Ageing of zinc in highly-weathered iron-rich soils

Abstract

BACKGROUND AND AIMS: The reactivity and bioavailability of soluble metal added to soil decreases with time. This process, called ageing, has mainly been investigated in temperate soils. This paper uses isotopic exchangeability to investigate Zn ageing in a range of highly weathered and/or oxide-rich soils. METHODS: Changes in lability of soluble added Zn (450 mg Zn/kg soil) over time was measured in six contrasting soils, with pH adjusted to give ten treatments per soil type ranging from pH 4 to 7. RESULTS: Decreasing extractability and isotopic exchangeability (lability) over time revealed substantial fixation of added zinc in six highly weathered/variable charge soils. Strong negative relationships between pH and solubility, and pH and lability were observed. In soils with pH > 6.5 a significant proportion of the added metal becomes non-isotopically exchangeable within 15 s of addition. Correlations between Mn solubility and Zn lability throughout the incubation demonstrated the role of redox conditions (and pH) in regulating Zn lability. CONCLUSIONS: Results showed zinc fixation was strongly related to pH and ageing time, and relatively unaffected by soil type and mineralogy. Very rapid reductions in radiolability immediately (<15 s) after spiking suggest that precipitation plays a role in fixation of added soluble zinc at near neutral pH, however spectroscopic studies are needed to confirm this. Radiolability of added zinc was also affected by changing redox conditions during incubation.