Conversion of tropical secondary forest into agricultural land: consequences for soil health and environmental stability

Abstract

The objective of the study was to address the impacts of tropical forest conversion into agricultural land on changes in invertebrates’ communities, as well as soil chemical properties across 2–4 texturally distinct soils in La Mé and Grand Lahou, Côte d’Ivoire. The fieldwork was carried out in the humid period on two study sites: 1– rubber landscape (secondary forest, 7-, 12- and 25-year-old rubber plantations) and 2– oil palm landscape (secondary forest, 13-, 20- and 39-year-old oil palm plantations). Three sampling stands were established on each land-use type and age class, for a total of 24 sampling stands. Over a 40–50 m transect, topsoil (0–10 cm) invertebrates were sampled by using monoliths (50 cm × 50 cm × 10 cm), following the modified ‘Tropical soil biology and fertility’ method. Soils chemical properties were also determined. The results highlighted a decrease in taxonomic richness (–35 % and –42 %), Shannon index (–32 % and –37 %), Margalef index (–34 % and –41 %), soil carbon stock (–41 % and –15 %), nitrogen stock (–38 % and –11 %) and carbon sequestration (–41 % and –15 %), respectively, after the transformation of secondary forests into rubber and oil palm plantations. The clay soils and clay sandy soils favored the development and emergence of the soil invertebrates, and allowed a higher carbon and nitrogen storage, as well as a greater carbon sequestration. Whatever the site, the soil quality was more degraded under the monoculture tree plantations compared to the secondary forests. Our findings suggest leaving residues of the previous crop on the soil surface and the incorporation of woody trees with an understory of smaller trees during the early establishment periods of monoculture tree plantations, which can create a sustainable agroforestry system with improved nutrient recycling and soil fertility.