Contributions of ant mounds to soil carbon and nitrogen pools in a marsh wetland of Northeastern China
Introduction
Ants can constitute a significant part of the animal biomass and also act as soil engineers in many ecosystems (Folgarait, 1998). Most ant species build their nests in mineral soil, but some species build mounds using organic material. The ability to build biogenic structures is a foundation of the soil engineering concept (Jones et al., 1997, Lavelle, 1997). In soil, the relative importance of regulation imposed by ecosystem engineers (sensu Jones et al., 1994) is likely to exceed regulation by foodweb complexity (Lavelle, 2002). Ants, as ‘extended phenotype engineers’, whose activities are concentrated at a few locations, have a large impact on the maintenance of ecosystem heterogeneity (Jouquet et al., 2006). The documented importance of ants to ecosystem functioning suggests that ants could affect soil structural heterogeneity (Folgarait, 1998, Boulton et al., 2003) and soil fertility (Holec and Frouz, 2006, Wagner and Jones, 2006, Ohashi et al., 2007, Jiménez et al., 2008). Besides soils, ants can also affect surface hydrological processes (Cammeraat and Risch, 2008), biological diversity and ecosystem functioning (see King, 1977, Moutinho et al., 2003, Wardle et al., 2011). At ecosystem scales, knowledge of ant mound densities and patterns will be crucial to understanding the impacts of ants, but this information is rarely available. Even less is known about the contribution of ant mounds to the total nutrient pools across ecosystems (Risch et al., 2005, Kilpeläinen et al., 2007).
Studies on the impacts of ant mounds to soil properties have been conducted in forests (Lenoir et al., 2001, Risch et al., 2005, Kilpeläinen et al., 2007), neotropical savannas (Jiménez and Decaëns, 2006), grasslands (Snyder et al., 2002, Wagner et al., 2004, Lane and BassiriRad, 2005) and agricultural lands (Amador and Görres, 2007). The ant species considered thus far include red wood ants (Formica rufa group, Lenoir et al., 2001, Domisch et al., 2008), leaf-cutting ants (Atta sexdens, Verchot et al., 2003; Atta colombica, Hudson et al., 2009), harvester ants (Pogonomyrmex rugosus, Whitford and DiMarco, 1995; Pogonomyrmex occidentalis, Snyder et al., 2002; Pogonomyrmex barbatus, Wagner et al., 2004) and red fire ants (Solenopsis invicta, Bender and Wood, 2003), because of their high densities and ecological importance. Ant mounds are also important biogenic structures in wetlands, where they can occur at high densities (Folgarait, 1998, Wu et al., 2010b). However, only a few studies have described ant mound characteristics in wetlands, or ant roles in the pedoturbation of wetland soils (Czerwinski et al., 1969, Pętal, 1980, Wu et al., 2013).
Wetland ecosystems comprise one of the most important carbon (C) and nitrogen (N) pools globally. Although wetlands occupy only 5–8% of the land's surface (Mitsch and Gosselink, 2007), they contain 20–25% of the terrestrial C and N pools (Batjes, 1996). Impacts of ant mounds on soil concentrations of C and N might potentially influence overall wetland C and N pools, but to what extent is unknown. Previous nutrient concentration studies have generally studied biogenic structures as a whole rather than analyzing the different components (Frouz et al., 2003, Jiménez and Decaëns, 2006, Hudson et al., 2009). For ant mounds, most research has focused on the above-ground portions of mounds, but Kilpeläinen et al. (2007) found soil nutrient concentrations under mounds were also influenced by ant mound-building.
In this study, we investigated the impacts and contributions of ant mounds to soil carbon and nitrogen pools in a marsh wetland in the Sanjiang Plain, Northeast China. The Sanjiang Plain is one of the largest and most intact freshwater wetlands in China (Liu and Ma, 2002). Numerous ant mounds occur in wetlands of the Sanjiang Plain and often cover a large proportion of the soil surface. Wu et al. (2010a) reported that ant mounds altered soil C, N and P distributions in a wet meadow ecosystem of the Sanjiang Plain. However, impacts and contributions of ant mounds to soil nutrient pools have not been studied in the marsh wetlands, another wide-spread habitat type in Sanjiang Plain. The goals of our study were to: (1) determine whether the soil C and N concentration profiles of ant mounds differed from those of the surrounding wetland soil, and whether such differences varied among ant species and (2) assess the contributions of ant mounds to the total pools of C and N in marsh soils by estimating ant mound nutrient storage and ant mound distribution density. We hypothesized that ant mound soils have enhanced both above and below ground nutrient contents compared to reference soil, and variation in soil nutrient concentrations and contributions to nutrient pools of ant mounds were dependent on the type of ant species present.
Section snippets
Study site
The Sanjiang Plain is a low floodplain, formed by the Heilong, Songhua and Wusuli rivers, located in Heilongjiang Province of Northeast China. The study was conducted in a Calamagrostis angustifolia–Carex schmidtii marsh at the Sanjiang Mire Wetland Experimental Station, Chinese Academy of Sciences (47°13″50′ N, 133°13″10′ E). In the Sanjiang Plain, wetland types vary with gradients in hydro-geomorphologic conditions. C. angustifolia–C. schmidtii marsh is seasonally inundated, and a dominant
Nutrient concentrations, bulk density
Overall concentrations of Corg (F = 12.34, df = 1, p = 0.001), DOC (F = 7.20, p = 0.010), TN (F = 13.95, p = 0.001), NO3− (F = 4.87, p = 0.033) and NH4+ (F = 8.26, p = 0.007) in above-ground ant mounds were significantly higher than in the surrounding reference soils (Fig. 2). However, Corg, TN, NO3− and NH4+ concentrations in soils under mounds and reference soils did not differ. DOC concentrations in reference soils were lower than in soils under L. niger mounds (F = 9.87, df = 3, p < 0.001), but not under F. candida or
Discussion
Our results confirmed that ant mounds contributed measurable amounts to soil nutrient pools in a marsh wetland. The contribution of Corg (5.3%) and N (TN 6.1%, NH4+ 6.9%, and NO3− 7.6%) was higher than that reported from European forests (C and TN 0.6–5.0% by Risch et al., 2005; C and TN <1% by Kilpeläinen et al., 2007). Our field investigations suggest that ant mound frequency and area tend to increase with marsh ecosystem degradation (Wu, unpublished data). Thus ant mounds can be important to
Conclusions
Our results confirm that ant mounds alter soil C and N concentrations and spatial characteristics among soil layers in marshes. Our study also demonstrated that formation of ant mounds can change overall soil C and N storage. Ant mounds contributed measurable amounts (5.3–7.6%) to the total nutrient pools of marsh soils. The impacts and contributions of ant mounds varied by ant species. Thus, ant mounds increased the spatial heterogeneity of nutrients pools, and are important for a more
Acknowledgments
We thank David C. Coleman in the University of Georgia, USA for providing useful suggestions. We thank Zhenghui Xu for identification of the ant species. We greatly appreciate three anonymous reviewers for their constructive comments on the manuscript. This work was supported by the programs of National Natural Science Foundation of China (Project 41171047, 40901036 and 41101468), Chinese Academy of Sciences visiting professorship for senior international scientists (grant no. 2012T1Z0017) and
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