335

Characteristics of Coral Cay Soils at Coringa-HeraldCoral Sea Islands, Australia1

George N. Batianoff,2 Gillian C. Naylor,2 Rod J. Fensham,2 and V. John Neldner2,3

Abstract: Coral cay soil chemical and physical properties were described fromCoringa-Herald National Nature Reserve, Australia. Soil A horizons under lit-toral herblands and Argusia argentea shrubs were shallow and coarse textured.Interior soil A horizons, particularly under Pisonia grandis closed forest, weredeeper (1.2 m) with finer textures. Average surface soil pH values ranged frompH 8.76 at the seashores to pH 8.09 in the interior. Average surface soil organiccarbon ranged from 2.4% to 4.8%; and phosphorus (Colwell-P) concentrationsranged from 467 mg/kg to 882 mg/kg within the interior areas. Chemical fer-tility of all A horizons increased from the seashore to the island interior. Thehigher fertility levels are attributed to high organic matter contributed by vege-tation, combined with activities of seabirds, particularly the burrowing wedge-tailed shearwater, Puffinis pacificus. Leaching of nutrients from surface soils isreflected in the rapid decline in soil fertility with depth. Deeper interior A hori-zons are interrupted by formation of an abrupt white C profile. It is speculatedthat the formation of this layer is the product of periodic ‘‘washing’’ by a sea-sonally high fresh/brackish water table.

Coringa-Herald National Nature Reserve(CHNNR) (Figure 1) is a remote localityof relatively high biodiversity in the north-eastern Australian Exclusive Economic Zone.The soils of the five vegetated cays atCHNNR were studied as part of an ongoingassessment of resilience and vulnerability ofterrestrial ecosystems. The soils of the low-lying coral cays and islets are products ofphysical, geological, and biological factors(Gourlay 1988). Coral cays in the PacificOcean formed during the Holocene periodbetween 3000 and 5000 B.P. (Smithers et al.2007). The coral cay soils of eastern Australiaare largely composed of sediments derivedfrom Foraminifera spp. and Halimeda spp.

(Yamano et al. 2000) that have developedthrough biotic interactions with soil microbessuch as bacteria and fungi. The coral caysands across the Pacific are essentially homol-ogous, although variability of soils is deter-mined by factors such as latitude and climate,geometries and age of the islands, the relativesea level histories of reefs, and the diversityof reef sediments of which they are com-posed (Smithers et al. 2007).

Most other coral island soil publicationshave reported on islands where human distur-bances have been high (e.g., clearing for agri-culture, extensive phosphate rock mining, thedetonation of nuclear weaponry, and so forth[Barry and Rayment 1997, Morrison andManner 2005, Deenik and Yost 2006]). Incontrast, the CHNNR islands have seen min-imal human disturbance. Therefore, the ma-jor factors influencing soil properties andconditions at CHNNR (vegetation type, avi-faunal associations, and environmental distur-bance events) are entirely natural.

Coral cay soils are generally considered tobe problematic for plant growth with theirhigh porosity and low water-holding capacity,being compounded by little cation exchangecapacity (Stone et al. 2000, Deenik and Yost

Pacific Science (2010), vol. 64, no. 2:335–347doi: 10.2984/64.2.335: 2010 by University of Hawai‘i PressAll rights reserved

1Manuscript accepted 10 June 2009.2 Queensland Herbarium, Department of Environ-

ment and Resource Management, Mt. Coot-tha Road,Toowong, Queensland 4066, Australia.

3 Corresponding author (e-mail: John.Neldner@derm.qld.gov.au).

Figure 1. Location of Coringa-Herald National Nature Reserve within the Australian Exclusive Economic Zone.

2006). The high carbonate, high pH soils ofcoral cays are predicted to be highly restric-tive on phosphorus and micronutrient solu-bility (Morrison 1990).

This paper draws largely on the study ofsoils at CHNNR from 2006 to 2007 thatformed part of a multidisciplinary survey ofterrestrial ecology (Batianoff et al. 2009).The main aims are to report chemical andphysical soil properties and describe the pro-cess of soil development from the seashoreareas to the island interiors. The roles of veg-etation and nesting seabirds that utilize differ-ent vegetation communities are highlightedas part of soil development from unvegetatedlow nesting beaches to high-usage nestingsites of Pisonia grandis closed forest.

Study Area

These low sand cays and islets have a maxi-mum elevation of about 5–6 m above sea lev-el (Batianoff et al. 2009). Island shapes rangefrom the cylindrical South West HeraldCay (19 ha) and Chilcott Islet (17 ha), theraindrop-shaped South West Coringa Islet(13.5 ha) and South East Magdelaine Cay(41 ha), and the half-moon-shaped NorthEast Herald Cay (60 ha) (Batianoff et al.2009). The dominant southeasterly tradewinds operating in the area lead to the devel-opment of ‘‘exposed’’ shores on the east tosoutheastern sides and ‘‘sheltered’’ shoreson the west to northwestern sides. Lithifiedlimestone ‘‘beach rock’’ platforms and rubbleare common along the exposed shores,whereas the sheltered shores feature sandybeaches up to 20 m wide. On all islands, ele-vated beach ridge areas have formed adjacentto shores, with lower plains within the islandinteriors. The seashores, beach ridges, andinterior plains make up the three distinctlandform units described for the islands.Seventeen plant communities have been de-scribed occurring on these three distinctlandforms (Batianoff et al. 2009).

CHNNR has a tropical and maritimeclimate characterized by seasonally variablerainfall, consistently high annual tempera-tures (21.9�C mean monthly minimum inJuly to 30.7�C mean monthly maximum in

January), and high evaporation rates (Farrow1984, Bureau of Meteorology 2008, Batianoffet al. 2010). Rainfall at nearby Willis Island(1,115 mm) occurs predominantly during thefour ‘‘wet’’ months of January–April. A ‘‘dry’’period lasts from May to December. Pro-longed drought periods lasting between 2and 6 yr are also common (Batianoff et al.2009). Southeasterly ‘‘trade’’ winds dominatethroughout most of the year except duringsummer (December to March), when thenortherly ‘‘monsoonal’’ influence bringsstorms and cyclonic conditions.

materials and methods

Field Studies

Fieldwork was conducted over three separatesurveys from 2006 to 2007. Soils were col-lected during the October 2007 field surveyfrom different vegetation communities identi-fied at three landform units: seashores, beachridges, and interior plains (Batianoff et al.2009). The seashore vegetation includesephemeral herblands of Lepidium engleria-num, Stenotaphrum micranthum and/or Leptu-rus repens, and the more permanent fringingArgusia argentea shrublands.

On the beach ridges the vegetation con-sists of mainly A. argentea open scrubs. OnNorth East Herald Cay (NE Herald) andSouth East Magdelaine Cay (SE Magdelaine),Cordia subcordata closed scrubs, windshorn Pi-sonia grandis closed scrubs, and immature P.grandis low closed forests are also found alongbeach ridges. The interior plains supportmixed herblands and Abutilon albescens shrub-lands, with P. grandis forests on NE Heraldand SE Magdelaine.

Fifty soil profiles were sampled using a4 cm diameter sand auger. Soils were sampledfrom distinct vegetation communities withinthe three landform units (seashore, beachridges, and interior plains). The seashoresoil samples included ephemeral herblands(n ¼ 4) and seashore Argusia (n ¼ 5). Sam-pling of the beach ridge unit was from theArgusia (n ¼ 3), Cordia (n ¼ 2), wind-exposedPisonia (n ¼ 6), and immature Pisonia (n ¼ 6)vegetation communities. The interior plains

Soils of Coringa-Herald Islands . Batianoff et al. 337

samples were from the interior herblandsand grasslands (n ¼ 7), Abutilon (n ¼ 8), andhealthy mature/old-growth Pisonia vegetation(n ¼ 9).

Soil profiles were sampled according todepth/horizon. The surface soils (Ah hori-zons) were sampled between 0 and 20 cm.The remainder of the A horizon was sampledin 20 cm intervals (i.e., 20–40 cm, 40–60 cm,60–80 cm, and 80–100þ cm) until the origi-nal parent material layer (C horizon) wasreached. C horizons were also sampled wherepossible. All soil samples were placed in zip-lock bags on site and oven-dried.

Analytical Studies

The dried soils were described based on soilconsistency, texture, color, structure, and thecoarse fragment component as outlined inFitzpatrick et al. (1999). Soil texture was esti-mated by the proportions of particles thatwere very coarse (1.00–2.00 mm), coarse(0.50–1.00 mm), medium (0.25–0.50 mm),fine (0.10–0.25 mm), to very fine (0.05–0.1 mm). Colors of the oven-dried soils weredescribed in accordance with the Munsell SoilColor Chart system (Munsell Color 2000).The coarse-fragment component of soils wasrecorded as those fragments larger than 2 mm.

The chemical properties of all soil pro-files were determined by the University ofQueensland’s Analytical Services unit. Analy-ses following standard procedures from Ray-ment and Higginson (1992) were conductedfor pH, electrical conductivity (dS/m) (usinga 1 :5 soil to water mixture), total nitrogen(wt%), total carbon (wt%), organic carbon(wt%), available phosphorus (Colwell-P)(mg/kg), extractable potassium (cmol(þ)/kg),extractable calcium (cmol(þ)/kg), extractablemagnesium (cmol(þ)/kg), extractable sodium(cmol(þ)/kg), total copper (wt%), total iron(wt%), total manganese (wt%), total sulphur(wt%), and total zinc (wt%). The physicaland chemical properties of soil samples wereaveraged for each horizon, vegetation com-munity, and landform type. Relationshipsbetween organic carbon and other nutrientswere explored with Pearsons Product Mo-ment Correlation. Differences between land-

form types were compared for each nutrientaccording to each horizon (where n > 3) us-ing parametric and nonparametric analysis ofvariance (ANOVA) depending on the distri-bution of the data. The nutrients with signif-icant differences were normally distributed,and Tukey’s test was used to compare indi-vidual means.

results

Soil Physical and Chemical Properties

The soils at CHNNR have AC profiles with-out B horizons. All soil profiles sampled con-sisted of loose and unconsolidated particlesthat were essentially structureless. Surfacesoils (Ah horizon) ranged in color from white(10YR 8/1) at the seashore areas to very palebrown (10YR 7/3) within island interiors.The A horizon layers below were darker incolor than the surface soils. Generally, theAh and C horizons had higher proportionsof very coarse to coarse coral fragments thanthe subsurface A horizons.

The results of chemical analyses are sum-marized by vegetation community and land-form unit in Tables 1, 2, and 3. The alkalinesoil pH values declined from the seashoreareas to the island interiors. Soil nutrientconcentrations were higher in the surfacesoils than in deeper horizons for each profileand with highest concentrations recordednear the island interiors (Figure 2). The nu-trients reflect typical characteristics of coralcay soils with a very high extractable Pthroughout that is attributable to the inputof guano from the resident seabird popula-tions. Potassium concentrations were low tomarginal in the beach and ridge sites andonly just adequate at the mature Pisonia sites.All soil nutrients were strongly positively cor-related with organic carbon content (all P <:001, except total Cu and total Fe P < :01).Organic carbon was strongly negatively cor-related with soil pH (P < :001).

seashore soils. The beach surface soilswere dominated by medium (45%) to coarse(5%) sized grains. The A horizons were upto 20 cm deep, composed mainly (50%) offine to very fine grains, and were predomi-

338 PACIFIC SCIENCE . April 2010

nantly white (10YR 8/1). The seashore Argu-sia soils had A horizons to a maximum depthof 30 cm, with around 10% medium to coarsegrains at the surface and up to 25% at a depthof 30 cm. The fine to very fine grains propor-tion was about 90% at the surface to 75% at

30 cm. The dominant color was white (10YR8/1) with a light brownish gray element(10YR 6/3). Some organic material (decom-posing plant material) was visible.

The beach surface soils were strongly alka-line (pH 8.8). Electrical conductivity was low(0.32 dS/m) for nearly all surface soil samples.Mean soil organic carbon value was 2.3%, andmean total nitrogen value was 0.35%. Thebeach averages for total nitrogen were higherthan those in the Argusia samples (Table 1).The mean concentration of phosphorus (P)for the surface soils of the seashore commu-nity was 467 mg/kg, though the minimumrecorded for one sample was 26 mg/kg. Thesurface soils of the seashore communities re-corded the lowest concentrations of potas-sium (K) of 0.11 cmol(þ)/kg (Table 1).

beach ridge soils. The maximum Ahorizon depths for the beach ridge commu-nities were as follows: beach ridge Argusia(0–30 cm); beach ridge Cordia (0–30 cm);wind-exposed Pisonia (0–90 cm), and imma-ture Pisonia (0–80 cm). The beach ridge soilshad a high fine (30–40%) to very fine grain(35–50%) component, with a lower coralfragment content than the seashore soils. Thebeach ridge soils from the exposed shoreswere often variable throughout the profile.For example, the wind-exposed Pisonia at NEHerald had 30–80% medium, 0–60% fine,and 10–60% very fine sized grains through-out the profiles.

The mean pH values for the surface soilsof the beach ridge vegetation communitieswere pH 8.2. The windshorn Pisonia was themost alkaline, whereas the immature Pisoniathe least alkaline (Table 2). The soil pH ofthe windshorn Pisonia C horizon was themost alkaline recorded (pH 9.28). Electricalconductivity was low: about 0.42 dS/m forsurface soil samples. The mean organic car-bon value for the beach ridge communitieswas 3.6%, with values ranging from 3.38%(immature Pisonia) to 4.51% (Cordia) (Table2). Total nitrogen was low, averaging 0.31,apart from a value of 0.54% in the Argusiashrubland. Phosphorus was extremely vari-able, ranging from 412 mg/kg within Cordiasurface soils to 978 mg/kg from the immaturePisonia surface soils. Potassium values were

Figure 2. Comparison of average chemical properties ofnutrients of surface soils with statistically significant dif-ference between landforms. Individual means that are notsignificantly different (P > :05) are annotated with thesame value. a, pH (F ¼ 7:41; df ¼ 2;43; P ¼ :002); b, or-ganic carbon (wt%) (F ¼ 6:48; df ¼ 2;43; P ¼ :003); c,Colwell-P (mg/kg) (F ¼ 3:36; df ¼ 2;43; P ¼ :044).

Soils of Coringa-Herald Islands . Batianoff et al. 339

very low but variable, ranging from 0.15cmol(þ)/kg) for Argusia to 0.64 cmol(þ)/kg)for immature Pisonia (Table 1).

interior soils. The interior soils arepart of older landforms represented by sandplains and relic beach ridges and swales. Themaximum depths of the interior plains soilswere Abutilon (0–70 cm), interior herblands(0–60 cm), and mature Pisonia (0–110 cm).Fine to very fine grain proportions werehigh, with some samples from the interiorherblands recording up to 100% fine to veryfine grains. The mature/old-growth Pisoniasoil profiles were the most developed atCHNNR, with A horizons up to 110 cmdeep. Ah and A horizon soils in the interiorwere darker than other soils. The mature/old-growth Pisonia soils were the darkest ofall samples (10YR 3/3, dark brown). Thetransition from A horizons to C horizonswas often abrupt, with the C horizon samplesresembling the white sands of the seashore.Litter cover was generally higher in the inte-rior areas than in the beach ridge and sea-shore areas. Moderate surface soil compactionand patchy organic crust layers were observedon the surface of some interior soils. Surface

runoff was evident in some areas of the Piso-nia forest floor, despite the freely draining na-ture of unconsolidated sands.

The mature P. grandis surface soils at SEMagdelaine recorded the lowest soil pHvalues, with the lowest 7.29. The mean inte-rior plains pH was 8.00 for surface soils. Amean electrical conductivity of 1.44 dS/mwas measured in mature Pisonia, consideredhigh by agricultural standards (Rayment andBruce 1984). The mature P. grandis profilesrecorded the highest mean organic carbonvalues (4.84%), with one sample recording12.08%. The interior communities organiccarbon mean value was 4.8%. Total nitro-gen concentrations were well maintainedthroughout the A horizons (0.36% to 0.69%)of interior soil profiles (Table 3). However,the average total nitrogen concentration inthe C horizons of mature Pisonia was consid-erably lower (0.28%).

discussion

Most coral island soils are described as havingAC profiles with an absence of B horizon(Fosberg 1954, Catala 1957, Wiens 1962,

TABLE 1

Comparison of Chemical Properties of Soils from Seashore Vegetation Communities Subdivided by Soil Horizon

Beach Argusiaa

Parameter Unit Ah (4,3) Ah (5,3) C (2,0)

pH 8.81G 0.50 8.72G 0.30 8.95G 0.07Electrical conductivity (dS/m2) 0.32G 0.14 0.34G 0.12 0.13G 0.00Total Nitrogen (wt%) 0.35G 0.31 0.15G 0.03 0.07G 0.00Total Carbon (wt%) 8.27G 1.77 5.01G 0.61 2.55G 0.13Organic Carbon (wt%) 2.29G 0.83 2.54G 0.29 1.77G 0.05Colwell-P (mg/kg) 378G 241 538G 329 226G 4Ca (cmol(þ)/kg) 14.9G 3.1 14.4G 2.3 12.2G 0.6K (cmol(þ)/kg) 0.1G 0.06 0.16G 0.11 0.02G 0.00Mg (cmol(þ)/kg) 1.87G 0.62 2.13G 0.8 1.12G 0.04Na (cmol(þ)/kg) 0.45G 0.19 0.54G 0.27 0.09G 0.00Total Cu (mg/kg) 5.7G 4.6 2.0G 0.8 naTotal Fe (mg/kg) 30G 23 8G 4 naTotal Mn (mg/kg) 8.49G 6.21 2.44G 0.09 naTotal S (mg/kg) 1,281G 155 1,123G 23 naTotal Zn (mg/kg) 53G 48 8G 3 na

a Sample size is given in parentheses, with the first value relating to rows 1–10 (pH through Na) and the second value relating torows 11–14 (Total Cu through Total Zn).

340 PACIFIC SCIENCE . April 2010

TABLE 2

Comparison of Chemical Properties of Soils from Beach Ridge Vegetation Communities Subdivided by Soil Horizon

Argusia Cordia Windshorn Pisonia Immature Pisonia

Parameter A (3,2) C (2,1) Ah (2,0) C (2,0) Ah (6,0) A (2,0) C (1,0) Ah (5,2) A (6,2) C (1,1)

pH 8.26G 0.15 9.02G 0.42 8.3G 0.03 8.81G 0.36 8.37G 0.10 8.81G 0.19 9.28 8.05G 0.15 8.32G 0.09 8.10Electrical Conductivity 0.36G 0.09 0.11G 0.03 0.29G 0.01 0.15G 0.04 0.48G 0.07 0.21G 0.07 0.15 1.16G 0.54 0.34G 0.09 0.53Total Nitrogen 0.54G 0.29 0.15G 0.04 0.28G 0.09 0.1G 0.07 0.28G 0.05 0.17G 0.03 0.02 0.39G 0.12 0.40G 0.11 0.60Total Carbon 7.73G 2.39 4.38G 1.07 5.97G 0.54 3.32G 0.31 6.12G 1.46 7.36G 3.5 2.59 6.79G 1.31 6.93G 1.39 6.18Organic Carbon 4.48G 1.36 1.98G 0.32 4.51G 0.3 2.52G 0.9 3.54G 0.33 2.7G 0.96 1.67 3.38G 0.45 3.5G 0.45 3.31Colwell-P 551G 121 293G 77 412G 68 288G 117 547G 74 412G 70 187 978G 220 832G 196 669Ca 16.3G 2.9 12G 2.8 16.6G 0.6 12.1G 0.5 14.3G 0.6 13.4G 0.1 12.1 18.0G 2.3 16.9G 2.0 24.2K 0.15G 0.05 0.03G 0.02 0.07G 0.01 0.02G 0.01 0.48G 0.23 0.17G 0.07 0.01 0.64G 0.17 0.66G 0.42 0.44Mg 2.24G 0.74 1.13G 0.15 1.92G 0.37 1.11G 0.05 3.12G 0.56 1.78G 0.19 1.14 3.21G 0.78 2.7G 0.9 6.69Na 0.52G 0.20 0.12G 0.01 0.19G 0.02 0.09G 0.02 1.1G 0.36 0.49G 0.20 0.14 1.43G 0.68 0.84G 0.46 2.92Total Cu 9.5G 7.8 na na na na na 0.0 20.7G 7.6 27.1G 6.1 32.2Total Fe 31G 25 na na na na na 0 88G 25 116G 17 131Total Mn 7.07G 3.56 na na na na na 0.00 8.65G 0.39 9.96G 0.33 9.74Total S 1,436G 219 na na na na na 0 1,802G 370 1,891G 560 2,401Total Zn 48G 40 na na na na na 0 94G 31 120G 26 145

Note: Units are provided in Table 1. Sample size is given in parentheses, with the first value relating to rows 1–10 (pH through Na) and the second value relating to rows 11–14 (Total Cuthrough Total Zn).

TABLE 3

Comparison of Chemical Properties of Soils from Interior Plains Vegetation Communities Subdivided by Soil Horizon

Herblands Abutilon Mature Pisonia

Parameter Ah (7,6) C (4,3) Ah (8,4) A (8,4) C (1,0) Ah (9,3) A (8,4) C (7,3)

pH 8.18G 0.12 8.38G 0.12 8.12G 0.08 8.38G 0.08 8.76 8.00G 0.14 8.01G 0.12 8.43G 0.16Electrical Conductivity 0.54G 0.14 0.3G 0.09 0.59G 0.20 0.29G 0.07 0.13 1.44G 0.54 1.17G 0.33 0.54G 0.17Total Nitrogen 0.65G 0.20 0.37G 0.09 0.45G 0.10 0.36G 0.09 0.05 0.69G 0.26 0.63G 0.25 0.28G 0.10Total Carbon 9.56G 1.7 6.58G 1.15 7.04G 1.10 5.95G 1.08 2.60 7.06G 1.42 6.04G 1.15 5.3G 1.0Organic Carbon 4.84G 0.8 3.13G 0.33 4.73G 0.42 3.69G 0.4 2.29 4.84G 1.01 4.24G 0.87 2.93G 0.43Colwell-P 949G 207 602G 36 833G 89 718G 94 364 873G 86 861G 90 634G 135Ca 17.4G 1.4 14.0G 0.7 16.4G 0.9 14.6G 0.8 11.6 18.5G 2.3 18.6G 2.4 15G 1.2K 0.48G 0.15 0.16G 0.06 0.39G 0.13 0.15G 0.07 0.00 0.84G 0.29 0.58G 0.18 1.1G 0.9Mg 3.01G 0.58 1.95G 0.25 2.66G 0.52 2.26G 0.52 0.75 2.14G 0.33 1.84G 0.33 1.53G 0.17Na 0.88G 0.18 0.45G 0.13 0.62G 0.18 0.44G 0.19 0.06 0.94G 0.32 0.7G 0.19 0.39G 0.1Total Cu 13.3G 2.7 10.4G 0.4 14.4G 3.4 17.6G 3.4 0.0 30.4G 3.2 26.5G 2.7 15.2G 9.1Total Fe 54G 9 34G 3 60G 18 68G 14 0 111G 10 100G 8 63G 22Total Mn 8.88G 2.5 5.35G 0.4 8.93G 1.07 8.56G 1.02 0.00 13.31G 2.01 11.05G 1.86 5.93G 2.10Total S 1,510G 180 1,383G 74 1,568G 80 1,452G 107 0 2,916G 217 2,400G 146 1,463G 310Total Zn 62G 11 42G 3 75G 20 96G 21 0 160G 20 123G 7 66G 37

Note: Units are provided in Table 1. Sample size is given in parentheses, with the first value relating to rows 1–10 (pH through Na) and the second value relating to rows 11–14 (Total Cuthrough Total Zn).

Morrison and Manner 2005). The parent Chorizons are derived from limestone rock orsediments from corals, forams (Foraminiferaspp.), green macroalgae (Halimeda spp.), cor-alline algae, and other invertebrate shells andhave a mineralogy dominated by calcium car-bonate (Barry and Rayment 1997, Yamano etal. 2000). Soil development on coral islands isdetermined by interactions between the par-ent materials and the surrounding marine en-vironment (Gourlay 1988, Gessel and Walker1992, Rodgers 1994). Most coral islands fea-ture freshwater or brackish lenses between1.5 and 2 m below the surface (Wiens 1962,Gessel and Walker 1992, Deenik and Yost2006). Such shallow-water table lenses limitsoil development to <2 m depth. Depth andsalinity of the freshwater lens is determinedby island size and elevation, with climatic in-fluences creating seasonal fluctuations (Wiens1962).

The upper part of the C horizons (whitesand) retained very low levels of nutrients, in-dicating frequent washing by tidal fluctua-tions of the freshwater lens. The freshwaterlens salinity often increases during dry sea-sons, when rainfall constancy and amountsare more critical (Stone 1951, Wiens 1962).However, during higher rainfall periods, theleaching of nutrients through the soil profileis enhanced (Wiens 1962). The salinity of thefreshwater lens in island interiors has beenmeasured as up to 50% of the salinity of sea-water (Gessel and Walker 1992).

A horizons form as a result of nutrient de-position on surface soils from the sea (dis-solved nutrients), the atmosphere (aerosols),seabirds (guano), and organic matter (vegeta-tion) (Morrison 1990, Gessel and Walker1992, Rodgers 1994). The character of the Ahorizon in a particular area is determined bythe placement in the landscape (exposure andshelter) and vegetation type (biomass), as wellas the interactions with wildlife (seabirds).Variances in soil color and depth of A hori-zons are also believed to be dependent onvegetation type, surface coverage, and slopeposition (Morrison and Manner 2005).

As a result, coral cay soil fertility as wellas soil structure is strongly related to plantcommunity biomass and the rate of organic

matter decomposition (Morrison and Manner2005). Our study shows strong evidence thatthe structural development and nutrient en-richment of soils at CHNNR are related tothe succession of plant communities. Thephysical and chemical properties of the vari-ous A horizon soils sampled at CHNNR actas a proxy for coral cay age and development.

The young age of the soils at the sea-shore and the low vegetation biomass of theephemeral herblands, as well as the limitedinteractions with seabirds and the high expo-sure to wind and waves, is not conducive tothe development of deep and fertile A hori-zons. The seashore and beach ridge areaswith limited plant biomass had low organiccarbon contents and low values of other nu-trients. Disturbance by marine turtles duringthe nesting season also limits establishment ofvegetation (Batianoff et al. 2009).

The beach ridge soils, though farther fromseashores, are still subject to the dynamicphysical environment. The variable grainsizes throughout profiles indicate the con-stant deposition of new sediments and windsorting. The grain size variability and deeperprofiles of wind-exposed and immature Piso-nia soils are also likely due to the burrowingactivities of wedge-tailed shearwaters (Puffi-nus pacificus), which were observed in moder-ate numbers in these communities. Along theexposed shores, Argusia argentea forms nar-row strips of woody vegetation one to twoshrubs wide. The majority of arboreal nestingspecies (Red-footed Booby [Sula leucogaster],Black Noddy [Anous minutus], Great Frigate-bird [Fregata minor], and Lesser Frigatebird[F. ariel ]) inhabit the seashore and beachridge Argusia areas. Brown Boobies and Red-tailed Tropicbirds (Phaethon rubricauda) nestbeneath shrubs.

The island interiors at CHNNR had thehighest fertility (Tables 1–3, Figure 2) andgreatest structural development of soil pro-files and featured more dense woody vegeta-tion (Batianoff et al. 2009). For example,mean phosphorus content for A horizon soilsof the interior was 882 mg/kg, a concentra-tion considered very high even for agricul-tural standards (Rayment and Bruce 1984,Moody and Bolland 1999). A sample of

Soils of Coringa-Herald Islands . Batianoff et al. 343

phosphate rock collected during surveys con-tained a Colwell-P concentration of 6,024mg/kg. This is consistent with the Jemo se-ries of soils described by Fosberg (1954)from under Pisonia grandis forests. Mean po-tassium levels within the tissue of a maturePisonia sample recorded 1.92 cmol (þ)/kg,also considered high by agricultural standards(Rayment and Bruce 1984, Gourley 1999).Coral cay soils have no clay, and organic mat-ter is crucial for the retention of water andnutrients within the soil profile (Wiens 1962,Morrison 1990, Gourley 1999, Deenik andYost 2006). The accumulation of organicmatter is considered the most important pro-cess for soil formation at Nauru (Morrisonand Manner 2005).

High nutrient concentrations in the inte-rior are due to the influence of more long-term deposition of seabird guano, higherorganic material from woody vegetation, andthe cycling of organic litter from soil surfacesto the rest of the profile with digging byburrowing wedge-tailed shearwaters. Biotur-bation processes increase the infiltration ofwater and incorporation of organic matterthrough increased porosity and the mixingof litter from both the surface and the rootbiomass (Field and Little 2008). The highernutrient values in the interior soils reflect thehigher biomass of woody plants (Rodgers1994), which in turn delivers higher organicmatter to the soil profile. Forests providenesting habitat for seabirds, which then inputextra nutrients into the soil through guano(Woodroffe and Morrison 2001). The greaterdepth of A horizons, the highest amounts ofsoil organic matter (indicated in the field bytheir darker color) and other nutrients, andthe highest proportions of fine to very finegrains reflect the greater stability of the soilson the interior plains at CHNNR. The as-similation of nutrients from the relatively fer-tile interior soils is enhanced by mycorrhizaeon Pisonia grandis roots (Ashford and Allaway1982, Sharples and Cairney 1998). Vesicular-arbusicular mycorrhiza associations betweenplant roots and fungi have been shown togreatly improve the uptake of minerals suchas zinc, phosphorus, and potassium, espe-cially when supplies are deficient such as at

CHNNR (McGee 1989, Chambers et al.2005, Fomina et al. 2005).

Activities of the burrowing wedge-tailedshearwaters (Puffinis pacificus) within the Piso-nia forests have an important influence on soildevelopment on coral cays (Bancroft et al.2005). According to Jones et al. (1994), thesebirds are considered ‘‘ecosystem engineers’’because they displace at least 11.5 t ha�1

(8.5 m3 ha�1) of soil annually ( Jones et al.1994). Bancroft et al. (2005) estimated annualsoil displacement of at least 22.0 t ha�1

(16.3 m3 ha�1). This bioturbation is an im-portant regolith process, comparable withsoil formation, and enriches the A horizonwith nutrients captured from aerosols, marinedebris, guano, and plant litter from themarine ecosystem (Field and Little 2008).However, for bioturbation to occur, suitablehabitat for wedge-tailed shearwaters is re-quired (i.e., well-compacted soils with a highproportion of fine roots that stabilize bur-rows). Although nutrient enrichment byseabirds may be substantial (Ellis 2005), nu-trient availability to plants may be low ifother factors are limiting uptake, such asthe high carbonate content, soil pH, andmoisture-stress (Morrison 1990). For exam-ple, the availability of zinc is markedly re-duced in alkaline soils especially whenphosphorus concentration is high (Armourand Brennan 1999), and both of these condi-tions are exaggerated on coral cays.

Soil organic carbon is determined by dis-tance from the shore and vegetation produc-tivity (Gessel and Walker 1992, Morrisonand Manner 2005, Deenik and Yost 2006).Organic carbon was highly correlated with Nand Ca in the Marshall Islands and less highlycorrelated with other soil nutrients (Deenikand Yost 2006). Organic carbon concentra-tion was most highly correlated with soilN values (R2 ¼ 0:74). Moderate correlations(R2 < 0:5) were also found for Ca and Mnwith organic carbon concentration. Giventhe neglible clay content of these soils, in-creases in organic carbon generally lead togreater water storage and high nutrient con-tent (Stone et al. 2000).

High alkalinity has been shown to retardthe uptake of Zn, Mn, Fe, and other nutrients

344 PACIFIC SCIENCE . April 2010

by plants (Armour and Brennan 1999, Mc-Farlane 1999, Uren 1999). Organic carbonreduces soil pH values; however with the low-est recorded value 8.0, nutrient uptake islikely to be restricted (Stone 1951, Morrison1990). The high calcium levels of coral caysubstrates have also been shown to affect po-tassium levels, which are generally very defi-cient (Morrison 1990, Stone et al. 2000). Ndeficiency is also reported as common in atollsoils (Deenik and Yost 2006).

In this study, we found that the organiccarbon values were at the lower end of therange recorded for atoll soils in the PacificOcean (Morrison 1990); however the nutri-ent levels of CHNNR were twice as high aslevels recorded at Capricornia Cays in centralQueensland (G.N.B., unpubl. data). Climateis also an influence, because organic matteraccumulation is better in wetter soils. Wetterclimates promote and sustain denser vegeta-tion, giving higher levels of organic matter(Deenik and Yost 2006).

conclusions

The soils of the CHNNR cays exhibit thecharacteristic features of coral cay soils: anabsence of clay, alkaline conditions, low po-tassium and nitrogen concentrations, and theoverabundance of phosphorus. Soil fertilityincreases from the seashore to the island inte-riors, which feature woody vegetation withhigher biomass. The parallel developmentof soils with increasing vegetation structureshows that high vegetation biomass assists insoil development. This is through the inputof organic matter from the vegetation but en-hanced by the input of excreta from nestingand/or burrowing seabirds. The depth of soildevelopment is enhanced by the burrowingwedge-tailed shearwaters’ activities but is lim-ited by the shallow water table. A seasonallyhigh fresh/brackish water table causing peri-odic ‘‘washing’’ is probably responsible for theformation of the abrupt white C horizons.

acknowledgments

We acknowledge the support and assistanceof the Commonwealth Department of the

Environment, Water, Heritage, and the Arts(DEWHA) for providing us with the oppor-tunity to study the terrestrial ecosystems atCoringa-Herald National Nature Reserve. Inparticular, Neil Gemmell and Astrida Mednisare acknowledged for their leadership on ex-peditions. Mark Hallam (ex-DEWHA) is alsoacknowledged for his collections of some soilsamples from NE Herald Cay. QueenslandHerbarium management, particularly Gor-don Guymer, is acknowledged for his supportand commitment. Hans Dillewaard and Da-vid Halford are acknowledged for assistancein fieldwork, and David Appleton at UQ An-alytical Services laboratory for assistance insoil tests and interpretations. The variousskippers and other field staff on the trips toCHNNR are thanked for providing safe pas-sage.

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