The Effects of Altitude and Land Use on Organic Matter and Integrated Fertility of Soils in the Northern Tropics Mountain

This study was conducted to gain insight into the distribution characteristics of Soil Organic Matter (SOM) and Soil Integrated Fertility (SIF) in various mountain soil zones and several typical land use types (secondary forest, banana plantation, and rubber plantation) on different altitude gradients in the northern tropics and the Butterfly Valley area in Jinping County, Yunnan Province, China. The surface (0-20 cm) and subsurface (20-40 cm) layers are distinguished. SOM, pH, total N, total P, total K, and available N, P, and K were measured. The SIF was evaluated using the modified Nemorow index method, and the key limiting factors were analyzed. First, the results show that the SOM content ranges from 21.91 to 120.67 g/kg for the surface layer and 16.38 to 101.88 g/kg for the subsurface layer, with the mean SOM of the surface layer (75.32 g/kg) greater than that of the subsurface layer (51.57 g/kg, P > 0.05). In terms of SIF, that of the surface layer was 1.15-1.59 and that of the subsurface layer was 0.94-1.44, with the mean SIF of the surface layer (1.35) greater than that of the subsurface layer (1.16). This shows that there is a significant positive correlation between the altitude gradient and SOM as well as SIF. Ranking of SOM and SIF for the surface layer (P < 0.05). Second, the SOM for the surface layer of the secondary forest, banana plantation, and rubber plantation were 26.61, 13.48, and 11.91 g/kg, respectively, while the SOM for the subsurface layer of these three typical land use types are 19.28, 9.61, and 8.91 g/kg, respectively. With regard to SIF, they were 1.48, 1.33, and 1.21 for the surface layer, and 1.17, 1.10, and 0.94, respectively. Notably, both in the surface and subsurface layers, the SOM and SIF were largest in the secondary forest, followed by the banana plantation, and then the rubber plantation. The SOM of the secondary forest was significantly greater than that of the banana and rubber plantations (P < 0.05), whereas there was no significant difference among the SIF of these three land use types. Finally, as the altitude increased, the factors limiting the SIF change from available N and available P to available P and available K, and the effect of pH gradually strengthened. The factors that maintain and limit the SIF of different land use types at low altitudes are generally consistent with natural soils at the same altitude. Therefore, in the process of future mountain development and utilization, it is important for low-altitude areas to supply nitrogen and phosphate to the soil. In medium- and high-altitude areas, supplementation with phosphate and potash is useful, and a suitable pH helps the soil release nutrients and promotes plant uptake and utilization.