Bioavailability of water extractable organic carbon fractions in forest and agricultural soil profiles

When forest ecosystems are converted to agriculture, there is generally a marked decline in the amounts of total and labile soil organic carbon (SOC). However, analysis of SOC changes induced by forest conversion to agriculture have generally been restricted to surface horizons and to pools of total and microbial biomass SOC. Changes in water-soluble SOC, which is likely the most labile and mobile form of SOC, have received much less attention. We have compared concentrations of total, water-soluble (both humic and acid-soluble fractions) and bioavailable SOC beneath long-term ( > 50 y) temperate forest and agricultural ecosystems located on the same soil type. Our objectives were (1) to determine if forest conversion to agriculture affects concentrations of total, water-soluble and bioavailable SOC throughout the soil profile and (2) to determine if amounts of water-soluble and bioavailable SOC show similar patterns as total SOC. As expected, total SOC was higher in forest than in crop systems, but the difference was restricted to the surface layers. Surprisingly, concentrations of water-soluble and bioavailable SOC were higher in agricultural soils than in forest soils, again only in the surface layers. The difference in water-soluble and bioavailable SOC between agricultural and wooded soils was largely caused by an increase in the soluble humic acid fraction in the agricultural soils. Our data suggest that while agricultural soils generally have lower amounts of total and microbial C than forest soils, they may support equal or greater rates of microbial activity than forest soils due to increased production of water-soluble carbon.