Original article The role of bacteria and protists in nitrogen turnover in ant nest and forest floor material: A laboratory experiment Veronika Jílkov  a a, b, * , Jan Frouz a , Tom  a  s Cajthaml a, c , Michael Bonkowski b a Institute for Environmental Studies, Charles University in Prague, Ben atsk a 2, CZ-12801 Prague, Czech Republic b Department of Terrestrial Ecology, Zoological Institute, University of Cologne, Zülpicher Str. 47b, D-50674 Cologne, Germany c Institute of Microbiology, Czech Academy of Sciences, Víde nsk a 1083, CZ-14220 Prague, Czech Republic article info Article history: Received 6 January 2015 Received in revised form 14 May 2015 Accepted 18 May 2015 Available online xxx Keywords: Ammonium Microbial loop Bacterial biomass Available carbon Coniferous forest abstract Wood ant nests are hotspots of nutrients and microbial activity in nutrient-limited coniferous forest ecosystems, as ants accumulate honeydew and nutrients in their nests due to foraging and building activities. In this study we carried out a microcosm experiment focussing on the role of bacteria and protozoa on carbon and nitrogen turnover in nutrient-rich and nutrient-poor litter materials. Two types of litter material, (i) ant nest material or (ii) surrounding forest floor material were sterilized and inoculated either with bacteria alone (B treatment) or with bacteria and protozoa in combination (BP treatment). The litter materials were subsequently incubated in laboratory microcosms for 21 days. Respiration of the microcosms was measured during the whole incubation period and leachates were sampled every week and analysed for ammonium and nitrate. Our results showed lower ammonium leaching and increased respiration in the BP treatment, which could be explained by higher microbial biomass in this treatment. The negative effect of protozoa on ammonium leaching was independent of the type of litter material suggesting that available carbon in nest material supported fixation of nitrogen in the bacterial biomass. Our data show that protozoan grazing can play a critical role in retaining ni- trogen in ant nests by increasing microbial activity and biomass, and thereby preventing nitrogen leaching. © 2015 Elsevier Masson SAS. All rights reserved. 1. Introduction Soils of temperate conifer forests that grow on granitic bedrock are acidic and have a low nutrient status [1]. Fulvic acids are released during the decomposition of needle litter material of coniferous trees thus acidifying the soil water and further reducing nutrient availability in litter materials as mineral nutrients are most available at neutral pH [2]. Although conifer forest ecosystems are usually nitrogen (N) limited [3], N availability is increased in nutrient-rich patches that occur in the forest floor, such as wood ant nests. However, N cycling in these nutrient-rich patches have not yet been investigated thoroughly. Wood ant (Formica s. str.) nests are typical for temperate and boreal forest ecosystems [4,5]. They can occur in densities of 10e20 nests per ha [6e8], and the mound nests of wood ants are large and permanent structures, being occupied for decades [5,9]. They are constructed from plant materials, mainly needles, as well as from mineral soil particles [10]. Since ants transport vast amounts of food and building material into their nests [7,11e 13], a part of these nutrient-rich organic materials afterwards succumbs to decompo- sition by microorganisms and, subsequently, mineral forms of nu- trients are released [10,14,15]. Respiration rates per m 2 of nest mounds can be 12 times higher than of the surrounding forest floor [8], and mineral nitrogen accumulates in the nests which consec- utively represent hotspots of carbon and nutrient turnover and significantly increase belowground heterogeneity in the otherwise nutrient-limited forest ecosystem. These nutrients can then be exploited for example by trees growing in nests' surroundings [16]. Although many studies deal with concentrations of nutrients in wood ant nests in comparison to the surrounding forest floor (e.g. Refs. [15,17e19]) or with carbon turnover [6,8,20], there is sur- prisingly little information on the nitrogen turnover in wood ant nest material. According to the general knowledge, one group of soil fauna that could have an impact in this respect are protists. Protists are abundant in soils of coniferous forest ecosystems and * Corresponding author. Institute for Environmental Studies, Charles University in Prague, Ben atsk a 2, CZ-12801 Prague, Czech Republic. E-mail address: jilkova.veronika@gmail.com (V. Jílkov a). Contents lists available at ScienceDirect European Journal of Soil Biology journal homepage: http://www.elsevier.com/locate/ejsobi http://dx.doi.org/10.1016/j.ejsobi.2015.05.004 1164-5563/© 2015 Elsevier Masson SAS. All rights reserved. European Journal of Soil Biology 69 (2015) 66e73