Spatial heterogeneity of soil quality around mature oil palms receiving mineral fertilization

The African oil palm (Elaeis guineensis Jacq.) is grown on a total area of 16 million ha; but data on soil quality in mature oil palm plantations are fragmentary and data concerning biota are almost nonexistent. Consequently, no well-tested sampling method is available for soil diagnoses. We studied the spatial heterogeneity of the soil around the palm by measuring comprehensive soil quality in a 24-yearold oil palm plantation. Soil quality and litter were assessed in five zones with different plant cover, and different applications of herbicide or fertilizer. Physical-chemical characteristics, macrofauna, and nematofauna were analysed. A sampling method was developed and adapted to the way the cultivation practices are implemented: sampling by zone and weighting the plot mean by the respective area of each zone. The total density of macrofauna in the litter and in the 0e15 cm soil layer followed a gradient from the harvest pathway (29 ind m_2) to the windrow (1003 ind m_2). Ants (13e237 ind m_2), earthworms (11e120 ind m_2), Dermaptera (0e35 ind m_2), Coleoptera (3e24 ind m_2) and Chilopoda (0 e43 ind m_2) were the main taxa. The termite population was very poor (3e4 ind m_2). The density of nematofauna was also heterogeneous (268e805 ind 100 g_1 of soil). Heterogeneity between zones was also reflected in the density of the functional groups, mainly soil engineers, detritivores and predators for macrofauna and bacterial feeders, and phytoparasites for nematofauna. The weeded circular zone around the palm had the highest soil nutrient content (P, K, Ca, Mg, Corg CEC, base saturation). Its biodiversity was average but it contained the highest density of earthworms and nematofauna. Possible relationships between chemicals and biological groups in the food web are discussed. (Résumé d'auteur)