Phosphorus distribution and fractionation during weathering of amphibolites and gneisses in different climatic setups of the Kaveri river catchment, India

Phosphorus distribution and fractionation during weathering of rocks are seminal to its availability to the life system. Here, the P fractionation pattern is reported in weathering profiles of two different rock types i.e. amphibolites and granitic gneisses, subjected to semi-arid and humid climatic conditions in the catchment of the Kaveri river, southern India, known for its fertile flood plain and delta. Weathering profiles developed on the two rock types were analysed for evaluating the intensity of chemical weathering using major ion geochemistry and for accompanying P fractionation. The Chemical Index of Alteration (CIA) and the mineralogy of different zones of the weathering profiles show that profiles under humid climatic conditions are more weathered than those under semi-arid conditions. It was also found that P, present dominantly as the apatite inorganic P (AIP) fraction in the bedrocks was converted into non-apatite inorganic P (NAIP) and organic P (OP), both of which increase with progressive weathering. The two major rock types show a very similar pattern of P fractionation during weathering which is similar to those observed in soil chronosequences. However, the transfer of P from AIP to OP and NAIP is near total in both rock types under humid conditions of weathering. Under semi-arid conditions, a part of AIP was also available in the most weathered zone. Thus the lithology of the bedrock has little, and climate a dominant, control on P fractionation during rock weathering. It was also observed that there is a net loss of total P during the weathering associated fractionation that could be attributed to the incorporation into biologically available forms and their subsequent removal to the fertile floodplains and delta in an undisturbed riverscape.