Organic carbon and plant nutrient dynamics under three land uses in the highlands of West Cameroon

Assessing land use-induced changes in soil systems is essential for addressing the problem of agroecosystem transformation and sustained land productivity. We compared organic carbon (OC) characteristics and plant nutrient levels in surface horizons (0–10 cm) of semi-permanent, mixed food crop fields (CF), Eucalyptus grandis plantations (EP) and tea (Camellia sinensis) plantations (TP) from an andic Palehumult in the highlands of West Cameroon. For more insight in organic matter transformations, OC, total nitrogen (TN) and NaHCO3 extractable phosphorus (NaHCO3–P) were analyzed on whole soils and size fractions (SF). Structural composition of soil organic matter (SOM) was assessed by solution 13C NMR spectroscopy. Land use conversion from CF to EP and ET increased OC stocks in surface horizons by a factor of 2.14 and 1.95 for EP and TP respectively after 15 years. In all land uses, O-alkyl C, aromatic C and carboxylic C were the dominant OC species, accounting for between 75.6 and 76.1% of total soil OC. The organic NaHCO3–P fraction was significantly higher (79.4±7.2 μg g−1) under TP and lowest under EP (58.8±4.3 μg g−1) as a result of phosphorus fertilization. The EP system recycled and accumulated substantial amounts of Ca and Mg in surface soil horizons. The inorganic NaHCO3–P fraction and K were significantly lower (P<0.05) under EP suggesting an immobilization in litter and standing biomass. TP had the lowest Mg level due probably to export through shoot harvest. Sites under EP and TP have significantly (P<0.001) higher proportion of silt and correlatively a lower proportion of clay. Soil size fractions exhibited distinct properties with respect to organic nutrient concentration, mineralization status and stability with silt and clay-sized fractions being enriched in OC and TN compared to whole soils. The highest quantitative changes after 15 years was recorded in the silt-sized fraction which accounted for 58–66% of total soil carbon.