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Enhancement of soil nutrients around nest entrances of the funnel ant Aphaenogaster barbigula (Myrmicinae) in semi-arid eastern Australia.

Introduction

Ants are probably the most conspicuous macro-invertebrates in arid and semi-arid rangelands because of their size and diversity. Through their foraging and seed harvesting activities, ants are active redistributors of organic matter (Briese 1982; Steinberger et al. 1991), and play a major role in soil and ecological processes (Hole 1961; Eldridge 1993; Eldridge and Pickard 1994). They also incidentally trap organic material in situ during nest building and bioturbation.

There is a large body of evidence to suggest that the large, generally long-lived, above-ground nests of some ants are associated with enhanced nutrients and water relations compared with nest-free soils (Czerwinski et al. 1969, 1971; Beattie and Culver 1983; Lee and Foster 1991). Although there is some evidence in Australia for nutrient enhancement by ants (Davidson and Morton 1981), the generally accepted view is that ants building small nest structures do not concentrate nutrients around their nests because of high nest turnover and thus the rapid relocation of nest entrances (Hughes 1991).

Large volcano-shaped structures surrounding entrances to the subterranean colonies of funnel ants (Aphaenogaster barbigula and A. longiceps) occur at high densities over many hundreds of square metres of the ground surface. Nest structures are a conspicuous feature of some coastal and semi-arid environments in eastern Australia, particularly where the soils are coarse-textured (Humphreys 1981; Mitchell 1988; Eldridge 1993; Eldridge and Pickard 1994; Nicholls and McKenzie 1994; Paton et al. 1995). From a pedological and geomorphic point of view, there are 2 main types of nest structures (Humphreys and Mitchell 1983; Paton et al. 1995). In Type-I mounds, the material is simply deposited on the ground and is very susceptible to erosion. These range in size from the small structures constructed by Pheidole spp., which are a few millimetres across, to larger structures produced by Aphaenogaster, which may be up to 30 cm across and 10 cm high. In contrast, Type-II mounds are larger, up to about 3 m in diameter, more compact, and therefore resistant to erosion, and mounds form an integral part of the nest (Humphreys and Mitchell 1983). Ants of the genus Aphaenogaster construct a Type-I nest that is intermediate in size and turnover between the long-lived (Type II) structures typical of Myrmicine species of the Northern Hemisphere, and the tiny Type-I structures with a high turnover described by Hughes (1991).

Observations at sites in semi-arid eastern Australia indicate that considerable quantities of organic material are incorporated into nest structures of Aphaenogaster barbigula during nest excavation. These observations prompted a study of the mechanisms whereby organic matter is trapped in situ by ant nest soil debris, and is therefore prevented from being transported off-site by overland flow. We hypothesised that, as nest building leads to the entrapment of organic material around Aphaenogaster nest entrances, it would lead to a build-up in soil fertility and therefore the development of nutrient-rich patches around nest entrances. We tested this by examining how soil chemical properties [nitrogen (N), organic carbon (C), and exchangeable cations] varied between surfaces with and without nest entrances.

Materials and methods

Study site

The study was conducted at a large colony of Aphaenogaster barbigula at Yathong Nature Reserve 140 km south west of Cobar, NSW (32 [degrees] 56' S, 145 [degrees] 35' E), between 1991 and 1994. The site was chosen because it is the centre of a long-term study of ant activity in relation to ground cover features, and has nest densities typical of eastern Australia. Nest entrances occupy up to 1% of the surface area of the landscape, and densities range from 0 to 40 entrances/[m.sup.2] (Eldridge 1993). The massive (structureless) soils at the site support a moderately dense stand of white cypress pine (Callitris glaucophylla). Apart from the timbered overstorey and scattered mosses (Barbula calycina, Didymodon torquatus) and lichens (Endocarpon pusillum and Heterodea beaugelholei), the soil surface supports only a sparse pasture cover dominated by members of the family Asteraceae.

The climate in the area is semi-arid, with a low and unreliable rainfall averaging 350 mm per annum. Rainfall is evenly distributed throughout the year, and average diurnal temperatures range from a maximum of 35 [degrees] C in January to 3.6 [degrees] C in July. Annual evaporation at Cobar is approximately 2575 mm (Bureau of Meteorology 1961). Soils at the site are massive red earths (Gn2.12, Northcote 1979) or Typic Durargids (Soil Survey Staff 1975), characterised by a dark, reddish brown sandy loam surface overlying a dark red clay hardpan at depths of 80-100 cm.

Field measurements

In August 1995, soils were collected from 10 ant nest entrances and 10 non-nest surfaces within a large area of about 4 ha. The 10 independent nest entrance sites were spaced randomly across the large colony and interspersed with the 10 non-nest sites. Nest-free surfaces were selected on the basis that they were [is greater than] 50 cm away from an existing active (i.e. currently occupied) or inactive/abandoned nest entrance (see Eldridge 1993 for explanations). Loose unincorporated soil and litter were carefully removed from the surface of both the nest and nest-free surfaces and discarded. At both nest and non-nest sites, a small pit was dug with a trowel and a sample of approximately 350 g of soil was taken from a depth of 0-2 cm and an additional sample was taken from a depth of 30-35 cm. The lower depth is considered to be the mean maximum depth of ant activity (Eldridge and Pickard 1994). The soil profile varied from a weakly coherent, earthy, fine sandy loam at depths of up to 40 cm, gradually grading to a moderately coherent, earthy, fine sandy clay to light clay at 80 cm. The material deposited around the nest entrances, which is typically a fine sandy loam, often consisted of granulated micro-aggregates, cemented together with mucus-like secretions (Eldridge and Pickard 1994). The resulting 40 samples (2 factorsx10 sitesx2 depths) were refrigerated, and transported to the laboratory for chemical analyses.

Soil analyses

Soil samples were air-dried, passed through a 2-mm sieve, and analysed according to the following methods. Electrical conductivity and pH were measured on a 1: 5 Soil water extract at a temperature of 25 [degrees] C (Rayment and Higginson 1992). Total soil N and total soil C (measured as the conversion of total C to [CO.sub.2]) were measured using the high frequency induction furnace technique with a LECO CNS 2000. As the soils were slightly acid (pH 5.5-6.5), most of the carbon detected in the soil using the LECO method equates to organically derived C. Exchangeable cations, sodium (Na), potassium (K), calcium (Ca), and magnesium (Mg), were measured using a Unicam Solaar 929 atomic absorption spectrophotometer.

Data analysis

Differences in soil chemical properties were analysed using a factorial treatment ANOVA model (Minitab 1989). Since the sample depths at each site were not independent, a split-plot blocking structure was necessary, where nest sites formed the whole plots and depths were the subplots.

Results

In situ trapping of organic matter

Field observations revealed that nest entrances commence as a tiny shaft in the soil, which is [is less than] 2 mm in diameter. Within a few days, this shaft is excavated into a large nest entrance (diameter 15.4 [+ or -] 0.37 mm; mean [+ or -] s.e.m.) with a typical conical-shaped torus (Fig. 1). Aphaenogaster barbigula ants maintain the integrity of the nest entrance by regular clearing of any litter, particularly leaves, which falls into the hole. This activity is generally greatest after rainfall events of [is greater than] 10 mm falling over a few days (Eldridge and Pickard 1994). After rain, any litter falling onto the soil surface meets 2 fates. Litter falling on or close to the torus is trapped in situ by soil which the ants excavate after rainfall. Litter falling at some distance from the nest entrance is eventually moved downslope by a process of rainsplash and entrainment by overland flow, and is either deposited as litter dams downslope where it is often colonised by termites and other soil invertebrates (Whitford et al. 1992), or is trapped on the upslope side of the nest entrances. Rain falling on the torus compacts the granular soil particles, increasing soil bulk density and reducing the height, and soil and trapped litter gradually extend out around the base of the torus. Subsequent reworking of the nest chamber and redeposition of litter on the surface result in successive layers of litter, separated by layers of soil. Some litter falls into the nest entrance where it is either cleaned out or remains. Within a few weeks the entrance is abandoned by the ants, and the torus rapidly loses its integrity and flattens out. Eventually all that remains is a small depression on the soil surface, until finally the location of the old nest entrance is indistinguishable from the surrounding soil. Observations suggest that, once the ants cease to excavate the nest entrance, the torus rapidly disappears, often within a month, although this depends on rainfall conditions. Eventually, reworking of torus material leads to a downward movement of decomposing litter until soil close to the entrances is dominated by organic material (Fig. 2).

[Figures 1-2 ILLUSTRATION OMITTED]

Soil nutrient levels

Data on soil chemical properties are given in Table 1. Ant nest soils contained significantly greater levels of total N (%) and total C (%) than the nest-free soils (P [is less than] 0.001). Furthermore, there was significantly more N in the surface samples than the samples from 30 cm, but only on the nest soils (Table 2). Mean electrical conductivity was very low in these sandy soils and was not significantly different between nest and nest-flee soils. There were slightly more free salts on the surface than at depth. The pH was significantly higher on ant soils and on the surface, but only on ant nest soils.

Table 1. Chemical characteristics of soils taken from nest entrances of Aphaenogaster barbigula and from nest-free (control) soils and the surface and subsurface
Chemical Nest entrances
property Surface Subsurface
 Mean s.e.m. Mean s.e.m.

Nitrogen (%) 0.09 0.01 0.02 0.01
Total carbon (%) 2.73 0.21 0.79 0.05
EC (dS/m) 22.72 3.13 23.40 3.59
pH 6.69 0.07 5.43 0.15
Exch. cations
 (mmol/kg)
 Na 0.38 0.06 0.43 0.06
 K 4.97 0.17 4.10 0.39
 Ca 23.04 1.10 13.43 1.84
 Mg 5.76 0.15 4.50 0.29

Chemical Nest-free surfaces
property Surface Subsurface
 Mean s.e.m. Mean s.e.m.

Nitrogen (%) 0.03 0.01 0.02 0.02
Total carbon (%) 0.72 0.05 0.65 0.03
EC (dS/m) 29.02 4.01 11.59 0.87
pH 5.79 0.11 5.24 0.05
Exch. cations
 (mmol/kg)
 Na 0.34 0.06 0.39 0.06
 K 4.25 0.15 3.02 0.20
 Ca 13.57 0.40 4.37 0.94
 Mg 5.42 0.09 3.04 0.20


Table 2. Breakdown of the effects of nest, depth, and nest x depth interactions on soil chemical properties associated with nest entrances of Aphaenogaster barbigula and nest-free soils A, ant soil; C, control soil; T, top (surface); B, bottom (depth, 30 cm)
Chemical Effect due to:
property Ant Depth

Nitrogen(%) P < 0.001 (A > C) P < 0.001 (T > B)

Org. carbon (%) P < 0.001(A > C) P < 0.001 (T > B)

EC (dS/m) n.s. P = 0.014 (T > B)

pH P < 0.001(A > C) P < 0.001 (B > T)

Exch. cations
(mmol/kg)
 Na n.s. n.s.
 K P = 0.007 (C > A) P < 0.001 (T > B)
 Ca P < 0-001 (A > C) P < 0.001 (T > B)
 Mg P < 0.001 (A > C) P < 0.001(T > B)

Chemical
property Ant x depth

Nitrogen(%) P < 0-001 (A: T > B)
 (C: T = B)
Org. carbon (%) P < 0.001 (A: T > B)
 (c: T = B)
EC (dS/m) P = 0.008 (A: T > B)
 (C: T = B)
pH P = 0.005 (A: T > B)
 (C: T = B)
Exch. cations
(mmol/kg)
 Na n.s.
 K n.s.
 Ca n.s.
 Mg P = 0-002 (A: T = B)
 (C: T > B)


n.s., not significant.

Trends for exchangeable cations were not as pronounced as for the other properties. Exchangeable Ca and Mg were generally higher in ant nest soils and at the surface, although there was a significant ant x depth interaction for Mg. Potassium levels were significantly higher in the control soils (P = 0.007) and on the surface. There was no significant relationship between either ant nest or depth and exchangeable Na levels (Table 2).

Discussion

In the present study, higher levels of N, organic matter, and most anions were associated with nest entrances of the funnel ant Aphaenogaster barbigula (Tables 1 and 2). Our field observations confirm that litter is trapped around nest entrances under successive layers of soil and nest debris reworked by Aphaenogaster barbigula. This ant activity, which is strongly tuned to rainfall events sufficient to activate overland flow and therefore entrainment of litter, leads to the development of zones of higher fertility around nest entrances.

The concentration of nutrients in the vicinity of ant nests appears to be a phenomenon widely reported for nests of Northern Hemisphere genera, such as Myrmica and Formica (e.g. Baxter and Hole 1967; Czerwinski et al. 1969; Gentry and Stiritz 1972; Beattie and Culver 1983; Culver and Beattie 1983). These ants construct large, generally circular nests up to a few metres in diameter, which are generally occupied continuously for many years, and consequently contain elevated levels of N, phosphorus (P), and organic matter. Whitford (1988) showed increased organic matter, total N, and organic C in nest soils of the harvester ant Pogonomyrmex rugosus compared with nest-free soils, and Carlson and Whitford (1991) demonstrated elevated concentrations of P, nitrates, and K in mound soils of Pogonomyrmex occidentalis compared with non-mound soils. Similarly, Rogers (1972) demonstrated increased levels of N and P in surface soils adjacent to ant nests on a North American prairie. In arid areas of New South Wales, Davidson and Morton (1981) demonstrated enhanced levels of N and P on large mounds of Rhytidoponera mayri.

Nest enrichment is thought to be evident only in species whose nests persist for many decades (Whitford 1996). However, unlike the large nests of the Northern Hemisphere genera, the nests of Aphaenogaster barbigula are smaller, approximately 25 mm across (external diameter; Fig. 2) with moderately high turnover rates (Eldridge 1993). Apart from the present study, we are aware of only one other Australian study reporting significantly increased nutrient levels associated with small-medium-sized nests. That study (Briese 1982) reported increased levels of C, N, and P from nests of the small ants Chelaner, Pheidole, and Iridomyrmex on clay soils in the Riverine Plain. Unfortunately, a lack of replication precluded any significance testing. Hughes (1991) reported no significant enhancement of nutrients associated with nests of 4 small ant species, Rhytidoponera metallica, Aphaenogaster longiceps, Iridomyrmex sp., and Pheidole sp., on infertile sandy soils near Sydney. The lack of increased fertility was attributed to the high turnover in nest structures and the continual excavation, abandonment, and reworking of soil around the nest entrances, which would have tended to dilute any small changes in nutrient levels. Conceivably, any increased fertility associated with ant nest structures may have been masked by the spatial variability in soils and hydrology at the site.

The storage of organic material within an ant nest is clearly a significant factor contributing to increased nutrient levels in these microsites. Whitford (1988) reported that decomposing plant material in shallow abandoned nest chambers is probably the initial source of organic matter and hence organic C responsible for enhanced nutrient levels in Pogonomyrmex rugosus nests. Aphaenogaster spp. are generalist foragers (Andersen 1990), and bring back plant material and seeds to their nests. In the present study, we believe that, compared with the large volumes of material trapped as a result of hillslope erosion processes, the contribution to nest fertility by seed harvesting is probably insignificant.

Apart from the increased nutrient levels associated with nests at the Yathong site, nests of these ants promote higher levels of infiltration of water due to the large biopore openings at the surface (Eldridge 1993, 1994). The depth to which this water permeates is significantly related to the size of the nest entrance openings (Eldridge 1993). Apart from the central large biopore conducting surface flows, numerous macropores, channels, and galleries allow free passage of water to the lower areas of these coarse-textured soils. The mixing of organic matter with soil around nest entrances, which have a potentially higher soil moisture status than nest-flee soils, probably stimulates populations of soil biota such as micro-arthropods and nematodes which are highly correlated with soil moisture levels (Whitford 1996). This in turn influences mineralisation rates and the distribution of essential minerals.

Although nest entrances in the Yathong landscape occupy [is less than] 1% of the surface area of the landscape (Eldridge 1993), their influence is clearly much greater. Using a conservative estimate that fertility is enhanced within a 20-mm radius of the centre of the entrance, these areas of higher nutrient concentration occupy approximately 7% of the landscape. These fertile sites probably represent sites for enhanced plant establishment and survival, as well as a refuge for small litter-inhabiting fauna, particularly during dry periods. Given the importance of water and nutrients, the 2 most limiting resources in arid environments, these small water- and nutrient-rich patches are critical to the maintenance of plant and animal diversity in arid systems.

Threshold rainfall required to reactivate nest clearing by Aphaenogaster barbigula is of a similar magnitude to that required to entrain organic matter deposited on the nest-flee soil surfaces. At this site, the interaction between ant activity and landscape processes resulted in a build-up of N and organic C on inherently infertile sandy soils. Although we hypothesise that enhanced fertility will result in a greater number of `safe sites' for vascular plants (Harper et al. 1961) and increased populations of soil fauna, there is clearly a need to document temporal and spatial changes in soil nutrients in this landscape and their implications for soil biota.

Acknowledgments

We thank Terry Koen for statistical advice, and Geoff Humphreys, Lesley Hughes, and Alister Spain for comments on an earlier draft. The work was carried out under a permit from the NSW National Parks and Wildlife Service.

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Manuscript received 17 April 1998, accepted 20 July 1998

D. J. Eldridge(A)(C) and C. A. Myers(B)

(A) Department of Land and Water Conservation, Centre for Natural Resources, c/- School of Geography, University of New South Wales, Sydney, NSW 2052, Australia.

(B) School of Geography, University of New South Wales, Sydney, NSW 2052, Australia.

(C) Corresponding author; email: D.Eldridge@unsw.edu.au
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Author:Eldridge, D. J.; Myers, C. A.
Publication:Australian Journal of Soil Research
Date:Nov 1, 1998
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