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

authors: Layton, Helena Georgette

subjects: Peat soils

abstract: 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.

text: 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.

year: 2022

title: A continuous 5300-yr Holocene cryptotephrostratigraphic record from northern New Zealand and implications for tephrochronology and volcanic-hazard assessment.

authors: Gehrels, Maria J.

subjects: crytotephra

abstract: A continuous  5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.

text: A continuous 5300-yr Holocene cryptotephrostratigraphic record from northern New Zealand and implications for tephrochronology and volcanic-hazard assessment. A continuous  5280 calendar (cal.) yr long cryptotephrostratigraphic record of a peat core from northern New Zealand demonstrates that cryptotephra studies can enhance conventional tephra records by extending the known distribution of ash fall and enabling re-assessment of volcanic hazards. A systematic sampling strategy was used to locate peaks in glass-shard concentrations and to determine loci of individual geochemical populations, and a palynological method involving spiking samples with Lycopodium spores was adapted to facilitate accurate counting of glass-shard concentrations. Using glass shard major element compositions, and a core chronology based on eight AMS 14C ages and two visible macroscopic tephra layers, Taupo Tephra (Unit Y) (1688-1748 cal. BP) and Tuhua Tephra (6800-7230 cal. BP) (2cr-age ranges), four cryptotephras were correlated with known eruptions: Whakaipo (Unit V) (2743-2782 cal. BP), Stent (Unit Q) (4240-4510 cal. BP), and Unit K (4970-5290 cal. BP), erupted from Taupo Volcanic Centre, and Whakatane Tephra (5470-5600 cal. BP) erupted from Okataina Volcanic Centre. Mixed glass populations were found in the core, most likely an artefact of post-depositional remobilization of shards vertically (both up and down) in the peat or on its surface by wind, or a result of closely spaced eruption events, or a combination of these. A secondary glass population identified within the macroscopic Taupo Tephra was tentatively attributed to either an earlier phase within that eruption or to mixing with a slightly older Taupo-derived eruptive or (less likely) a currently unknown Okataina-derived eruptive. These results indicate that, in the absence of continuous cryptotephrostratigraphic analysis, a peak in shard concentrations may not in itself be indicative of the ‘true’ stratigraphic (ie, isochronous) level of a tephra layer. For cryptotephra studies of peat cores, we recommend (1) using a detailed sampling strategy for the analysis of distal tephra-derived glass to detect and account for any mixed populations and possible vertical spread of glass shards through the peat, and (2) analysing more shards from larger samples to help ‘capture’ sparsely represented cryptic andesitic tephra deposits.

year: 2006-02

title: Implications and opportunities for Maori land in the Rotorua catchment

authors: Tahana, Arapeta

abstract: Arapeta is the Portfolio Manager, Waiariki Region, at Te Tumu Paeroa. He works with Maori land owners to enable them to achieve higher aspirations for their land and assets. He also collaborates with a wide range of other people and organisations, from owners in Māori land blocks, to government agencies, private sector companies and experts in many fields to find innovative and pragmatic solutions to the challenges of growing Māori. He is an elected member on the Māori Advisory Committee to RDC working to develop a meaningful partnership between Te Arawa and the Council.

text: Implications and opportunities for Maori land in the Rotorua catchment Arapeta is the Portfolio Manager, Waiariki Region, at Te Tumu Paeroa. He works with Maori land owners to enable them to achieve higher aspirations for their land and assets. He also collaborates with a wide range of other people and organisations, from owners in Māori land blocks, to government agencies, private sector companies and experts in many fields to find innovative and pragmatic solutions to the challenges of growing Māori. He is an elected member on the Māori Advisory Committee to RDC working to develop a meaningful partnership between Te Arawa and the Council.

year: 2013