Does the Acid Hydrolysis–Incubation Method Measure Meaningful Soil Organic Carbon Pools?

The literature was reviewed and analyzed to determine the feasibility of using a combination of acid hydrolysis and CO 2 –C release during long‐term incubation to determine soil organic carbon (SOC) pool sizes and mean residence times (MRTs). Analysis of 1100 data points showed the SOC remaining after hydrolysis with 6 M HCl ranged from 30 to 80% of the total SOC depending on soil type, depth, texture, and management. Nonhydrolyzable carbon (NHC) in conventional till soils represented 48% of SOC; no‐till averaged 56%, forest 55%, and grassland 56%. Carbon dates showed an average of 1200 yr greater MRT for the NHC fraction than total SOC. Long‐term incubation, involving measurement of CO 2 evolution and curve fitting, measured active and slow pools. Active‐pool C comprised 2 to 8% of the SOC with MRTs of days to months; the slow pool comprised 45 to 65% of the SOC and had MRTs of 10 to 80 yr. Comparison of field 14 C and 13 C data with hydrolysis–incubation data showed a high correlation between independent techniques across soil types and experiments. There were large differences in MRTs depending on the length of the experiment. Insertion of hydrolysis–incubation derived estimates of active (C a ), slow (C s ), and resistant pools (C r ) into the DAYCENT model provided estimates of daily field CO 2 evolution rates. These were well correlated with field CO 2 measurements. Although not without some interpretation problems, acid hydrolysis–laboratory incubation is useful for determining SOC pools and fluxes especially when used in combination with associated measurements.