Occurrence forms of inorganic phosphorus in soils of karst wetland under different landuses and comparison of two analysis methods

The content and speciation of phosphorus（P）determine soil fertility and the risk of P leaching from soils. This work uses standard, measurement and testing（SMT）and seven-step sequential extraction methods（“seven-step method”）to investigate the occurrence of inorganic P（In-P）in typical karst soils（paddy land，orchard，barren land）and river sediment in the Huixian karst wetland of Guilin, to analyze the effects of different land utilization on the distribution of In-P in karst soils and compares the two methods on this issue. The results show that total P content in the karst wetland soils from high to low is river sediment，orchard，paddy land，and barren land. This value is the highest in the river sediment（416.97 mg?kg-1 ），in which highly bioavailable exchangeable P（Ex-P）and iron P（Fe-P）are dominant. In the barren land soil，organic P（Or-P）and residual P（Res-P） are the major forms；while In-P content（64.45 and 47.56 mg?kg-1 for 0-20 cm and 20-40 cm depth）is the lowest in all four soils. The autologous P（Au-P），which has low bioavailability，has the highest proportion in In-P of this soil compared to that in other soils. In comparison，the surface soil of the paddy land and orchard has relatively high In-P content（276.04 and 418.19 mg?kg-1， respectively）in which Fe-P is the dominant form. In this analysis, the seven-step method has advantages in the extraction of P with complex binding states. Specifically for the karst soils，which are characterized by high Ca content and alkalinity，the seven-step method is more effective in the extraction of Ca-P. The In-P constitutes a great share in the total P in karst soils, therefore the P pool is more active. Human disturbance greatly increases the In-P content and its bioavailability. Especially，the surface orchard soil has a great potential risk of P leaching. The barren soil has low In-P content and bioavailability，indicating its high P buffering capacity. The enrichment of P in sediment via leaching suggests the sediment is a risk source for river eutrophication. In general，the seven-step method is a more suitable approach for P speciation analysis in karst soils.