Effects of Altitudinal Gradient and Primary Forest Conversion on Soil Organic Carbon Density in the Northern Tropical Mountains

Objective The paper aimed to approach the response and mechanism of soil organic carbon density (SOCD) to mountain elevational gradients and lowland native forest conversion.

Method The effects of altitudinal gradients and forest conversion on SOCD (0 ~ 40 cm) were investigated in the Butterfly Valley tropical rainforest of southwestern China. Meanwhile, the driving role of climatic and soil physicochemical factors were further explored based on LARS regression, Partial and Pearson correlation analyses.

Result ① Altitudinal gradients had a significant positive correlation with SOC, and SOCD was increased by 452.1% from the laterite zone at a low altitude to the brown earth zone at a high elevation in the 0 to 40 cm soil layers, which the variation was mainly driven by BD, pH, TP, AN and MAT. Partial correlation analysis showed that the effect of MAT on SOC did not vary with soil physicochemical factors, but the effect of soil physicochemical factors on SOCD was controlled by MAT. ② The conversions of lowland native forests to rubber and banana plantations resulted in SOCD losses of 49.7% and 43.5%. The loss of SOC was associated with an increase in soil BD and a decrease in C/N. both of them reflects two important ecological processes, the reduction of litter return and the intensification of erosion, following the conversion of native forests.

Conclusion The SOCD in the northern tropical alpine elevation zone were highly susceptible to MAT factors, while the SOCD dynamics at the lower elevations probably were subject to a combination of human disturbance. Future assessments of SOCD in tropical soils should take into account the loss of SOC due to conversion of native forests to plantations and the altitudinal gradient differentiation.