Land use effects on the geochemical soil properties and their control on organic carbon in volcanic soils, near Bandung area (Indonesia)

Land use change can significantly influence both mineralogy and chemical soil properties. This conversion, particularly from forest to agricultural system occurs often in volcanic soils due to their favorable properties for food production. Under agriculture, minerals can weather faster than in natural vegetation and this also impacts soil functioning. We aim to assess the impact of land use on geochemical soil properties and soil organic carbon across soils of different age. This study was conducted in Mt. Tangkuban Perahu and Mt. Burangrang where the soils were derived from similar andesitic parent material and have different ages based on their lithology. Five sites were selected representing land uses that have been converted (pine forest and agricultural land) and one site of natural forest as the origin of land use. The results showed that land use management enhances the mineral transformation. Pine forest and agricultural sites displayed higher weathering degree than natural forests as indicated by higher clay content, iron crystallinity index and the presence of gibbsite. The weathering degree of soils in agricultural sites might result from the length of cultivation period and soil age. Land use conversion also altered chemical properties such as pH, CEC, basic cations, and the proportion of amorphous materials. Non-crystalline Al and Fe minerals as indicated by Al_o+1/2Fe_o were highly correlated with organic carbon and specific surface area (SSA) in the subsoils of all land uses. However, we did not see the accumulation of organic carbon in subsoils compared to topsoils as the amount of non-crystalline Al and Fe minerals increases with depth, especially in agricultural lands where the organic fertilizer input is very high. In addition, a significant proportion of carbon was stored in sand aggregate fractions in agricultural land which have longer cultivation period, while it was more readily found in silt and clay fractions in the site with shorter period.