Speciation of phosphorus accumulated in fertilized cropland of Aichi prefecture in Japan with different soil properties by sequential chemical extraction and P K-edge XANES

Fertilizer-derived P is accumulated in cropland soils and their phytoavailability is variable depending on the speciation of P, which is affected by soil properties. In this study, we aimed to clarify how soil properties affect the speciation of P accumulated in fertilized soils. Soil samples were collected from the cultivated soil layers of 137 vegetable fields in Aichi prefecture, Japan. The speciation of soil P was evaluated by Truog method, sequential extraction by acetic acid, NH4F, and NaOH (Sekiya method), P K-edge X-ray adsorption near-edge structure (XANES). Phosphorus accumulated in cropland consisted predominantly of the inorganic form. Phosphorus extracted by the Truog method and acetic acid was correlated with exchangeable Ca and total C. The XANES analysis indicated that the P species extracted by Truog and acetic acid were associated with Ca, while those extracted by acetic acid also included crystalized calcium phosphate with higher crystallinity than CaHPO4, for example, hydroxyapatite. Due to the high application of P fertilizer and manure with lime, P was accumulated as calcium phosphate except for soils with relatively high contents of oxalate extractable Al and Fe. The amounts of NH4F-extracted P were higher in soils with larger amounts of oxalate extractable Al and Fe. Based on the XANES analyses, the P species extracted by NH4F were associated with Al and Fe, which is inconsistent with the operational definition of NH4F-soluble species. The P in soils extracted by NaOH was correlated with various soil properties, especially with clay content. The P species extracted by NaOH could not be identified. Our results revealed that the important factors controlling available P associated with Ca were total C and exchangeable Ca, whereas those controlling the P extracted by NH4F were oxalate extractable Al and Fe, because this fraction of P was associated with Al and Fe.

Automated summary

The study found that phosphorus accumulated in cropland soils is mainly in the form of calcium phosphate due to the high application of P fertilizer and manure with lime. Phosphorus extracted by NH4F is associated with oxalate extractable Al and Fe, while phosphorus species associated with Al and Fe are extracted by NH4F contradicting the operational definition of NH4F-soluble species. Additionally, the P extracted by NaOH is correlated with various soil properties, especially clay content.