Long-term monitoring of soil quality and trace elements to evaluate land use effects and temporal change in the Wellington region, New Zealand

Abstract

Soil quality monitoring is used to assess the soil's ability to maintain agricultural productivity, ecological, and environmental quality. Very few soil quality monitoring studies have reported on multiple samplings over the long-term. Several regional authorities in New Zealand have monitored soil quality since the late 1990s. In the Wellington region, dairy, mixed cropping, market garden, drystock (sheep/beef), horticulture, exotic forestry and indigenous land use systems, and four soil orders have been monitored over 19 years, with up to five repeat samplings per site. This study reports on key soil quality indicators and Cu, Zn, and Cd concentrations. For the most recent sampling per land use, all land use system sites, except drystock, had Zn concentrations below recommended ecological toxicity guidelines. Dairy land use had 21%, and 36% of sites within recommended soil quality target ranges, for Olsen P, and macroporosity, respectively. Compared with indigenous land, across all samplings, Cu concentrations were elevated in horticultural and market gardens sites, while several land uses had lower total nitrogen and higher Olsen P concentrations. Across all samplings, significant increases over time were observed in Zn for dairy, total nitrogen for drystock, and Olsen P for mixed cropping. Significant decreases over time were observed for Cu in forestry, Cd for indigenous and forestry, and bulk density for drystock. No changes over time were detected for macroporosity, anaerobically mineralised nitrogen, or organic carbon. This study shows the programme and our analysis of multiple samplings are valuable for detecting significant trends as an early warning, e.g. Zn and Olsen P changes. The study provides evidence for recommending additional sites for several land uses and increased sampling frequency to ensure future robust statistical analysis. This study included only sites where land use systems did not change, providing a robust basis for detecting change over time, for informing policy, resource and environmental decision-making.