Effects of mound building Lasius flavus on organic carbon and nutrient fluxes in soils of temperate grassland ecosystems

Highlights

• Lasius flavus activity increases K fluxes in soil solutions under mounds.

• Reduced water fluxes accounted for reduced DOC and DN fluxes under L. flavus mounds.

• Water fluxes between mounds and controls vary with precipitation amounts.

Abstract

Ants that build long-lasting mounds and move large amounts of mineral soil may influence the composition and fluxes of soil solutions, and thus soil nutrient cycling. However, studies quantifying such effects are scarce. In central Europe, the yellow meadow ant (Lasius flavus) is a common species that changes the character of extensively managed grasslands by its mound building activity.

In total, sixteen mound-control pairs were selected within four plots on two grassland sites in Thuringia, Central Germany and monitored from 2013 to 2016. On each plot, bulk precipitation and soil solutions were collected via two open funnel samplers and eight suction plates to determine input and loss fluxes of dissolved organic carbon (DOC), nitrogen (DN), and base cations under L. flavus mounds and control locations. Effects of L. flavus mounds on soil microclimatic conditions were also investigated.

In comparison to the controls, L. flavus mounds were characterised by slightly lower soil temperatures and always lower soil moisture. The main driver of lower fluxes of DOC and DN from mounds was related to the reduced seepage water flux under mounds compared to the adjacent grassland. Lasius flavus activity increased potassium (K) concentrations and fluxes in soil solutions from mounds in comparison to the controls, independent from plot-specific soil and seepage water flux variability. Increased concentrations of calcium (Ca) and magnesium (Mg) were measured in the soil solutions of the mounds but were generally not associated with higher Ca or Mg fluxes. In conclusion, L. flavus present unique micro-sites within grassland ecosystems by creating mounds that may function as ecosystem control points for the cycling of certain elements, such as K.

Introduction

Ants, through their nesting and foraging activities, change the availability of resources and alter environmental conditions (e.g., temperature, soil moisture) that control the rates of important biogeochemical processes in ecosystems, such as decomposition and the turnover of organic matter and elements (Gutierrez and Jones, 2006; Lenoir et al., 2001; Ohashi et al., 2005). Their presence may lead to increased element concentrations and exceptionally high reaction rates in space and time, referred to as hot spots and hot moments (McClain et al., 2003).

Soil solutions represent the phase which characterizes such processes in soils and quickly reacts to changes (Ranger et al., 2001). Dissolved organic matter and nutrients in soil solutions are mobile, available and reactive fractions, and thus provide sensitive indicators for current and long-term soil biogeochemical processes (Kaiser and Kalbitz, 2012; Kalbitz et al., 2000). These fractions are able to reflect differing conditions caused by land-use, as well as ecosystem disturbances, e.g. fire (Näthe et al., 2018; Potthast et al., 2017). Despite a large body of knowledge on the influence of ant activities on soil physical properties and nutrient contents (Cammeraat and Risch, 2008; Frouz and Jilkova, 2008), studies of ant mediated effects on the composition and fluxes of soil solutions, as responsive components of the nutrient cycle, are seldom (Bierbaß et al., 2015; Finér et al., 2013; Stadler et al., 2006). Furthermore, studies investigating the effect of species activities on nutrient cycling may bring insight to the currently reviewed hot spots and hot moments concept (Bernhardt et al., 2017).

The effect of ants on nutrient cycling may be variable, depending on factors, such as the nesting strategy of the respective species. Stadler et al. (2006) observed significant increases of dissolved organic carbon (DOC) and nitrogen (DN) in solutions from wood ant nests-mounds in European forests, composed of organic material such as twigs and needles and only minor amounts of mineral soil (Seiffert, 2007). However, future climatic conditions, with forecasts of warmer temperatures, lower rainfall amounts and hence drier soil moisture conditions and more open vegetation structures, are predicted to favour ant species building mineral soil mounds over organic matter mound building species (Jurgensen et al., 2008). The significance of such mound building ants to nutrient cycling is closely related to the amount and properties of soil moved during nest construction, as well as to the longevity of the nest site (Lobry de Bruyn, 1999).

The yellow meadow ant Lasius flavus, a common species in Central Europe and beyond, changes the character of grasslands, sand dunes and salt marshes by building mounds of mineral soil, also referred to as “antscapes” or “hummock pastures” (Elmes, 1991; Schreiber, 1969; Woodell, 1974). On a yearly average, this subterranean-living ant species may bring up to seven tonnes of mineral soil material per hectare close to or on top of the ground surface (Seiffert, 2007). Observations of grassland age indirectly indicate that L. flavus mounds can last for over hundred years (King, 1981). This makes L. flavus an ecosystem engineer, a species which alters the environment and the availability of resources for other organisms through its activity (Jones et al., 1994; Jouquet et al., 2006). Out of three mound building ant species groups, L. flavus has the most pronounced and long-lasting effects on the vegetation (Kovar et al., 2000). Also, the stage of L. flavus mound development has important influence on plant species growth (Ehrle et al., 2017).

Mound building activity changes the transmission and dissipation of heat and the flow of matter. During summer, diurnal temperature gradients are larger on L. flavus mounds than on adjacent locations (Streitberger and Fartmann, 2015). The excavation of galleries and brood rearing chambers during nest construction increases soil porosity. Consequently, L. flavus activity may indirectly alter water infiltration and increase evaporation (Cammeraat and Risch, 2008) and thus affect seepage water fluxes and the contact time of water during percolation within grassland soils.

As a consequence of the reallocation and mixing of topsoil and deeper mineral soil, L. flavus mound soil is depleted in organic matter and enriched with inorganic nutrients derived from mineral soil and bedrock weathering (Dostal et al., 2005; Ehrle et al., 2019; Holec and Frouz, 2006). Studies based on soil extraction methods indicate that bioturbation during mound building lowers the release of soluble DOC and DN and increases soluble calcium (Ca) in established L. flavus mounds (Ehrle et al., 2017, 2019). On the other hand, Bierbaß et al. (2015) found higher concentrations of DOC and DN in soil leachates under active L. flavus mounds. Furthermore, L. flavus cover their energy requirement by consuming root aphids or their excreted honeydew (Pontin, 1978; Seiffert, 2007). Concentrations of potassium (K) and other nutrients in soil solutions may change through this food acquisition, as well as excreta and leaching from dead ant bodies (Frouz et al., 2003).

It remains unclear, whether effects of L. flavus mound building activity can also be detected in element fluxes obtained by measuring in situ soil solutions and their chemical properties. Consequently, the influence of the mounds constructed by this ant species to nutrient cycling is almost unknown. From their investigations during a two-month field observation period in summer, Bierbaß et al. (2015) suspected that L. flavus mounds are locations of enhanced N-mineralisation and DN leaching and may present hot spots of N-cycling within grassland ecosystems. If such patterns hold for different sites and longer time periods, has not yet been investigated.

Therefore, we investigate how soil microclimatic conditions as well as soil solution composition and element fluxes vary over space and time to make clear statements about the influence of L. flavus mound building on carbon and nutrient cycling in extensively managed temperate grassland ecosystems. Further, we investigate the inputs of elements by bulk precipitation as this deposition pathway can potentially drive soil element fluxes and ecosystem dynamics. The specific objectives of our study are to examine:

• soil temperature, soil moisture and seepage water fluxes,

• DOC and nutrient concentrations in soil solutions and

• DOC and nutrient fluxes in soil solutions of L. flavus mounds and control soils.

The investigations were conducted at two sites on four different plots varying in their soil physico-chemical properties to increase the explanatory power of our study. We hypothesize that L. flavus mounds provide warmer and drier soil conditions compared to the control locations. These specific soil physical conditions and the lower organic matter contents of the mounds lead to generally lower organic matter turnover as indicated by diminished leaching of DOC and DN from mounds in comparison to the surrounding soil. On the other hand, the combination of soil mixing and feeding activities increases especially base cation (e.g. Ca, K) concentrations under L. flavus mounds compared to the control locations. The effects of L. flavus mound building on carbon and nutrient fluxes in soil solutions and thus, nutrient cycling depend on the potential of such biogenic structures to alter the water fluxes of temperate grasslands.