ARE PEDOGENIC CARBONATES IN DRYLAND SOILS A SOLUTION TO CLIMATE CHANGE? EVALUATING THE AMOUNTS, RATES AND PROCESSES OF CARBONATE FORMATION IN AGRICULTURAL AND NATURAL CRITICAL ZONE SYSTEMS

An estimated 40% of all soil carbon is stored as inorganic carbonate minerals, but the age, amount, and flux of carbon from soil is still poorly constrained. This study examines controls on the origin, amount and flux of Soil Inorganic Carbon (SIC) in the Reynolds Creek Critical Zone Observatory and Northwest Soils and Irrigation Lab in Southwestern Idaho. Prior studies from Reynolds Creek (Stanbery et al., 2017) sagebrush steppe ecosystem demonstrate gravel coatings contain up to 44% of total SIC at a given site and an average of 13% of the total SIC is stored as carbonate coasts within the gravel fraction. The threshold for SIC accumulation is ~500 mm mean annual precipitation, and rainfall is the largest predictor of the presence or absence of SIC. Variability in SIC below that value is significant; SIC content from ~1 m deep soil pits and cores at 71 sites show that 64 sites contained less than 10 kg/m2 SIC, 5 contained between 10-20 kg/m2, and 2 sites had between 24 and 29 kg/m2. Parent material consistently ranks as an important predictor in random forest analysis; however, we were unable to quantify the importance of wind-blown dust in the soil profiles which we believe to play a vital role in SIC accumulation.

Kimberly Idaho is a semi-arid xeric ecosystem ( ~300 mm of precipitation annually). The parent material is loess on top of wavy basalt bedrock. In ~1904, irrigation from the Snake River transformed this dryland ecosystem into an agricultural community with an average of ~900 mm of additional water input from irrigation annually. Research as part of a new NSF CZO project at Kimberly Idaho reveals intriguing results. Despite irrigation well in excess of the 500 mm pedocal/pedalfer threshold identified by Hans Jenny and others, and the observed annual threshold for SIC accumulation in Reynolds Creek, preliminary results from Kimberly Idaho show an average of ~11% CaCO₃ in the top meter of irrigated cropland soils and ~24% CaCO₃ from pasture soils. Notably, SIC in irrigated soils is concentrated at the top of the soil profile and just above the loess/basalt boundary.

Ongoing and future research will further quantify amounts of SIC in irrigated soils, as well as CO₂ flux, water flux and chemistry, dust flux and composition. Preliminary findings suggest irrigated farmlands have been sequestering inorganic carbon for >100 years, with important implications for mechanisms of both addressing climate change and supporting agricultural communities.

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