Quantifying the influences of free-living nematodes on soil nitrogen and microbial biomass dynamics in bare and planted microcosms Mesfin Tsegaye Gebremikael * , David Buchan, Stefaan De Neve Department of Soil Management, University of Gent, Coupure Links 653, 9000 Gent, Belgium article info Article history: Received 28 August 2013 Received in revised form 23 November 2013 Accepted 11 December 2013 Available online 27 December 2013 Keywords: Free-living nematodes Gamma irradiation Nitrogen Planted microcosms Mineralization PLFA abstract Several ecosystem processes such as nitrogen mineralization are mainly controlled by complex multi- trophic interactions in which nematodes play major roles. Despite the abundance and diversity of these nematodes, studies on their contribution to N mineralization have been limited to a few selected species and often in completely sterilized media. Such simplified experiments are unrealistic and usually give inaccurate results as part of the interaction is missed. Therefore the contributions of nematodes to N mineralization need to be quantified more accurately in realistic conditions where native microbes, nematodes and plants interact. To do this, we set up a microcosm incubation experiment in which the whole nematode communities were extracted from fresh soil and reinoculated into the soil defaunated by 5 kGy dose gamma irradiation that leaves the microbial community largely intact. Three treatments namely control, þNem (with nematodes) and Nem (without nematodes), with or without plants were incubated for 86 days. In bare microcosms, þNem cores increased total mineral nitrogen with 32% compared to Nem. However, no significant (p > 0.05) differences were observed on total mineral ni- trogen and plant N uptake between þNem and Nem treatments in planted microcosms. Nematode abundances and community structures were significantly changed over time and differed in bare and planted microcosms. Bacterivores dominated in bare, while herbivores dominated in planted soils throughout the incubation period. This could explain the contrasting effects of nematodes in N miner- alization in bare and planted microcosms. Optimization of gamma irradiation technique is required to further reduce its side effects on nutrient flush and non-targeted organisms. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Free-living soil nematodes are extremely abundant (3.5e5 million m 2 ) and diverse (33e384 species m 2 ) in terrestrial eco- systems (Neher and Powers, 2004; Yeates, 2003). Nematodes have evolved a diversity of feeding mechanisms and occupy key posi- tions in the soil food web that enabled them to highly interact with microbes, other soil fauna, plants and among themselves, and thus contribute to the crucial ecosystem services such as nutrient cycling (de Ruiter et al., 1993; Ferris, 2010). Nitrogen (N) mineral- ization and N availability in the soil are mainly controlled by these complex multitrophic interactions in which nematodes of different feeding groups play direct and indirect roles. Plant-feeding nema- todes increase the release of exudates into the soil stimulating more microbial growth (Denton et al., 1999; Yeates et al.,1999a,b). Bacterivorous and fungivorous nematodes feed on these microbes and excrete excess N directly into the soil mainly as NH 4 þ (Bouwman and Zwart, 1994; Ferris et al., 1998; Freckman, 1988). The abun- dances and activities of these microbivorous nematodes may also be controlled by predatory nematodes and other fauna indirectly influencing N mineralization (Neher, 2001; Wardle and Yeates, 1993). Despite their abundance, diversity and the complex multi- trophic interactions between nematodes and other soil biota, most of the studies conducted so far have been limited to simplified experiments in which few selected species of nematodes and mi- crobes are often inoculated in completely sterilized soil or inert media in the absence of plants (Anderson et al., 1981; Chen and Ferris, 1999; Ferris et al., 1998). The problems of such simplified experiments dealing with a specific feeding group at a time lie not only with the difficulties of extrapolating the results to the actual field conditions but also in that part of the actual interactions are omitted, resulting in a possibly less accurate estimation of their contribution. For example Tu et al. (2003) have estimated the contribution of plant-feeding * Corresponding author. Tel.: þ32 926 460 66; fax: þ32 926 462 47. E-mail addresses: mesfintsegaye.gebremikael@ugent.be, mgebremi@gmail.com (M.T. Gebremikael), david.buchan@ugent.be (D. Buchan), stefaan.deneve@ugent. be (S. De Neve). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2013.12.006 Soil Biology & Biochemistry 70 (2014) 131e141