Eucalypt litter quality and sandy soils : addressing two cumulative effects on topsoil organic-matter and soil faunal activity in African plantations

A constraint in many sandy soils is their low organic matter content. Originating mainly from litterfall in forests, soil organic matter (SOM), besides its importance for soil fertility, is the feeding resource for soil fauna, essential for soil functioning. Because eucalypt are known for their low quality litter, their influence on SOM and soil fauna on sandy soils was investigated in comparison with a loamy-clay soil (Senegal) and a clay soil (Congo) and with tree species with contrasting litter quality. For this purpose particle size fractionations at the soil-litter interface were performed, and invertebrate density was assessed with the TSBF method. Eucalypt litter has a low N content and a high phenolic content. Both soil texture and tree species controlled SOM at the soil-litter interface, and low amounts of SOM were observed in the fine particulate fraction and the organo-mineral fraction under eucalyptus on sandy soils. High phenolic content in the litter might decrease particulate SOM. Tree species and soil texture influenced earthworm density, whereas termite and ant densities were mainly dependent on soil structure. The other litter-dwelling invertebrates were mainly dependent on tree species. Eucalypts and sandy soils present together some adverse effects on soil fertility, through both organic matter and biological activity. More extensive sampling in clay soils and experimental studies on the chemical influence of litters are required for a better understanding of soil structure-SOM-soil invertebrate relationships. However silvicultural practices which are able to increase organic matter in eucalypt plantations on sandy soils are essential for their sustainability. Logging residue management are currently being studied by UR2PI (Unit de Recherché usr la Productivity Des Plantations Commercials) who also tests the input of organic matter from non-eucalypt vegetation by inter-planting acacias with eucalyptus. 1 IRD, Tropical Soil Ecology Laboratory, 32 rue Henri Varagnat, 93143 Bondy, France. France.Reversat@bondy. ird.fr 2 IRD, Ecology Laboratory, BP 1286, Pointe Noire, Congo 3 University Marien Ngouabi, Department of Biology and Plant Physiology, BP 69, Brazzaville, Congo Introduction Eucalypts are extensively grown in the tropics, either as farmer forestry or as industrial plantations mainly for paper pulp production. In Africa, many eucalypt plantations are grown on sandy soils, usually nutrient poor, which however are able to support tree growth. Eucalypt plantations were tried in the semi-arid Senegal in the years 1970-80s, on sandy soil and on loamy sand soil. In the Congo, commercial plantations are grown since 1978 near Pointe Noire, covering now more than 40,000 ha on sandy soils, under a wet climate, and a few experimental plantations are also grown on clay soils. The main constraint of African sandy soils is their low soil organic matter (SOM) content and its decrease with cultivation (Feller et al. 1991, Walker and Desanker 2004). However SOM provides exchange sites, which contribute to nutrient conservation. Besides its importance for soil fertility and structure, SOM is the primary feeding resource for most soil living organisms and especially soil invertebrates, which in turn are essential for soil functioning (Lavelle and Spain 2001). So for sustainability of fast growing tree plantations, it is of significant importance to manage sandy soils in order to increase SOM. In a forest environment, SOM originates mainly from litterfall and litter decomposition. It is well known that the litter quality of eucalypts is low (Woods 1974). Eucalypt leaf litter contains large amounts of phenolics (Bernhard-Reversat et al. 2001) which are antibiotic and anti-feeding agent for invertebrate and vertebrate fauna (Waterman and Mole 1994; Harborne 1997). Eucalypt litter quality leads to low biological activity and results in a low litter decomposition rate in eucalypt plantations. Whether this low decomposition rate results in SOM accumulation in the topsoil is not clearly