Inorganic Phosphorus Fractions and Phosphorus Availability in a Calcareous Soil Receiving 21-Year Superphosphate Application

A long-term (21-year) field experiment was performed to study the responses of soil inorganic P fractions and P availability to annual fertilizer P application in a calcareous soil on the Loess Plateau of China. Soil Olsen-P contents increased by 3.7, 5.2, 11.2 and 20.6 mg P kg−1 after 21-year annual fertilizer P application at 20, 39, 59, and 79 kg P ha−1, respectively. Long-term fertilizer P addition also increased soil total P and inorganic P (Pi) contents significantly. The contents of inorganic P fractions were in the order of Ca10-P > Ca8-P > Fe-P > Al-P > occluded P > Ca2-P in the soil receiving annual fertilizer P application. Fertilizer P application increased Ca8-P, Al-P and Ca2-P contents as well as their percentages relative to Pi. Pi application increased Fe-P and occluded P contents but nor their percentages. Soil Ca10-P content remained unchanged after fertilizer P application while its percentage relative to Pi declined with increasing fertilizer P rate. All Pi fractions but Ca10-P were correlated with Olsen-P significantly. 90% of variations in Olsen-P could be explained by Pi fractions, and the direct contribution of Ca8-P was predominant. Long-term annual superphosphate application would facilitate the accumulation of soil Ca8-P, and thus improve soil P availability.