title: The effects of irrigation on soil carbon and nitrogen stocks of pumice soils

authors: Millar, Jamie Alexandra

subjects: Carbon

abstract: The use of irrigation has become important for sustaining the production of food, fibre and biofuel. In New Zealand (NZ), irrigation of pastures is increasing in areas with seasonal water deficits, such as the Central North Island. Nationally, there was an estimated 794,443 ha of land under irrigation in 2017. A synthesis of the literature showed that soil C and N stocks response to irrigation was dependent on the initial soil organic matter content and climatic zone. However, most of the literature focused on arid/semiarid climate regimes where cereals and other crops were grown, with few studies conducted on pastoral land in temperate climates. However Mudge et al. (2017), measured a decrease in soil carbon (C) (6.99 t C ha⁻¹) and nitrogen (N) (0.58 t N ha⁻¹) to 0.3 m depth in irrigated grazed pastures relative to adjacent non-irrigated grazed pastures. Of their 34 paired sites across NZ, only seven were sampled from the Pumice Soil order, which was insufficient to determine whether this soil order was vulnerable to soil C and N loss.
The main objective of this thesis was to compare the total soil C and N stocks of irrigated and non-irrigated Pumice Soils. A second objective was to determine where there were any trends in changes in soil C and N in relation to irrigation duration.
Fourteen paired irrigated and adjacent non-irrigated sites were sampled in the Reporoa Basin, north of Taupo and soil C and N stocks were measured accounting for differences in soil bulk density. Most sites where sampled to 0.6 m depth, however, due to high water tables and buried soils, some sites were only sampled to 0.4 m or 0.5 m depth. One site was omitted from statistical analysis due to high residuals (F11-35) and was considered an outlier that potentially represented a poorly matched pair of sites. On average, for the remaining 13 paired sites, irrigated soils had significantly (P < 0.05) lower soil C stocks (5.93 t C ha⁻¹) to 0.2 m depth and lower N stocks (0.24 t N ha⁻¹) to 0.1 m depth in comparison to adjacent non-irrigated soils. When considering the full depth of the sampled soils (0-0.6 m), there were no significant changes in soil C and N stocks (P > 0.05). Using this coring method, I was able to detect a change of about 6.9 % in soil C stocks and a 4.8 % in soil N stocks. This study also found no evidence of a relationship between soil response to irrigation and duration of irrigation.
With the loss of soil C and N under irrigation comes potential release of CO₂ to the atmosphere and N leaching into surrounding water bodies. Based on the findings in this thesis, there is a need for further investigation on the effects of irrigation for different Soil Orders. This research will aid farmers in understanding their land management on-farm and inform them of their potential effects on greenhouse gas emissions on a national scale.

text: The effects of irrigation on soil carbon and nitrogen stocks of pumice soils The use of irrigation has become important for sustaining the production of food, fibre and biofuel. In New Zealand (NZ), irrigation of pastures is increasing in areas with seasonal water deficits, such as the Central North Island. Nationally, there was an estimated 794,443 ha of land under irrigation in 2017. A synthesis of the literature showed that soil C and N stocks response to irrigation was dependent on the initial soil organic matter content and climatic zone. However, most of the literature focused on arid/semiarid climate regimes where cereals and other crops were grown, with few studies conducted on pastoral land in temperate climates. However Mudge et al. (2017), measured a decrease in soil carbon (C) (6.99 t C ha⁻¹) and nitrogen (N) (0.58 t N ha⁻¹) to 0.3 m depth in irrigated grazed pastures relative to adjacent non-irrigated grazed pastures. Of their 34 paired sites across NZ, only seven were sampled from the Pumice Soil order, which was insufficient to determine whether this soil order was vulnerable to soil C and N loss.
The main objective of this thesis was to compare the total soil C and N stocks of irrigated and non-irrigated Pumice Soils. A second objective was to determine where there were any trends in changes in soil C and N in relation to irrigation duration.
Fourteen paired irrigated and adjacent non-irrigated sites were sampled in the Reporoa Basin, north of Taupo and soil C and N stocks were measured accounting for differences in soil bulk density. Most sites where sampled to 0.6 m depth, however, due to high water tables and buried soils, some sites were only sampled to 0.4 m or 0.5 m depth. One site was omitted from statistical analysis due to high residuals (F11-35) and was considered an outlier that potentially represented a poorly matched pair of sites. On average, for the remaining 13 paired sites, irrigated soils had significantly (P < 0.05) lower soil C stocks (5.93 t C ha⁻¹) to 0.2 m depth and lower N stocks (0.24 t N ha⁻¹) to 0.1 m depth in comparison to adjacent non-irrigated soils. When considering the full depth of the sampled soils (0-0.6 m), there were no significant changes in soil C and N stocks (P > 0.05). Using this coring method, I was able to detect a change of about 6.9 % in soil C stocks and a 4.8 % in soil N stocks. This study also found no evidence of a relationship between soil response to irrigation and duration of irrigation.
With the loss of soil C and N under irrigation comes potential release of CO₂ to the atmosphere and N leaching into surrounding water bodies. Based on the findings in this thesis, there is a need for further investigation on the effects of irrigation for different Soil Orders. This research will aid farmers in understanding their land management on-farm and inform them of their potential effects on greenhouse gas emissions on a national scale.

year: 2018

title: Tubular Carbonate Concretions from North Island, New Zealand: Evidence for Hydrocarbon Migration and the Subsurface Plumbing System of Cold Seeps

authors: Nyman, Stephanie Leigh

subjects: hydrocarbon migration

abstract: Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated.

In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements.

As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation.

The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general.

text: Tubular Carbonate Concretions from North Island, New Zealand: Evidence for Hydrocarbon Migration and the Subsurface Plumbing System of Cold Seeps Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated.

In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements.

As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation.

The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general.

year: 2009

title: A newly-available resource for historians of early New Zealand: The Marist Missionary Letters (1838-54)

authors: Jennings, William

abstract: The 7000 pages of primary documents of the Lettres reçues d’Océanie constitute a vast new source for the study of the Pacific during the period 1838-54. Published in 2009 after sixteen years of transcription, the Lettres are the collected correspondence of the first French Marist missionaries to New Zealand and other Pacific islands. In the words of Pacific historian Hugh Laracy, they are ‘the single most important foundational contribution to Pacific history in its fullest extent since J. C. Beaglehole’s magisterial editions of James Cook’s Journals. About 2000 of those pages are transcripts of letters written in New Zealand that open up a fresh perspective of life before the wars of the 1860’s. This paper will locate the early French Marist Māori mission within the context of New Zealand public history before exploring how the Lettres reçues d’Océanie can complement existing views of early colonial New Zealand. To better contextualise the correspondence, it would be useful to begin with a brief background of the Marists’ presence in New Zealand.

text: A newly-available resource for historians of early New Zealand: The Marist Missionary Letters (1838-54) The 7000 pages of primary documents of the Lettres reçues d’Océanie constitute a vast new source for the study of the Pacific during the period 1838-54. Published in 2009 after sixteen years of transcription, the Lettres are the collected correspondence of the first French Marist missionaries to New Zealand and other Pacific islands. In the words of Pacific historian Hugh Laracy, they are ‘the single most important foundational contribution to Pacific history in its fullest extent since J. C. Beaglehole’s magisterial editions of James Cook’s Journals. About 2000 of those pages are transcripts of letters written in New Zealand that open up a fresh perspective of life before the wars of the 1860’s. This paper will locate the early French Marist Māori mission within the context of New Zealand public history before exploring how the Lettres reçues d’Océanie can complement existing views of early colonial New Zealand. To better contextualise the correspondence, it would be useful to begin with a brief background of the Marists’ presence in New Zealand.

year: 2012