Temporal changes in soil carbon and nitrogen in west African multistrata agroforestry systems: a chronosequence of pools and fluxes

The conversion of forests to agroecosystems or agroforests comes with many changes in biological and chemical processes. Agroforestry, a tree based agroecosystem, has shown promise with respect to enhanced system nutrient accumulation after land conversion as compared to sole cropping systems. Previous research on tropical agroforestry systems has revealed increases in soil organic matter and total organic nitrogen in the short term. However, research is lacking on long-term system level sustainability of nutrient cycles and storage, specifically in traditional multi-strata agroforestry systems, as data on both the scope and duration of nutrient instability are inconclusive and often conflicting. This study, conducted in Ghana, West Africa, focused on carbon and nitrogen dynamics in a twenty-five year chronosequence of cacao ( Theobroma cacao Linn.) plantations. Three treatments were selected as on-farm research sites: 2, 15 and 25-year-old plantations. Soil carbon ( C, to a depth of 15 cm) varied between treatments ( 2 years: 22.6 Mg C ha(-1); 15 years: 17.6 Mg C ha(-1); 25 years: 18.2 Mg C ha(-1)) with a significant difference between the 2- and 15- and the 2- and 25-year-old treatments ( p < 0.05). Total soil nitrogen in the top 15 cm varied between 1.09 and 1.25 Mg N ha(-1) but no significant differences were noted between treatments. Soil nitrification rates and litter fall increased significantly with treatment age. However, photosynthetically active radiation ( PAR) and soil temperature showed a significant decrease with age. No difference was found between decay rates of litter at each treatment age. By 25 years, system carbon sequestration rates were 3 Mg C ha(-1) y(-1), although results suggest that even by 15 years, system-level attributes were progressing towards those of a natural system.