Effect of forest type conversion on soil organic carbon mineralization and its abiotic regulation factors in humid subtropics.

Forest harbors the largest carbon reservoir in the terrestrial ecosystems. The conversion of forest types may lead to significant changes in soil carbon pools and carbon mineralization. We collected soil samples at 0-10 cm depth converted from <i>Castanopsis carlesii</i> natural forests (NF) to <i>C. carlesii</i> plantations (CC) and <i>Cunninghamia lanceolata</i> plantations (CF) at both Chenda and Xinkou sites in Sanming, Fujian Province to analyze the differences in soil organic carbon mineralization among the three forest types by laboratory incubation. We examined the effects of abiotic variations caused by forest conversion on soil organic carbon (SOC) mineralization, including SOC, total nitrogen (TN), iron and aluminum oxides, mean weight diameter (MWD) of aggregates, and particle size composition. The results showed that the contents of SOC, TN, and MWD differed significantly among the three forest types at both sites, which followed the order of NF>CC>CF. The iron and aluminum oxide contents in CF were significantly lower than those in NF and CC. The sand content in NF was significantly higher than those in CC and CF, while the silt content showed an inverse pattern. The clay content in CC was significantly higher than those in NF and CF. The cumulative carbon mineralization per unit soil (<i>C</i>_m-soil) was significantly affected by stand type, with the <i>C</i>_m-soil in CC and CF stands being 11.3% and 23.3% lower than in NF stands, respectively. The cumulative carbon mineralization per unit soil organic carbon (<i>C</i>_m-SOC) was significantly affected by the interaction between forest types and sites, in that the <i>C</i>_m-SOC in NF stands was 32.8% lower than that in CF at the Chenda site but without differences among forest types at Xinkou site. The <i>C</i>_m-soil positively and significantly correlated with SOC content at both sites, with MWD on the Chenda site, with iron oxide on the Xinkou site, respectively. However, the correlations between <i>C</i>_m-soil and MWD or iron oxides became no significance after introducing SOC as the controlling factor in partial correlation analysis, which suggested that the SOC mediated the relationships between <i>C</i>_m-soil and MWD or iron oxides at both sites. Overall, the conversion of natural forest to plantations could decrease soil C mineralization, due to the decline of SOC content. The effect of aggregate stability, iron and aluminum oxides on soil carbon mineralization varied depending on the site.