Carbon and nitrogen mineralization from light- and heavy-fraction additions to soil

Mineralization of C and N from soil organic matter (SOM) can be altered whe n natural ecosystems are transformed for food and fiber production. We exam ined C and N dynamics in adjacent long-term minimally disturbed and disturb ed soils from agricultural and forest sites. Light and heavy fractions (LF and HF, respectively) of SOM were collected by physical density separation using sodium polytungstate. Aerobic C and N mineralization of soil (WS), so il plus HF (S + HF) and soil plus LF (S + LF) mixtures were determined. Bet ween 0.8% and 1.7% of C and 0.3% and 1.2% of N from WS was mineralized afte r 28 days. The proportion of C mineralized from HF was negligible in all si tes, suggesting that the HF component of soils could be a major sink for C storage in soils. Larger proportions of N from HF were mineralized in distu rbed than minimally-disturbed soils, suggesting greater protection of N in the HF of disturbed soils. The proportion of C mineralized from LF ranged f rom -0.3% to 3.2%, and was not consistent with C mineralization dynamics fr om the HF component of soils. It appeared that, while the LF component of s oil contained C that was chemically and, to a lesser extent, physically pro tected from decomposition, more C was potentially mineralizable from the LF than the HF component of the agricultural and forest soils examined. In mo st soils, LF additions resulted in N immobilization rather than N mineraliz ation. Our results indicate that HF is the main source of potentially miner alizable N whereas LF is a potential sink for mineral N, regardless of land management practices, in the agricultural and forest soils we examined. (C ) 2000 Elsevier Science Ltd. All rights reserved.