The drainage and conversion of peatlands to productive agro-ecosystems leads to ongoing surface subsidence because of densification (shrinkage and consolidation) and oxidation of the peat substrate. Knowing the ra0te of this surface subsidence is important for future land-use planning, carbon accounting, and economic analysis of drainage and pumping costs. We measured subsidence rates over the past decade at 119 sites across three large, agriculturally managed peatlands in the Waikato region, New Zealand. The average contemporary (2000s-2012) subsidence rate for Waikato peatlands was 19 ± 2 mm yr (± SE) and was significantly less ( = 0.01) than the historic rate of 26 ± 1 mm yr between the 1920s and 2000s. A reduction in the rate of subsidence through time was attributed to the transition from rapid initial consolidation and shrinkage to slower, long-term, ongoing oxidation. These subsidence rates agree well with a literature synthesis of temperate zone subsidence rates reported for similar lengths of time since drainage. A strong nonlinear relationship was found between temperate zone subsidence rates and time since initial peatland drainage: Subsidence (mm yr) = 226 × (years since drained) ( = 0.88). This relationship suggests that time since drainage exerts strong control over the rate of peatland subsidence and that ongoing peatland subsidence rates can be predicted to gradually decline with time in the absence of major land disturbance.
An ongoing programme of soil quality assessment in the Waikato region of New Zealand is providing baseline data and allowing identification of the impacts of land use and associated key soil quality issues. In this paper, we review the strengths and weaknesses of key soil quality indicators and provide summary statistics for chemical, physical and biochemical parameters in both background and production soils of the Waikato Region. Statistical comparison of production to background soils, time related trends, relative surface enrichments, and correlations enable us to infer likely and potential impacts of human activity on the region's soil resource. Five key issues that cause loss of soil resource have been identified. These are soil compaction, loss of soil organic matter, excessively high fertility levels, erosion risk, and accumulation of contaminants. In addition, evidence exists for accelerated attack or weathering of aluminosilicate minerals in farmed soils [dealumination, Australian Journal Soil Research (2009) vol. 47, 828-838], a process previously only recognized in soils adjacent to aluminium smelters.
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