Impacts of Tropical Rainforest Conversion on Soil Nutrient Pools in Viti Levu, Fiji

Abstract: Among the impacts of human disturbance on forests, changes in soil nutrients and carbon stocks are of prime concern. Soil physicochemical properties were investigated at three depths in tropical rainforest converted to degraded secondary forest and mahogany plantation in Naitasiri province, Fiji. The mean values of pH, EC and CEC were 5.13, 0.06 mS/cm and 23.70 me/ 100 g respectively. Soils were low in total carbon (3.10%), total nitrogen (0.23%), Olsen available phosphorous (0.87 mg/kg), exchangeable calcium (483.83 mg/kg), exchangeable potassium (163.57 mg/kg) and sodium (10.67 mg/kg), while exchangeable magnesium (193.50 mg/kg) was moderate in concentration. This study showed that forest disturbance affects soil physicochemical properties (pH, EC, Ca, Mg, Na, Mn, Cu, Zn), and soil physicochemical properties vary with soil depth (EC, total C%, total N%, Ca, K, Na, Fe, Mn, Cu, Zn). Though not all pairwise differences were detectable statistically, yet undisturbed primary rainforest and plantation had higher pH, EC, CEC, total C %, soil organic matter %, total N%, Ca, Mg, Mn, Cu, Zn and Fe than the disturbed secondary forest. All examined soil parameters except pH, decreased with increase in soil depth. Relatively poor nutrient status of the disturbed secondary forest was associated with higher human interference, invasive species and faulty land management practices. Long undisturbed plantations managed for conservation purposes can regain nutrient pools of native forests in time. Understanding implications of human modification on soil nutrients in tropical ecosystems is therefore imperative for conservation, climate change mitigation and informed decision making in the land management sector.