5:15 PM - 6:30 PM
[HTT16-P10] Soil food webs after more than a year since litter-removal experiment: an evaluation based on carbon and nitrogen stable isotope ratios
Keywords:soil microarthropods, food web, ecology
Organic layer on soil surface is vertically heterogeneous habitat and one of the important endmembers; i.e. their source of energy and nutrients, for soil fauna. The vertical heterogeneity of soil organic matter is primarily ascribed to decomposition process from plant litter to different matters so called fumus and humin. Many studies have shown that species composition of soil fauna is changed by perturbations of soil organic matter, mainly because the spatial heterogeneity is disrupted. On the other hand, effects of the disturbances to biological interactions between predator and prey; food web structures, are little described. In this study, we aimed to elucidate response of diets of soil microarthropods to ecological disturbances and compared carbon and nitrogen stable isotope ratios of them between those from litter removal plots and those from control plots.
Litter removal manipulation was conducted at a Japanese cypress forest (Chamaecyparis obtusa) in Kamigamo Experimental Station, Kyoto, Japan on July 12th, 2019. Litter manipulation plot was comprised of three 4 m x 4 m quadrates settled in one-line, and two 2 m x 4 m quadrates on both sides of the litter removal plot were defined as control plot. All quadrates were split into 2 m x 2 m subplots, and at least two soil cores were sampled from every subplot for the extraction of soil microarthropods by Tullgren apparatus. The extracted microarthropods in September 2019, were identified to order or suborder for the measurements of stable isotope ratios of carbon and nitrogen (δ13C and δ15N) for the following taxa; Oribatida (detritivore), Collembola (detritivore), Prostigmata (Predator or microbivore), and Gamasida (predator). Part of the soil cores were used for the δ13C and δ15N measurements of the following four collembola species; Tetracanthella sylvatica, Tomocerus varius, Folsomia octoculata, and Isotoma carpenteli. In 2020, only Gamasida and the four collembola species were chosen for measurements of the stable isotope ratios.
To test the effect of manipulation and sample type on the δ13C or δ15N values, ANOVA-like GLMM considering effect of plot as block factor, were conducted for each of soil organic layer, three Acari suborders with Collembola, and four Collembola species. Goodness of fit was evaluated by AIC, and in case singular fit was returned, block random factor was removed and GLM was re-selected. Post-hoc test was also conducted by model selection of GLMM for every sample type.
Based on the models, effect of litter removal was selected for δ13C, δ15N of Collembola species and δ15N of Acari suborders and Collembola. δ15N of organic layers were slightly different between manipulated plots and control plots, although the effects were not consistent between the sample types: δ15N of litter layer was increased, while δ15N of humus was decreased by litter removal. Collembola species living in the deeper organic layer showed in higher δ13C and δ15N than surface-living species, as shown in previous studies. Gamasida in the manipulated plot showed on average 1.7‰ and 0.7‰ higher δ15N values in 2019 and 2020, than those from control plot. These results indicate that predators approached to prey in deeper organic layers. Collembola species sampled in manipulated plot showed up to 0.5‰ lower δ13C values and up to 0.7‰ higher δ15N values than controlled plot in 2019. The difference in the δ13C value expanded up to 1.1 ‰ in a surface-dwelling Collembola (Tomocerus varius). Based on the effect of litter removal on δ13C of some Collembola species, phytophagous-oriented diet shift was suggested. On the other hand, relative abundance of Tomocerus varius was higher in the litter exclusion plot, while relative abundance of Folsomia octoculata and Isotoma carpenteli, species inhabiting deeper organic-layer (Upper F, lower F), decreased. These results indicate that ecological plasticity both on food and habitat is a key to survive in a perturbed system.
Litter removal manipulation was conducted at a Japanese cypress forest (Chamaecyparis obtusa) in Kamigamo Experimental Station, Kyoto, Japan on July 12th, 2019. Litter manipulation plot was comprised of three 4 m x 4 m quadrates settled in one-line, and two 2 m x 4 m quadrates on both sides of the litter removal plot were defined as control plot. All quadrates were split into 2 m x 2 m subplots, and at least two soil cores were sampled from every subplot for the extraction of soil microarthropods by Tullgren apparatus. The extracted microarthropods in September 2019, were identified to order or suborder for the measurements of stable isotope ratios of carbon and nitrogen (δ13C and δ15N) for the following taxa; Oribatida (detritivore), Collembola (detritivore), Prostigmata (Predator or microbivore), and Gamasida (predator). Part of the soil cores were used for the δ13C and δ15N measurements of the following four collembola species; Tetracanthella sylvatica, Tomocerus varius, Folsomia octoculata, and Isotoma carpenteli. In 2020, only Gamasida and the four collembola species were chosen for measurements of the stable isotope ratios.
To test the effect of manipulation and sample type on the δ13C or δ15N values, ANOVA-like GLMM considering effect of plot as block factor, were conducted for each of soil organic layer, three Acari suborders with Collembola, and four Collembola species. Goodness of fit was evaluated by AIC, and in case singular fit was returned, block random factor was removed and GLM was re-selected. Post-hoc test was also conducted by model selection of GLMM for every sample type.
Based on the models, effect of litter removal was selected for δ13C, δ15N of Collembola species and δ15N of Acari suborders and Collembola. δ15N of organic layers were slightly different between manipulated plots and control plots, although the effects were not consistent between the sample types: δ15N of litter layer was increased, while δ15N of humus was decreased by litter removal. Collembola species living in the deeper organic layer showed in higher δ13C and δ15N than surface-living species, as shown in previous studies. Gamasida in the manipulated plot showed on average 1.7‰ and 0.7‰ higher δ15N values in 2019 and 2020, than those from control plot. These results indicate that predators approached to prey in deeper organic layers. Collembola species sampled in manipulated plot showed up to 0.5‰ lower δ13C values and up to 0.7‰ higher δ15N values than controlled plot in 2019. The difference in the δ13C value expanded up to 1.1 ‰ in a surface-dwelling Collembola (Tomocerus varius). Based on the effect of litter removal on δ13C of some Collembola species, phytophagous-oriented diet shift was suggested. On the other hand, relative abundance of Tomocerus varius was higher in the litter exclusion plot, while relative abundance of Folsomia octoculata and Isotoma carpenteli, species inhabiting deeper organic-layer (Upper F, lower F), decreased. These results indicate that ecological plasticity both on food and habitat is a key to survive in a perturbed system.