Adsorption-desorption kinetics and phosphorus loss standard curve in erosive weathered granite soil: Stirred flow chamber experiments

Weathered granite has a high soil adsorption rate and it is difficult to apply phosphate fertilizers. In order to effectively use phosphorus, it is very important to study the intensity and speed of soil retention and desorption of phosphorus. The purpose of this study is to evaluate the kinetics of adsorption and desorption in soils with different erodibility. We selected three weathered granite soils with hugely different erosions: topsoil layer (TL-Tillage layer), laterite layer (LL-Laterite layer) and sand layer (SL-Sand layer). Phosphorus adsorption and desorption experiments were successively carried out in the stirred flow reaction chamber, and the stirred flow experiment under unsaturated adsorption conditions was carried out for the first time. The maximum absorption capacity (qamax) and desorption capacity (qdmax) of phosphorus in TL soil was both greater than that in LL and SL soil. The adsorption process mainly occurs within the first 250 min of the reaction. The desorption ratio (Dr) in the TL soil is smaller than the LL and SL soil. The order of the fast reaction sites proportion (F) is SL > LL > TL in adsorption process and LL > SL > TL in desorption process. Seven key soil property indicators can fit qamax and qdmax values well. In the determination coefficient (R2) of the standard curve of phosphorus loss, SL soil is the larger than TL and LL. The qamax and Dr values measured in the stirred flow experiment are both larger than those in the batch experiment. Among the three soils, LL has the lowest adsorption capacity for P and the highest accessibility. The slow adsorption sites in TL account for the main part, and the adsorption in LL and SL mainly occurs at fast adsorption sites. Through the establishment of the phosphorus loss standard curve, the maximum phosphorus loss in the entire region can be predicted and calculated. This study can provide theoretical basis and reference for phosphorus management in similar soil types or physical geographical areas with severely eroded soils.