Soil carbon and bio-physicochemical properties dynamics under forest restoration sites in southern Ghana

Forest degradation and conversion to agriculture and other land uses usually leads to a reduction in soil carbon stocks and negatively impacts key soil bio-physicochemical properties. The recovery of these soil properties following reforestation has been unclear and/or has not been well studied in the tropics. We analysed the soil carbon stocks and soil physicochemical characteristics of 48 sample plots across 11 sites in 42–47 year-old tropical hardwood plantations and secondary forests of similar ages, and primary reference forests. In addition, we assessed soil microbial biomass and litter decomposition. The study was conducted in the moist and wet climatic zones of Ghana.

Climate was the main determinant of soil characteristics, while the effect of forest type was minor. After 40 years of restoration, the soil carbon stocks and key soil physicochemical properties in plantations and secondary forests reached similar levels to those in the primary forests within the respective climatic zones. We observed that forests in the wet zone had higher soil carbon stocks and higher above ground biomass with much lower soil pH. Soil carbon stocks (0–50 cm) ranged from 51.16 Mg ha⁻¹ to 122.84 Mg ha⁻¹, with the mean values for the moist and wet zones being 56.9 Mg ha⁻¹ and 106.8 Mg ha⁻¹respectively.

There were no differences in soil microbial biomass between the forest types within zones, however, it was notably much higher in the moist compared to the wet zone. The decomposition of two types of tea (green and rooibos) with contrasting decomposability was analysed using a two-component exponential decomposition model. The results indicated that the decomposition of recalcitrant material in the tea bags was slower in the wet climatic zone. Further analysis showed that soil organic carbon content was inversely related to the recalcitrant organic matter's decomposition rate. In addition, there was a positive relation between the soil carbon stocks and above-ground biomass. Our results indicate that soil biophysico-chemical properties and carbon stocks were restored in plantations and secondary forests after 40 years of restoration. However, a number of our restoration sites were cleared and farmed for a relatively short period (≈ 3–4 years) but are representive of many recently deforested areas in the tropics.