DISTRIBUTION AND VARIATION OF SOIL INORGANIC CARBON IN NATIVE AND IRRIGATED SOILS OF A COLD SEMI-ARID DRYLAND SYSTEM

Carbon stored in soils provides natural carbon sequestration and therefore a potential climate change mitigation strategy. Recent studies quantify global soil inorganic carbon (SIC) stocks but the rate of SIC accumulation, particularly in human-altered agroecosystems, is poorly constrained. Since irrigation of drylands may render drylands a net carbon source, determining carbon stocks in unmanaged dryland soils and agricultural lands is important. This study aims to determine i) SIC stocks in native and agricultural semi-arid dryland soils of south-central Idaho, and ii) the difference (if any) in SIC stocks between native and irrigated soils.

This study is focused on the Kimberly Northwest Irrigation and Soils Research Laboratory (NWISRL) in south-central Idaho, which transitioned from a native sagebrush ecosystem to agricultural lands with the advent of irrigation in the early 1900s. In addition to precipitation (~240 mm/yr), ~900 mm of irrigation water from the Snake River is added annually to Kimberly soils (85% of total water). Most of the Kimberly area was converted to agricultural land, so it was difficult to find native soils; therefore, less-managed, non-cultivated, and unmanaged/native soils are categorized as native and compared with intensely irrigated soils. SIC has been measured from seven irrigated and four native soils. The mean and standard deviation of SIC was calculated for 2 m soil depth assuming the wetting front from irrigation water extends to 2 m depth. Preliminary results show the mean of SIC for irrigated and native soils is 60 and 69 (kg/m²), respectively. Irrigated soils have a lower SIC standard deviation (7 kg/m²), which likely indicates homogeneity and less variability than native soils due to tilling, irrigation, and erosion of the A horizon. The larger standard deviation for native soils (18 kg/m²) reflects natural variability due to the presence of an A-horizon at the surface and heterogeneity due to buried horizons and paleosols. Based on our current number of study sites (n=11) the mean SIC for irrigated soils is within the range of the standard deviation for native soils. Future work will use hydrologic modeling and a larger sample size to help determine if irrigated and native sites demonstrate a statistically significant difference in SIC values.