Mineralization and speciation of organic phosphorus in a sandy soil continuously cropped and phosphorus-fertilized for 28 years

Phosphorus (P) is essential to plants and many crops require P fertilizers to optimize yields. However, there are concerns about future scarcity of the rock phosphates used to produce P chemical fertilizers. Soil organic phosphorus (SOP) might play a significant role in plant nutrition and soil P availability, reducing fertilizer requirements, but SOP mineralization is poorly understood. This study assessed rates of SOP mineralization and SOP speciation using a long-term field experiment in France, continuously cropped (1972–2000) and fertilized at three rates (triple superphosphate). The SOP concentration was determined every 2–3 years using the Saunders and Williams ignition method and converted to stock considering the soil mass per hectare; P speciation was determined using solution P nuclear magnetic resonance spectroscopy. The SOP stocks slightly increased with time, irrespective of P fertilization rates. Inorganic P stocks decreased without fertilization, but increased at the high P fertilization rate. By adapting a model describing soil organic carbon dynamics, the gross mineralization coefficient (k) was calculated with initial SOP stock (368 kg P ha−1), P in maize residues (average of 17.0 kg P ha−1 yr−1, differences across yields, plant parts and P fertilization rates) and their 17% incorporation rates into SOP (i.e. 2.9 kg P ha−1 yr−1). Irrespective of P fertilization rates, the model calibration using SOP time series gave a k-value of 0.0047 yr−1 (212 yr residence time) and a mineralization rate of 1.7 ± 1.2 kg P ha−1yr−1. Speciation of SOP was 77% of orthophosphate monoesters, and 20% of orthophosphate diesters regardless of the P fertilization rates. The predominance of inositols hexakisphosphates, known to be strongly stabilized through sorption, might explain the low SOP mineralization rate. The results for these soils suggest that SOP may only play a minor role in soil P availability and crop nutrition.