Comparison of labile soil organic matter fractionation techniques

Labile soil organic carbon (SOC_L), soil organic carbon with a relatively short turnover time, is an important source of energy for the belowground portion of ecosystems and is sensitive to land management changes. Many techniques exist to differentiate and quantify labile SOC, but rarely have these been directly compared. Here we compare the results of four common chemical, physical, and biological methods of empirically measuring labile SOC with soils taken from 33 restored grasslands that differ in length of time since cessation of agriculture. Among sites, microbial biomass C, acid-hydrolyzable C, the amount of C respired after 12 d of a laboratory incubation, and light fraction carbon (LFC) were all positively correlated with one another, although there were large differences in the sizes of the pools estimated with each method. Acid-hydrolyzable C consistently provided the largest estimate and 12-d incubations the smallest estimate of labile SOC. The quantity of labile SOC obtained by fitting respiration data from a laboratory incubation with a two-pool model with separate decay constants for each pool was also positively correlated with the three measures of labile soil C not derived from respiration data, although this technique was sensitive to whether the decay constant of the recalcitrant pool was constrained or not. All methods showed increases in labile SOC pools with increases in total soil organic carbon (SOC_T) pools, although the rate of change varied between techniques. The size of stable aggregates correlated positively with hydrolyzable C and SOC_T, supporting the idea that aggregates may physically protect soil C from decomposition, although the degree to which this C is labile is unclear.