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Carbon sequestration uncertainty: Is grazing-induced soil organic carbon accrual offset by inorganic carbon loss?

Kurt Reinhart , Matthew Rinella, Richard Waterman, Hilaire Sanni Worogo, Lance Vermeire

Abstract

In drylands, soil inorganic carbon (SIC) represents the largest terrestrial carbon sink, and observational studies indicate a negative relationship and possible trade-off between SIC (e.g. calcium carbonate [CaCO3]) and soil organic carbon (SOC). Now some rangeland managers aim to increase SOC stocks (mass SOC × area-1) to help decarbonize the atmosphere. Unfortunately, the fate of SIC (and SOC) is uncertain, and grazing-induced SOC accrual may correspond with CaCO3 dissolution which can produce CO2 emissions. An added concern is whether carbon sequestration schemes focused solely on SOC stocks need to be discounted for putative CO2 emissions due to CaCO3 dissolution. We used data from a 5-year grazing experiment in the Northern Great Plains of the US. We tested whether grazing management treatments affect SIC, and whether grazing-induced SOC accrual was potentially offset by SIC loss. The experiment had a randomized complete block design and pre-treatment data. Response variables were SOC and SIC stocks (0-60 cm depth increment). Moderate summer grazing (control) is regionally common and treatments that may alter soil stocks included: no grazing, severe summer grazing, moderate fall grazing, and severe fall grazing. We also tested for a negative relationship between SOC and SIC across all soil cores (n= 244). Severe grazing (summer and fall) increased SOC by 0.83 and 0.88 kg × m-2 relative to moderate summer grazing, respectively. However, no treatments affected SIC. Conversely, we found an overall weak but significant (r2= 0.04, P= 0.002) near 1-to-1 negative relationship between SIC and SOC stocks of soil cores. Our findings suggest severe grazing can increase SOC without affecting SIC, at least over the short-term (5 years). This finding mirrors results from an observational study elsewhere in the Northern Great Plains that also failed to detect grazing effects on SIC. Long-term grazing experiments (> 5 years) with pre-treatment data may be required to detect grazing effects on SIC.

RJ24006  Accepted 16 July 2024

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