Accumulation of micronutrients (Cu, Zn) in vineyard soils and transport via soil erosion

Copper (Cu) and zinc (Zn) are important micronutrients for vine plants; however, the long-term use of Cu-fungicides and micronutrient fertilizers can lead to their accumulation in the topsoil of vineyards. Water erosion on sloping vineyards transports sediments downslope, a processus that may redistribute micronutrients in the topsoils. Our study aims at assessing the rate of enrichment in Cu and Zn of vineyard topsoils compared to the geochemical background and their downhill transport during rainfall events attached to sediments.

The study was conducted in 2019 in a 1,8 ha sloping vineyard at Tokaj (mean slope: 8°) and a 0.4 ha plot near Tállya (mean slope: 18°), both in the historical vinegrowing region of Tokaj-Hegyalja (in northern Hungary). The vineyards at Tokaj have been converted to organic farming, where Cu-based fungicides are repeatedly used in a typical dose of 4 kg/ha/year, supplemented with fertilizers containing micronutrients. The soil samples from the top layers (0-10 cm and 10-20 cm) have been collected using a hand auger from the two vineyards and from local forested sites, the latter accounting for the local geochemical background. Additionally, sediment traps have been deployed for collecting eroded sediment samples. The examined soil type is a Regosol at Tokaj with sandy loam texture, while the Cambisol at Tállya displays slightly heavier soil texture (sandy loam/loam). The soils are characterized by a slightly acidic pH(d.w.) of 6.36±0.27 at Tállya and a moderately alkaline pH(d.w.) of 8.03±0.04 at Tokaj. The differing pH is due to the soil forming parent rocks, that are loess at Tokaj and rhyolite at Tállya. The topsoils (0-20 cm) bear a low to medium organic matter (OM) content (1.5±0.5% OM at Tállya and 1.4±0.2% OM at Tokaj) and a low carbonate content at Tállya (3.1±0.2%), while a low to medium carbonate content at Tokaj (4.4±1.5%).

The micronutrient (Cu, Zn) concentrations have been determined by an inductively coupled plasma-optical emission spectrometer, following microwave-assisted digestion of powdered soil samples in aqua regia (hydrochloric acid:nitric acid = 3:1). At Tállya, our results show a considerable Cu enrichment and a slight Zn enrichment in the topsoil (mean±se: 127±37 mg/kg Cu, 47±4 mg/kg Zn) due to the repeated use of pesticides and fertilizers, compared to a local forested soil displaying 5 mg/kg Cu and 28 mg/kg Zn. The lower Cu enrichment in the vineyard topsoil at Tokaj (49±14 mg/kg in vineyards, 17 mg/kg at the local forested site) is probably due to the more recent plantation of grapevines. The soil-bound Zn at Tokaj also displayed to some degree higher concentrations in the top 20 cm layers in vineyards (64±6 mg/kg) as to the forest soil, exhibiting 41±3 mg/kg Zn. At both sites, eroded sediments tend to display higher Cu and Zn concentrations relative to the vineyard topsoils with mean enrichment factors (sediments/topsoil) of 4.2 (Cu) and 1.4 (Zn). Additionally, we evidenced that soil erosion significantly affects the topsoil Cu concentrations at Tokaj, as higher Cu concentrations have been found downslope, where the eroded sediments accumulate, compared to the erosion bases upstream.