Molecular speciation and transformation of soil legacy phosphorus with and without long-Term phosphorus fertilization: Insights from bulk and microprobe spectroscopy

Phosphorus (P) is an essential element for crop growth, but the rock sources used to produce P fertilizer are non-renewable. Therefore, more sustainable soil P management for crop production has become a global concern. Not all fertilizer P that is applied to soil each year is used by crops. What remains, called “soil legacy P”, represents a substantial secondary P resource to reduce the need to add P fertilizer. However, more research is needed to understand the forms of legacy P in soil, how they are bound and cycle in soil, and how to make them available to plants. Using advanced chemistry techniques, this study analyzed the forms of legacy P from two experimental treatments in black soil in China; one with and one without P fertilization for 27 years. Results showed that P bound to calcium (Ca-P) acts as a reserve of plant-available P in response to soils with or without P fertilization. Cropping for 27 years depleted Ca-P, while iron-bound P in soils was enriched over the same period. Soil organic P from root residue inputs also occurred in soils with and without P fertilization. These results provide vital information for agronomic and environmentally sustainable P management by demonstrating the potential crop availability of legacy soil P, which could reduce future P fertilization.