Phosphorus speciation and colloidal phosphorus response to the cessation of fertilization in lime concretion black soil

Long-term excessive mineral fertilizer application led to soil acidification and phosphorus (P) accumulation, increasing the risk of P loss and environmental pollution. The cessation of fertilization was widely considered as a cost-effective management strategy to relieve the situation, however, how the fertilization cessation influences the speciation and concentrations of P in bulk soil and the colloidal fractions, and whether the declining P concentration could maintain soil fertility are unclear. In this study, the effect of long-term fertilization (~40 years) and short-term cessation of fertilization (~16 months) on inorganic P, organic P, and colloidal P in lime concretion black soil were investigated with P sequential fractionation and 31P nuclear magnetic resonance spectroscopy (NMR). After long-term fertilization, available P, Ca2-P, Fe-P, orthophosphate monoesters, and orthophosphate diesters increased significantly, but soil pH decreased by ~2.8 pH units, indicating that long-term fertilization caused soil acidification and P accumulation as well as changed P speciation dramatically. In contrast, fertilization cessation in short term brought the increase of soil pH (~0.8 pH units) and decreased slightly for available P and inorganic P. The available P after fertilization cessation was 22.9-29.8 mg kg−1, which was still sufficient for crop growth requirements. In addition, fertilization cessation brought increases in the proportions of fine colloids (100-450 nm, including nontronite and some amorphous Fe oxides) and drove the significant release of Fe/Al oxides nanoparticles (1-100 nm) and associated P with the species of orthophosphate and pyrophosphate. In summary, fertilization cessation in short term could alleviate soil acidification and inorganic P accumulation effectively, at the same time maintain soil P fertility and improve the potential mobilization of P associated with microparticles.