ACCUMULATION AND DISTRIBUTION OF TRACE ELEMENTS IN AGRICULTURAL SOILS IMPACTED FROM LONG-TERM PHOSPHATE FERTILIZER APPLICATION

Excessive application of phosphate fertilizers can result in the accumulation of nutrients (P) and trace metals (U, Cd) in agricultural soils, which might cause chronic harms to the environment. Here we investigate soils in a long-term trial corn/soybean field at the Tidewater Research Station, North Carolina, where both surface soils (~ 20 cm) and soil cores (~ 150 cm) were collected from five plots with different application rates of P-fertilizer since 1966. We focus on trace elements including Cd, U, V, Cr, As, and Sr – all are notably enriched in the used P-fertilizer relative to background soil. We aim to study their accumulation and distribution in bulk soils and four-step sequentially extracted soil fractions (i.e., F1: exchangeable, F2: reducible, F3: oxidizable, and F4: residual). The results show that the impact of long-term P-fertilizer application was mostly manifested in the surface soils (~ 30 cm) compared to deeper soils. Among the five plots, bulk soils applied with P-fertilizer had higher concentrations of P and trace elements than that without P fertilization. The concentration of Cd was significantly correlated with that of P (r = 0.92, p < 0.001), 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 bulk soils. The distribution of trace elements in different soil fractions varied from different plots, in which 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%), redox-sensitive elements were higher in the reducible (As, V) and oxidizable (U, Cr) fractions, reflecting their redox-dependent mobilization potential. The potential bioavailability of elements in soils was assessed via 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%) and P (up to 56%), whereas the other trace elements had much lower bioavailability (0.4 – 12%). Overall, our data showcase the effect of long-term P-fertilizer application on the accumulation of trace elements in soils, and further study is needed to evaluate their uptake by crops and mobilization to the underlying groundwater resources.