Changes of soil C stocks and stability after 70-year afforestation in the Northeast USA.

Background and aims: Forest restoration has been proposed as an effective strategy for the sequestration of atmosphere CO. Understanding the mechanisms of soil organic carbon (SOC) dynamics in afforestation is important to quantifying and enhancing carbon (C) sequestration. Methods: After 70 years afforestation with two conifer and three broadleaf tree species in monocultures in Northeast USA, we measured soil C pools as well as C fractions in these plantations and nearby pasture control. Results: Soil C stored in forest floor was increased by afforestation, especially in conifer stands. Total C stock in mineral soils was not statistically different among species, but C fractions had been altered. Compared to pasture land, afforestation decreased coarse particle organic C (cPOC) fraction, while increased recalcitrant mineral associated SOC (mSOC) fraction. Afforestation decreased the macro-aggregate C fraction in the 0-5 cm soil, but increased the micro-aggregate C fraction in the 5-15 cm soil. Conclusions: 1) Afforestation using conifers could improve the whole soil-profile carbon stock compared to broadleaves when the forest floor is included; 2) Even though the overall mineral soil C stock was not changed, afforestation could improve soil C stabilization through increasing mSOC fraction and forming more micro-aggregate C fraction in deeper soils. [ABSTRACT FROM AUTHOR]