an important role in increasing and decreasing the amount of sequestered inorganic carbon. However, the CO2 exchange from atmosphere to land, the age, amount and flux of inorganic carbon associated with natural dryland soils and dryland agriculture is poorly known. This research examines the origin, amount and flux of SIC in the pasture and cropland soils of the Kimberly Agricultural Research Station in southwestern Idaho.

The Kimberly site in southwestern Idaho is a semi-arid dryland ecosystem with annual precipitation of ~300 mm. The parent material is loess on top of wavy basalt bedrock. This dryland region was converted to agricultural land in ~1904, including ~900 mm of supplemental irrigation water annually from the Snake River. Our preliminary results indicate the upper 100 cm of soil have accumulated SIC on an average of ~11% in the irrigated cropland and ~24% in the pasture land. The SIC appears concentrated at shallower depths in the soil profile and just above the loess/basalt boundary in irrigated soils.

The Kimberly soils have been irrigated for more than 100 years and whether this irrigation has caused an increase or decrease in SIC is still unknown. Addressing agriculture impacts on accumulation of SIC has its implications for climate change and agricultural communities. The present dataset and future research will further quantify SIC concentration in irrigated soils, CO₂ and water flux, and will address the issues of carbon and calcium provenances. Moreover, all the acquired results from Kimberly will be compared with prior SIC studies by Stanbery et al. (2017) as well as with the abundance and stable isotope composition (ẟ13C, ẟ18O) of CaCO3 determined from the nearby Reynolds Creek natural dryland ecosystem.