Soil Abandonment as a Trigger for Changes in Zn Fractionation in Afforested Former Vineyard Acidic Soils

Zinc is an essential element for plant nutrition, but it may cause toxicity depending on its bioavailability and potential transformation in soil. In vineyard soils, high concentrations of Zn are usually found, mainly due to agricultural practices. However, a great abandonment of vineyards has recently occurred, leading to changes in the total and bioavailable Zn concentrations, as well as Zn fractionation. We analyzed Zn concentrations (total, ZnT, and bioavailable, ZnED) and fractionation in the soil of three paired sites (PM, PT, and AR) up to depths of 50 cm in active and adjacent abandoned vineyards that were already transformed into forests. The ZnT averaged at 210 mg kg-1 among all studied vineyards. The results showed changes in the vertical pattern ZnT concentrations after vineyard abandonment at the PM and PT sites, while at the AR site, no great variation occurred. The ZnED (mean values = 7 mg kg-1) decreased after abandonment at PM and AR in the uppermost surface layers, while it increased in the top 10 cm at the PT site, reaching up to 60 mg kg-1. Regarding Zn fractionation in active vineyards, the residual fraction (ZnR) was the most abundant, followed by Zn bound to crystalline Fe and Al oxy-hydroxides (ZnC) and Zn bound to soil organic matter (ZnOM). After abandonment, the ZnR slightly increased and the ZnC slightly decreased at the PM and AR sites at all depths, while the ZnOM showed a noticeable variation in the uppermost 10 cm of the PT site. These results suggest that the soil organic matter that is provided during afforestation may play an important role in Zn fractionation and mobilization, depending on its humification degree and chemical stability. Zn mobilization could result in a positive nutrient supply for plants, but caution must be taken, since an excess of Zn could cause toxicity in long-term abandoned vineyards.