Changes in soil organic carbon and total nitrogen as affected by primary forest conversion

Numerous studies have addressed the effects of forest conversion on soil organic carbon (SOC) and total nitrogen (TN); however, knowledge of SOC and TN stocks and SOC stability after primary forest conversion to secondary forest is still lacking. Such an understanding is critical for the accurate estimation of the potential of forest soils to act as carbon sinks. To learn more about the changes in SOC and TN stocks and SOC stability after primary forest conversion, we used a strict paired-plots method and measured bulk-soil and aggregate-associated OC and TN as well as four SOC stability indices, i.e., heterotrophic respiration, alkyl/O-alkyl C ratio, aggregate-occluded OC, and mineral-associated OC, in paired primary and secondary forest plots in Liangshui National Nature Reserve, Heilongjiang Province, Northeast China. The SOC and TN stocks at the 0-15 cm depth significantly increased with the conversion of primary to secondary forest, by 17.11 Mg ha(-1) and 2.91 Mg ha(-1), respectively, which was mainly due to increases in soil aggregate ( > 0.053 mm)-associated OC and TN concentrations. The primary forests dominated by conifer tree species had more low-quality plant OC in the organic layer (i.e., higher amounts of litter and forest floor and higher litter C/N ratio), while the secondary forests composed of broadleaf tree species with higher litter and fine root TN and lower C/N ratios accumulated more stable OC (as indicated by the reduced respiration per g of SOC and the greater proportion of aggregate-occluded OC) and TN in the mineral soil. We did not observe significant differences in the alkyl/O-alkyl C ratio or the proportion of mineral-associated OC (< 50 mu m) between the two forest types, reflecting the insensitivity of some stability indicators to decadal-scale shifts in primary forest conversion. Tree species-specific differences in litter and fine root traits (i.e., C and N) might be the main driving factors affecting the accumulation and stability of SOC and TN. Thus, it is important to identify and select tree species that are beneficial for the sequestration of OC in mineral soil for forest management or afforestation from the perspective of ecosystem carbon sequestration.