Adsorption-desorption of copper (Cu2+) at contaminated levels in two red soils was investigated. The red soil derived from the Quaternary red earths (clayey, kaolinitic thermic plinthite Aquult) (REQ) adsorbed more Cu2+ than the red soil developed on the Arenaceous rock (clayey, mixed siliceous thermic typic Dystrochrept) (RAR). The maximum adsorption values (M(A)) that are obtained from the simple Langmuir model were 25.90 and 20.17 mmol Cu2+ kg(-1) soil, respectively, for REQ and RAR. Adsorption of Cu2+ decreased soil pH, by 0.8 unit for the REQ soil and 0.6 unit for the RAR soil at the highest loadings. The number of protons released per Cu2+ adsorbed increased sigmoidally with increasing initial Cu2+ concentration for the RAR soil, but the relationship was almost linear for the REQ soil. The RAR soil released about 2.57 moles of proton per mole of Cu2+ adsorbed at the highest Cu2+ loading and the corresponding value for the REQ soil was 1.12. The distribution coefficient (Kd) decreased exponentially with increasing Cu2+ loading. Most of the adsorbed Cu2+ in the soils was readily desorbed in the NH4Ac. After five successive extractions with 1 mol L(-1) NH4Ac (p 5.0), 61 to 95% of the total adsorbed Cu2+ in the RAR soil was desorbed and the corresponding value for the REQ soil was 85 to 92%, indicating that the RAR soil had a greater affinity for Cu2+ than the REQ soil at low levels of adsorbed Cu2+.

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Red soils are widespread in Southern China and other subtropical regions in the world. An improved management of phosphorus (P) is crucial for sustainable agriculture and environmental quality in red soil regions. Plantavailability of P in red soils mainly depends on fertilization and biological cycling. Both laboratory analyses and greenhouse experiments were conducted to examine the relationships between plant P uptake, chemical index of P, and microbial biomass P in red soils with different fertility levels. Microbial biomass P ranged from 2.1 to 43 mg kg -1 in the red soils and was significantly correlated with total P (r=0.84*), organic P (r=0.87*), or Bray I extractable P (r=0.94**). Extractable P plus organic carbon accounted for >85% of the variation in microbial biomass P in the red soils. The significant relationship between microbial biomass P and extractable P suggests that microbial biomass P has a great potential in predicting P-supply ability in soil. Greenhouse experiments showed that there were close relationships between ryegrass dry matter yield, plant P uptake or tissue P concentration and microbial biomass P in the red soils. The corresponding correlation coefficients were 0.79*, 0.90*, and 0.91*, respectively. These results imply that microbial biomass P plays an important role in the availability of P to plants, and is a potential biological index of P availability in the red soils. 655

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