Land use shapes soil organic matter stabilization and chemistry in tropical soils: insights from density fractionation and X-ray photoelectron spectroscopy.

Although land use influences on soil organic matter (SOM) dynamics are well recognized, stabilization and chemical components of SOM under diverse land uses in tropical soils are not fully understood. Herein, we aimed at investigating SOM stabilization and chemical components across contrasting land uses in tropical soils. Soil samples were collected from seven representative land uses: forest, pasture, corn plantation, sugarcane plantation, cassava plantation, orchard, and abandoned land. SOM was physically isolated into free particulate SOM (fSOM), occluded light SOM (oSOM), weakly bound SOM (wSOM), and strongly bound SOM (sSOM) fractions. The SOM chemical components were characterized using X-ray photoelectron spectroscopy (XPS). Among different land uses, total soil organic carbon (SOC) content was highest in pasture (18.5 g C kg-1 soil), followed by cassava plantation, forest, and orchard. The SOC contents in fSOM (1.26-2.37 g C kg-1 soil), oSOM (0.57-2.33 g C kg-1 soil), and wSOM (1.24-4.52 g C kg-1 soil) fractions were highest under forest. The SOC in sSOM fraction (6.66-10.63 g C kg-1 soil) was higher under pasture and cassava plantation. XPS data showed that aliphatic C was the most predominant C functional group, accounting for the larger C content in bulk soil, fSOM, wSOM, and sSOM fractions. On the opposite, the larger C content in oSOM was attributed to physically protected ether/alcohol C and/or carboxylic C inside soil aggregates. These findings highlight that management practices enhancing aliphatic C-rich organic inputs may be beneficial for improving C sequestration in tropical agricultural landscapes.