Ecosystem Resilience and Limitations Revealed by Soil Bacterial Community Dynamics in a Bark Beetle-Impacted Forest

Time outweighs the effect of host developmental stage on microbial community composition

Thousands of microbial taxa in the soil form symbioses with host plants, and due to their contribution to plant performance, these microbes are often considered an extension of the host genome. Given microbial effects on host performance, it is important to understand factors that govern microbial community assembly. Host developmental stage could affect rhizosphere microbial diversity while, alternatively, microbial assemblages could change simply as a consequence of time and the opportunity for microbial succession. Previous studies suggest that rhizosphere microbial assemblages shift across plant developmental stages, but time since germination is confounded with developmental stage. We asked how elapsed time and potential microbial succession relative to host development affected microbial diversity in the rhizosphere using monogenic flowering-time mutants of Arabidopsis thaliana. Under our experimental design, different developmental stages were present among host genotypes after the same amount of time following germination, e.g. at 76 days following germination some host genotypes were flowering while others were fruiting or senescing. We found that elapsed time was a strong predictor of microbial diversity whereas there were few differences among developmental stages. Our results support the idea that time and, likely, microbial succession more strongly affect microbial community assembly than host developmental stage.

A comparison of lodgepole and spruce needle chemistry impacts on terrestrial biogeochemical processes during isolated decomposition

This study investigates the isolated decomposition of spruce and lodgepole conifer needles to enhance our understanding of how needle litter impacts near-surface terrestrial biogeochemical processes. Harvested needles were exported to a subalpine meadow to enable a discrete analysis of the decomposition processes over 2 years. Initial chemistry revealed the lodgepole needles to be less recalcitrant with a lower carbon to nitrogen (C:N) ratio. Total C and N fundamentally shifted within needle species over time with decreased C:N ratios for spruce and increased ratios for lodgepole. Differences in chemistry correlated with CO₂ production and soil microbial communities. The most pronounced trends were associated with lodgepole needles in comparison to the spruce and needle-free controls. Increased organic carbon and nitrogen concentrations associated with needle presence in soil extractions further corroborate the results with clear biogeochemical signatures in association with needle chemistry. Interestingly, no clear differentiation was observed as a function of bark beetle impacted spruce needles vs those derived from healthy spruce trees despite initial differences in needle chemistry. These results reveal that the inherent chemistry associated with tree species has a greater impact on soil biogeochemical signatures during isolated needle decomposition. By extension, biogeochemical shifts associated with bark beetle infestation are likely driven more by changes such as the cessation of rhizospheric processes than by needle litter decomposition.

Response of Soil Surface Respiration to Storm and Ips typographus (L.) Disturbance in Boreal Norway Spruce Stands

Disturbances such as storm events and bark beetle outbreaks can have a major influence on forest soil carbon (C) cycling. Both autotrophic and heterotrophic soil respiration may be affected by the increase in tree mortality. We studied the effect of a storm in 2010 followed by an outbreak of the European spruce bark beetle (Ips typographus L.) on the soil surface respiration (respiration by soil and ground vegetation) at two Norway spruce (Picea abies L.) dominated sites in southeastern Finland. Soil surface respiration, soil temperature, and soil moisture were measured in three types of plots—living trees (undisturbed), storm-felled trees, and standing dead trees killed by I. typographus—during the summer–autumn period for three years (2015–2017). Measurements at storm-felled tree plots were separated into dead tree detritus-covered (under storm-felled trees) and open-vegetated (on open areas) microsites. The soil surface total respiration for 2017 was separated into its autotrophic and heterotrophic components using trenching. The soil surface total respiration rates at the disturbed plots were 64%–82% of those at the living tree plots at one site and were due to a decrease in autotrophic respiration, but there was no clear difference in soil surface total respiration between the plots at the other site, due to shifts in either autotrophic or heterotrophic respiration. The soil surface respiration rates were related to plot basal area (living and all trees), as well as to soil temperature and soil moisture. As storm and bark beetle disturbances are predicted to become more common in the future, their effects on forest ecosystem C cycling and CO2 fluxes will therefore become increasingly important.

Increased Forest Soil CO2 and N2O Emissions During Insect Infestation

Forest soils are major sinks of terrestrial carbon, but this function may be threatened by mass outbreak events of forest pests. Here, we measured soil CO2-C and N2O-N fluxes from a Scots pine (Pinus sylvestris L.) forest that was heavily infested by the nun moth (Lymantria monacha L.) and an adjacent noninfested (control) forest site during one year. In the infested forest, net emissions of CO2-C were higher during main defoliation, summer and autumn, while indications of increased N2O-N emissions were found at one sampling date. On basis of this, a microcosm incubation experiment with different organic matter treatments was conducted. Soil treatments with needle litter, insect feces plus needle litter, and insect feces showed 3.7-, 10.6-, and 13.5-fold higher CO2-C emissions while N2O-N of the insect feces plus needle litter, and insect feces treatment was 8.9-, and 10.4-fold higher compared with soil treatments without added organic matter (control). Hence, the defoliation in combination with high inputs of organic matter during insect outbreaks distinctly accelerate decomposition processes in pine forest soils, which in turn alters forests nutrient cycling and the functioning of forests as carbon sinks.

The Abundance of Fungi, Bacteria and Denitrification Genes during Insect Outbreaks in Scots Pine Forests

Outbreaks of defoliating insects may affect microbial populations in forests and thereby mass balances and ecosystem functioning. Here, we investigated the microbial dynamics in Scots pine (Pinus sylvestris L.) forests during outbreaks of the nun moth (Lymantria monacha L.) and the pine-tree lappet (Dendrolimus pini L.). We used real-time PCR (polymerase chain reaction) to quantify genes that characterize bacterial and fungal abundance and the denitrification processes (nirK, nirS, nosZ clades I and II) in different forest compartments and we analyzed the C and N content of pine needles, insect feces, larvae, vegetation layers, organic layers, and mineral soil horizons. The infestation of the nun moth increased the bacterial abundance on pine needles, in the vegetation layer, and in the upper organic layer, while fungal populations were increased in the vegetation layer and upper organic layer during both outbreaks. In soil, the abundance of nirK increased after insect defoliation, while the C/N ratios decreased. nosZ clades I and II showed variable responses in different soil layers and to different defoliating insects. Our results illustrate changes in the microbial populations in pine forests that were infested by defoliating insects and changes in the chemical soil properties that foster these populations, indicating a genetic potential for increased soil N2O emissions during the defoliation peak of insect outbreak events.

Factors Affecting the Leaching of Dissolved Organic Carbon after Tree Dieback in an Unmanaged European Mountain Forest

Forest disturbances affect ecosystem biogeochemistry, water quality, and carbon cycling. We analyzed water chemistry before, during, and after a dieback event at a headwater catchment in the Bohemian Forest (central Europe) together with an un-impacted reference catchment, focusing on drivers and responses of dissolved organic carbon (DOC) leaching. We analyzed data regarding carbon input to the forest floor via litter and throughfall, changes in soil moisture and composition, streamwater chemistry, discharge, and temperature. We observed three key points. (i) In the first 3 years following dieback, DOC production from dead biomass led to increased concentrations in soil, but DOC leaching did not increase due to chemical suppression of its solubility by elevated concentrations of protons and polyvalent cations and elevated microbial demand for DOC associated with high ammonium (NH₄⁺) concentrations. (ii) DOC leaching remained low during the next 2 years because its availability in soils declined, which also left more NH₄⁺ available for nitrifiers, increasing NO₃^- and proton production that further increased the chemical suppression of DOC mobility. (iii) After 5 years, DOC leaching started to increase as concentrations of NO₃^-, protons, and polyvalent cations started to decrease in soil water. Our data suggest that disturbance-induced changes in N cycling strongly influence DOC leaching via both chemical and biological mechanisms and that the magnitude of DOC leaching may vary over periods following disturbance. Our study adds insights as to why the impacts of forest disturbances are sometime observed at the local soil scale but not simultaneously on the larger catchment scale.