Quantifying the Contribution of Entire Free-Living Nematode Communities to Carbon Mineralization under Contrasting C and N Availability

RESEARCH ARTICLE Quantifying the Contribution of Entire FreeLiving Nematode Communities to Carbon Mineralization under Contrasting C and N Availability Mesfin Tsegaye Gebremikael1*, Hanne Steel2, Wim Bert2, Peter Maenhout1, Steven Sleutel1, Stefaan De Neve1 1 Department of Soil Management, University of Ghent, Ghent, Belgium, 2 Department of Biology, Nematology Research Unit, University of Ghent, Ghent, Belgium * Abstract OPEN ACCESS Citation: Gebremikael MT, Steel H, Bert W, Maenhout P, Sleutel S, De Neve S (2015) Quantifying the Contribution of Entire Free-Living Nematode Communities to Carbon Mineralization under Contrasting C and N Availability. PLoS ONE 10(9): e0136244. doi:10.1371/journal.pone.0136244 Editor: Wenju Liang, Chinese Academy of Sciences, CHINA Received: July 10, 2015 Accepted: July 16, 2015 Published: September 22, 2015 Copyright: © 2015 Gebremikael et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: Funding was provided by The Research Foundation-Flanders (FWO): research grant G.0426.13N. http://www.fwo.be/; Special Research Fund-Ghent University (BOF): grant number BOF10/ 2JO/227. http://www.ugent.be/nl/onderzoek/ financiering/bof; Special Research Fund-Ghent University (BOF): (HS).http://www.ugent.be/nl/ onderzoek/financiering/bof. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. To understand the roles of nematodes in organic matter (OM) decomposition, experimental setups should include the entire nematode community, the native soil microflora, and their food sources. Yet, published studies are often based on either simplified experimental setups, using only a few selected species of nematode and their respective prey, despite the multitude of species present in natural soil, or on indirect estimation of the mineralization process using O2 consumption and the fresh weight of nematodes. We set up a six-month incubation experiment to quantify the contribution of the entire free living nematode community to carbon (C) mineralization under realistic conditions. The following treatments were compared with and without grass-clover amendment: defaunated soil reinoculated with the entire free living nematode communities (+Nem) and defaunated soil that was not reinoculated (-Nem). We also included untreated fresh soil as a control (CTR). Nematode abundances and diversity in +Nem was comparable to the CTR showing the success of the reinoculation. No significant differences in C mineralization were found between +Nem and -Nem treatments of the amended and unamended samples at the end of incubation. Other related parameters such as microbial biomass C and enzymatic activities did not show significant differences between +Nem and -Nem treatments in both amended and unamended samples. These findings show that the collective contribution of the entire nematode community to C mineralization is small. Previous reports in literature based on simplified experimental setups and indirect estimations are contrasting with the findings of the current study and further investigations are needed to elucidate the extent and the mechanisms of nematode involvement in C mineralization. PLOS ONE | DOI:10.1371/journal.pone.0136244 September 22, 2015 1 / 17 Nematodes and Carbon Mineralization Competing Interests: The authors have declared that no competing interests exist. Introduction Organic matter (OM) decomposition processes and the three primary energy pathways that exist in the soil food web, i.e., the bacterial, fungal and root channels are regulated by free-living nematodes [1–3]. For example, the fastest decomposition pathway in which bacteria are the primary decomposers, OM decomposition is affected by bacterivorous and omnivorous nematodes as they feed on and disperse these microbes [4–6]. Plant-root feeding and fungi feeding nematodes regulate the root and fungal energy channels, respectively. As they pierce the cell wall of plants or hyphae, part of the cell contents is transferred to and increases the size of the labile organic matter pool [2] which is a rate limiting factor for C and N mineralization [7, 8]. Two main mechanisms have been reported how nematodes are involved in C and N mineralization and nutrient flow in the soil food web: 1) C:N ratio differences between nematodes and their prey, particularly bacteria [6, 9, 10] and 2) high respiration efficiencies, wherein most of the assimilated C is respired as CO2 [6, 11–13]. Respiration efficiencies, defined as the amount of C respired per unit of C assimilated, have been consistently reported to be high (60–85%) for several nematode species in various feeding groups [11], indicating that it may be the main mechanism how nematodes are involved in energy and nutrient flows in soils. Despite such high respiration efficiency, previous studies reported that the contribution of nematodes to the total soil heterotrophic respiration is low. For instance, [14] estimated that nematodes contribute 0.8% and [15] 2% of the total heterotrophic soil respiration in a Japanese coniferous and US deciduous forest, respectively. These respiration values are not often determined by measuring CO2 experimentally, but calculated using the relation between the estimated weight of nematode population, their oxygen consumption and estimated values of the respiration quotient (RQ) [16]. Due to these methodological limitations, the reported low contribution of nematodes to the total heterotrophic soil respiration may thus be questioned. In contrast to the above estimates, studies which directly measured the relative CO2 respired by nematodes in soil microcosms reported significantly higher contributions of nematodes to C mineralization. For instance, [17] found that the presence of a bacterivorous nematode increased cumulative CO2-C mineralized by 50% and 27% over the presence of bacteria only in glucose amended and unamended treatments, respectively. While such kinds of experiments based on a single species of nematodes and bacteria are useful in investigating specific mechanisms in detail, the findings cannot be considered as the effect of the entire nematode community. Such findings obviously are not representative of field conditions where many species of nematodes interact amongst each other and with a multitude species of microbes. Moreover, the contribution of individual species to nutrient cycling may vary considerably as each nematode taxon exhibits a wide range of metabolic activity, and their energetics may depend on the food availability (C and N) and the nematode growth stage [18]. Thus, to obtain reliable estimates, the contribution of t (...truncated)