TEMPESTX

A SOIL EXCLUSION EXPERIMENT

Question: Do biotic interactions control coastal forest resilience to extreme storm events?

Authors: A. Hopple, B. Bond-Lamberty, S. Pennington, and J.P. Megonigal

Project Overview: The dynamic accumulation and distribution of forest soil organic matter arises from stabilization and destabilization processes that, in turn, are affected by biotic and abiotic factors. Of particular importance are interactions between microbes, soil fauna, and root rhizospheres which exert strong controls on the rates, pathways, and fates of plant litter decay through (1) direct consumption and transformation of plant detritus, (2) physical fragmentation and binding of organo-mineral aggregates, and (3) microbial and plant root exudate accumulation and composition1. Together, these interactions enhance soil pore space physicochemical complexity, diversifying belowground niche space and increasing species taxonomic and functional diversity2. Biodiverse soil environments have a higher probability of including species that (1) have strong ecosystem effects, (2) have a higher resource use efficiency, and (3) have differences in environmental sensitivity among functionally similar species3. Changes in the abundance and diversity of species affects the structure and functioning of ecosystems2, as well as their ability to respond to and buffer the direct effects of abiotic drivers, such as increases in temperature4, nitrogen deposition4, atmospheric [CO2]5, and salinity6,7, highlighting the importance of investigating bottom-up biotic controls with the capacity to regulate structure-function relationships in ecosystems facing climate change. Finally, interactions between the rhizosphere and mycorrhizal mycelium have been shown to affect ecosystem-scale CH4 emissions8, an important measurement being made by the TEMPEST project.

Figure 1. Conceptual diagram of the anticipated ecological cascade that will be initiated by bottom-up ecosystem responses to short-term flooding events. Soil physicochemical, biotic, and functional responses are shown in blue, green, and red, respectively.

Here, we propose the addition of a manipulative field experiment to the PNNL-SERC TEMPEST Project that assesses the regulatory role of belowground biotic interactions in coastal forest resilience to extreme storm events. The TEMPEST project is investigating the initial series of events that occur as an upland forest transitions to a wetland state by simulating extreme freshwater and seawater storm events. We expect the consequences of increasing soil saturation, whether freshwater or saline, to have bottom-up cascading ecological effects, the severity of which will be regulated in part by the belowground network of biotic interactions. Briefly, we expect short-term flooding events to fundamentally shift the soil pore space physicochemical environment through a combination of decreased redox potential and aggregate stability (details outlined in Figure 1). This fundamental change will subsequently alter the dominant pathways of microbial respiration in soil and tree stems, increasing plant and microbial physiological stress and, ultimately, increasing carbon storage and CH4 emissions. However, we hypothesize that the rate and magnitude of ecosystem resilience will be regulated by the degree of belowground biodiversity and interaction.

We specifically hypothesize that: -H1: While storm events will consistently trigger ecosystem state shifts, greater belowground species diversity and interaction will result in higher ecosystem resilience to perturbation (Figure 2a). -H2: The rate and magnitude of functional change in the soil environment will be inversely related to belowground species diversity and interaction (Figure 2b).

Figure 2. Conceptual diagrams of the hypothesized role of belowground biodiversity and interaction on (a) physiological stress and (b) rate and magnitude of change in ecosystem respiration rates (ecosystem function). Note that in both panels, increased biodiversity results in reduced physiological stress and functional shifts. +/- denotes the inclusion or exclusion of a belowground biotic component and AMBIENT/STORM refers to either ambient or flooded conditions.

Research methods: To manipulate belowground biodiversity, we will install 16 1-m2 soil biotic exclusion sub-plots in each TEMPEST plot (Figure 3) as one of four treatments: (1) +bacteria/archaea, +fungi, +rhizosphere (control, no mesh/trenching); (2) +bacteria/archaea, +fungi, +rhizosphere (control, trenched with permeable mesh to assess trenching disturbance effect); (3) +bacteria/archaea, +fungi, -rhizosphere (45 µm mesh, 1m deep trench); (4) +bacteria/archaea, -fungi, -rhizosphere (1 µm mesh, 1m deep trench). Exclusion sub-plots will be established at least 6 months prior to the first flooding event to allow time for soil recovery following installation disturbance. Biotic exclusion plots are commonly used throughout the ecological literature to disentangle belowground soil-fungi-plant interactions4,8-11. While we recognize that our 1 µm mesh treatment may be unlikely to completely prevent ectomycorrhizal fungi growth, this treatment will suppress the growth of the dominant cord-forming fungi at the site12 (Dr. McCormick: resident SERC mycologist, pers comm.). Additionally, although our intention is to separate bacterial/archaeal and fungal interactions, our hypotheses ultimately concern total belowground biodiversity. As microbial taxonomy maps onto distinct size classes, a 1 µm mesh exclusion will successfully exclude a larger size class the soil community relative to the 45 µm mesh treatment, further decreasing belowground biodiversity. Finally, we will quantify the changes in bacterial, archaeal, and fungal composition and abundance due to our exclusion treatments through 16S rRNA and ITS rRNA qPCR.

Soil functional responses will be used as our primary metric of ecosystem resilience. Specifically, we will determine changes in carbon storage and composition by measuring (1) soil surface CH4 and CO2 flux, (2) soil CH4 and CO2 porewater concentrations, and (3) soil carbon quantity and composition. We will also determine the effects of belowground biodiversity and inundation on soil physicochemical properties, including pH, water content, [O2], [NO3-], [SO42-], and [PO43-]. Additionally, we will investigate the underlying biotic mechanisms regulating these ecosystem functions by measuring (1) the dominant bacterial/archaeal and fungal oxidative and hydrolytic exo-enzyme concentrations and activities and (2) bacterial, archaeal, and fungal composition and abundance prior to and following flooding events.

Figure 3. Soil biotic exclusion installation.

Impact on site: Overall, our experimental design will have minimal impacts on other variables of interest in the context of TEMPEST experiment, such as hydrology or tree physiology. The TEMPEST plots encompass 2,000 m2. Here, we proposed the addition of eight 1-m2 exclusion sub-plots (we are not counting the control plots since they are not trenched). These sub-plots would cover only 0.4% of the TEMPEST plot surface area and, thus, will have minimal effects on other ecosystem measurements. Additionally, while the mesh is designed to exclude certain biotic organisms, it will allow for water and solute transfer under ambient and flooded conditions.

Benefits to TEMPEST: Closing the ecosystem carbon cycle requires partitioning the sources of belowground respiration–in particular, its heterotrophic and autotrophic components. As TEMPEST is not employing eddy covariance towers, it must use biometric approaches to do this, and thus the autotrophic/heterotrophic partitioning must occur in the treatment plots, and not outside. Our experiment will provide a robust and multifaceted estimate of heterotrophic respiration (via soil surface CO2 as well as CH4 fluxes) which, when combined with net primary production, can be used to estimate net ecosystem production and thus the carbon loss/gain of the system.

References:

1. Jackson, Robert B., et al. "The ecology of soil carbon: pools, vulnerabilities, and biotic and abiotic controls." Annual Review of Ecology, Evolution, and Systematics 48 (2017): 419-445.
2. Lehmann, Anika, Weishuang Zheng, and Matthias C. Rillig. "Soil biota contributions to soil aggregation." Nature ecology & evolution 1.12 (2017): 1828.
3. Chapin, F. Stuart, et al. "Biotic control over the functioning of ecosystems." Science 277.5325 (1997): 500-504.
4. Crowther, Thomas W., et al. "Biotic interactions mediate soil microbial feedbacks to climate change." Proceedings of the National Academy of Sciences 112.22 (2015): 7033-7038.
5. Eisenhauer, Nico, et al. "Global change belowground: impacts of elevated CO2, nitrogen, and summer drought on soil food webs and biodiversity." Global Change Biology 18.2 (2012): 435-447.
6. Evelin, Heikham, Rupam Kapoor, and Bhoopander Giri. "Arbuscular mycorrhizal fungi in alleviation of salt stress: a review." Annals of botany 104.7 (2009): 1263-1280.
7. Al-Amri, S. M. "Mitigation of salinity stress of pepper (Capsicum annuum L.) by arbuscular mycorrhizal fungus, Glomus constrictum.” Applied Ecology and Environmental Research. (2019): 9965-9978.
8. Subke, Jens-Arne, et al. "Rhizosphere activity and atmospheric methane concentrations drive variations of methane fluxes in a temperate forest soil." Soil Biology and Biochemistry 116 (2018): 323-332.
9. Paterson, Eric, et al. "Arbuscular mycorrhizal hyphae promote priming of native soil organic matter mineralisation." Plant and Soil 408.1-2 (2016): 243-254.
10. Vollsnes, A. V., C. M. Futsaether, and A. G. Bengough. "Quantifying rhizosphere particle movement around mutant maize roots using time‐lapse imaging and particle image velocimetry." European journal of soil science 61.6 (2010): 926-939.
11. Ruehr, Nadine K., and Nina Buchmann. "Soil respiration fluxes in a temperate mixed forest: seasonality and temperature sensitivities differ among microbial and root–rhizosphere respiration." Tree Physiology 30.2 (2009): 165-176.
12. Teste, François P., et al. "Methods to control ectomycorrhizal colonization: effectiveness of chemical and physical barriers." Mycorrhiza 17.1 (2006): 51-65.