Changes in soil organic carbon fractions in a tropical Acrisol as influenced by the addition of different residue materials

Residues of Leucaena (Leucaena leucocephala (L), Senna siamea (S) and maize stover (M) were tested to evaluate their effect on soil organic matter accumulation and composition under sub-humid tropical conditions. On an Imperata cylindrica (I) dominated grass fallow, a total amount of 30 Mg ha(-1) DM were applied within 18 months. Two months after the last application, changes in the light and heavy soil organic carbon fraction (LF and HF) and in the total soil organic carbon content (LF + HF) in the topsoil were observed. All organic materials increased the proportion of the LF fraction in the soil significantly. The increase in HF was 39 to 51% of the increase in total organic carbon, depending on the source of the organic material. The potential of the tested organic materials to increase total soil organic carbon content (including all soil organic carbon fractions) was in the order L > S > M > I, whereas the order of increase of the HF fraction was L = S > I > M. Cation exchange capacity of the newly formed heavy soil organic carbon was highest with L and lowest with M. Ranking of the transformation efficiency of applied plant residues into the heavy soil organic carbon fraction was I > L = S > M. Transformation efficiency of the residues could neither be explained by lignin nor lignin/N ratio, but rather by extractable polyphenols (Folin-Denis extraction). The results show that accumulation of the HF fraction in tropical soils is feasible through the application of large quantities of plant residues, but depends strongly on the composition of the applied materials.