Canterbury Plains Research Articles

Soil macroaggregate-occluded mineral-associated organic carbon drives the response of soil organic carbon to land use change

Understanding land use effects on carbon sequestration in various soil fractions is vital to mitigating climate change and restoring soil functions. The objective of this study was to explore the effects of land use on soil organic carbon (SOC) fractions in different soil types. For this purpose, we studied the effects of long-term (>20 years) land use including dryland pasture (DP), irrigated pasture (IP) and irrigated cropland (IC) on SOC in water-stable aggregates, particle-size fractions, and their coupling relations at the surface soils (0–7.5 cm) in the Canterbury Plains, New Zealand. For each land use, three typical soil types with contrasting drainage levels (i.e. well drained Lismore soil, LIS; imperfectly drained Templeton soil, TEM; and poorly drained Waterton/Temuka soil, WAT) were selected. Macroaggregate-occluded mineral-associated organic carbon (M-MAOC) contributed to the majority of the total SOC difference and drove the response of SOC to land use change. On average, M-MAOC followed an order of IP > DP > IC. The effects of land use change from DP to IP and IC on M-MAOC varied, and these variations were dependent on soil type. The relative gain in M-MAOC with change in land use from DP to IP was the greatest in the well drained LIS soil, while both the relative and absolute loss in M-MAOC following the land use change to IC was the greatest in the poorly drained WAT soil. The interactive effects of managements (e.g. irrigation and cultivation) and soil type (e.g. soil water condition) on aggregate size distribution and macroaggregate-associated C concentration were important in explaining the responses of M-MAOC to land use change. This study advances the mechanistic understanding of total SOC dynamics in response to land use (changes) in different soil types. It also highlights the potential of M-MAOC to serve as a diagnostic fraction to reflect changes in total SOC, which may have application to global warming mitigation.

Limited role of present-day onshore freshwater recharge in the emplacement of offshore freshened groundwater in the Canterbury Bight, New Zealand

ABSTRACT Offshore freshened groundwater (OFG) represents a significant, yet underexplored, component of the global freshwater system as it is found across various continental margins. In this study, a 3-D geological and groundwater model was developed for the Canterbury Plains and Bight (New Zealand) to assess the role of present-day onshore recharge in the emplacement of OFG. Topographic and bathymetric, seismic reflection and well data have been integrated to construct an onshore-offshore geological model. The facies property distribution and associated hydrodynamic parameters were determined using stochastic modelling techniques. The geological reconstruction was imported within a variable density groundwater model. Simulations, under transient regime, were run until equilibrium conditions between the involved aquifer portion and the adjacent ocean have been reached. Our results show that the geological model accurately captures the stratigraphic and sedimentary features of the area, and that onshore recharge at present contributes to OFG emplacement up to 15 km from the coast, which is equivalent to 25% of its maximum extent. Onshore recharge during sea-level lowstands is therefore the dominant OFG emplacement mechanism in the Canterbury Bight.

Decision making in complex land systems: outline of a holistic theory of agency

ContextModels of human agency within research on land systems and landscapes do not fully account for social and cultural factors in decision making. Conversely, within social theory, parallel concepts of agency do not fully take biophysical and spatial factors into account. This calls for a synthesis of conceptual models addressing human decision making in land systems.ObjectivesThe review identifies parallels between social and ecological perspectives on humans as co-constituent parts of complex land systems. On this basis selected models of agency combining insights from social theory and land systems research are outlined and compared, and improved concepts are outlined.MethodsElements of agency in modern agricultural land systems are reviewed. A case study illustrating the application of agency concepts in an analysis of decision making among farmers on the Canterbury Plains (New Zealand) is presented. On this basis it is discussed how to improve understandings of human agency in land systems.ResultsThe review identifies and compares parallel conceptions of agency, practice and holism in landscape ecology and social theory. Taking the agency of farmers in contemporary agricultural landscapes as an example, theories currently used to characterise and interpret the agency of farmers are discussed and improvements considered. Potentials for improvement of current conceptual models are indicated and discussed, and an improved model of agency is suggested.ConclusionsBased on the review, the article presents an improved conceptual model of agency in land systems emphasizing the position of agents in social-ecological contexts of action.

Response of soil organic carbon stock to land use is modulated by soil hydraulic properties

Understanding the effects of land use on soil organic carbon (SOC) stock is important to develop practices that promote agricultural sustainability and mitigate climate change. The objective of this study was to explore whether the effects of land use on SOC stock differed with soil type and, particularly, with soil hydraulic properties. For this purpose, 90 sites from the Canterbury Plains, New Zealand, were sampled to a depth of 30 cm. The sampled sites consisted of long-term (>20 years) dryland pasture [DP], irrigated pasture [IP], and irrigated cropping [IC] covering three contrasting soil types with different soil drainage levels, including well drained Lismore (LIS) soil, imperfectly drained Templeton (TEM) soil, and poorly drained Waterton/Temuka (WAT) soil. On average, compared with DP, IP increased SOC and hot water extractable carbon (HWEC) stock at all depths; IC decreased SOC and HWEC at 0–15 cm. The results highlighted that the effects of land use change from DP to IP and IC on total SOC and HWEC stock significantly differed between soil types. The greatest gains in SOC and HWEC stocks following change from DP to IP were found in LIS soil; the greatest losses in SOC and HWEC stocks following change from DP to IC were found in WAT soil; and no significant differences in SOC stocks with land use changes were found in TEM soil. The results implied that the interaction between land use and soil type on SOC and HWEC stocks was associated with soil hydraulic properties (e.g. available water capacity and field capacity) that regulate the response of C inputs (net primary productivity) and outputs (microbial decomposition) to land use and management practices (e.g. irrigation and cultivation). Therefore, soil hydraulic properties should be taken into account in designing land use for the purpose of SOC sequestration.

The influence of multiple impedance contrasts on mHVSR site period estimates in the Canterbury Plains of New Zealand and implications for site classification

An experimental study of the fundamental period of vibration of geologic strata above multiple impedance contrasts is presented for the Canterbury Plains, a deep, complex sedimentary basin on the South Island of New Zealand. Microtremor Horizontal-to-Vertical Spectral Ratio (mHVSR) analysis was used at 152 locations to investigate the fundamental period of the strata above three significant impedance contrasts in the region: the Riccarton Gravel, the Banks Peninsula volcanic (BPV) rock, and the deep graywacke basement bedrock. Up to three mHVSR peaks were observed in many of the mHVSR curves, with the long-period (2–7 s) peaks controlled by the BPV and/or the underlying bedrock. As the thickness of the BPV increases, the response of the geologic strata above the basement bedrock was masked by the shallower BPV, effectively changing the definition of what constitutes the reference condition (i.e. engineering bedrock) for site response analyses. The mHVSR peaks matched the long-period amplification peaks observed in the response spectra calculated from ground motions recorded during the Canterbury Earthquake Sequence events, underscoring the importance of the identification and consideration of deep impedance contrasts in site classification.

Four new species of Craspedia (Compositae/Asteraceae, Gnaphalieae) from the South Island of New Zealand, all characterised by dark red-purple anthers

ABSTRACT In New Zealand, Craspedia (Compositae/Asteraceae, Gnaphalieae) has much more morphological variation than previously recognised, as demonstrated by suggestions of about 50 informally named putative species and varieties. As part of our research towards a revision of New Zealand Craspedia we have focused here on those plants that can be segregated from other New Zealand species of Craspedia by their dark red-purple anthers. We are describing the four species C. argentea, C. diversicolor, C. rugosa, and C. thinicola from the South Island of New Zealand as new to science. They are recognised based on a morphological assessment of scape, rosette leaf, inflorescence, floret, and achene characters following detailed herbarium, cultivation, and field studies. All four species are threatened. Each is known from a single locality only. Craspedia argentea, endemic to Mahaka Katia Scientific Reserve (Pisa Flats) in Central Otago, has rosette leaves with a dense, silvery, appressed to floccose lanate indumentum. Craspedia diversicolor was once more widespread in the Canterbury plains but is now restricted to Wakanui Gully in Canterbury, where only two plants were found recently. The colour of its rosette leaves varies from olive-green to grey, to dark red. Craspedia rugosa, known only from Cameron Fan, near Lake Heron, in Canterbury, can be distinguished from all other species of Craspedia by its rugose, dark-red-mottled rosette leaves. Craspedia thinicola is confined to the stable deflation hollows at Kaitorete Spit, adjoining Banks Peninsula in Canterbury. Its rosette leaves are covered with a dense, appressed, lanate indumentum. When in the open its leaves are appressed to the ground, with the upper rosette leaves strongly recurved. The formal taxonomic treatments for each species are provided, together with illustrations and descriptions of their morphology, distribution, habitat, phenology, etymology, and cytology, as well as an assessment of their conservation status.

A review of legume research and extension in New Zealand (1990–2022)

Legumes have underpinned transformational change on New Zealand sheep and beef farms over the last 30 years. This was through an emphasis on ewe nutrition based on lucerne or red clover dominant pastures, and increased use of subterranean and white clovers on uncultivatable hill country. Pre- and post-weaning lamb growth rates have increased, and enabled earlier slaughter of heavier lambs. The farm systems results include greater numbers of hoggets mated, higher lambing percentages and greater ewe efficiency (kg lamb weaned/kg ewe mated). Extension packages to support legume use have compared growth rates of resident and legume-based pastures, economic analyses of successful farms and management packages for the most appropriate legume in different environments. Over the same period, the dairy industry rapidly expanded in cow numbers and area onto flat irrigated land on the Canterbury Plains. The nitrogen deficiency of perennial ryegrass was overcome by a linear increase in nitrogen fertiliser use. Environmental concerns from this intensification has led to a legislated nitrogen cap of 190 kg/ha.year. This, coupled with a recent trebling in urea price, has returned attention to increasing the white clover content of these pastures. Nitrogen applications can be minimised by using diverse pastures sown with a legume, herb and <8 kg/ha of perennial ryegrass. Work on other legumes, including annuals and those with condensed tannins, has to date failed to increase their use in most pastoral settings, with the exception of the perennial lupin which is adapted to high-aluminium soils in the South Island High Country.

Predicting field capacity in undisturbed stony soils

An increasing number of studies around the world are showing that a long-held assumption that rock fragments (RFs) are inert with respect to water retention is incorrect. Yet very few pedotransfer functions (PtFs) account for water held by RFs or the effect RFs have on the water retention of the fine earth. The few PtFs that incorporate the water content (WC) of RFs have relied upon measurement methods that may not be representative of field conditions. This indicates a gap in research regarding the characterisation of the water holding behaviour of stony soils in situ using soil volumes that adequately represent the soil. We address this gap in research by developing PtFs that predict the field capacity WC of stony soils using soil water storage measurements from 52 pits excavated into stony soils on the Canterbury Plains, New Zealand. These soils comprise sediment derived from a Mesozoic hard sandstone. The soils at each site were watered to saturation, and then after two days of drainage (a proxy for field capacity), a 30 × 30 cm pit was excavated in 10 cm increments to a depth of 60 cm. Matric potential was measured in situ for each increment, and soil WC was calculated from samples taken back to the laboratory. Our results showed it was possible to accurately predict the field capacity WC of stony soils using only explanatory variables that could be easily measured or estimated from a minimalistic field survey. An existing PtF calibrated on NZ soils (logit PtF), which was constructed on the assumption that RFs had no effect on WC at FC other than reducing the volume of the fine earth, performed worse than our models. By modifying the logit PtF, we conclude that its poorer performance stems from its inability to account for deviations from 1) the matric potential it assumes for field capacity (−10 kPa), 2) water held by RFs, and 3) the effect of RFs on the water retention characteristics of the fine earth. Our results demonstrate that even the low porosity RFs measured in this study can significantly affect model performance, but by including two variables (depth and volumetric proportion of RFs) that are routinely measured or estimated in most soil sampling projects, it is possible to improve prediction accuracy in established models.

Contextualizing the relative importance of habitat connectivity for metapopulation persistence: A case study of a critically endangered fish

AbstractLocal population characteristics and habitat connectivity both have important influences on metapopulation persistence; however, the relative importance of each can vary depending on the ecological context, making it difficult to apply general ‘rules‐of‐thumb’ for conservation actions. This is particularly true in dendritic networks, where habitat connections are constrained.By constructing a metapopulation model that specifically accounts for the unique characteristics and ecological system of a target species, the relative influence of local population characteristics and habitat connectivity on metapopulation persistence can be assessed more accurately.This modelling approach was used to improve conservation outcomes for the Canterbury mudfish (kо̄waroNeochanna burrowsius), a critically endangered and poorly understood species that is endemic to the Canterbury Plains of Aotearoa/New Zealand. A spatially structured metapopulation model ofN. burrowsiuswas used to investigate the relationship between habitat patch connectivity, rate of population growth as limited by habitat quality (rconditional), and metapopulation persistence.N. burrowsiusmetapopulation persistence increased most strongly under increases inrconditional, but increases in habitat connectivity also had positive effects (94% metapopulation persistence for highly connected networks vs. 39% metapopulation persistence for poorly connected networks whererconditional = 1.3).Although rates of population growth were more influential than increases in habitat connectivity in this case study, both will be important to consider for effectively allocating conservation resources for metapopulations in dendritic networks, depending on what managers could effectively manipulate to enhance metapopulation persistence.

Seismicity Rate Change as a Tool to Investigate Delayed and Remote Triggering of the 2010–2011 Canterbury Earthquake Sequence, New Zealand

ABSTRACTCrustal earthquakes in low-strain-rate regions are rare in the human life span but can generate disastrous consequences when they occur. Such was the case in the Canterbury earthquake sequence that began in 2010 and eventually led to almost 200 fatalities. Our study explores this earthquake sequence’s origins by producing an enhanced earthquake catalog in the Canterbury Plains and Otago, South Island, New Zealand. We investigate seismicity rate changes from 2005 to before the 2010 Mw 7.2 Darfield earthquake. During this time, major subduction-zone earthquakes, such as the 2009 Mw 7.8 Dusky Sound earthquake, created measurable coseismic and postseismic strain in the region. We use template matching to expand the catalog of earthquakes in the region, and use a support vector machine classifier to remove false positives and poor detections. We then compare the newly obtained seismicity rates with the coseismic and postseismic crustal strain fields, and find that seismicity rate and crustal strain are positively correlated in the low-stress, low-seismicity region of the northern Canterbury Plains. In contrast, near fast-moving plate-boundary faults, the seismicity rate changes rise without much change in the strain rate. Our analysis reveals a substantial seismicity rate decrease in the western rupture area of the Darfield earthquake, which we infer to be an effect of coseismic and postseismic deformation caused by the Dusky Sound earthquake. We show in low-strain-rate regions, stress perturbation of a few kPas creates substantial seismicity rate change. However, the implication that such seismic quiescence is responsible for the nucleation of the Darfield earthquake requires further studies.

Variation in matric potential at field capacity in stony soils of fluvial and alluvial fans

Field capacity of soil is fundamental for many agronomic management decisions, as well as for hydrological and environmental models. Around the world, field capacity is assumed to occur at defined matric potentials, −10 kPa and –33 kPa being common criteria. However, these criteria have not been tested in fluvial and alluvial fan stony soils (FAFSS). In this project, 57 pits on FAFSS located on the Canterbury Plains were watered to near saturation. After two days of drainage (proxy for field capacity), a 30 × 30 cm pit was excavated in 10 cm increments to a depth of 60 cm for 52 of the pits. At each increment, matric potential was measured. For the remaining 5 pits, matric potential was measured after 4–5 days to a depth of 1.5 m. Matric potential was generally higher than −10 kPa and the matric potential-depth profile of the pits could be characterised by one of five modes. The most common mode was hydrostatic equilibrium, which is normally associated with soils that have a shallow water table (~<2 m). Our results indicate that a capillary break due to a coarse sandy gravel layer at ~ 1 m depth causes a near zero matric potential boundary condition that allows hydrostatic equilibrium to occur regardless of the depth to groundwater. This capillary break was linked to two soil conditions: a layer of open framework gravels or soil fines with specific surface area < 15 m2g−1. Our results provide a basis for better approximations of the matric potential at field capacity, and improved modelling of soil water behaviour in FAFSS.

Investigations on the luminescence properties of quartz and feldspars extracted from loess in the Canterbury Plains, New Zealand South Island

Investigations on the luminescence properties of quartz and feldspars extracted from loess in the Canterbury Plains, New Zealand South Island

The influence of rock fragments on field capacity water content in stony soils from hard sandstone alluvium

Worldwide, rock fragments (RFs) are generally considered inert with respect to bulk soil hydraulic properties, such that all soil water retention properties predicted by national pedotransfer functions (such as S-map) are based on the volumetric fraction of the fine earth (<2 mm fraction) only. Research findings contradict those assumptions, but studies commonly focus on porous RFs, and rely on repacked cores and lab studies, leaving uncertainty as to how low porosity RFs characteristic of common strongly indurated lithologies affect soil in the field. We address this question by examining soil water storage in 52 pits excavated into stony soils on the Canterbury Plains, New Zealand, which are formed in sediment derived from a Mesozoic hard sandstone. The soils at each site were watered to saturation, and then after two days of drainage (a proxy for field capacity), a 30 × 30 cm pit was excavated in 10 cm increments to a depth of 60 cm. From each increment, soil samples were collected and analysed to determine the volumetric size distribution of RFs, the water content of the fine earth and the water content of the RFs themselves. Our results indicated that RFs could influence the fine earth bulk density, porosity, and soil chemistry. RFs could also retain water: 2–20 mm RFs (0.07 m3 m−3) retained twice as much water as >20 mm RFs (0.03 m3 m−3). The water retention of the hard sandstone was low compared to other lithologies, but the volumetric abundance of RFs in the stony soils we sampled meant that they accounted for ~10% of the water retained to a depth of 60 cm at field capacity. Our results demonstrate that ~13 mm of water retained by RFs at field capacity is not currently considered in water budgets and nutrient leaching predictions, which may be relevant to best practice land management.

Sustained Changes in Well Water Levels Following a Large Earthquake: Possible Evidence of Permeability Decreases in a Shallow Groundwater System

AbstractObservation of earthquake‐induced changes in well water levels provides an opportunity to study the effects of seismic activity on the groundwater system. In this study, we used data from a well network in the alluvial Canterbury Plain of New Zealand’s South Island to document sustained and complex changes in well water levels following the 2010 Darfield earthquake. Sustained increases in well water levels in midstream areas, as well as sustained decreases in downstream areas, resulted from decreases in permeability. These decreases in permeability were possibly caused by consolidation of liquefied sediments following the earthquake.

Investigations on the Luminescence Properties of Quartz and Feldspars Extracted from Loess in the Canterbury Plains, New Zealand South Island

Abstract The applicability of the single-aliquot regenerative-dose (SAR) protocol, by using the optically stimulated luminescence (OSL) signal of quartz as well as the post-infrared–infrared (pIRIR) signals of polymineral fine grains, namely pIRIR225 and pIRIR290, was assessed for dating loess in New Zealand South Island. OSL signals of quartz grains displayed low sensitivity. However, the application of repeated irradiation/bleaching cycles did not result in an increase in sensitivity; annealing in the 300–500°C temperature range generated the sensitisation of both the 110°C thermoluminescence (TL) peak as well as the OSL signal, likely by activation of yet unidentified luminescence centres. After heating, the quartz signal is comparable to that of ideal samples, but the annealing is precluding successful dating. On the other hand, feldspar infrared-stimulated signals displayed satisfactory properties, allowing estimation of ages ranging from 14 ± 1–29 ± 3 ka for the investigated deposit. It was shown that pIRIR225 and pIRIR290 methods have potential for dating loess in the South Island of New Zealand, based on the following observations: (i) Dose recovery tests were successful with recovered-to-given dose ratios with a &lt;10% deviation from unity, (ii) constant residual values of about 4 Gy and about 10 Gy were obtained after exposures for 48 h in the case of pIRIR225 signals and 96 h in the case of pIRIR290 signals, respectively, (iii) while a slight dose-dependence of the residual was reported, and for a dose as large as 1600 Gy the residual values are ≅9 Gy and ≅19 Gy for pIRIR225 and pIRIR290 signals, respectively.

New Zealand as a source of mineral dust to the atmosphere and ocean

The chemical and isotopic compositions of sediments and dust can be used to trace their provenance, providing insights into many Earth surface processes. During past glacial climates, much of the New Zealand (NZ) South Island was blanketed by erosive glacier systems that produced large volumes of sediment. We estimate the expansion of glacial outwash plains based on a sea level lowering of 130 m at the Last Glacial Maximum (LGM), and find that the Canterbury Plains in the central South Island likely expanded by 30,000 km2, a nearly five-fold increase, while the Southland/southern Otago region may have extended southward to cover an additional ∼45,000 km2, an eight-fold increase of the coastal plain area. Considering NZ’s extreme uplift and erosion rates (∼10 m kyr−1), the South Island, though limited in extent compared to larger Southern Hemisphere landmasses, may serve as an important dust source to the high-latitude atmosphere and ocean.To facilitate accurate tracing of the extent of aeolian and oceanic transport of NZ dust, this study presents major/trace element and Sr-Nd-Pb isotope ratios on sediments from the major present-day dust and sediment producing regions of the South Island. The sediment compositions strongly reflect the regional geology. For example, compared to the central South Island, Nd isotope ratios in the southern South Island are more variable and show younger crustal residence ages. The combined Sr-Nd-Pb isotopic ratios show that the central NZ South Island can be distinguished geochemically from many other Southern Hemisphere dust sources. Although isotopic similarities between the central NZ South Island and more northerly regions of South America, including Central Western Argentina and the Puna-Altiplano Plateau, and Kati Thanda-Lake Eyre in Australia (based on new data in this study) hinder downstream source attribution, a key finding is that these isotopes successfully discriminate NZ from other locations in Australia, such as the Murray-Darling Basin, Northern Territory, and Western Australia, as well as southern Africa and regions of South America south of ∼37°S (Patagonia and Tierra del Fuego). A comparison of the NZ data with East Antarctic ice core dust samples indicates that NZ was not a significant dust supplier to East Antarctica; rather, the East Antarctic dust compositions can be explained by dust supplied by Patagonia, Tierra del Fuego, and Central Western Argentina in South America, and West Antarctic Rift System volcanism. In contrast, the compositions of marine sediments in the Pacific sector of the Southern Ocean are compatible with mixing of South and North Island NZ dust sources, consistent with NZ’s role as an active dust supplier to the Southern Ocean. New data from Kati Thanda-Lake Eyre in Australia show that it may also be a contributor, if dust from this region is able to reach the Pacific sector independent of other Australian sources.

Unintended consequences to groundwater from improved irrigation efficiency: Lessons from the Hinds-Rangitata Plain, New Zealand

In many countries, including New Zealand, limits on freshwater availability have led to a push toward improved irrigation efficiency via changes in irrigation application technologies, so as to increase the ‘crop per drop’. In this paper, we provide evidence of unintended consequences to groundwater arising from improved irrigation efficiency in parts of the Canterbury Plains, New Zealand. We compare groundwater levels, groundwater nitrate concentrations and stable isotope values from before and after the change in irrigation efficiency. We show that changes in irrigation methods from low efficiency border-dyke irrigation to high efficiency sprinkler irrigation has resulted in reduced recharge to groundwater and reductions in groundwater levels. Groundwater level reductions of up to 17 m have occurred in some areas. The reduced groundwater levels have impacted spring flows and required groundwater users to drill new wells to access the now deeper groundwater. The reduction in irrigation recharge has also likely contributed to increased nitrate concentrations in groundwater. In response to the unintended consequences documented in this study, managed aquifer recharge is being implemented as a means of augmenting groundwater levels, quality, and spring-fed waterways. As such, increased irrigation efficiency may not be reducing water use in this region. A regional water balance would be required to determine if water savings are occurring overall.

Woodchip denitrification wall technology trialled in a shallow alluvial gravel aquifer

Woodchip denitrification walls are a tried and tested groundwater nitrate remediation concept in shallow sandy aquifer conditions. There are however no published case studies of them having been applied in heterogeneous, fast-flowing gravel aquifers. Such a pilot study is being made in a shallow alluvial gravel aquifer on the Canterbury Plains, New Zealand, as part of an assessment of whether denitrification walls represent a viable edge-of-field nitrate mitigation option for the New Zealand hydrological landscape. Hydrogeological conditions at the field study site were characterised using a suite of investigative methods, the results from which informed design and placement of an experimental woodchip denitrification wall that was installed in November 2018. The average specific flux in the target gravel aquifer is estimated at 2.7 m/d, and 3.1 m/d through the woodchip wall itself, owing to its hydraulic efficiency. These groundwater fluxes are significantly higher than conditions reported for pre-existing denitrification wall case-studies. Monitoring of the groundwater chemistry over the first year of the denitrification woodchip wall's operational life has shown how the woodchip initially leached labile dissolved organic carbon and created a redox plume in which methanogenic conditions existed. Even though dissolved organic carbon concentrations have restored to background levels, the woodchip wall remains effective at nitrate reduction. The measured nitrate removal rate of between 4.2 and 5.4 g N removed/m3 wall/d is higher than what had previously been predicted from controlled lab-scale studies of the wall media and ranks towards the higher end of published removal rates for denitrification walls. Whilst there is direct evidence that heterotrophic denitrification is contributing to the observed nitrate removal, on the basis of chemical indicators, it is assumed other reactive process, such as dissimilatory reduction to ammonia, anammox, and possibly nitrate-reducing Fe(II)-oxidising reduction reactions may also be contributing to the overall removal of nitrogen in the system. Indications are the woodchip wall is enhancing emission of methane gas, albeit at rates less than what is typically reported for constructed wetlands that are an alternative nitrate-remediation option. Emission of the more potent greenhouse gas nitrous oxide from the woodchip denitrification wall has so far been immeasurably low. Longer-term study of the woodchip denitrification wall is continuing.

Shifts in population size structure for a drying‐tolerant fish in response to extreme drought

AbstractFor freshwater systems, climate change‐induced alterations to drought regimes are a considerable threat to already threatened species. This is particularly poignant for kōwaro (or Canterbury mudfish, Neochanna burrowsius), a critically endangered fish largely restricted to drying‐prone waterways on the Canterbury Plains, New Zealand. By comparing three catchment‐wide surveys (2007, 2010, 2015) within the Waianiwaniwa Valley, we assessed the scale and magnitude of population change induced by 2 years of consecutive drought (2014–15), when compared to surveys during wetter conditions (2007, 2010). The droughts triggered a catchment‐wide switch from adult‐dominated populations to populations comprised of juveniles indicated by a significant reduction in median size (~95 mm during the wet to ~60 mm after drought). In comparison, population abundances were highly variable, indicated by no significant change in catch‐per‐unit‐effort. The large variation in catch rates and connection of median size to reproductive potential mean median size will be useful to measure in monitoring to infer potential changes to population resilience, particularly during extreme events. Furthermore, because N. burrowsius could be regarded as extremophile fish, already restricted to harsh habitats, they are likely to become increasingly threatened by climate change. Thus, tools that allow for insightful comparisons between populations, such as a population resilience framework based on both abundance and body size distribution, will be increasingly important for pragmatic decision‐making for targeted conservation measures.

Settlers of the Marsh: settler desire and its vicissitudes

ABSTRACTFrederick Philip Grove’s Settlers of the Marsh (1925) is a settler-colonial farm novel. Grove’s novel is an exemplary Canadian instance of a mode of fiction which attended the ‘settler revolution’ – the agricultural colonisation of temperate range lands in North and South America, Eurasia, Australasia and Southern Africa in the second half of the nineteenth century and first half of the twentieth. The defining feature of this iconic form of settler colonialism was the presence of what the Canadian historian Peter A. Russell terms a ‘large scale open land frontier’. The settler farm – whether it was in the South African High Veld, the Western Australian wheatbelt, the prairies of Manitoba or the Canterbury Plains of New Zealand – was the main instrument in a mode of colonisation in which seized indigenous land was made available to mainly European immigrants for agricultural enterprise. In the literary sphere, the particular aura and ideological valency of the pioneer farm find expression in the appearance of novels that dramatise the taking up of land by settlers in these frontier communities. This essay uses Grove’s Settlers of the Marsh to trace how the particular forms of desire mobilised by settler colonialism are modulated in the settler-colonial farm novel.