Soil organic matter and the development of oil palm using different methods for handling frond prunings. Oil palm village plantations in the Plateau department of Benin
Thesis defended on 16 December 2013 at the University of Abomey-Calavi, Benin
The cultivation of oil palms in village plantations is becoming increasingly widespread in tropical areas, representing a significant part of the economy of the countries. In Benin, oil palm village plantations are expanding in a climate characterised by high annual water deficiency. The particular characteristics of these oil palm village plantations, such as the association of subsistence crops in young plantations, the lack of mineral fertilisers throughout the cropping cycle and the export of all or part of the recyclable biomass, raise doubts about the sustainability of the production system. The study was carried out in the Plateau department in south east Benin (2°30 - 2°45 E and 6°35 - 7°45 N).
The first stage of classifying the oil palm plantations showed that most of the village oil palm plantations belonged to small producers with limited financial resources who were unable to plant large areas or to apply mineral fertilisers to the oil palms. Most of the land had previously been used for subsistence crops. When young, the palm plantations are associated with a wide range of subsistence crops. Village oil palm plantations differ from the few large producers in that by-products from processing are not returned to the land and the frond prunings are managed in different ways. Fronds are cut when the oil palms are 7 years old and management practices vary from leaving nearly all the fronds banked in windrows to leaving no fronds on the ground. The management of frond prunings in village oil palm plantations can have various effects on the soil / plant system and its agronomic and environmental effects merit being studied.
The classification of the oil palm plantations led to the choice of a chronosequence of plantations representing the two different frond pruning management methods: recycling all fronds in windrows and exporting all fronds. This chronosequence was used to compare the effect of these practices on the soil and plant compartments of the production system. In the soil compartment, the organic matter stocks, the physical and chemical parameters, the microbial activity and the status of the organic matter were studied. For the plants, the development parameters (density, length and area) of the primary, secondary and fine roots were studied initially. Then the plant biomass and mineral content of the aerial mass were studied. Non-destructive methods based on allometric equations for estimating the biomass of the fronds and trunk had to be developed for studying the aerial biomass.
Recycling fronds had a significant effect on the soil fertility parameters after 10 years of leaving fronds on the ground. The stocks of organic matter and nitrogen in the top 50 cm horizon were 70% and 50% greater respectively beneath the windrows than in the interrows without fronds. Surprisingly, zero recycling of fronds did not lead to a reduction in organic matter stocks over time. The increase in carbon and nitrogen stocks after 10 years of leaving fronds on the ground only improved the soil parameters in the top 20 cm. The soil was enriched with organic matter (20gC.kg-1) and nitrogen (1.5gN.kg-1). The mean values (7meq.100g-1) for total exchangeable cations and cation exchange capacity (CEC) were double those in areas with zero frond restitution. The calcium and magnesium concentrations increased, but the potassium concentrations remained very low. The pH increased by 0.5, changing the soil from acid to slightly acid. Recycling fronds intensified microbial activity which changed the quality of soil organic matter. The carbon concentrations in the fine organic fractions (< 20μm) increased by 40% in the top 5 cm and by 15% at a depth of 20-30 cm, compared to zero frond restitution.
These positive effects of recycling clearly benefited the oil palm root development. The density and length of the secondary and fine roots increased in the top 20 cm soil horizon. The fine roots were the most sensitive to frond recycling. Their density was three times greater than for zero recycling and their specific length reduced, confirming the enrichment of the land under the windrows. Recycling fronds created a layer of litter on the top of the soil beneath the windrows in which a significant mass of fine and secondary roots of density 2 kg.m-3 and 0.13 kg.m-3 developed. Despite these effects on the soil and root system, frond pruning management had no significant effect on the aerial biomass or on the nutritional state of the oil palms. Only the trunk biomass increased significantly when all fronds were recycled. The oil palm plantations had low yield and suffered from mineral deficiency. The oil production (6 t.ha-1 in mature plantations) and concentrations of the main mineral elements N (< 2%), and K (< 0.5%) were below the optimum for oil palm plantations.
From a global environmental point of view, the soil in the oil palm plantations could be considered as “a potential carbon sink” depending on the management of frond prunings: carbon was stored in the top 50 cm of soil beneath the windrows and in the total biomass of the windrows on the soil. The studies also showed the role of carbon storage provided by the oil palm in the fronds, trunk and root biomass. The total biomass was estimated at 88 tonnes dry matter per hectare. This carbon storage is higher than for other agrosystems or forest land use change but lower than for forestry systems. Given their agronomic characteristics, the performance of village oil palm plantations is unlikely to improve in the long term. Without significant N and K amendment, the serious deficiency of these minerals will continue. Future action to rectify this situation must associate several agricultural practices including minimum use of nitrogen and potassium fertilisers, setting up an agroforestry system with oil palms and leguminous shrubs and recycling fronds and oil palm bunch waste whenever possible.