Zealand Soils Research Articles

Rapid soil attribute evaluation for soil security assessments in data-poor environments in the Pacific region

Many global environments face increasing pressures on soil resources, and effective, scalable methods for assessment of soil condition and capital are essential to respond to tangible soil threats. This situation is common across Pacific Island Countries and Territories (PICTs), where high throughput soil analysis laboratories are limited, and issues such as soil organic carbon decline, acidification and fertility declines are present. Soil spectral inference presents an opportunity in such regions to provide rapid insights into soil capital and condition, though the need for robust calibration libraries remains a limiting factor. This work investigates the utility of a regionally appropriate spectral library, the New Zealand Soil Spectral Library (NZSSL) to support the development of soil spectral inference in data-poor environments, such as PICTs, through a case study on the island of Tongatapu in The Kingdom of Tonga. We contrast the performance of existing partial least squares regression (PLSR) models developed for New Zealand soils on soils from Tongatapu and explore the opportunities for enhancement of predictions formed through memory-based learning (MBL) supplemented with local data. Our work shows the potential for cost-effective and timely soil monitoring through soil spectral inference in PICTs. The work further underscores the importance of regional cooperation and data-sharing for addressing soil security.

An Improved Pedotransfer Function for Soil Hydrological Properties in New Zealand

This paper describes a new pedotransfer function (PTF) for the soil water content of New Zealand soils at seven specific tensions (0, −5, −10, −20, −40, −100, −1500 kPa) using explanatory variables derived from the S-map soil mapping system. The model produces unbiased and physically plausible estimates of the response at each tension, as well as unbiased and physically plausible estimates of the response differences that define derived properties (e.g., macroporosity and total available water content). The PTF is a development of an earlier model using approximately double the number of sites compared with the earlier study, a change in fitting methodology to a semi-parametric GAM Beta response, and the inclusion of sample depth. The results show that the new model has resulted in significant improvements for the soil water content estimates and derived quantities using standard goodness-of-fit measures, based on validation data. A comparison with an international PTF using explanatory variables compatible with variables available from S-map (EUPTF2) suggests that the model is better for prediction of soil water content using the limited information available from the S-map system.

Derivation of physically based soil hydraulic parameters in New Zealand by combining soil physics and hydropedology

AbstractField‐characterised soil morphological data (to 1 m depth) and modelled soil water release characteristics are recorded in the S‐map database for soils covering approximately 40% of New Zealand's soil area. This paper shows the development of the Smap‐Hydro database that estimates hydraulic parameters by synergising soil morphologic data recorded in S‐map and soil physics. The Smap‐Hydro parameters were derived using the bi‐modal Kosugi hydraulic function. The validity of the Smap‐Hydro parameters was tested by applying them within an uncalibrated physically based hydrological model (HyPix) and comparing results with soil water content, θ, measured with Aquaflex soil moisture probes (0–40 cm deep) at 24 sites across New Zealand. The HyPix model provided an excellent fit with observed soil water content for 25% of the sites, a good fit for 33% of the sites and a poor fit for 42% of the sites. Applying the model to all soils in the S‐map database required adjustments for the occurrence of rock fragments, hydraulic discontinuities caused by soil pans and required the addition of boundary conditions for water tables and the occurrence of impermeable rock. A discussion on how we can further synergise the development of pedotransfer functions with knowledge of soil physics is provided.

A soil spectral library of New Zealand

Diffuse reflectance spectroscopy, both in the visible and near-infrared (vis-NIR: 350–2500 nm) and the mid-infrared (MIR: 2500–25,000 nm) ranges, has been increasingly employed as an alternative to obtaining a multitude of soil information and data non-destructively, rapidly, and cost-effectively. Soil spectral libraries (SSLs) have been developed across the world to calibrate prediction models for rapid and non-destructive assessment of soils at local, regional, national and global scales. Several continental or national libraries exist, for example, vis-NIR SSL in Europe, Australia, Czech Republic, Denmark, France, China and Brazil, and MIR SSL in the U.S., Switzerland and Brazil. In this study, we document the development of a vis-NIR and MIR SSL of New Zealand based on legacy and modern samples that have been sieved to 2 mm and air-dried, and explore the application of such libraries for predicting a wide range of soil properties, including soil carbon (total carbon or organic carbon), total nitrogen, pH, soil texture (sand, silt and clay), phosphate retention and available water capacity (field capacity and permanent wilting point). The vis-NIR and MIR spectral features were compared between New Zealand Soil Classification (NZSC) soil orders and the performances of both vis-NIR and MIR SSLs were compared with other national SSLs. Results of the study indicate that both vis-NIR and MIR SSL demonstrated reliable modelling performance using the PLSR model. However, MIR outperforms vis-NIR for all soil attributes. Furthermore, the study found that both vis-NIR and MIR have the potential to distinguish different soil orders within the NZSC.

An Indigenous-assisted cultural perspective on conservation of New Zealand soils and the biota dependent on them

Soil habitat quality is an important indicator of environmental health. New Zealand soils are in critical condition because of various land-use practices, such as intensification of agriculture and urban growth, causing increased erosion and loss of fertile soil. Soil consists of organic (carbon-rich, microbe-laden detritus) and inorganic (weathered rock particles, minerals, water, air) components that are vital for sustaining flora and fauna, including humans. Soil in and of itself is alive with various forms of biota. Soil conservation takes an anthropocentric, utilitarian approach to policy and planning, whereas soil preservation takes an ecocentric or nature-centred approach. When the catchment boards of the earlier eras [governed by the Soil Conservation and Rivers Control Act 1941 (NZ)] were decommissioned to make way for regional councils around the time when the Resource Management Act 1991 (NZ) came into force, the soil scientific expertise was largely lost. At the same time, soil quality in New Zealand has been deteriorating ever since. In this paper, I argue for the establishment of community-based soil conservancies and conservancy officers and, ultimately, the legal endowment of personhood on soil in order for kaitiaki oneone (soil guardians) to be appointed. These measures will assist in the protection of soils, which will lead to better outcomes for the environment and generations to come.

Soil moisture is a primary driver of comammox Nitrospira abundance in New Zealand soils

The objectives of this study were to investigate the abundance and community composition of comammox Nitrospira under: (i) pasture-based dairy farms from different regions, and (ii) different land uses from the same region and soil type. The results clearly showed that comammox Nitrospira were most abundant (3.0 × 106 copies) under the west coast dairy farm conditions, where they were also significantly more abundant than canonical ammonia oxidisers. This was also true in the Canterbury dairy farm. The six land uses investigated were pine monoculture, a long term no input ecological trial, sheep + beef and Dairy, both irrigated and non-irrigated. It was concluded that comammox Nitrospira was most abundant under the irrigated dairy farm (2.7 × 106 copies). Contrary to the current industry opinion, the relatively high abundance of comammox Nitrospira under fertile irrigated dairy land suggests that comammox Nitrospira found in terrestrial ecosystems may be copiotrophic. it was also determined that comammox Nitrospira was more abundant under irrigated land use than their non-irrigated counterparts, suggesting that soil moisture is a key environmental parameter influencing comammox abundance. Comammox abundance was also positively correlated with annual rainfall, further supporting this theory. Phylogenetic analysis of the comammox Nitrospira detected determined that 17 % of the comammox community belonged to a newly distinguished subclade, clade B.2. The remaining 83 % belonged to clade B.1. No sequences from clade A were found.

Low copper levels measured in the aortic wall of New Zealand patients with non-syndromic ascending thoracic aortic aneurysm.

OBJECTIVESStudies in animals have shown causal relationships between copper (Cu) deficiency and the development of thoracic aortic aneurysms (TAAs) [1, 2]. Cu deficiency is widespread in New Zealand (NZ) soils; the high soil pH from the use of lime fertilizers reduces the bioavailability of Cu for grazing animals and growing plants; this, in turn, reduces Cu availability in the NZ human food chain. Our study is a pilot study to explore associations between Cu and TAA. We measured Cu levels in aneurysmal aortic tissues in patients undergoing Bentall procedures and non-aneurysmal aortic tissue from coronary artery bypass graft patients.METHODSAortic samples were collected from 2 groups of patients during elective open-heart surgery over 4 months between November 2017 and February 2018. The groups were a TAA group, patients with non-syndromic aortic aneurysm and without the bicuspid aortic valve or known infectious or inflammatory condition (ANEURYSM; n = 13), and a control coronary artery bypass graft group (CONTROL; n = 44). Standardized digested dry tissue weighed samples were analysed from both groups. Tissue extraction of trace elements was carried out using HCl-H2O2 digestion and a highly sensitive analytical technique, inductively coupled plasma mass spectrometry—used to measure elemental concentrations.RESULTSCu concentration (mean ± SD) was significantly lower in ANEURYSM (3.34 ± 0.16 µg/g) when compared to the CONTROL group tissues (4.33 ± 0.20 µg/g) (dry weight; mean ± SD; Student's t-test, P < 0.05). Over 46% of the Aneurysm patients were Maori and live in a geographically Cu-deficient NZ territory.CONCLUSIONSCu deficiency may play a role in the development or progression of non-syndromic ascending aortic aneurysms in NZ. Maori patients are more at risk as they commonly live in rural NZ, dependent on locally grown nutritional sources. Further studies are required to confirm this exciting finding and to establish cause and effect relationship.

Wetting and drainage cycles in two New Zealand soil types: Effects on relative gas diffusivity and N2O emissions

Nitrous oxide (N2O) is a potent greenhouse gas generated in agricultural soils by microbial processes that vary according to soil redox. Soil oxygen (O2) supply and demand strongly influence soil redox. Migration of O2 into the soil primarily occurs via gas diffusion, expressed as relative gas diffusivity (Dp/Do), and is influenced by soil structure (air-filled porosity and tortuosity of pores) and soil water content. Soil N2O emissions have been shown to increase at low values of Dp/Do but detailed studies examining the relationship between Dp/Do and soil N2O emissions remain limited, with relatively few soil types examined, and no studies of repeated wetting-drainage cycles. Thus, the objectives of this study were to examine how successive wetting-drainage cycles affected both Dp/Do dynamics and associated N2O emissions in two New Zealand soils; a pallic silt loam and an allophanic loam, with the latter also having a higher organic matter content. Soil cores, repacked to varying density, were wetted up with 15N enriched NO3− solution and placed on tension tables where they underwent two consecutive 12-day wetting-drainage cycles from saturation to field capacity (0 to −10 kPa). Over time measurements were made of N2O, N2, inorganic-N and soluble carbon, while Dp/Do was modelled using soil physical characteristics. For both soils each wetting-drainage cycle induced N2O fluxes but with 5-fold lower fluxes in the allophanic soil. Greater aggregation and sand content in the allophanic soil generated higher porosity and Dp/Do values that were almost always greater than recognized anaerobic limits. Thus, wetting-induced N2O fluxes observed in the allophanic soil during early drainage were concluded to result from anaerobic or hypoxic pathways of N2O production potentially within the intra-aggregate zone. While wetting-drainage events induce N2O emissions by altering Dp/Do and the soil aeration status, the draining of soils, especially soils high in organic matter, may enhance O2 demand generating anaerobic zones conducive to denitrification. Further detailed studies examining the interaction between soil structure and soil organic matter content and their effect on N2O emissions under wetting-drainage events, with measures of soil O2, are needed.

Soil Moisture is a Primary Driver of Comammox Nitrospira Abundance in New Zealand Soils

Soil Moisture is a Primary Driver of Comammox Nitrospira Abundance in New Zealand Soils

Sensitivity of organic matter mineralisation to water availability: role of solute diffusivity and the ‘Birch effect’

Context Several functions are used to describe the effects of soil water content on organic matter mineralisation. A meta-analysis of published studies identified relative water content (RWC; available water relative to the soil’s available water holding capacity) as the best water descriptor for N mineralisation. Aims To evaluate RWC as a predictor of C and N mineralisation in New Zealand soils; and to investigate how solute diffusivity and the ‘Birch effect’ may help to explain this relationship. Methods Three agricultural soils (0–15 cm), differing in water holding capacity were incubated (8-week; 20°C) under a range of RWCs to measure carbon (respiration) and net N mineralisation. After 4 weeks, a subset of samples from each treatment were re-wetted to field capacity for a further 4-weeks to quantify the respiration response to re-wetting. Key results For all three soils, there was a linear relationship between respiration and RWC where the C respired at the wilting point (RWC = 0) was ∼25–30% of that at field capacity (RWC = 1.0). Results from a solute diffusivity model suggested that a decrease in microbial substrate supply, owing to restricted diffusion of dissolved organic compounds, contributed to moisture-induced decline in respiration. A respiration flush was not observed when RWC was &gt;0 at re-wetting. Nitrogen mineralisation was non-linearly related to RWC, with small decreases in RWC below 1.0 (optimum) having a greater effect on N, than C, mineralisation. Conclusions RWC may be a reliable ‘water modifier’ to describe the influence of soil moisture on respiration. Further work is recommended to verify the RWC vs net N mineralisation relationship observed in this study.

Factors associated with suppression of Fusarium basal rot of onion in New Zealand soils: literature review and greenhouse experiments

ABSTRACT Fusarium basal rot, caused by Fusarium oxysporum f.sp. cepae, is an important onion disease worldwide. Greenhouse pot experiments were conducted during two growing seasons to determine if field soils from New Zealand’s main onion growing regions had different fusarium basal rot suppressive capabilities, and identified factors involved in disease suppression This study demonstrated that soils naturally suppressive to fusarium basal rot were present in New Zealand and that soil organic matter content, total soil carbon, total nitrogen and soil phosphorous concentrations were associated with disease incidence. Soil microorganisms were shown to be involved in disease suppression since when they were eliminated or reduced by soil heat treatment, fusarium basal rot incidence markedly increased. This study provided an increased understanding of how soil physical, chemical and biological factors affect the suppressiveness of selected New Zealand soils to fusarium basal rot of onion, and demonstrated that certain bacterial and fungal taxa may be involved in suppression of this disease.

Contributions of soil organic carbon to soil water repellency persistence: Characterization and modelling

Soil water repellency (SWR) affects hydrological processes such as water infiltration, preferential flow, run-off, and evapotranspiration. Understanding the relationship between SWR and volumetric soil water content (θ) is critical for predicting hydrological processes in hydrophobic soils. In this study, water droplet penetration time (WDPT) was used to investigate the SWR persistence (SWRP) in three soil types with contrasting drainage levels, namely a well-drained Lismore (LIS) stony silt loam, a moderately well-drained Templeton (TEM) silt loam, and a poorly drained Waterton/Temuka (WAT) clay loam under three long-term (>20 years) land uses (irrigated pasture [IP], dryland pasture [DP], and irrigated crop [IC]) in Canterbury, New Zealand. The three soils belong to different soil orders (i.e., Brown, Pallic, and Gley, respectively) in the New Zealand soil classification. Within each land use, 9, 12, and 9 sites were sampled in the LIS, TEM, and WAT soils respectively. There was strong evidence of SWR in all three soil types under three land uses. The maximum SWRP (WDPTmax), ranging from 6 to 1470 s with the median value of 56 s, differed with land use by following descending order of IP > DP > IC. The effects of land use on SWRP differed with soil type. Compared with other two soils, LIS soil had higher differences in WDPTmax between IP and DP, but lower differences in WDPTmax between DP and IC. Most of the SWRP parameters were significantly correlated with soil organic carbon (SOC) content. In soils with higher SOC content, SWR usually developed under drier conditions and declined as soils were maintained under wetter conditions. This study suggests that the WDPTmax appears at soil water contents of 0.17 to 0.19 cm3 cm−3 (equivalent to 0.15 to 0.16 g g−1). Soil water repellency persistence was positively related to SOC content. Soils with SOC content < 2% tended to be wettable, while soils with SOC content > 4% tended to be water-repellent. A Gaussian model with three physically meaningful parameters was developed to successfully fit the SWRP curve (SWRPC, LnWDPT as a function of θ). Our study also showed the possibility to predict the model parameters and SWRPC using SOC content. The model needs to be tested across other soil types, land uses, and climates before it can be widely applied to predict the SWR occurrence from SOC data. Future work should also focus on the impacts of SOC composition on SWRP parameters and other hydrological properties for predicting soil water processes in water-repellent soils.

Protecting the unseen majority: Land cover and environmental factors linked with soil bacterial communities and functions in New Zealand

The biodiversity in soil ecosystems is simultaneously incredibly rich and poorly described. In countries such as New Zealand, where high endemism in plant species emerged following extended geographical isolation, it is likely similar evolutionary pressures extended to soil microbial communities (our biodiversity ‘dark matter’). However, we have little understanding of the extent of microbial life in New Zealand soils, let alone estimates of endemism, rates of species loss or gain, or implications for systems where plants and their microbiomes have co-evolved.

Metathramycin, a new bioactive aureolic acid discovered by heterologous expression of a metagenome derived biosynthetic pathway

Bacterial natural products have been a rich source of bioactive compounds for drug development, and advances in DNA sequencing, informatics and molecular biology have opened new avenues for their discovery. Here, we describe the isolation of an aureolic acid biosynthetic gene cluster from a metagenome library derived from a New Zealand soil sample. Heterologous expression of this pathway in Streptomyces albus resulted in the production and isolation of two new aureolic acid compounds, one of which (metathramycin, 6) possesses potent bioactivity against a human colon carcinoma cell line (HCT-116, IC50 = 14.6 nM). As metathramycin was a minor constituent of the fermentation extract, its discovery relied on a combination of approaches including bioactivity guided fractionation, MS/MS characterisation and pathway engineering. This study not only demonstrates the presence of previously uncharacterised aureolic acids in the environment, but also the value of an integrated natural product discovery approach which may be generally applicable to low abundance bioactive metabolites.

Extractable aluminium in New Zealand Andisols and Inceptisols

It is critical to have a soil test that allows for the measurement of potential toxicity. A laboratory experiment was undertaken to investigate the effects of changing the concentration of salt in the extract and extraction time of the standard CaCl2 and KCl soil Al tests on the Al concentrations extracted from 13 soils from four New Zealand soil orders. Al extracted by KCl (AlKCl) was 13 times higher than extracted by CaCl2 (AlCaCl2) across all soils. The effect of changing extract salt concentration and extraction time on Al extracted differed among the four soil orders tested for the two extraction methods. Increasing the concentration of CaCl2 in the extract increased (P < .001; P < .05) the amount of AlCaCl2 (by 0.02–0.13 cmolc/kg) for the four soil orders, while increasing extraction time resulted in a difference only in the Allophanic soils, where AlCaCl2 decreased (P < .01). The interaction of the concentration of salt in the extract and extraction time for the CaCl2 extraction did not affect (P > .05) the AlCaCl2 extracted from all soils. An increase in the concentration of KCl in the extract up to 1 M increased AlKCl (P < .01) (by 0.2–0.8 cmolc/kg) on the Allophanic, Brown and Pumice soils, with no increase (P > .05) with a further increase in concentration. Extraction time affected AlKCl (P < .001) for Pallic soils, while the interaction of concentration of KCl in the extract and extraction time resulted in differences (P < .001) only for the Pallic soils. These findings suggest that the Al concentrations measured by the two extraction methods are affected by specific soil properties in the topsoil related to soil order. This means that when measuring the Al bioavailability in soils, extreme care must be taken when interpreting soil Al test results.

&lt;p&gt;&lt;strong&gt;&lt;em&gt;Bimuria omanensis&lt;/em&gt; sp. nov. (Didymosphaeriaceae, Pleosporales) from Oman&lt;/strong&gt;&lt;/p&gt;

Bimuria is a monotypic genus in Didymosphaeriaceae, typified by B. novae-zelandiae collected in terrestrial habitats from New Zealand soil. In our study, Bimuria omanensis, a novel species was isolated from unidentified decaying leaves in Oman. The phylogenetic placement of B. omanensis is resolved based on maximum likelihood, maximum parsimony and Bayesian analyses of combined LSU, ITS and TEF1-α sequence data of Didymosphaeriaceae. The placement of Bimuria omanensis as a distinct species, is confirmed based on phylogeny. This is the first record of an asexual morph in Bimuria and first record of a Bimuria species from Oman. The relationship of this taxon with other phylogenetically closely related Didymosphaeriaceae species is shown.

Educator's Heart Leads to Legacy—the Kirkham Conference

Educator's Heart Leads to Legacy—the Kirkham Conference

Radium in New Zealand agricultural soils: Phosphate fertiliser inputs, soil activity concentrations and fractionation profiles

Phosphate ores can contain high levels of 238U and its decay products. Of these decay products 226Ra is an important environmental contaminant, while 228Ra from 232 Th day may also be present, albeit at lower activity concentrations. Acid processing of phosphate ore to triple superphosphate elutes a large proportion of the 226Ra from the final product. However, fertiliser production in New Zealand generally avoids acid processing and instead uses single superphosphate and reactive phosphate rock to maintain crop yields, meaning that 226Ra is retained in the final product. As a first step towards characterising the human health impacts from fertiliser-borne radium, research was undertaken to identify loading and long-term accumulation of 226Ra and 228Ra in New Zealand agricultural soils, as well as the fractionation of 226Ra into different soil phases.Activity concentrations for 226Ra of up to 1.6 kBq/kg were determined in phosphate-containing fertilisers used in New Zealand. In contrast, 228Ra did not exceed 75 Bq/kg. Analysis of 40 New Zealand soils, covering a range of agricultural uses, showed activities of between (27–88) Bq/kg 226Ra and (21–102) Bq/kg 228Ra. Unexpectedly, there was also a strong correlation between the two radium isotopes. In 13 of the agricultural soils, all with very high available phosphate levels, the fractionation profile of 226Ra was determined. These data indicated that 226Ra largely remains immobile in the residual phase of the soil. Calcium and available phosphate were significantly correlated with binding of 226Ra into labile and non-labile fractions. Barium is also hypothesised to play a significant role in co-precipitating 226Ra into non-labile soil fractions. While a high percentages of 226Ra immobile in the non-labile fraction would allow for marked accumulation over time it may limit the availability for uptake into crops and thus the ionising radiation dose for consumers.

Using Soil Stratigraphy and Tephrochronology to Understand the Origin, Age, and Classification of a Unique Late Quaternary Tephra-Derived Ultisol in Aotearoa New Zealand

In this article, I show how an Ultisol, representative of a globally-important group of soils with clay-rich subsoils, low base saturation, and low fertility, in the central Waikato region in northern North Island, can be evaluated using soil stratigraphy and tephrochronology to answer challenging questions about its genesis, age and classification. The Kainui soil, a Typic Kandiudult (Soil Taxonomy) and Buried-granular Yellow Ultic Soil (New Zealand Soil Classification), occurs on low rolling hills of Mid-Quaternary age mainly in the Hamilton lowlands in, and north and northeast of, Hamilton city. It is a composite, multi-layered tephra-derived soil consisting of two distinct parts, upper and lower. The upper part is a coverbed typically c. 0.4–0.7 m in thickness (c. 0.6 m on average) comprising numerous late Quaternary rhyolitic and andesitic tephras that have been accumulating incrementally since c. 50 ka (the age of Rotoehu Ash at the coverbed’s base) whilst simultaneously being pedogenically altered (i.e., forming soil horizons) via developmental upbuilding pedogenesis during Marine Oxygen Isotope Stages (MOIS) 3-1. Any original depositional (fall) bedding has been almost entirely masked by pedogenic alteration. Sediments in lakes aged c. 20 ka adjacent to the low hills have preserved around 40 separate, thin, macroscopic tephra-fall beds mainly rhyolitic in composition, and equivalent subaerial deposits together form the upper c. 30 cm of the coverbed. Okareka (c. 21.8 ka), Okaia (c. 28.6 ka), Tāhuna (c. 39.3 ka) and (especially) Rotoehu tephras make up the bulk of the lower c. 30 cm of the coverbed. Tephra admixing has occurred throughout the coverbed because of soil upbuilding processes. Moderately well drained, this upper profile is dominated by halloysite (not allophane) in the clay fraction because of limited desilication. In contrast, Otorohanga soils, on rolling hills to the south of Hamilton, are formed in equivalent but thicker (&gt;c. 0.8 m) late Quaternary tephras ≤c. 50 ka that are somewhat more andesitic although predominantly rhyolitic overall. These deeper soils are well drained with strong desilication and thus are allophanic, generating Typic Hapludands. Ubiquitous redox features, together with short-lived contemporary reduction observed in the lower coverbed of a Kainui soil profile, indicate that the Kainui soil in general is likely to be saturated by perching for several days, or near saturation for several months, each year. The perching occurs because the coverbed overlies a slowly-permeable, buried, clay-rich paleosol on upper Hamilton Ash beds, &gt;c. 50 ka in age, which makes up the lower part of the two-storeyed Kainui soil. The coverbed-paleosol boundary is a lithologic discontinuity (unconformity). Irregular in shape, it represents a tree-overturn paleosurface that may be c. 74 ka in age (MOIS 5/4 boundary). The buried paleosol is markedly altered and halloysitic with relict clay skins (forming paleo-argillic and/or paleo-kandic horizons) and redoximorphic features. It is inferred to have formed via developmental upbuilding pedogenesis during the Last Interglacial (MOIS 5e). The entire Hamilton Ash sequence, c. 3 m in thickness and overlain unconformably by Rotoehu Ash and underlain by c. 340-ka Rangitawa Tephra at the base, represents a thick composite (accretionary) set of clayey, welded paleosols developed by upbuilding pedogenesis from MOIS 10 to 5.

A meta-analysis of exchangeable aluminium in New Zealand soils using the National Soils Database

Soil acidification and associated aluminium toxicity are critical issues in New Zealand, particularly in high and hill country areas. However, there have been few studies that have examined the key drivers of exchangeable Al concentrations in New Zealand soils. The National Soils Database was used to investigate the relationship between soil chemical, physical and environmental variables and KCl-extractable Al (AlKCl) for New Zealand soils. Soil AlKCl concentrations were strongly associated with base saturation, soil pH, cation exchange capacity, total N, total C and soil order. However, the relationships differed among the three depth zones (0–20cm, 20–50cm and 50–120cm). Soil acidity and high cation exchange capacity contributed to high concentrations of AlKCl in the soil, whereas high base saturation and total C had the opposite effect. Total N decreased with increasing AlKCl in the topsoil (0–20cm), which is likely a response to the effects of Al toxicity on biological N fixation by pasture legumes. Across a pHH2O range of 3.8–6.4, AlKCl was measured at concentrations that can be toxic to sensitive plants (&amp;gt;1.0 cmolc kg−1). Brown Soils and Podzols are likely more susceptible to Al toxicity, with the highest mean concentrations of AlKCl measured across all depth zones.