Effects of moderate forest fires on soil organic carbon density in secondary forests of Pinus massoniana

Abstract

Aims As a discontinuous disturbance factor, forest fire is one of the drivers of biogeochemical cycles. It significantly changes the structure and function, nutrient cycling, and energy transfer of ecosystems and alters the forest carbon pools and carbon distribution patterns, consequently affecting the processes of forest succession and carbon sequestration capacity. This study aims to determine the impacts of incidental forest fire on soil organic carbon density, and to explore the mechanisms of forest fire impacts on soil organic carbon fixation.Methods The study was conducted in secondary Pinus massoniana forests of different ages in Guangdong Province, using the method of space for time substitution. The sampling plots were set up on adjacent sites of burned and control stands, and soil samples (0-60 cm) were collected from each plot for indoor tests and analysis of the physical and chemical properties. The soil organic carbon components were measured and calculated for density. Changes in soil physical and chemical properties and soil organic carbon with forest ages were quantified.Important findings Fire reduced the soil organic carbon density in secondary P. massoniana forests; the level of reduction in soil organic carbon density decreased with forest age and soil depth. Compared with the controls, the soil organic carbon density in the burned plots of young, mid-age and mature forests were 10.93%, 8.52% and 7.56% lower, respectively. The soil organic carbon density in the burned plots of young, mid-age and mature forests varied in the range of 5.04-7.76, 5.26-10.27 and 6.33-13.58 t·hm-2, respectively, along the soil profile of 0-60 cm, which were 2.51%-16.83%, 1.31%-11.85% and 1.09%-12.50% lower, respectively, than the controls. Fire significantly reduced the soil organic carbon density of the young and the mid-age forests in the 0-30 cm soil layer, and of the mature forest in the 0-20 cm soil layer. There were significant correlations between soil organic carbon density and soil physical and chemical properties. Path analysis revealed the greatest direct effect of soil total nitrogen content on soil organic carbon density in both the control and burned plots; fine root biomass had a smaller direct effect, but imposed an indirect effect on soil organic carbon density via its controls on soil total nitrogen content. Nested ANOVA showed that soil depth accounted for 70.60% of the variations in soil organic carbon density, forest age 25.35%, and fire 2.34%.