Laura Castaneda-Gomez: Roots may not be key drivers of soil responses to elevated CO2 in a phosphorus-limited forest

In this new post, Laura Castañeda-Gómez, a new ecological researcher working at The University of Toronto, Canada, shares their paper: The influence of roots on mycorrhizal fungi, saprotrophic microbes and carbon dynamics in a low‐phosphorus Eucalyptus forest under elevated CO2—recently shortlisted for the Haldane Prize for Early Career Researchers.

Last February, the Earth’s atmosphere reached a new record high CO2 concentration of almost 422 ppm. We are all witnesses and architects of the speed and persistent rising trend of this greenhouse gas that is already negatively impacting life on our planet. Although the impacts of rising atmospheric CO2 concentrations have been widely studied, we are still missing a great deal of information to fully understand the current and future effects of CO2 on Earth’s ecosystems—we must find ways to mitigate and reduce its negative effects.

About the research

Soil carbon (C) sequestration has become a key strategy for fighting climate change; however, the capacity of soils to accumulate C might be altered with elevated CO2. Rising CO2 concentrations can increase plant photosynthetic activity, thereby increasing plant biomass production and C transfer to the soil, which subsequently increases the chance for soil C accumulation. However, higher C availability in soils can also promote soil microbial activity and SOM (soil organic matter) decomposition and C losses. Most of the effects of elevated CO2 on soil C and SOM are mediated by changes in the C allocated belowground, to which both roots and symbiotic (mycorrhizal) fungi contribute. Mycorrhizal fungi can also play a relevant contribution to altered SOM dynamics with elevated CO2, either by promoting or limiting soil microbial activity, or via their direct impacts on SOM decomposition—a role that can vary depending on soil nutrient availability.

One of the rings at the EucFACE site in Western Sydney (Credit: Johana Pihlblad)

Most of the information available about root and mycorrhizal contributions to SOM dynamics in forests are mainly focused on nitrogen (N)-limited plantation forests, even though changes in SOM dynamics due to elevated CO2 are also known to depend on phosphorus (P) availability. Therefore, to fill this gap, I set up an experiment at the Eucalyptus Free-Air CO2 facility (EucFACE) in Western Sydney, Australia. These woodlands are characterized by having inherently low nutrient availability, particularly that of P. This is also the first FACE facility located in a natural, mature forest. Therefore, I was expecting to observe different responses, around SOM dynamics and microbial communities, to those from predecessor studies in N-limited plantation forests that are more actively growing and are thus likely to be more responsive to higher CO2 concentrations.

About the paper

I wanted to directly quantify the influence of roots on mycorrhizal fungi, free-living soil microbial communities and SOM dynamics in this mature, P-limited woodland which was exposed to high CO2 concentrations. For this, I adapted the ingrowth core method to be able to answer our research questions. I needed to be able to control the access of roots, mycorrhizal fungi, and free-living microorganisms to a C substrate as ingrowth cores do; however, I also needed to be able to contain different C substrates and account for their mass and C losses over time. After several prototypes, the idea to utilise nested mesh bags was born. Affectionally called “soil burritos”, these nested litter bags also allowed us to reduce frequent visits to the site and minimize trampling.

Preparing and assembling my army of soil burritos (nested mesh bags) with two different inner C substrates and different outer mesh sizes to exclude different belowground components. (Credit: Laura Castañeda-Gómez)

Designing and implementing these nested mesh bags was the most exciting phase of this experiment. However, keeping them buried in the site was challenging at first, as animals at the EucFACE site thought these were tasty treats and dug a few of them out just a few weeks after I buried them! Enclosure mesh around the buried mesh bags solved the issue in the long term.

Burying the nested mesh bags in the field with an auger. Sure they looked tasty for the animals that dug them out! (Credit: Laura Castañeda-Gómez)

One of the key messages of our study is that although roots have a relevant role in shaping soil microbial communities and SOM dynamics in this P-limited woodland, they may not have a great effect on the belowground responses of this ecosystem to elevated CO2, nor will they significantly alter mycorrhizal-mediated mechanisms for SOM decomposition. These results divert from prior knowledge and understanding of N-limited plantation forests, and challenges the notion that rhizosphere mediated mechanisms are the main contributors to altered SOM dynamics under elevated CO2 conditions in this P-limited forest. At this study site, other factors beyond belowground C inputs, such as altered understory plant diversity and water availability, may act as drivers of soil responses to elevated CO2. These findings also call for increased attention to the nutrient status dependency of rhizosphere effects on SOM dynamics, and for increased consideration of  ecosystem-specific mechanisms of elevated CO2 influence.

Nested mesh bags after one year incubation in the field with roots (left) or fungal hyphae colonizing them (middle) and soil nutrient extractions for all mesh bags (right). Credit: Laura Castañeda-Gómez)

This study is relevant for everyone interested in how elevated CO2 conditions can impact different functional guilds of soil microorganisms, and how this effect can influence soil organic matter dynamics in natural mature forests under climate change stressors. It is also for anyone interested in exploring the mechanisms that lead to altered SOM dynamics and for those seeking to better represent soil microbial and biogeochemical processes in Earth System Models. This study is particularly useful for understanding the dependence of nutrient availability, particularly that of P, on belowground responses.

About the author

Laura in a forest hike in Tasmania, Australia (Credit: Camilo López-Aguirre)

I am currently a Postdoctoral Research Fellow at the University of Toronto, Canada, at Prof. Myrna Simpson’s lab. I am investigating mechanisms for soil C accumulation in different forests across North America in response to altered C input quantity and quality, within the Detrital Input and Removal Treatment (DIRT) network.

I have always been amazed and intrigued by the complexity of soil processes and how soil microbial communities can adapt and shape soil functionality. With the current climate change crisis, the role of soils as a C reserve is becoming more relevant than ever, and understanding the mechanisms for soil C accumulation is now more crucial than ever. As soil scientists, I believe we have the responsibility to share our knowledge and help guide efforts and strategies focused on regenerating the Earth’s soils and preserving natural ecosystems and their C stocks.

As a Latina in STEM, and a Colombian, I feel a very personal need to support and further develop soil C knowledge in my home country. It is now my long-term goal to find ways to support initiatives that help preserve and regenerate one of the most biodiverse countries on Earth and their soil resources.

Enjoyed the blog? Read the research here.

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