Assessment of Two Methods for Determination of Quantity and Isotopic Composition of Soil C in Arid Soils.

Soils play a critical role in regulating global carbon (C) cycles through a combination of biological, physical and chemical processes. Soil organic matter contains approximately 1,500 Pg C but in many arid ecosystems inorganic C (IC) often represents the dominant soil C pool and global secondary or pedogenic C pools are estimated to be up to 940 Pg. Stable isotopes are commonly used to study processes involving organic and inorganic C in soils. We tested two methods, loss on ignition (400⁰C for 12 hours) and acid dissolution (1M HCl), to quantify organic C (OC) and IC as well as their isotopic signatures using mixtures of carbonate-free soil and carbonates. The amount of OC as a fraction of total soil C in the soil-carbonate mixtures ranged from 0 (carbonate only) to 1 (soil only). This approach allowed us to compare measured with calculated C and δ¹³C values. The loss on ignition technique did not remove all OC but IC was not affected and at fraction of OC< 0.3 isotopic values of the remaining mass equaled those of the IC. Surprisingly, the measured amount of organic matter in the soil decreased with increasing fraction of OC even though weight loss of the IC was negligible indicating that presence of IC affects measured organic matter values. Acid dissolution was effective in removing all carbonates irrespective of the fraction of OC present and did not affect the %C or the δ¹³C values of the remaining soil. The δ¹³C values of the evolved CO₂ were about 1‰ higher than those of the IC for reasons that are unclear. Our results show that loss on ignition does not affect isotopic ratios of the residues in soils rich in carbonates whereas the acid dissolution technique performed well in soils both high and low in carbonates.