‘Light’ fraction mineralization potentials of humid tropical soils

Abstract

Soil samples from a rainforest, a papaya plantation, a pure Brachiaria humidicola pasture and a B. humidicola/Desmodium ovalifolium mixture were analyzed for fertility parameters. Soils were sieved to >100 µm and the organic matter light fraction was obtained by 1.8 g mL⁻¹ density separation in a sodium polytungstate solution which was subsequently incubated under anaerobic conditions. The effect of different density agents (sodium polytungstate, Ludox and sodium iodide) on mineralization was also tested on a temperate and a tropical soil and on maize tissue.

Anaerobic incubation of light fractions of tropical topsoils (0-2 cm) demonstrated a good differentiation between the compared management treatments whereas mineralization potentials of whole soil samples were less contrasting. Samples from the rainforest showed the highest mineralization values and those from the plantation the lowest values. With an increasing proportion of legumes in the pasture the mineralization potential increased and the C:N ratio of the light fraction decreased. Thus, although D. ovalifolium is considered to have poor quality attributes (relatively low %N and high tannin content), its litter contributed to increased soil fertility. Light fraction mineralization potentials of samples from 5–15 cm depth were much lower than the topsoil and not clearly affected by soil management. This confirms that rainforest soil fertility is very much restricted to the upper soil layer and the maintenance of soil fertility depends largely on the quantity and quality of above-ground litter inputs. None of the density agents tested were without interference on mineralization potential. Nevertheless, light fraction parameters appeared to be more sensitive to alterations in soil management than parameters of whole unfractionated soil samples and could be used as early warning indicators of changes in soil fertility.