Carbon and nutrient cycling in tree plantations vs. natural forests: implication for an efficient cocoa agroforestry system in West Africa

Nutrient cycling has been widely studied in tree plantations. However, studies on the observed negative changes relative to natural vegetations and how these could be capitalized toward setting up ecofriendly agroecosystems are rare. This study was conducted in Central-West Cote d’Ivoire to establish the changes in carbon and macronutrient cycling occurring in full-sun cocoa (Theobroma cacao L.) and Teak (Tectona grandis) relative to primary forest and subsequently make cocoa agroforestry-based recommendations. Leaf litterfall and associated carbon and macronutrient inputs, rates of leaf litter decomposition, and macronutrient release were assessed. In addition, soil (0–10 cm depth) chemical and microbial parameters were evaluated. Litterfall yields were 10.6 ± 2.0, 9.3 ± 0.8, and 10.1 ± 0.4 Mg dry mass ha−1 year−1 in forest, cocoa, and teak, respectively. Compared to the forest, the cocoa plantation supplied lower inputs of C (−736 kg ha−1 year−1 or −15.5%) and N (−75 kg ha−1 year−1 or −27%), similar P, but greater K. Similar quantities of C and N were recorded in the teak plantations and the forest. However, the teak plantation supplied lower K but higher P inputs than the forest. Cocoa leaf litters decomposed at the same rate as those of the forest (k = 0.3 month−1) but faster than the teak’s, the initial leaf litter N:P ratio being the most influencing factor. Except for P, the macronutrient release from cocoa and forest litters exhibited similar patterns and rates, which were significantly different from those observed in the teak leaf litter. Soil C mineralization rate and mineral N concentration drastically declined in both tree plantations, the greatest gap occurring in cocoa (Cmin: −40%, mineral N: –54.2%) due to lower litter Ca input and soil acidity. Teak appears to be a good candidate for shade as it may compensate for the deficit in litter C (and N, to a lesser extent) supply exhibited by cocoa relative to the forest. The trends in microbial activity underscore the need to grow cocoa in association with trees that provide quality litter materials, for improved cocoa nutrient availability and faster C storage in soil. In line with this, some suggestions were made and discussed. This study can be used in support of developing an efficient cocoa agroforestry system in West Africa.