A Multi-Disciplinary Study to Quantify Terroir in Central Otago and Waipara Pinot Noir Vineyards

Abstract

Soils form the critically important and extremely complex interface between the geosphere and the atmosphere, and provide vital nutrients and habitats for a wide variety of plant, animal and microbial ecosystems. This complexity forms a part of terroir, which is becoming increasingly important for wine producers. The basic notion is that environmental conditions impart particular flavour and aromas in wine. New Zealand is internationally recognised as a producer of premium wines, yet there is a distinct lack of analyses on New Zealand terroir. This research is the first to quantify New Zealand terroir on a regional-national level, and examines the soil component of terroir in five Pinot Noir vineyards. Each wine growing region in New Zealand has a unique terroir that influences regional wine styles. The wine growing regions have from 900 growing degree days in Central Otago to over 1600 in Auckland. Vineyards are mainly located on flat alluvium and gravels with slopes less than 3°. White varieties are generally grown to premium standard in climates that are similar to other areas internationally. Most trophy winning red varieties, however, are grown in areas that are cooler than where similar quality grapes are grown internationally. Rapid growth of the industry is pushing new plantings up hillsides onto different soil types, and this may have an influence on overall wine profiles from different regions in the future. Five case study vineyards in two regions were selected for further, more detailed terroir study. Grapevine trunk circumference measurements can vary by up to 65% in a uniformly managed vineyard block, signifying that grapes of different brix and other attributes may be produced within these blocks, yet harvested and vinified together. Approximately 500 samples from 102 trenches were collected and analysed for grain size, and a subset of 288 samples was analysed for nutrient availability. The soil samples have a unique geochemical signature, and it is possible to differentiate region, and sometimes vineyard, based on a combination of geochemical attributes. Nutrient analysis indicates that some concentrations such as Mg, Ca and Na increase with depth at some sites by 20-50%, 100%, and 80-100%, respectively. Deep penetration of fine roots indicates that these reserves may be available to the grapevine. Concentrations of some elements in shallow soils are occasionally below recommended levels of 4-10% Total C, 156-312 ppm K, 1200 ppm Ca, 120-360 ppm Mg for vineyards in New Zealand, yet the vines do not suffer from obvious nutrient deficiencies, indicating that vines may be utilising deep soil nutrients. This may have implications for global viticulture where roots penetrate deep into the soil profile. The variable nature of soils means that conducting meaningful soil surveys is generally a time consuming and expensive process. For example, soils at the study sites vary from minimal gravel content of <4% to 80%, and depth to gravel is as low as 25cm at one site. ii Clay contents range from <1% to 19% at the most clay-rich site. Proximal sensing surveys can provide data on soil properties on a vineyard block scale. Electromagnetic induction and ground penetrating radar surveys are commonly used techniques. New multi-frequency units such as the GEM-2 can measure soil ECa at different depths simultaneously. A comparison with the better understood GPR surveys indicates that changes in radar reflectance as measured by the GPR can also be measured by conductivity changes in the EMI surveys, effectively eliminating the need for multiple geophysical survey techniques in some instances. EMI surveys, when combined with dGPS surveys, significantly correlate with grapevine vigour at most study blocks. Multi-frequency EMI surveys can also be used to predict soil type more accurately (~70% correct) than single frequency surveys (~40% correct) because of specific soil ECa changes with depth exhibited by different soil types. Because grapevine root penetration can exceed several meters, the multi-frequency EMI surveys are a more valid approach to geophysical surveying in New Zealand vineyards, and may provide an additional dimension to traditional EMI surveys. Links between changes in soil properties and wine attributes are difficult to prove. Microvinification trials that eliminate winemaker influence in three neighbouring sites with different soils produce significantly different wines. Vineyard management practices such as trellis design and crop yield are similar. Due to the close proximity of the sites, many environmental properties are similar. Detailed soil analysis shows texture varies from <5-80% gravel content between sites. Nutrient content can vary between sites from 0.05-0.35% total N, 40-100 ppm P, 1-6 ppm Cu, and 1000 - 10,000 ppm Ca. Soil variability has an impact on grape properties and wine tannins, polyphenols, colour and aroma compounds. Soil is a valid component of terroir, and can significantly influence concentrations of wine aroma compounds. Soil properties should therefore be included when analysing vineyard terroir.