Rational land-use types in the karst regions of China: Insights from soil organic matter composition and stability

Composition and stability of soil organic matter (SOM) affect the sustenance and productivity of soil over the long-term. This issue is particularly important for karst regions in China where the water supply and fertilizer use are limiting factors. Here, we used four indicators to evaluate changes in the composition and stability of SOM quantitatively in five main land-use types in karst area, including primary forest [PF], 15-year secondary forest [SF], grazing secondary forest [GF], abandoned farmland [AF] and farmland [FL]. We collected soil samples at a depth of 0-20 cm to conduct the analyses. Four indicators were used: soil physical and chemical properties, active organic carbon (C), humus C composition, and SOM functional groups. Our results showed that the content of SOM, total nitrogen, and easily oxidized organic C at 0-20 cm soil depth differed among the five land use types (P PF > GF > FL > AF. Solid-state C-13 NMR spectra showed that the highest ratio of Alkyl C/O-alkyl C was in AF, while the lowest was in SF. Overall, the comprehensive quality of SOM in different land-use types was PF (setting 100%) > SF (83.1%) > GF (58%) > AF (30.9%) > FL (29.9%). For karst areas, we suggest that farmlands in sloping area should be converted back to forests, with only moderate grazing being permitted, whereas farmlands in the plains should implement grain-forage rotation and grain-soybean rotation to meet the needs of the growing population and economic development. In conclusion, our findings provide a scientific basis from which to delineate rational land-use types for different land (geographical and geological) formations