Soil acidity and exchangeable cations in remnant natural and plantation forests in the urbanised Pearl River Delta, China

Increasing urbanisation and industrialisation have led to a dramatic reduction in forest area, and now only culturally protected remnants of natural forests and some new plantations remain in most areas of the Pearl River Delta (PRD), China. To investigate the status of soil acidity and exchangeable cations under these remnant forests and assess the possible impacts of reforestation on soil nutrients in the plantation forests, soils at 0-0.03, 0.03-0.13, and 0.13-0.23m depths were sampled from 16 forest patches (eight natural and eight plantations), and soil pH in water, organic matter content, and exchangeable cation (H, Al, Ca, Mg, K, and Na) contents were determined. Results showed that 90% of the soils were strongly acid (pH <4.5) and 70% of the soils had a base saturation <15%. About 50% of the soils had <0.5 mmol (+)/kg of exchangeable Mg. Soil exchangeable K and Mg contents were significantly lower under plantation forests than under native forests in all layers, whereas exchangeable Ca and Na contents showed little difference between two types of forests. Moreover, contents of all exchangeable cations except Al showed a significant decrease with depth. These results suggest that remnant forests in the PRD generally experience a high risk of Al and acidity stresses and non-acidic cation deficiencies for plant growth. Reforestation may cause further decline in soil exchangeable K and Mg contents, but is unlikely to affect soil exchangeable Ca and Na contents.