Long-term copper application in an organic vineyard modifies spatial distribution of soil micro-organisms
Introduction
Viticulture is an important perennial cropping system in Europe. In certain regions, however, these crops are blighted by downy mildew, Plasmopara viticola, a mold that in turn stresses the vine and reduces grape quality (Salinari et al., 2006, Jermini et al., 2010). Copper-based fungicides have been used to combat P. viticola in different production systems worldwide for more than a century. Today, synthetic fungicides play a major role in conventional farming; these fungicides replace copper products and, thus, reduce this input of the heavy metal in vineyard soils, although copper-based fungicides continue to be applied in restrained amounts. In organic farming, copper-based fungicides are still the only effective method permitted to treat grapes against P. viticola. As organic methods are becoming more common, especially in Europe, where 100,000 ha of grapes are under organic management (IFOAM, 2009), the total use of copper-based fungicides remains important even though the maximum amount of copper input per year is limited to a maximum of 6 kg y−1 ha−1 according to the EU Regulation on Organic Production and Labeling of Organic Products (European Commission, 2007). As copper is known to accumulate within topsoil following fungicidal sprays (Pietrzak and McPhail, 2004, Rusjan et al., 2007) and can never be degraded (McBride et al., 1981), its potential to have adverse eco-toxicological effects on the environment is large. In environments, such as vineyards, where pH is often above neutral, copper is immobile, accumulating over years of use. However, soluble fractions are always present (McBride et al., 1981) and both low organic matter content (OM) and low cation exchange capacity (CEC) often found in vineyards encourage copper mobilization (Maier et al., 2000). When copper becomes mobile and more available to organisms, it stresses macro fauna (such as earthworms), microorganisms and their enzyme activities and, in high amounts, becomes toxic to plants, thereby disturbing essential elemental cycles (McBride et al., 1981, Moolenaar, 1998, Paoletti et al., 1998, Maier et al., 2000, White, 2009, Hinojosa et al., 2010).
The focus of this study was the influence of copper use in grape cultivation on the spatial abundance and function of soil microorganisms. Copper is said to reside in the topsoil and decrease with distance from the vine (Wightwick et al., 2006, Komárek et al., 2010, Mackie et al., 2012). The question remains, how does copper affect soil quality within the vineyard system and where is its impact greatest? Soil microbial abundance and activity are heterogeneously distributed within the soil matrix at different scales ranging from millimeters to meters (Berner et al., 2011). This distribution is affected by the carbon/nitrogen ratio, the chemical composition, the porosity, the location of organic substrates and plant biomass, the grade of humification, the water content, the pH, and the heavy metal content of the soil (Buscot and Varma, 2005). Investigation of meso-scale (field-scale) distribution can be helpful in understanding multivariable interactions, which are difficult to observe on a larger scale (Philippot et al., 2009, Berner et al., 2011, Keil et al., 2011). Heavy metals, particularly, can modify spatial distribution of microbial abundance and activity by altering soil characteristics and reducing microbial biomass (Kandeler et al., 1996). In vineyard soils, copper has been found to diminish the rate of ammonification, which signifies an alteration in bacterial presence and/or functions (McBride et al., 1981). However, field knowledge regarding copper impacts specifically on soil microorganisms is limited (Dell'Amico et al., 2008, Wang et al., 2009, Brandt et al., 2010, Fernández-Calviño et al., 2010) and, as these communities maintain essential processes and support soil fertility, their upkeep is vital for viticultural and other agricultural systems. We expected that even in areas of low to moderate copper pollution, such as vineyards, soil microorganisms and enzymes involved in the nitrogen (N), phosphorus (P) and sulfur (S) cycling will be compromised. Therefore, it was the goal of this study to investigate meso-scale distribution of copper, soil properties and microorganisms within an organic vineyard and to identify the interactions between copper accumulation and the soil eco-system. We hypothesized that the spatial distribution of the activity (i.e. enzyme activity) and abundance of soil microorganisms is correlated with the spatial distribution of copper in the soil, thus, (i) areas of high copper will indicate areas of low soil microbial activity and (ii) copper distribution will influence the spatial distribution of the microbial community composition (i.e. phospholipid fatty acid composition) in the soil.
Section snippets
Study area
Samples were taken from an organic vineyard in Brackenheim, Baden Württemberg, Germany (49°5′43.53″N, 9°2′57.93″E). The site has an elevation of approximately 230 m, an average precipitation of 650 mm and an average temperature between 7 °C and 14 °C. The vineyard was established in 1988 and has been under certified organic management since 1993. The soil type sampled on the site is an Umbric Leptosol. It is planted with Trollinger vines. Various species of grass were growing between each row.
Chemical properties and distribution
Table 1 shows means and significant differences (p < 0.05) based on tillage and distance from the vine row. CuT (Table 1) ranged from 43 to 142 mg kg−1, where the vine row had significantly higher values than the interrows. CuT (Fig. 2D), CuDTPA (Fig. 2E) and (Table 1) were significantly higher in the vine rows, while the interrows had a significantly lower mean (Table 1). However, the map shows (Fig. 2D) that CuT was amassing in the upper slope. CuDTPA had even significantly lower
Spatial distribution of soil microorganisms related to copper treatment
Although Cu was hypothesized to modify the spatial distribution of soil microbial activity and composition, not all results could be explained by Cu contamination. Results showed that a second factor, tillage, also played a role in influencing the spatial placement of microorganisms, which follow carbon substrate availability. CuT concentration in the vineyard site was highest at the upper slope of the grid with maximum of 142 mg kg−1. Comparing results from our vineyard to those from other
Conclusion and outlook
This study is one of the first to illustrate the spatial distribution of soil microorganisms and soil enzyme activities within a vineyard after long-term copper application on agricultural soils. The trends displayed in the organic vineyard identify copper loading and subsequent speciation below the vines, predominately in the DTPA exchangeable fraction, and a total copper concentration that is considered low to moderate pollution levels, as defined by the EC, and requiring copper prevention by
Acknowledgments
Thank you to J. Winkler for allowing us to sample within his vineyard. To Emma Petersson, Pol Tock and Barbara Eickhoff thank you for your interest in the topic and the additional analyses and help with interpolation. Thank you to Karin Hartung and Ralph Gäbler for their assistance with statistical and spatial analyses. Finally, many thanks to the anonymous reviewers, your assistance improved this paper greatly. Funding from the Carl Zeiss Stiftung supported writing this manuscript.
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