This study compares soils from Rangitāhua (29°15'S, 177°55'W) and nearby North Meyer Island. They are located approximately 1,000 km north of Aotearoa, New Zealand and a similar distance south of Tonga. Rangitāhua is a remote, volcanically active island of immense cultural significance to many Māori as a traditional stopping point in ancestral Polynesian migrations (Brown 2024). It lies within the rohe of Ngāti Kuri.

The soils of Rangitāhua are composed from a mixture of basalt, andesitic ash, and pumice. They are generally young and relatively fertile, shaped by recent volcanic activity and ongoing erosion (Wright and Metson 1959). The terrain of the island is steep, with the highest peak, Moumoukai, reaching 516 m above sea level, and is dominated by mature Kahika (Metrosideros kermadecensis) forest (Towns 2023). However, its ecosystem has been historically impacted by mammalian pests such as goats, rats, and cats, and transformer weed species, introduced by European settlers in the 1900’s (Cieraad 2006; Gentry 2013). Since the eradication of mammalian pests, completed by 2003, and conservation efforts to remove transformer weeds, Rangitāhua’s ecosystem has recovered substantially (Cieraad 2006). However, seabird breeding populations have yet to return to their pre-disturbance levels (Gaskin 2011).

In contrast, North Meyer Island (130 m asl.), is a remnant of older volcanic activity in the region and has remained pest-free, serving as a refuge for seabirds and other terrestrial birds (Lloyd and Nathan 1981; Veitch et al. 2004). The soils are developed on weathered andesitic tuff that has been strongly modified by nesting seabird species. (Oliver 1910). North Meyer is predominantly a seabird colony (Towns 2023), supporting 13 seabird species, including surface-nesting and burrowing types (Gaskin 2011). The topography is steep, and the dominant tree is Ngaio Myoporum rapense, with a number of kahika trees at higher elevations.

Sampling was carried out beneath kahika (Metrosideros kermadecensis) which is the dominant canopy tree species on Rangitāhua. To ensure consistency in overlying vegetation structure across sites on both islands, all soil samples were collected from beneath kahika trees (see Figure 1).

Soils were collected beneath kahika trees within five permanent plots in forests (30 × 30 m) on Rangitāhua and from beneath 10 kahika trees at a single location on North Meyer Island (Figure 2). The Rangitāhua plots were established in 1993 to track the impact of rat eradication on vegetation on the island, and the canopy is dominated by kahika (Cieraad 2006). All five plots have been dominated since their establishment by a mature kahika canopy (Cieraad 2006).

Soil samples were collected between 10–19 November 2023 using a 25 mm diameter soil auger to a depth of 10 cm. At each Rangitāhua plot, soil from 15 trees was combined into a single pooled sample for analysis. On North Meyer, samples were pooled from beneath 10 trees (see Suppl. material 1: table SS2), and five of these were analysed individually (see Suppl. material 1: table SS1).

On North Meyer, nesting Kermadec petrels (Pterodroma neglecta) and masked boobies (Sula dactylatra) were observed near the sampled kahika trees. Burrowing wedge-tail shearwaters (Ardenna pacifica) were also present. Seabird burrows and nesting birds were ubiquitous and difficult to avoid while sampling. In contrast, no seabird burrows or signs of recent nesting were observed at any of the Rangitāhua sites, and burrow density was not quantified. However, seabird burrows have been reported elsewhere on Rangitāhua, though at a lower density (Gaskin 2011).

The soil nutrient concentrations of the Rangitāhua samples and the North Meyer samples were analysed by the Manaaki Whenua Environmental Chemistry Laboratory in Palmerston North. Rangitāhua soils were analysed as composite samples only. For North Meyer, ten samples were collected in total. Five were analysed individually, and an additional composite sample was prepared by combining equal amounts of soil from all ten samples and analysed separately (Table 1.). All analyses were performed on air-dried soils.

Each soil sample was air-dried at 35 °C and then ground and sieved to <2 mm (Metson 1971). Soil pH was measured in a 1:2.5 soil-to-water suspension after an overnight equilibration using a pH electrode (Blakemore et al. 1987). Organic carbon and total nitrogen were measured using a CN98 analyser, which uses the Dumas principle of dry combustion (Leco Corporation 2003). Samples were combusted in pure oxygen at 1050 °C. An aliquot was subsampled and passed through a heated copper catalyst, which measures forms of nitrogen (N₂) and organic carbon (CO₂), and was measured using an infrared detector cell. C:N was calculated based on the ratio of nitrogen to organic carbon. Total phosphorus was determined by igniting the soil at 550 °C for 60 minutes, extracting the residue with 0.5 M sulphuric acid for 16 hours, and analysing the extract using a flow injection analyser (Blakemore et al. 1987).

Additional soil testing was conducted on the composite samples at RJ Hill Laboratories (Hamilton) using their General Soil and Organic Soil profiles. These samples were also air-dried at 35–40°C overnight (Hill Laboratories 2025b). Soil pH was determined using a 1:2 (v/v) soil to water slurry, followed by potentiometric measurement (Hill Laboratories 2025b). Soil total carbon and total nitrogen were determined using the Dumas dry combustion analyser (Hill Laboratories 2025a). C:N was calculated based on the ratio of total carbon to total nitrogen (Hill Laboratories 2025c). Olsen phosphorus was measured using a 0.5 M sodium bicarbonate (pH 8.5) extraction, followed by molybdenum blue colorimetry (Hill Laboratories 2025b). Total phosphorus was determined by heating soil at 550 °C for 60 minutes, extracting with 0.5 M sulphuric acid for 16 hours and analysing with a flow injection analyser (Blakemore et al. 1987).

Concentrations of the cations calcium (Ca), potassium (K), magnesium (Mg), and sodium (Na) were measured in 1 M neutral ammonium acetate extracts using inductively coupled plasma optical emission spectrometry (ICP-OES) (Hill Laboratories 2025b). Cation Exchange Capacity (CEC) was calculated as the sum of these four cations and reported in me/100 g (Blakemore et al. 1987; Hill Laboratories 2025b).

Mean values and standard errors were calculated from the soils tested by Manaaki Whenua. Welch’s t-tests were then used to assess whether pH, total carbon, total nitrogen, C:N ratio, and total phosphorus differed significantly between Rangitāhua and North Meyer. All statistical analyses were conducted in R version 4.4.1 (R Core Team 2024). Data and R script related to the statistical analyses can be found in Suppl. materials 2–4.

All soils on Rangitāhua were mildly acidic, with a mean pH of 6.16 ± 0.22 (SE). In contrast, soils on North Meyer had a lower mean pH of 4.12 ± 0.25 (Figure 3). A Welch’s t-test showed that pH was significantly lower on North Meyer than on Rangitāhua (Table 2).

Rangitāhua sites had a mean organic soil carbon level 10.36% ± 1.09 and North Meyer soils a mean of 11.39 ± 3.51 (Figure 4). A Welch’s t-test indicated that soil carbon levels were not significantly different at both locations (Table 2). CC BY 4.0.

The mean total soil nitrogen was 0.58% ± 0.04 on Rangitāhua and 0.77% ± 0.23 on North Meyer (Figure 5). A Welch’s t-test indicated that total nitrogen levels did not differ significantly between the two sites (Table 2).

The mean C:N ratio was 17.75 ± 0.94 for Rangitāhua soils and 14.16 ± 0.72 for North Meyer soils (Figure 6). A Welch’s t-test indicated that North Meyer had a significantly lower C:N ratio than Rangitāhua (Table 2).

The mean total phosphorus was 856.80 ± 81.53 mg/kg on Rangitāhua and 3462.80 ± 908.75 mg/kg on North Meyer (Figure 6). A Welch’s t-test indicated that total phosphorus was significantly higher on North Meyer than on Rangitāhua (Table 2).