Leaf-cutter ants engineer large nitrous oxide hot spots in tropical forests

Larvae of an invasive scarab increase greenhouse gas emissions from soils and recruit gut mycobiota involved in C and N transformations

Background

Soil-derived prokaryotic gut communities of the Japanese beetle Popillia japonica Newman (JB) larval gut include heterotrophic, ammonia-oxidizing, and methanogenic microbes potentially capable of promoting greenhouse gas (GHG) emissions. However, no research has directly explored GHG emissions or the eukaryotic microbiota associated with the larval gut of this invasive species. In particular, fungi are frequently associated with the insect gut where they produce digestive enzymes and aid in nutrient acquisition. Using a series of laboratory and field experiments, this study aimed to (1) assess the impact of JB larvae on soil GHG emissions; (2) characterize gut mycobiota associated with these larvae; and (3) examine how soil biological and physicochemical characteristics influence variation in both GHG emissions and the composition of larval gut mycobiota.

Methods

Manipulative laboratory experiments consisted of microcosms containing increasing densities of JB larvae alone or in clean (uninfested) soil. Field experiments included 10 locations across Indiana and Wisconsin where gas samples from soils, as well as JB and their associated soil were collected to analyze soil GHG emissions, and mycobiota (ITS survey), respectively.

Results

In laboratory trials, emission rates of CO₂, CH₄, and N₂O from infested soil were ≥ 6.3× higher per larva than emissions from JB larvae alone whereas CO₂ emission rates from soils previously infested by JB larvae were 1.3× higher than emissions from JB larvae alone. In the field, JB larval density was a significant predictor of CO₂ emissions from infested soils, and both CO₂ and CH₄ emissions were higher in previously infested soils. We found that geographic location had the greatest influence on variation in larval gut mycobiota, although the effects of compartment (i.e., soil, midgut and hindgut) were also significant. There was substantial overlap in the composition and prevalence of the core fungal mycobiota across compartments with prominent fungal taxa being associated with cellulose degradation and prokaryotic methane production/consumption. Soil physicochemical characteristics such as organic matter, cation exchange capacity, sand, and water holding capacity, were also correlated with both soil GHG emission, and fungal a-diversity within the JB larval gut. Conclusions: Results indicate JB larvae promote GHG emissions from the soil directly through metabolic activities, and indirectly by creating soil conditions that favor GHG-associated microbial activity. Fungal communities associated with the JB larval gut are primarily influenced by adaptation to local soils, with many prominent members of that consortium potentially contributing to C and N transformations capable of influencing GHG emissions from infested soil.

Precipitation-drainage cycles lead to hot moments in soil carbon dioxide dynamics in a Neotropical wet forest

Soil CO₂ concentrations and emissions from tropical forests are modulated seasonally by precipitation. However, subseasonal responses to meteorological events (e.g., storms, drought) are less well known. Here, we present the effects of meteorological variability on short-term (hours to months) dynamics of soil CO₂ concentrations and emissions in a Neotropical wet forest. We continuously monitored soil temperature, moisture, and CO₂ for a three-year period (2015-2017), encompassing normal conditions, floods, a dry El Niño period, and a hurricane. We used a coupled model (Hydrus-1D) for soil water propagation, heat transfer, and diffusive gas transport to explain observed soil moisture, soil temperature, and soil CO₂ concentration responses to meteorology, and we estimated soil CO₂ efflux with a gradient-flux model. Then, we predicted changes in soil CO₂ concentrations and emissions under different warming climate change scenarios. Observed short-term (hourly to daily) soil CO₂ concentration responded more to precipitation than to other meteorological variables (including lower pressure during the hurricane). Observed soil CO₂ failed to exhibit diel patterns (associated with diel temperature fluctuations in drier climates), except during the drier El Niño period. Climate change scenarios showed enhanced soil CO₂ due to warmer conditions, while precipitation played a critical role in moderating the balance between concentrations and emissions. The scenario with increased precipitation (based on a regional model projection) led to increases of +11% in soil CO₂ concentrations and +4% in soil CO₂ emissions. The scenario with decreased precipitation (based on global circulation model projections) resulted in increases of +4% in soil CO₂ concentrations and +18% in soil CO₂ emissions, and presented more prominent hot moments in soil CO₂ outgassing. These findings suggest that soil CO₂ will increase under warmer climate in tropical wet forests, and precipitation patterns will define the intensity of CO₂ outgassing hot moments.

First field estimation of greenhouse gas release from European soil-dwelling Scarabaeidae larvae targeting the genus Melolontha

Arthropods are a major soil fauna group, and have the potential to substantially influence the spatial and temporal variability of soil greenhouse gas (GHG) sinks and sources. The overall effect of soil-inhabiting arthropods on soil GHG fluxes still remains poorly quantified since the majority of the available data comes from laboratory experiments, is often controversial, and has been limited to a few species. The main objective of this study was to provide first insights into field-level carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) release of soil-inhabiting larvae of the Scarabaeidae family. Larvae of the genus Melolontha were excavated at various sites in west-central and southern Germany, covering a wide range of different larval developmental stages, larval activity levels, and vegetation types. Excavated larvae were immediately incubated in the field to measure their GHG production. Gaseous carbon release of individual larvae showed a large inter- and intra-site variability which was strongly correlated to larval biomass. This correlation persisted when upscaling individual CO₂ and CH₄ production to the plot scale. Field release estimates for Melolontha spp. were subsequently upscaled to the European level to derive the first regional GHG release estimates for members of the Scarabaeidae family. Estimates ranged between 10.42 and 409.53 kt CO₂ yr^-1, and 0.01 and 1.36 kt CH₄ yr^-1. Larval N₂O release was only sporadically observed and not upscaled. For one site, a comparison of field- and laboratory-based GHG production measurements was conducted to assess potential biases introduced by transferring Scarabaeidae larvae to artificial environments. Release strength and variability of captive larvae decreased significantly within two weeks and the correlation between larval biomass and gaseous carbon production disappeared, highlighting the importance of field measurements. Overall, our data show that Scarabaeidae larvae can be significant soil GHG sources and should not be neglected in soil GHG flux research.

How Much Human-Caused Global Warming Should We Expect with Business-As-Usual (BAU) Climate Policies? A Semi-Empirical Assessment

In order to assess the merits of national climate change mitigation policies, it is important to have a reasonable benchmark for how much human-caused global warming would occur over the coming century with “Business-As-Usual” (BAU) conditions. However, currently, policymakers are limited to making assessments by comparing the Global Climate Model (GCM) projections of future climate change under various different “scenarios”, none of which are explicitly defined as BAU. Moreover, all of these estimates are ab initio computer model projections, and policymakers do not currently have equivalent empirically derived estimates for comparison. Therefore, estimates of the total future human-caused global warming from the three main greenhouse gases of concern (CO2, CH4, and N2O) up to 2100 are here derived for BAU conditions. A semi-empirical approach is used that allows direct comparisons between GCM-based estimates and empirically derived estimates. If the climate sensitivity to greenhouse gases implies a Transient Climate Response (TCR) of ≥ 2.5 °C or an Equilibrium Climate Sensitivity (ECS) of ≥ 5.0 °C then the 2015 Paris Agreement’s target of keeping human-caused global warming below 2.0 °C will have been broken by the middle of the century under BAU. However, for a TCR < 1.5 °C or ECS < 2.0 °C, the target would not be broken under BAU until the 22nd century or later. Therefore, the current Intergovernmental Panel on Climate Change (IPCC) “likely” range estimates for TCR of 1.0 to 2.5 °C and ECS of 1.5 to 4.5 °C have not yet established if human-caused global warming is a 21st century problem.

Integrating host plant phylogeny, plant traits, intraspecific competition and repeated measures using a phylogenetic mixed model of field behaviour by polyphagous herbivores, the leaf-cutting ants

Herbivores use a wide range of factors to choose their host, including their own physiological states, physical characteristics of plants and the degree of competition. Field observations of herbivores in their native habitats provide a means for simultaneously estimating the relative importance of these factors, but statistical analysis of all these factors may be challenging. Here we used a 7-week dataset of leaf-cutting ant (Atta cephalotes) foraging in a diverse Neotropical arboretum containing 193 tree species (822 trees) to examine the relative role of tree phylogeny, territoriality and tree functional characteristics using a phylogenetic generalized least squares (PGLS) model. We observed that 54 tree species (117 trees) were foraged by the ants. This pattern was not random, but reflected known features of leaf-cutting ant foraging behaviour, such as a preference for larger trees and the decreased likelihood of foraging at the periphery of a colony’s territory. However, random effects such as tree phylogeny, the identity of individual trees and colony-specific effects explained most of the variation in foraging data. A significant phylogenetic effect on foraging likelihood (λ = 0.28), together with repeated measures of foraging on the same tree species, allowed estimation of relative palatability for each plant species. PGLS models can be flexibly scaled to include other covariates for even more complex investigation of foraging behaviour, and the link function can be modified to include the amount of plants foraged. As a result, PGLS can be used as a flexible framework for the study of LCA foraging.

Diel pattern driven by free convection controls leaf-cutter ant nest ventilation and greenhouse gas emissions in a Neotropical rain forest

Leaf-cutter ant nests are biogeochemical hot spots where ants live and import vegetation to grow fungus. Metabolic activity and (in wet tropical forests) soil gas flux to the nest may result in high nest CO₂ concentrations if not adequately ventilated. Wind-driven ventilation mitigates high CO₂ concentrations in grasslands, but little is known about exchange for forest species faced with prolonged windless conditions. We studied Atta cephalotes nests located under dense canopy (leaf area index > 5) in a wet tropical rainforest in Costa Rica, where wind events are infrequent. We instrumented nests with thermocouples and flow-through CO₂ sensing chambers. The results showed that CO₂ concentrations exiting leaf-cutter ant nests follow a diel pattern with higher values at night. We developed an efflux model based on pressure differences that evaluated the observed CO₂ diel pattern in terms of ventilation by (1) free convection (warm, less dense air rises out the nest more prominently at night) and (2) episodic wind-forced convection events providing occasional supplemental ventilation during daytime. Average greenhouse gas emissions were estimated through nest vents at about 78 kg CO₂eq nest^-1 year^-1. At the ecosystem level, leaf-cutter ant nest vents accounted for 0.2% to 1% of total rainforest soil emissions. In wet, clayey tropical soils, leaf-cutter ant nests act as free convection-driven conduits for exporting CO₂ and other greenhouse gases produced within the nest (fungus and ant respiration, refuse decay), and by roots and soil microbes surrounding the nest. This allows A. cephalotes nests to be ventilated without reliable wind conditions.