Impacts of slash burning on soil carbon pools vary with slope position in a pine plantation in subtropical China

Slash burning is commonly used to clear harvest residues and improve forest health in tropical and subtropical forest plantations. However, whether the effects of slash burning on soil carbon pools (C) varied with slope position remains unknown. In this study, soils samples were collected at three depths (0-5, 5-10 and 10-20 cm) in three slope positions (upper, middle, and lower) one day before the slash burning and one week afterwards in a Pinus massoniana plantation in southeastern China. We measured soil physiochemical properties (bulk density, pH and moisture) and six soil C pools (total C, dissolved organic C (DOC), microbial biomass C (MBC), coarse particulate organic C (CPOC), fine particulate organic C (FPOC), and mineral-associated organic C (MAOC)). The results showed that slash burning increased soil pH (p < 0.001) and total C content (p < 0.001), but decreased soil DOC content (p < 0.001). Soil BD and moisture were not altered by the slash burning (p > 0.05). Slope position significantly affected soil moisture, DOC, MBC and MAOC. Soil moisture at 0-5 cm depth was significantly higher in the lower than upper slope position. The effects of slash burning on soil MBC varied with slope position and soil depth. Slash burning significantly decreased soil MBC at the 5-10 cm depth in the middle and lower slope position, but not in the upper slope position. Slash burning significantly decreased soil MBC at the 0-5 cm depth, while had no influence on soil MBC at the 10-20 cm depth irrespective of slope position. Averaged across slope positions for each soil depth, slash burning resulted in 48.0%, 41.3% and 25.7% decrease in soil MBC content at the 0-5, 5-10 and 10-20 cm depth, respectively. Slash burning had no effect on the distribution of the mass, but altered the C content in soil physical fractions. Slash burning increased FPOC (p < 0.01), but had no influence on CPOC and MAOC. The effects of slash burning on soil FPOC did not vary with slope position (p > 0.05). Our findings suggested that slope position should be considered as an important factor influencing the impact of slash burning on soil microbial mediated C pools, and future research is needed to investigate the post-fire recovery of vegetation and soil C dynamics.