Impacts of landscape recontouring on soil properties in Marlborough, New Zealand

Landscape recontouring, used to reduce slope gradients and relief to increase the ease of mechanised vineyard management, was studied in the Awatere Valley, Marlborough. The aim was to determine whether soil degradation had occurred as a result of recontouring on viticultural sites. Two hilly sites, the Hardcase and Duelling Banjos vineyards, with similar virgin soils (Argillic Sodic Pallic) were studied. The lithology of the Hardcase site was slightly calcareous siltstone of Starborough formation, whereas the Duelling Banjos site lithology was the Upton formation which included siltstone, mustone, and conglomerate facies. Recontouring was carried out similarly on both sites and involved a ‘double stripping’ technique in which topsoil and subsoil was collected into piles followed by recontouring of underlying bedrock, followed by reapplication of topsoil and subsoil in a mimicry of the virgin soil profile. These sites were investigated with a variety of techniques including opening of four deep (to 1-1.4 m) representative soil pits on each site, auger transect surveys, topsoil sampling, and irrigation pond water sampling. Soil morphology, chemical properties, and physical properties were determined for each soil pit and compared between virgin and reconstructed soils (2 of each per site). These analyses highlighted mixing and simplification of soil profile morphology as a result of recontouring, which required a re-classification of reconstructed soils to Mixed Anthropic soils. Soil structure was degraded in most reconstructed soils, relative to the strongly structured virgin soils. Soil exchangeable bases in both virgin and reconstructed soils often contained a high percentage of sodium, which was typically found in clay-rich subsoil horizons with some attendant dispersion. Soil physical properties showed differences which amounted to higher available water capacity values in the reconstructed soils, although the differences were apparent primarily on the Hardcase site where fill material was derived from fine-textured siltstone. There was also a shift in reconstructed soil towards lower subsoil bulk density and decreased aggregate stability, which may result in soil consolidation. Topsoil organic C and N profile masses were not significantly different between reconstructed and virgin soils on the Hardcase property, and recontouring resulted in lower spatial variability of both C and N. Salts leached from siltstone and integrated into the soil as a consequence of recontouring have concentrated in infilled gullies on Hardcase vineyard. Saline seeps proximal to a prior gully thalweg on Hardcase have resulted in vine death and high soil salinity. Accumulation of salts in the irrigation pond on this site has reached harmful levels (2.5 to 4 mS/cm), and was found to be higher during the summer, when irrigation is abstracted for irrigation. There is the potential to decrease viticultural production if this high salinity water continues to be used. Recontouring on the two sites studied was implemented carefully and with consideration for long-term primary production. Many soil properties that may be negative for plant growth in reconstructed soils are inherited from the parent virgin soils, such as sodicity. The short-term effects of recontouring on these sites have been neutral, with positive and negative impacts being balanced. Longer-term issues of site stability and susceptability to erosion and compaction may arise over time.