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 when natural ecosystems are transformed for food and fiber production. We examined C and N dynamics in adjacent long-term minimally disturbed and disturbed 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), soil plus HF (S+HF) and soil plus LF (S+LF) mixtures were determined. Between 0.8% and 1.7% of C and 0.3% and 1.2% of N from WS was mineralized after 28 days. The proportion of C mineralized from HF was negligible in all sites, 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 disturbed than minimally-disturbed soils, suggesting greater protection of N in the HF of disturbed soils. The proportion of C mineralized from LF ranged from −0.3% to 3.2%, and was not consistent with C mineralization dynamics from the HF component of soils. It appeared that, while the LF component of soil contained C that was chemically and, to a lesser extent, physically protected from decomposition, more C was potentially mineralizable from the LF than the HF component of the agricultural and forest soils examined. In most soils, LF additions resulted in N immobilization rather than N mineralization. Our results indicate that HF is the main source of potentially mineralizable N whereas LF is a potential sink for mineral N, regardless of land management practices, in the agricultural and forest soils we examined.