ACID SULFATE SOIL LANDSCAPE RELATIONSHIPS IN THE PEARL RIVER DELTA, SOUTHERN CHINA

Relationships between the development of acid sulphate soil and landsc ape characteristics were examined in a fluvial-dominated, estuarine-de ltaic plain of the South China coast. It was found that the accumulati on of pyrite (potential acid sulphate material) has been limited gener ally by rapid sedimentation rates and insufficient supply of sulphate ion. The occurrence of the pyritic sediments is, however, closely asso ciated with a particular depositional pattern, i.e. ''surrounding isla nd deposition''. The acid sulphate soils in the area frequently occur within narrow zones surrounding present-day's rocky hills (previously bedrock islands). This provides a guide for predicting the occurrence of potential acid sulphate soils in the area. Apart from the naturally large input of fluvial sediment, human activities have also accelerat ed the delta progradation and limited the vertical accretion of the de ltaic surface. These effects have had significant impacts on the thick ness and occurrence depth of the pyritic layer. With a history of more than 1000 years of land reclamation, the intensive cultivation in the area has turned most of the potential acid sulphate soils into actual acid sulphate soils, however, the soil properties vary markedly with landuse types, The case of the Pearl River Delta is generally in agree ment with Pons and van Breemen's (1982) conclusion in that the accumul ation of pyrite is inversely related to the rate of sedimentation. Our results show that the existence of bedrock islands in the estuary bas in provided zones of slow sedimentation and therefore enhanced pyrite accumulation. On the other hand, human intervention over the past 2000 years has acted to reduce pyrite accumulation both through landward c atchment clearing that increased sediment discharge to the estuary and through empolderment and associated land management practices.