BIOAVAILABILITY OF WATER-EXTRACTABLE ORGANIC-CARBON FRACTIONS IN FOREST AND AGRICULTURAL SOIL PROFILES

When forest ecosystems are converted to agriculture, there is generall y a marked decline in the amounts of total and labile soil organic car bon (SOC). However, analysis of SOC changes induced by forest conversi on to agriculture have generally been restricted to surface horizons a nd to pools of total and microbial biomass SOC. Changes in water-solub le SOC, which is likely the most labile and mobile form of SOC, have r eceived much less attention. We have compared concentrations of total, water-soluble (both humic and acid-soluble fractions) and bioavailabl e SOC beneath long-term (> 50 y) temperate forest and agricultural eco systems located on the same soil type. Our objectives were (1) to dete rmine if forest conversion to agriculture affects concentrations of to tal, water-soluble and bioavailable SOC throughout the soil profile an d (2) to determine if amounts of water-soluble and bioavailable SOC sh ow 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 bio available 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 cau sed by an increase in the soluble humic acid fraction in the agricultu ral soils. Our data suggest that while agricultural soils generally ha ve 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. Copyright (C) 19 96 Elsevier Science Ltd