ObjectiveNatural restoration and artificial reconstruction of vegetation are the main measures to accelerate the restoration of karst ecosystems and improve soil quality. Studying the soil organic carbon fraction and enzyme activities under different restoration patterns in karst area can reveal the effect of different restoration methods on soil quality, and aims to provide scientific basis for the screening of ecological restoration patterns and evaluation of restoration effects in karst areas.

MethodBased on the naturally restored secondary forest and artificially restored Pinus yunnanensis needle forest and Eucalyptus robusta broadleaved forest in Jianshui County, Yunnan Province of southwestern China, the soil organic carbon fraction, carbon pool management indexes and enzyme activity distribution characteristics and their correlations in different soil layers were analyzed.

ResultThe soil organic carbon(SOC) content was 9.076-56.855 g/kg, the dissolved organic carbon(DOC) content was 822.311-1 175.778 mg/kg, the microbial biomass carbon(MBC) content was 332.933-2 035.244 mg/kg, and the easy oxidation organic carbon (EOC) content was 2.381-6.094 g/kg. Under the same vegetation restoration mode, except for the content of EOC under the Pinus yunnanensis forest, the content of each organic carbon component decreased with the increase of soil depth, and the local soil depth fluctuated. The content of EOC in the subsurface (10-20 cm) soil of Pinus yunnanensis was significantly higher than that in the surface layer (0-10 cm) and deep layer (20-30 cm). The impacts of different vegetation restoration methods on DOC, EOC and SOC contents were roughly as follows: natural secondary forest>Eucalyptus robusta forest>Pinus yunnanensis forest, and the MBC content of Pinus yunnanensis forest was always significantly higher than that in the Eucalyptus robusta forest. The organic carbon components and SOC showed very significantly (P < 0.01) or significantly (P < 0.05) positive correlations; the ability of different vegetation restoration methods to improve the soil carbon pool management indexes was: secondary forest> eucalypt forest>Pinus yunnanensis forest. SOC had very significant and significant correlations with CPI and CPMI, and EOC also had very significant (P < 0.01) or significant (P < 0.05) correlations with CPAI, CPI, and CPMI. Each vegetation restoration method increased soil SOC, EOC, etc., thereby increasing the soil carbon pool management indexes. As a whole, the soil enzyme activities decreased with the deepening of the soil layer, and there was fluctuation in the local range. Soil enzyme activities varied differently under different vegetation restoration methods. The activities of catalase and amylase were shown as secondary forest>Pinus yunnanensis forest>Eucalyptus robusta forest. The SOC, MBC, DOC, and EOC contents were positively correlated with the four soil enzyme activities under various vegetation restoration patterns, and most of them were significant or very significant.

ConclusionThe three vegetation restorations improved soil organic carbon fraction, carbon pool management indexes and soil enzyme activities to varying degrees. Among them, secondary forests have the highest ability to improve the overall soil quality. Eucalyptus robusta forests have a significant increase in the content of total organic carbon and active carbon components, while the Pinus yunnanensis forest has a significantly greater ability to increase the activities of catalase and amylase. Therefore, it is necessary to speed up the transformation of land management methods in suitable forest lands in karst areas and give priority to natural restoration, and pay attention to the utilization and optimization management of broadleaved trees when selecting artificial afforestation.