Nematodes in deep soil profiles under young mycorrhizal forest stands

Mycorrhizal symbiosis of plants is widely distributed worldwide, and most of the forests form either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungal associations [1]. Therefore, mycelium of mycorrhizal fungi is being discussed as one of the important functional components of soil microbiome, and also as a feeding resource for soil biota. Nematodes are one of the key consumers of both roots and mycorrhizal fungi [2, 3]. In soil horizons deeper than 20-30 cm or subsoils [4], the mycelium of mycorrhizal fungi can be an even more important driver and source of nutrients compared to topsoil, due to the lack of plant-derived resources [5, 6]; but our knowledge on its functional role in subsoil is extremely scarce. Therefore, the high reliance of subsoil-living nematodes on the mycorrhizal fungi as a feeding substrate was hypothesized.

The research was carried out based on ten-years-old MyDiv tree diversity experiment, Bad Lauchstädt, Germany. The soil at the site is classified as Haplic Chernozem. The experiment manipulates tree species richness and mycorrhizal types; the detailed site description is given in Ferlian et al., 2018 [7]. We exclusively focused on plots exclusively represented by either ectomycorrhizal (ECM, four plots) or arbuscular mycorrhizal (AM, four plots) tree stands. Soil cores, 5 cm in diameter were sampled up to the depth of 1 meter using motorized soil auger. Soil profile was studied divided by genetic horizons: Ap1 (~0-10 cm), Ap2 (~10-35 cm), Ah (~40-60 cm) and C (~60-100 cm). Nematodes were extracted within a week of sampling using a modified Baermann method. Soil parameters (water content, % and bulk density, g*cm^-3) were estimated in parallel. Nematodes were counted and biomass estimated using an inverse microscope (Leica DM) under ×400 magnification, and then 100 specimens per sample were identified to genus level. Feeding types of nematodes were assigned to genera following Yeates et al. (1993) [8].

Our data show that the nematode community of only top soil horizon Ap1 (~0-10 cm) differ (MANOVA, F = 2.525; p = 0.0332) between ECM and AM dominated ecosystems. Both biomass and density of nematodes significantly decreased with depth, being the most pronounced for bacterivores in ECM systems (Least-squared Means test, t-ratio ≤ -5.293; p < 0.0001). The shown effects are likely related to the legacy of post-agricultural soil and high buffer capacity of Chernozem soils. The importance of further subsoil studies in mature forest stands, as well as the research on the nematode communities of post-agricultural reforestation successions is proposed.

References:

• Soudzilovskaia NA, et al. (2019). Nat Commun 10:5077. https://doi.org/10.1038/s41467-019-13019-2

• Kudrin AA, et al. (2021). Soil Biol Biochem 155:108184. https://doi.org/10.1016/j.soilbio.2021.108184

• Li Y, et al. (2009). Soil Biol Biochem 41:877–882. https://doi.org/10.1016/j.soilbio.2008.07.031

• Frelih-Larsen A, et al. (2018). Sustainability 10:3006. https://doi.org/10.3390/su10093006

• Callesen I, et al. (2016). For Ecol Manag 359. https://doi.org/10.1016/j.foreco.2015.08.019

• Dietzel R, et al. (2017). Soil 3:139–152. https://doi.org/10.5194/soil-3-139-2017

• Ferlian O, et al. (2018). Ecosphere 9:e02226. https://doi.org/10.1002/ecs2.2226

• Yeates GW, et al. (1993). J Nematol 25(3):315-331.