Assessments of the impacts of Chinese fir plantation and natural regenerated forest on soil organic matter quality at Longmen mountain, Sichuan, China

Three soil labile organic carbon indicators, i.e., particulate organic carbon (POC) by size fractionation, light fraction organic carbon (LFOC) by density fractionation and permanganate-oxidizable carbon (MnoxC) oxidized by 333 mM KMnO4, were employed to identify the soil organic carbon (SOM) pools of a Chinese fir plantation (CF) and a regenerated forest (RF) that were converted originally from the natural evergreen broad-leaved forest (NF) 18 years ago after clear cutting on an upland yellow soil in Longmen mountain, southwest China. Bulk soil organic carbon and nitrogen concentration at the soil depth of 0–20 cm were significantly lower in CF and RF than in NF, while no significant C/N ratio change was found. Top soil MnoxC concentration of CF showed a significant increase while RF demonstrated a slight decline compared to NF, whereas, non-MnoxC of CF and RF both presented a significant decline compare to NF. A different indication between chemical oxidization and physical fractionation method was found. POC, particulate organic nitrogen (PON), mineral-associated organic carbon (MOC) and mineral-associated organic nitrogen (MON) were significantly reduced in CF and RF compared to NF. CF, POC and PON showed significant reductions than that in RF. The losses of TOC and N in the particulate separates were more than that in the mineral-associated separates for both CF and RF. A significant decline of LFOC at 0–10 cm, 10–20 cm and 20–30 cm soil depths were found in CF and RF compared to NF, and a significant larger loss of LFOC in CF than in RF was also found. The heavy fraction organic carbon (HFOC) at the 0–10 cm soil depth was also significantly reduced in CF and RF, while there was no significant difference between CF and RF. The absence of vegetation cover in CF and RF, especially at the early stage of restoration, was contributed to the reductions in both labile and non-labile carbon. Timely thinning after canopy closure or establishing CF mixing with native broad-leaved species should be encouraged to mitigate the labile carbon loss.