Assessment of Decomposition Rate and Soil Nutrient Status under Different Woody Species Combination in a Tree Plantation

Abstract

Forest Litter is the major input determining the nutrient accumulation within the forest soil ecosystem which goes a long way in determining forest stand productivity. To better understand this, the study investigated the litter decomposition rate and soil nutritional status under different woody species combinations in tree plantation established in 1998. Four different pocket of tree combinations Terminalia sp and Tectona grandis (1); Gmelina arborea and Tectona grandis (2); Khaya sp and Tectona grandis (3); Theobroma cacao and Cola sp. (4) were used, while undisturbed natural forest served as the control. Three plots (25 m x 25 m) were randomly mapped out of each site in which fresh litter were collected with litter trap (1 m x 1 m ) and 45 litter bags were placed and 90 composite soil samples to the depths of 0-15 cm and 15-30 cm collected using a stainless steel auger. These collections followed the principle of co-location in each of the plots. Litter bag technique was used for Litter decomposition rate. The results of the litter accumulation in the forest plantations were in the magnitude of Tectona grandis and Gmelina arborea (1249.2 kgha-1) > Teak and Khaya sp. (899.42 kgha-1) > Teak and Terminalia sp., (867.58 kgha-1) > natural forest (489.96 kgha-1) Cocoa and Cola (199.87 kgha-1). The decomposition rates under Tectona grandis and Khaya sp., Tectona grandis and Gmelina arborea mixtures were higher than other tree species mixtures. The rate of decomposition under Tectona grandis and Gmelina arborea mixtures was 5.3 times higher than that of Tectona grandis and Terminalia sp., Cocoa and Cola combinations and natural forest at 6 weeks. At 15-30 cm soil depth, the C/N ratio was in the magnitude of Tectona grandis and Gmelina arborea (8.6:1) < Cocoa and Cola (9.3:1) < Tectona grandis and Khaya sp. (9.8:1) < Tectona grandis and Terminalia sp. Natural forest (11.7:1). The organic carbon and available nitrogen at 0-15cm soil depth under Tectona grandis and Khaya sp. combinations were significantly lower compared with other trees species combinations. However, the available phosphorus was significantly higher under Tectona grandis and Terminalia sp. compared with other tree species combinations. The dendograme indicated that the soil characteristics in the various tree species combinations plot were similar up to 50% with four clusters. The observed relative nutrient availability within the structurally different forested ecosystem in the study area might not be unconnected to the litter mixtures emerging from different tree combinations.