Soil organic matter decomposition and turnover in a tropical Ultisol: evidence from δ¹³C, δ¹⁵N and geochemistry

Soil organic matter (SOM), leaf litter, and root material of an Ultisol from the tropical rainforest of Kakamega, Kenya, were analyzed for stable carbon (δ 13 C) and nitrogen (δ 15 N) isotopic values as well as total organic carbon (TOC) and total nitrogen (TN) contents in order to determine trends in SOM decomposition within a very well-developed soil under tropical conditions. In addition, we quantified mineralogy and chemistry of the inorganic soil fraction. Clay mineralogical variation with depth was small and the abundance of kaolin indicates intense weathering and pedoturbation under humid tropical conditions. The soil chemistry was dominated by silica, aluminium, and iron with calcium, potassium, and magnesium as minor constituents. The relative depletion of base cations compared with silica and aluminium is an indicator for intense weathering and leaching conditions over long periods of time. Depth profiles of δ 13 C and δ 15 N showed a distinct enrichment trend down profile with a large (average 13 ΔC = 5.0% and average 15 ΔN= 6.3%) and abrupt offset within the uppermost 10-20 cm of the soil. Isotopic enrichment with depth is commonly observed in soil profiles and has been attributed to fractionation during decomposition. However, isotopic offsets within soil profiles that exceed 3% are usually interpreted as a recent change from C 4 to C 3 dominated vegetation. We argue that the observed isotopic depth profiles along with data from mineralogy and chemistry of the inorganic fraction from the Kakamega Forest soil are a result of intense weathering and high organic matter turnover rates under humid tropical conditions.