Peat soil properties and seasonal surface elevation changes of drained Waikato peatlands

Peat surface oscillation (PSO) is the elastic seasonal change in peatland surface elevation that occurs as a response to changes in the water balance, and is an important self-regulating behaviour for sustaining hydrological functioning of intact peatlands. Drainage of peatlands leads to subsidence caused by peat consolidation, shrinkage and oxidation, which results in a non-reversible lowering of the peat surface. In the Waikato, 80% of peatlands have been drained and are now under agricultural and horticultural land uses, with subsidence rates around 20 mm y-1. Ongoing monitoring of subsidence is challenged by a lack of knowledge of PSO behaviour. This study is the first of its kind to investigate PSO in drained peatlands at a regional scale. This study investigated peat properties and hydrological and PSO regimes of the eleven study sites that comprise a Waikato regional drained peatland monitoring. These sites were under three different land uses, being dairy and dry stock farms, and blueberry orchards. Peat soil physical and chemical properties were used to determine the effect land use has on both peat soil properties and PSO. PSO was measured using the paired pressure transducer method and soil cores were taken from each site to measure bulk density, volumetric moisture content (VMC), porosity, shrinkage characteristics, carbon to nitrogen ratios (C/N), organic matter fractions (OMf), with additional grab samples taken for pH analysis. There were general trends of decreasing pH and bulk density, and increasing porosity, C/N and OMf with increasing depth. The agricultural sites (dairy and dry stock) had greater degradation than the blueberry sites in the near surface peat but there was less of a difference in peat properties between land uses deeper in the profile. All of the study sites had higher bulk densities, and lower porosities and OMf than commonly reported in intact peatlands. These results show that land management can have a major effect on near surface peat properties and can degrade peat further than just drainage alone. Reversible surface elevation changes were observed at all study sites, which followed a similar seasonal pattern but with different magnitudes of shrink and swell. There did not appear to be a relationship between land use or drainage history and magnitude of PSO. Although the seasonal surface elevation changes broadly followed changes in water table depth, there did not appear to be an association between magnitude of surface elevation change and depth to the water table, in contrast to PSO studies in wetlands. The magnitude of PSO was reduced at the majority of the sites compared to previous measurements in intact peatlands, but some sites were relatively large compared to international studies of drained peatlands. There was some evidence that subsidence was occurring at all of the study sites. The results of this study have provided more detail than previously given in the literature for short-term, seasonal and annual PSO regimes. Overall, this study found that PSO in drained peatlands is highly variable and the drivers of PSO are likely very complex. These results can be used to help further improve long-term monitoring of peat subsidence demonstrate the importance of high-resolution measurements for accurate monitoring of annual PSO cycles