Estimation of slow- and fast-cycling soil organic carbon pools from 6N HCl hydrolysis

Acid hydrolysis is used to fractionate the soil organic carbon pool into relatively slow- and fast-cycling compartments on soils from Arizona, the Great Plains states and Michigan collected for carbon isotope tracer studies related to soil carbon sequestration, for studies of shifts in C-3/C-4 vegetation, and for ''pre-bomb'' soil-carbon inventories. Prior to hydrolysis, soil samples are first treated with cold 0.5-1N HCl to remove soil carbonates if necessary. Samples are then dispersed in a concentrated NaCl solution (p approximate to 1.2 g cm(-3)) and floated plant fragments are skimmed off the surface. After rinsing and drying all remaining recognizable plant fragments are picked from the soil under 20x magnification. Plant-free soils, and hot, 6N HCl acid-hydrolysis residue and hydrolyzate fractions are analyzed for carbon content, delta(13)C and C-14 age, and the carbon distribution is verified within 1-2% by stable-carbon isotope mass balance. On average, the recalcitrant residue fraction is 1800 yr older and 2.6 parts per thousand more C-13-depleted than total soil organic carbon. A test of hydrolysis with fresh plant fragments produced as much as 71-76% in the acid-hydrolysis residue pool. Thus, if plant fragments are not largely removed prior to hydrolysis, the residue fraction may date much younger than it actually is.