Accumulation and Distribution of Trace Elements and Radionuclides in Agricultural Soils Impacted from Long-term Phosphate Fertilizer Application

Excessive application of phosphate fertilizers can result in the accumulation of both phosphorus (P) and trace metals (U, Cd) in agricultural soils, which could end up in crops and cause chronic harms to the environment. Here we investigate the quality of soils in a long-term trial corn/soybean field at the Tidewater Research Station, North Carolina, where both surface soils (top 20 cm) and subsurface soils (up to 150 cm) were collected from five plots with different application rates of P-fertilizer since 1966. We analyzed a broad range of major nutrients and trace elements with focus on metals and metalloids including Cd, U, V, Cr, As, and Sr, which are notably enriched in the used P-fertilizer relative to local background soil. The study aims to investigate the trace elements accumulation, distribution, and mobilization in the soils. The results show that the impact of long-term P-fertilizer application was mostly manifested in the top layers of the soils compared to deeper soils, with the exception of As showing accumulation in the deeper soils. Among the five plots, bulk soils applied with higher rates of P-fertilizers had higher concentrations of P and trace elements than soils without using P-fertilizers. The concentration of Cd was significantly correlated with that of P (r = 0.97, p = 0.005) in the bulk surface soils, indicative of its direct contribution from P-fertilizer and accumulation in the soil. In contrast, other trace elements exhibited weaker or little correlations with P in the bulk surface soils. The potential bioavailability of elements in soils was assessed via the Mehlich III extraction method, showing that the higher application rate of P-fertilizer, the higher percent of bioavailability was found for Cd (up to 65% of the bulk soil) and P (up to 56%), whereas the other trace elements had much lower bioavailable fractions (0.4 – 12%). Strong correlations (r > 0.9, p < 0.05) were observed between the bioavailable concentrations of P and that of Cd, U, Cr, V, As, Sr in the surface soils. This indicates that the bioavailable form of trace elements is more sensitive in reflecting the impacts from P- fertilizer on surface soils. Four-step sequential leaching tests (i.e., F1: exchangeable, F2: reducible, F3: oxidizable, and F4: residual) conducted for the surface soils indicated differential mobilization of trace elements under different P-fertilizers application rates. Greater portions of Cd were found in the mobile fractions (F1 – F3) of soils with higher P-fertilizer input, while Sr was dominantly present in the residual fraction (F4: 95 – 97%), and redox-sensitive elements were higher in the reducible (As, V) and oxidizable (U, Cr) fractions than Sr, reflecting their redox-dependent mobilization potential. Overall, our systematic data analysis shows the effect of long-term P-fertilizer application on the accumulation of trace elements in soils. Further studies should evaluate the uptake of trace elements by crops and their mobilization to the underlying groundwater resources.