Wood ant nests as potential hot spots for carbon and nitrogen mineralisation

Red wood ants (the Formica rufa group) build large nests which can last for many years. The ants often bring large quantities of conifer litter to their mounds. In this study we compared chemical properties of the forest floor and ant-nest materials of red wood ants at two different sites in Sweden. We also did an incubation study in the laboratory to determine C and N mineralisation rates of soil and nest materials at two moisture levels. Some chemical properties, e.g. pH and C/N ratio, differed between sites. Nests were always drier, had a higher C/N ratio and often had higher NH₄⁺ concentrations than the surrounding forest floor. This indicates that the nests increase spatial heterogeneity of the forest floor not only because they contain large amounts of organic matter but also because of qualitative differences in nest composition. In the incubation study, experimentally increased moisture levels in the nest material resulted in net N immobilisation, while surrounding litter and humus materials showed net N mineralisation. The CO₂ evolution was clearly increased by increased moisture. Dry conditions in the nest material prevented it from being decomposed and, since there were no plants to take up N, inorganic N accumulated. However, based on our results we hypothesise that ant nests do not produce large amounts of inorganic N as long as the ants bring fresh litter with high C/N ratio to the nests and keep the nests dry. However, the nests can be considered as potential "compost heaps" which probably decompose faster and will be a source of N leaching when the ants no longer maintain the specific dry conditions.     

Correlation between the morphogenetic types of litter and their properties in bog birch forests

A formalized arrangement of morphogenetic types of litter according to the physicochemical parameters provided their significant grouping in three genetic associations. The litter group (highly decomposed + moderately decomposed) is confined to the tall-grass group of bog birch forests. The rhizomatous (roughly decomposed) litter is formed in the sedge-reed grass bog birch forests. The litter group (peaty + peatified + peat) is associated with the bog-herbaceous-moss group of forest types. The genetic associations of the litters (a) reliably characterize the edaphic conditions of bog birch forests and (b)correspond to formation of the peat of certain ecological groups. We found highly informative the acid-base parameters, the exchangeable cations (Ca²⁺ + Mg²⁺) and the total potential acidity, which differentiated the genetic associations of litter practically with 100% probability. The expediency of studying litters under groups of forest types rather than under separate types of bog birch forests was demonstrated.     

Variability in soils and vegetation associated with harvester ant (Pogonomyrmex rugosus) nests on a Chihuahuan Desert watershed

The effects of harvester ant (Pogonomyrmex rugosus) nests on the density and cover of spring annual plants and on soil characteristics were measured at three locations characterized by different soils and dominant vegetation on a desert watershed. There were few differences in vegetation and soils associated with harvester ant nests at locations at the base of the watershed where brief periods of flooding and sediment deposition occur at periodic intervals. At mid-slope locations, there were significant increases in total nitrogen, inorganic phosphorus, and cover (biomass) of four species of spring annuals at the edges of nest disks when compared with reference sites. The spring annuals that exhibited increased cover were species that increase biomass as a function of available nitrogen. At a clay-loam, Scleropogon-Hilaria, grassland site, there were significant reductions in the concentrations of Ca²⁺ and Mg²⁺, significant increases in nitrate and total nitrogen, but a significant increase in cover in only one species of annual plant. The data demonstrate that the effects of ants on soil properties and vegetation vary with site location and soil type.     

Respiration in wood ant (Formica aquilonia) nests as affected by altitudinal and seasonal changes in temperature

Wood ants build large, long-lasting nests and maintain stable high temperatures in nest centers from April to September to support brood development. Similar nest temperatures have been recorded regardless of latitude and altitude, suggesting that nests from colder environments produce more heat than nests from warmer environments. We measured changes in temperature and in situ respiration in nests from 700 and 1000 m a.s.l. We also sampled ants and nest materials from the same nests and measured their respiration at 10 and 20 °C in the laboratory. Both ant and microbial respiration increased greatly as temperature increased in spring, especially at 1000 m, resulting in the increase in nest temperature in spring. Metabolic activity greatly increased for ants in March and for nest microorganisms in April when nutrient contents also increased because of input of plant material and food. Nests from 1000 m maintained similar temperatures as nests from 700 m in summer but were colder in winter. Ants were responsible for the maintenance of high temperatures during summer but metabolic activity did not differ between the two altitudes, suggesting that the increased respiration and heat production in summer by ants at the higher altitude resulted from an increase in numbers of ants per colony.     

Unique soil microbial assemblages associated with grassland ant species with different nesting and foraging strategies

Ants are important ecosystem engineers and can be abundant in extensively managed grassland ecosystems. Different ant species create nests varying in structure and size, and tend to have a variety of feeding strategies. Differences in food imported to the nest and contrasting nesting behaviour may control soil microbial community structure in nest soil, with cascading effects on nutrient cycling, but this has not been tested in grassland ants. Soil and ants were sampled from nests of three ant species: two formicines; Lasius flavus (aphid farmer/scavenger, mound builder) and Formica lemani (scavenger/hunter, non-mound builder), and a myrmicine; Myrmica sabuleti (hunter/scavenger, non-mound builder), in an extensively grazed temperate grassland and compared to similar soils without ants. Microbial assemblages were determined using molecular approaches (terminal restriction length polymorphism and automated ribosomal intergenic spacer analysis). Both aboveground (vegetation diversity) and belowground (soil physico-chemical properties) components were measured to assess the potential of the different ant species to modify the environment. Stable isotope ratios (δ¹³C and δ¹⁵N) of ant tissues and nest soil organic matter confirmed differences in trophic distances. Significant changes in soil pH, moisture content, total C and total N, and in vegetation composition, demonstrated ant ecosystem engineering effects. In turn, nests of L. flavus, M. sabuleti and F. lemani had different microbial activities and harboured significantly different microbial assemblages (total bacteria, total fungi, ammonia-oxidising bacteria and nitrogen-fixing bacteria), but the diversity was similar. These findings suggest that grassland ants can control microbial assemblages via changes in physical and biological soil characteristics in their nests, and as such, different ant species harbour unique microbial assemblages in nests.     

Variability in soil chemistry and arbuscular mycorrhizal fungi in harvester ant nests: the influence of topography, grazing and region

Harvester ants are important disturbance agents across western North America, but the effects of ant disturbances on soils may vary considerably with topography and land use. We examined how soil properties and arbuscular mycorrhizal (AM) fungi in harvester ant nests varied across spatial scales according to topography, grazing regime and region. Soils from undisturbed areas were compared with nest disturbances created by two species of harvester ants, Pogonomyrmex occidentalis on shortgrass steppe in Colorado and P. rugosus on Chihuahuan desert grassland in New Mexico, in 1996 and 1997. Nests of both ant species were enriched with NO₃^--N, total P and roots colonized by AM fungi. Soil moisture was higher in P. rugosus' nests and lower in P. occidentalis' nests compared to surrounding areas. Soil pH was consistently lower in ant nests. Broad-scale factors such as grazing, topography and site affected most soil properties in and away from ant nests. Site exerted a strong influence on soil organic matter, pH and moisture. Within sites, topography had a significant affect on pH. Mycorrhizal colonization was influenced by site and topography in 1996 only, a substantially wetter year at both sites. Lastly, nutrient levels were largely determined by the fine-scale effects of ant disturbances. Principal components analysis revealed that, after removing site-level effects, harvester ants have similar functional roles in creating soil heterogeneity in these two different semiarid ecosystems.     

Ants mediate soil water in arid desert ecosystems: Mitigating rainfall interception induced by biological soil crusts?

As vital components of desert systems, the roles of ants in arid ecological processes have been well documented, while little attention has been given to their effects on soil water. We conducted a six-year investigation in sand dune systems stabilized via revegetation, to explore the hydrological role of ants through comparing the influence of ant nests on rainfall infiltration in different-aged revegetated dunes. The presence of ant nests markedly enhanced infiltration due to weakening the rainfall interception by biological soil crusts (BSCs) in revegetated dunes. The distribution of ant nest was denser in older revegetated areas, due to better developed BSCs of later successional stages, compared to younger revegetated areas. Ants prefer later to early successional BSCs because the later lichen–moss dominated crusts were thicker and their surface was more stable than the early cyanobacteria dominated crusts. Conversely, the crustal rainfall interception was positively correlated with BSC thickness. These findings suggest that the occurrence of ant nests in older revegetated areas benefited to the planted shrubs with deeper root systems and maintain a relative constant cover of shrubs in artificial sand-binding vegetation following an increase in infiltration to deeper soil layers.     

Leaf-cutting ant (Atta Sexdens) and nutrient cycling: deep soil inorganic nitrogen stocks, mineralization, and nitrification in Eastern Amazonia

Nest excavation and agricultural activities of the leaf-cutting ant Atta sexdens create complex belowground heterogeneity in secondary forests of Eastern Amazonia. We examined the effects of this heterogeneity on inorganic-N stocks, net mineralization, and net nitrification to test the hypothesis that the bulk soil of the nests has higher net rates of mineralization and nitrification than soil that was not affected by the influences of ant nests, throughout the profile. This study was conducted in a secondary forest at Fazenda Vitoria, near Paragominas in the Eastern Brazilian Amazon, where a previous study showed that the bulk soil of ant nests had elevated NO₃⁻. The results of the inorganic-N measurements were consistent with the previous study, showing elevated NO₃⁻ deep in the soil profile of the nests. However, neither net mineralization nor net nitrification were significantly greater at depth in the mineral soil of the nests compared to soil that was not influenced by nests (P=0.05), although variability was higher in the nest soil. These results suggest that the NO₃⁻ may have diffused into the surrounding mineral from the N-rich organic matter buried by the ants in chambers within the deep soil.     

Ant mounds as a source of sediment on citrus orchard plantations in eastern Spain. A three-scale rainfall simulation approach

Ants are widely found in Mediterranean soils, where they increase water infiltration rates by forming soil macropores during nest construction. While higher water infiltration usually results in lower soil erosion rates, new soil brought to the surface by ant activity could increase sediments available for erosion. This could be especially important in intensively-managed citrus orchards, where surface mineral soil is exposed due to the lack of vegetation cover as a consequence of herbicide treatments. In the summer of 2009 rainfall simulations of low frequency–high intensity rainstorms were conducted in an orange orchard in eastern Spain on plots that contained ant nests and adjacent paired-plots without ant nests. Since soil erosion is a scale-dependent process, we used three plot sizes (0.25 m², 1 m², and 12 m²) to determine the effect of ant burrowing and nesting on soil and water losses. Ant nests decreased water losses from 22.5% at 0.25 m² to 10.6% at 12 m², but soil erosion rates were nearly double in areas with ant activity (0.56 to 0.59 Mg ha^− 1 h^− 1), as compared to soil with no ants (0.31 to 0.36 Mg ha^− 1 h^− 1). Our results indicate that the presence of ants can increase soil erosion when rainfall intensity is greater than the infiltration capacity of the ant macropores.     

Ant-mediated effects on spruce litter decomposition, solution chemistry, and microbial activity

Forest management practices often generate clear-cut patches, which may be colonized by ants not present in the same densities in mature forests. In addition to the associated changes in abiotic conditions ants can initiate processes, which do not occur in old-growth stands. Here, we analyse the effects of ants and aphid honeydew on litter solution of Norway spruce, microbial enzyme activities, and needle decomposition in a field and greenhouse experiment during summer 2003. In the field, low ant densities had relatively little effects on litter solution 30 cm away from a tree trunk, but significantly increased organic carbon concentrations and decreased inorganic nitrogen concentrations next to a trunk where ants tend to build their nests. In a greenhouse experiment, the addition of ants to lysimeters containing spruce litter significantly increased dissolved organic carbon (DOC), dissolved organic nitrogen (DON), NH₄-N, NO₃-N and K concentrations in litter solutions compared to the control treatment, while the simulation of aphid infestation (addition of honeydew) significantly increased DOC as a direct result of honeydew leaching, and decreased inorganic N concentrations in leachates. The presence of ants resulted in a changed composition of dissolved organic matter (DOM) with more aromatic and complex compounds, and microbial enzyme activity was significantly higher in litter extracts from the ant treatment compared to the honeydew and control treatment. However, mass loss, litter %C and %N were not affected by ants or honeydew. Our results suggest that ants have a distinct and immediate effect on solution composition and microbial activity in the litter layer indicating accelerated litter decay whereas the effect of honeydew was insignificant.     

Nest predation in a fragmented Afrotropical forest: evidence from natural and artificial nests

Nest predation accounts for a substantial share of nest failure and low reproductive success in most tropical songbirds. Normally, forest fragmentation leads to an increase in nest predation pressure due to reduced cover, fewer (and poorer) nest sites and predator influxes from the surrounding habitats. To test this hypothesis, we studied natural nesting behaviour and nest success of the white-starred robin (Pogonocichla stellata) in seven Afrotropical forest fragments differing in size and level of habitat disturbance. Based on data from 12 nests, we estimate that 29% of all natural nests initiated by the robins survive to produce fledglings across all fragments. We also conducted an experiment using artificial (plasticine) model-eggs to reveal potential predators and compare relative predation rates amongst fragments. This experiment revealed that small mammals might be the major predators on robin nests at the egg-stage. In addition, it showed that the highest incidences of nest disturbance during this stage were in the most heavily disturbed fragment. This was presumably attributable to an influx of mammalian predators from the surrounding habitats as forest degradation created suitable habitats for them. Such an infiltration was recently reported in this study site. Both nest placement and microhabitat did not significantly affect depredation levels in our experiment. This suggests that depredation was predominantly incidental (i.e., predators mainly encountered nests fortuitously while foraging for other food items), where the likelihood of encountering a nest largely depended upon the prevalence of the principal potential predators - the small mammals.     

Temporal patterns of nutrient availability around nests of leaf-cutting ants (Atta colombica) in secondary moist tropical forest

Leaf-cutting ants consume up to 10% of canopy leaves in the foraging area of their colony and therefore represent a key perturbation in the nutrient cycle of tropical forests. We used a chronosequence of nest sites on Barro Colorado Island, Panama, to assess the influence of leaf-cutting ants (Atta colombica) on nutrient availability in a neotropical rainforest. Twelve nest sites were sampled, including active nests, recently abandoned nests (<1 year) and long-abandoned nests (>1 year). Waste material discarded by the ants down-slope from the nests contained large concentrations of nitrogen and phosphorus in both total and soluble forms, but decomposed within one year after the nests were abandoned. Despite this, soil under the waste material contained high concentrations of nitrate and ammonium that persisted after the disappearance of the waste, although soluble phosphate returned to background concentrations within one year of nest abandonment. Fine roots were more abundant in soil under waste than control soils up to one year after nest abandonment, but were not significantly different for older sites. In contrast to the waste dumps, soil above the underground nest chambers consistently contained lower nutrient concentrations than control soils, although this was not statistically significant. We conclude that the ‘islands of fertility’ created by leaf-cutting ants provide a nutritional benefit to nearby plants for less than one year after nest abandonment in the moist tropical environment of Barro Colorado Island.     

Carbon,nitrogen and phosphorus dynamics of ant mounds (Formica rufa group) in managed boreal forests of different successional stages

Wood ants (Formica rufa group) are ubiquitous in European boreal forests and their large long-lived mound nests, which mainly consist of forest litter and resin, accumulate carbon (C) and nutrients. The C and nutrient dynamics of wood ant mounds in response to forest succession have received minor attention in boreal forests. We aimed to study whether the C, nitrogen (N) and phosphorus (P) concentrations and the bulk density of ant mounds differ from those of the surrounding forest soil, to estimate the C, N and P pools in ant mounds, and to test whether the concentrations and pools change with forest age. Norway spruce (Picea abies (L.) Karst.) stands on medium-fertile sites in 5-, 30-, 60- and 100-year stand age classes were studied in eastern Finland. Carbon and P concentrations in the above-ground mound material were higher than those in the surrounding organic layer. The C, N and extractable P concentrations were higher in the soil under the ant mounds than in the surrounding mineral soil (0–21 cm). The low bulk densities in the ant mounds and the soil below them could be a result of the porous structure of ant mounds and the soil-mixing activities of the ants. The C/N ratios were higher in the mounds than in the organic layer. Carbon concentrations in the ant mounds increased slightly with stand age. Carbon, N and P pools in the ant mounds increased considerably with stand age. Carbon, N and P pools in ant mounds were <1% of those in the surrounding forest soil. Nevertheless, the above- and belowground parts of the ant mounds contained more C, N and P per sampled area than the surrounding forest soil. Wood ants therefore increase the spatial heterogeneity in C and nutrient distribution at the ecosystem level.     

Decomposition of organic matter and nutrient mineralisation in wood ant (<Emphasis Type="Italic">Formica rufa</Emphasis> group) mounds in boreal coniferous forests of different age

Wood ants (Formica rufa group) are dominating ecosystem elements of the boreal region due to their wide and abundant occurrence. They collect and concentrate organic material from the surrounding forest floor by building large above-ground mounds. These mounds have higher temperature and lower water content than the surrounding forest floor. We studied how these peculiar environmental conditions affected mass loss and carbon (C), nitrogen (N), phosphorus (P) and potassium (K) mineralisation of organic matter in boreal Norway spruce (Picea abies L. Karst.)-dominated mixed forest stands of four different age classes (5-, 30-, 60-, and 100-year-old) situated in eastern Finland using the litter bag technique. Norway spruce needle litter was incubated in inhabited and abandoned wood ant mounds as well as on the surrounding forest floor. We expected decomposition to be extremely slow due to the dryness of the mounds. Mass losses inside inhabited mounds were lower compared to the surrounding forest floor (on average 30 vs 50% after 2 years) but not as low as we expected, which might be a result of ant and microbial activity in the mounds. Decomposition in the abandoned mounds proceeded similarly as on the forest floor. Nutrient mineralisation proceeded more slowly in the ant mounds than on the surrounding forest floor. Mineralisation occurred for all studied nutrients in the ant mounds, except for N, which net amount remained stable during the years of the experiment. When wood ant mounds are abandoned and their porous and dry structure is no longer maintained by the ants, their decomposition is accelerated, and nutrients may be available for uptake by plants, although the nutrient mineralisation seems still to remain lower compared to the surrounding forest floor. However, eventually the mound material will be decomposed and nutrients mineralised, thus providing a nutrient hot spot increasing the heterogeneity of forest floor nutrient availability.     

The impact of harvester ants on decomposition, N mineralization, litter quality, and the availability of N to plants in the Mojave Desert

In arid areas of North America, nests of the seed-harvesting ant Pogonomyrmex rugosus tend to be elevated in mineral nitrogen and other soil nutrients relative to other microhabitats. We investigated the roles of decomposition, N mineralization, and plant nutrient uptake in maintaining high standing stocks of nutrients in P. rugosus ant nests. Decomposition rates of standard cellulose substrates placed on the surface of ant nests and other desert microhabitats suggest that conditions found in ant nests and bare areas are conducive to higher rates of decomposition than conditions under shrubs. In laboratory incubations of moist soil, net N mineralization rates were significantly higher in soil from ant nests than from bare areas and under two of three plant species. Net N mineralization rates measured in situ were much lower than those measured in laboratory incubations, but ant nest soil still exhibited higher rates at one of two sites. Litter collected from ant mounds, composed chiefly of seed chaff, was similar in N content to litter collected from underneath the dominant plant species, but had a significantly higher mean δ¹⁵N. Using this distinctive isotope signature as a tracer, we found no evidence that large perennial shrubs tap ant nests as a source of N. An invasive, annual grass species was significantly enriched in ¹⁵N, had higher leaf %N, and produced more seeds when growing on the mound than when growing several meters away; however P. rugosus nest surfaces are typically free of such annuals. We conclude that both high rates of nutrient cycling relative to other Mojave Desert microhabitats and low N utilization by the surrounding vegetation contribute to high standing stocks of mineral N in P. rugosus nests.     

Cation exchange in forest soils: the need for a new perspective

The long‐term sustainability of forest soils may be affected by the retention of exchangeable nutrient cations such as Ca²⁺ and the availability of potentially toxic cations such as Al³⁺. Many of our current concepts of cation exchange and base cation saturation are largely unchanged since the beginnings of soil chemistry over a century ago. Many of the same methods are still in use even though they were developed in a period when exchangeable aluminium (Al) and variable charge were not generally recognized. These concepts and methods are not easily applicable to acid, highly organic forest soils. The source of charge in these soils is primarily derived from organic matter (OM) but the retention of cations, especially Al species, cannot be described by simple exchange phenomena. In this review, we trace the development of modern cation exchange definitions and procedures, and focus on how these are challenged by recent research on the behaviour of acid forest soils. Although the effective cation exchange capacity (CECe) in an individual forest soil sample can be easily shown to vary with the addition of strong base or acid, it is difficult to find a pH effect in a population of different acid forest soil samples. In the very acidic pH range below ca 4.5, soils will generally have smaller concentrations of adsorbed Al³⁺. This can be ascribed to a reduced availability of weatherable Al‐containing minerals and a large amount of weak, organic acidity. Base cation saturation calculations in this pH range do not provide a useful metric and, in fact, pH is modelled better if Al³⁺ is considered to be a base cation. Measurement of exchangeable Al³⁺ with a neutral salt represents an ill‐defined but repeatable portion of organically complexed Al, affected by the pH of the extractant. Cation exchange in these soils can be modelled if assumptions are made as to the proportion of individual cations that are non‐specifically bound by soil OM. Future research should recognize these challenges and focus on redefining our concepts of cation retention in these important soils.     

丹江口湖北库区不同林分类型枯落物储量及持水性能

对丹江口湖北库区马尾松林、柏木林、松柏混交林、针阔混交林、栎类林5种主要林分类型的枯落物储量、持水量、吸水速率进行研究。结果表明:不同林分类型枯落物现存量具有一定的差异,松柏混交林枯落物储量最大(29.26t/hm2),其次为马尾松林(24.49t/hm2)、针阔混交林(21.93t/hm2)、栎类林(6.56t/hm2),以柏木林枯落物储量最小(9.47t/hm2)。各林分不同层次持水量、吸水速率与浸水时间之间的动态变化规律基本相似,随着浸泡时间的增加,枯落物吸水速率具有差异,0～1h枯落物吸水最快,1～2h逐渐减缓,而到了2～10h枯落物吸水基本饱和,逐渐趋向于0。拟合回归发现,枯落物持水量与浸泡时间按指数方程Q=aln t+b增加,吸水速率与浸泡时间按幂函数V=ktn递减。同时,最大持水量均是半分解层>已分解层>未分解层,而吸水速率则是针叶林分半分解层>已分解层>未分解层,阔叶林为已分解层>半分解层>未分解层。     

Predation of artificial nests in a fragmented landscape in the tropical region of Los Tuxtlas, Mexico

Predation rates of artificial nests were investigated in a fragmented landscape in the lowlands of Los Tuxtlas in southern Mexico. Hen and plasticine eggs were used to assess predation pressure in four habitats: the interior of forest fragments, the forest-pasture edge, corridors of residual forest vegetation and linear strips of live fences across pastures. Three sites per habitat were used in three experimental trials. Hen and plasticine ground nests with three eggs each were alternated every 50 m along transects at each site. Predation rates on each type of nest were monitored for 9 days. Survey of potential avian and mammalian potential nest predators were conducted at each site prior to the experimental trails. Readings of amount of light illuminating the ground were taken by each nest at each site to assess exposure of nests. In general, average predation rates were significantly higher for both hen and plasticine nests in the forest-pasture edge and in the corridors than in the interior of the forest fragments. While birds and mammals were the principal predators on hen eggs in the forests, mammals were responsible for the majority (?70%) of eggs damaged at the other habitats. Surveys of potential nest predators showed that avian and mammalian potential nest predators were significantly more common at the forest-pasture edges and at the other habitats than in the forests. Readings of light reaching the ground suggest that concealment of nests by the vegetation may play an important role in predation risk. Our results are consistent with reports from other Neotropical rainforests indicating an increase of artificial nest predation pressures from forest interior to open habitats. Restoration of forest fragments, allowing the vegetation to grow along the forest-pasture edge and the planting of arboreal crops at the forest-pasture edges may be measures that could increase cover and nest protection.     

Nitrogenous constituents in nest soils of harvester ants (messov ebeninus) and their influence on plant growth

Abstract

As a corollary of a detailed study on the ecology of the Harvester ants in soils of semi‐arid natural pastures, samples from a loessial Arid Brown soil were analyzed for the content of nitrogenous constituents (humic compounds, amino acids, nitrates, ammonia) as well as phenols and carbohydrates and the nutrient status of the soil.

While there was a significant increase in fulvic and humic acids in the ant nests, the simultaneous decrease in amino acids ‐ in relation to the surrounding soil ‐ indicates that the humic substances have been synthesized from amino acids and the abundantly present phenols and possibly carbohydrates.

The much higher concentration of nitrogen compounds in the ant mounds supports the idea that the soil mixing activity of the Harvester ants enhances microbiological processes and in turn the fertility of the mounds, as evident in the better growth of the pasture plants.     

Nest-mounds of the yellow meadow ant (Lasius flavus) at the “Alter Gleisberg”, Central Germany: Hot or cold spots in nutrient cycling?

Nests of the yellow meadow ant (Lasius flavus) occur at high densities in grasslands worldwide. Although many studies have shown that L. flavus nests influence soil nutrient contents, little is known about their effect on soil nutrient cycling rates. The aim of this study was to examine the role of nest-mounds inhabited by L. flavus as potential ‘hot spots’ for soil nutrient cycling. Six pairs of nest-mounds and control soils were selected at a grassland site at the plateau of the Alter Gleisberg (Thuringia, Central Germany). L. flavus significantly modified the soil environment within the nest. In comparison to the control soils, nest-mounds were characterized by slightly higher soil temperatures during the summer months. In addition, we found that nests were related to decreased potential C mineralization rates and increased potential net N mineralization rates. Nest-mound soil exhibited lower amounts of SOC, hot-water extractable DOC and DN, and higher concentrations of leachable DOC and DN. Moreover, ants promoted the enrichment of base cations in the nest. Differences in the soil environment between nests and control soils were possibly a result of the burrowing activity of ants, soil mixing, accumulation of aphid honeydew, and decreased plant-derived nutrient inputs into the nest-mound soil. In conclusion, L. flavus nest-mounds had a significant but element dependent effect on the soil nutrient cycling and may represent cold spots for C cycling and hot spots for N cycling. Thus, L. flavus nests increase the spatial heterogeneity of soil properties and create unique micro-sites within grassland ecosystems.