Of mammals and bacteria in a rainforest: Temporal dynamics of soil bacteria in response to simulated N pulse from mammalian urine

Changes in soil bacterial community diversity following the removal of invasive feral pigs from a Hawaiian tropical montane wet forest

Nonnative, invasive feral pigs (Sus scrofa) modify habitats by disturbing soils and vegetation, which can alter biogeochemical processes. Soil microbial communities drive nutrient cycling and therefore also play important roles in shaping ecosystem structure and function, but the responses of soil microbes to nonnative ungulate removal remains poorly studied. We examined changes in the soil bacterial community over a ~25 year chronosequence of feral pig removal in tropical montane wet forests on the Island of Hawai’i. We extracted bacterial eDNA from soil samples collected inside and outside of ungulate exclosures along this chronosequence and sequenced the eDNA using the Illumina platform. We found that ungulate removal increased diversity of soil bacteria, with diversity scores positively correlated with time since removal. While functional and phylogenetic diversity were not significantly different between pig present and pig removed soils, soil bulk density, which decreases following the removal of feral pigs, was a useful predictor of dissimilarity among sites and correlated to changes in functional diversity. Additionally, increases in soil porosity, potassium, and calcium were correlated to increases in functional diversity. Finally, sites with greater mean annual temperatures were shown to have higher scores of both functional and phylogenetic diversity. As such, we conclude that feral pigs influence overall bacterial community diversity directly while influencing functional diversity indirectly through alterations to soil structure and nutrients. Comparatively, phylogenetic differences between communities are better explained by mean annual temperature as a climatic predictor of community dissimilarity.

Soil biological responses to, and feedbacks on, trophic rewilding

Trophic rewilding-the (re)introduction of missing large herbivores and/or their predators-is increasingly proposed to restore biodiversity and biotic interactions, but its effects on soils have been largely neglected. The high diversity of soil organisms and the ecological functions they perform mean that the full impact of rewilding on ecosystems cannot be assessed considering only above-ground food webs. Here we outline current understanding on how animal species of rewilding interest affect soil structure, processes and communities, and how in turn soil biota may affect species above ground. We highlight considerable uncertainty in soil responses to and feedbacks on above-ground consumers, with potentially large implications for rewilding interactions with global change. For example, the impact of large herbivores on soil decomposers and plant-soil interactions could lead to reduced carbon sequestration, whereas herbivore interactions with keystone biota such as mycorrhizal fungi, dung beetles and bioturbators could promote native plants and ecosystem heterogeneity. Moreover, (re)inoculation of keystone soil biota could be considered as a strategy to meet some of the objectives of trophic rewilding. Overall, we call for the rewilding research community to engage more with soil ecology experts and consider above-ground-below-ground linkages as integral to assess potential benefits as well as pitfalls.This article is part of the theme issue 'Trophic rewilding: consequences for ecosystems under global change'.

Spatio-temporal dynamics of soil bacterial communities as a function of Amazon forest phenology

Most tropical evergreen rain forests are characterised by varying degrees of precipitation seasonality that influence plant phenology and litterfall dynamics. Soil microbes are sensitive to soil water:air ratio and to nutrient availability. We studied if within-year seasonality in precipitation and litterfall-derived nutrient input resulted in predictable seasonal variation in soil bacterial diversity/microbial functional groups in an Amazonian forest. We characterised the spatio-temporal dynamics of microbial communities from the plot to the stand scales and related them to precipitation seasonality and spatial variability in soil characteristics. Community composition and functional diversity showed high spatial heterogeneity and was related to variability in soil chemistry at the stand level. Large species turnover characterised plot level changes over time, reflecting precipitation seasonality-related changes in soil nutrient and moisture regimes. The abundance of decomposers was highest during the rainy season, characterised also by anaerobic saprophytes and N₂-fixers adapted to fluctuating redox conditions. In contrast, Beijerinckiaceae, likely derived from the phyllosphere, were found at higher abundances when litter inputs and accumulation were highest. We showed that in a mildly seasonal rain forest, the composition of soil microbial communities appears to be following canopy phenology patterns and the two are interlinked and drive soil nutrient availability.