1:45 PM - 3:15 PM
[HTT15-P06] Carbon and nitrogen isotope natural abundance as an indicator of litter derived organic matter input to surface soil: a case study in a field manipulation experiment
Keywords:δ13C, δ15N of soil organic matter, Soil disturbance, Nested exclusion experiment, Biodiversity and Ecosystem Functioning
Soil organisms, in particular macrofauna like earthworms, are known to play significant ecosystem function to incorporate organic matter into soil through their food intake. Due to the foraging behavior of mammals, typically wild boar decreases the abundance of soil macrofauna through disturbance of soil habitat by turning up, trampling for their foraging, and even forage the fauna. Then, overabundance of the mammals has been speculated to restrain the ecosystem function played by the soil macrofauna. On the contrary, turning up the soil should at the same time increase the input of fresh litter into surface soil. Such antagonistic effects of mammals on the soil organic matter (SOM) input lead to the complex and context-dependent aspects of mammal — carbon dynamics interaction.
In this study, a mammal exclusion experiment was conducted at the wild boar overabundance site, in Sadayama, Kochi Prefecture, Japan and tested if there is any sign of wild boar to inhibit SOM input by way of adverse effects on soil macrofauna. To exclude wild boar, wire baskets with approximately 40 cm x 35 cm square opening were bound on the soil surface, which were defined boar exclusion site, and a quadrate adjacent to the basket were defined as a control site. 10 boar exclusion-control pairs were set in the experimental area and these pairwise sites were placed in a line, with each pair were 10 m apart. For each of in a exclusion site and a control site, 2 types of mesocosm (ambient litter and soil were enclosed in a mesh container) were set and incubated during experiment. One mesocosm was enclosed by 5 mm mesh and allowed moving in and out of macrofauna, the other mesocosm was enclosed by 0.1 mm mesh and thus macrofauna was excluded from it.
Based on the gradual enrichment of δ13C and δ15N values associated with the SOM decomposition, we used δ13C and δ15N values of SOM as a mass-weighted average of "extent of decomposition". In that understanding, 13C and 15N enrichment in the way of decomposition were a few per-mill, which are comparable values to the δ13C and δ15N ranges of primary materials (i.e. litter input) in an area. Therefore, δ13C and δ15N measurements were conducted for leaf litter input, top soil (0 - 3 cm in depth) in the mesocosms, and lower soil (3 - 6 cm in depth) in the mesocosms, and tested at the first step of statistical analyses if spatial variability of litter δ13C and δ15N affected the δ13C and δ15N of SOM. Semivariogram visualization revealed spatial autoregression of litter δ13C and δ15N until 20 m distance between sampling sites. In addition to the autoregression, soil isotopic values were positively correlated with the values of leaf litter at the same sampling site, which indicate the isotopic values of leaf litter, a typical primary material of SOM, significantly affected SOM isotopic composition. Based on these analytical results, effects of wild boar exclusion and soil macrofauna exclusion on the SOM δ13C or δ15N were evaluated by Generalized Linear Mixed-effect Model which designated leaf litter δ13C or δ15N as random slope coefficient. Based on the AIC model selections, both topsoil and lower soil were more 13C and 15N enriched in soil macrofauna exclusion mesocosms, compared to the not-soil macrofauna exclusions. On the contrary to the macrofauna, wild boar exclusion sites were more 13C or 15N depleted at least in topsoil. Considering gradual enrichment of 13C and 15N along with the organic matter decomposition, these results should indicate that soil macrofauna facilitated organic matter input into 0 - 6 cm soil, while wild boar inhibited organic matter input, at least for 0 - 3 cm soil. More importantly, no-interaction term between the effect of macrofauna and the effect of wild boar was selected, which indicated the wild boar effect on the SOM isotopic values were not mediated by soil macrofauna. A speculation to explain this result should be, that the boar exclusion we conducted was obviously too small to incubate and increase in the abundance of macrofauna. On the other hand, it is plausible to argue that size of exclusion sites was enough to raise abundance of soil microfauna which can access the macrofauna exclusion mesocosms, hence our experiment suggests inhibition of organic matter input by wild boar mediated by microfauna.
In this study, a mammal exclusion experiment was conducted at the wild boar overabundance site, in Sadayama, Kochi Prefecture, Japan and tested if there is any sign of wild boar to inhibit SOM input by way of adverse effects on soil macrofauna. To exclude wild boar, wire baskets with approximately 40 cm x 35 cm square opening were bound on the soil surface, which were defined boar exclusion site, and a quadrate adjacent to the basket were defined as a control site. 10 boar exclusion-control pairs were set in the experimental area and these pairwise sites were placed in a line, with each pair were 10 m apart. For each of in a exclusion site and a control site, 2 types of mesocosm (ambient litter and soil were enclosed in a mesh container) were set and incubated during experiment. One mesocosm was enclosed by 5 mm mesh and allowed moving in and out of macrofauna, the other mesocosm was enclosed by 0.1 mm mesh and thus macrofauna was excluded from it.
Based on the gradual enrichment of δ13C and δ15N values associated with the SOM decomposition, we used δ13C and δ15N values of SOM as a mass-weighted average of "extent of decomposition". In that understanding, 13C and 15N enrichment in the way of decomposition were a few per-mill, which are comparable values to the δ13C and δ15N ranges of primary materials (i.e. litter input) in an area. Therefore, δ13C and δ15N measurements were conducted for leaf litter input, top soil (0 - 3 cm in depth) in the mesocosms, and lower soil (3 - 6 cm in depth) in the mesocosms, and tested at the first step of statistical analyses if spatial variability of litter δ13C and δ15N affected the δ13C and δ15N of SOM. Semivariogram visualization revealed spatial autoregression of litter δ13C and δ15N until 20 m distance between sampling sites. In addition to the autoregression, soil isotopic values were positively correlated with the values of leaf litter at the same sampling site, which indicate the isotopic values of leaf litter, a typical primary material of SOM, significantly affected SOM isotopic composition. Based on these analytical results, effects of wild boar exclusion and soil macrofauna exclusion on the SOM δ13C or δ15N were evaluated by Generalized Linear Mixed-effect Model which designated leaf litter δ13C or δ15N as random slope coefficient. Based on the AIC model selections, both topsoil and lower soil were more 13C and 15N enriched in soil macrofauna exclusion mesocosms, compared to the not-soil macrofauna exclusions. On the contrary to the macrofauna, wild boar exclusion sites were more 13C or 15N depleted at least in topsoil. Considering gradual enrichment of 13C and 15N along with the organic matter decomposition, these results should indicate that soil macrofauna facilitated organic matter input into 0 - 6 cm soil, while wild boar inhibited organic matter input, at least for 0 - 3 cm soil. More importantly, no-interaction term between the effect of macrofauna and the effect of wild boar was selected, which indicated the wild boar effect on the SOM isotopic values were not mediated by soil macrofauna. A speculation to explain this result should be, that the boar exclusion we conducted was obviously too small to incubate and increase in the abundance of macrofauna. On the other hand, it is plausible to argue that size of exclusion sites was enough to raise abundance of soil microfauna which can access the macrofauna exclusion mesocosms, hence our experiment suggests inhibition of organic matter input by wild boar mediated by microfauna.