Impact of Mineral P Fertilization on Trace Elements in Cropland Soils

Abstract

Phosphorus (P) is indispensable for crop production, and mineral fertilizers processed from rock phosphates, such as superphosphates, represent an important P source. In the past, P fertilizers from iron and steel slags (basic slag) have also contributed to increased biomass productivity. Both types of fertilizer contain heavy metals as potentially adverse environmental contaminants: they can accumulate in the soil, be taken up by food crops or transferred into aquatic ecosystems. This article reviews the past and current use of P fertilizers and presents the results of two Austrian long-term P field experiments (Fuchsenbigl and Rottenhaus) on superphosphate and basic slag fertilization. The focus is on its impact on the accumulation of cadmium (Cd), uranium (U), chromium (Cr) and vanadium (V) in the soil and their uptake into plants (grain of winter wheat, Triticum aestivum L.). The mining of rock phosphate first began in the mid- to late nineteenth century, before which mainly organic fertilizers were used as P sources. From the 1950s to the  1980s, phosphate rocks were extensively applied to agricultural soils in Central European countries. This marked the onset of more intensive research on the adverse environmental effects of P fertilization. In Austria, 48 years of very high phosphate fertilization (175 kg P ha⁻¹ year⁻¹) significantly increased the aqua regia soluble trace elements in the soil. Superphosphate application led to a +25%/+22% increase of Cd and a +34%/+21% increase of U in 0–25 cm/25–50 cm soil depth at the Fuchsenbigl site. At the Rottenhaus site, which is rich in silt and clay, only U was significantly enhanced (+37%/+25% in 0–25 cm/25–50 cm) after long-term, very high superphosphate fertilization. In Fuchsenbigl, long-term basic slag fertilization also significantly enriched Cr and V at lower application amounts (44 kg P ha⁻¹ year⁻¹) in 0–25 cm soil depth, in 25–50 cm only with very high P fertilization (175 kg P ha⁻¹ year⁻¹). In Rottenhaus, high fertilization significantly increased the Cr concentrations in 0–25 cm (+34%) and the V concentrations in both 0–25 cm (+80%) and 25–50 cm (+50%). After long-term, very high superphosphate fertilization (175 kg P ha⁻¹ year⁻¹), winter wheat grains showed the highest but non-critical Cd concentrations at both experimental sites. Finite natural P resources and the adverse environmental effects of P overuse increasingly require sustainable agricultural P management options to optimize P use efficiency. Careful consideration of the amount and type of P fertilizers and well-planned crop rotations are known as promising strategies. Another potential way forward is P recovery from phosphate salts, municipal waste treatment plants, manure and digestates. These approaches are necessary to meet the challenges of global change and a circular P economy.