Friday, March 29, 2013

Getting to the Roots (and Fungi) of Carbon Sequestration

This week, I found a paper that I’m calling the best of both worlds. Well, for me at least. This paper combines my past (and lingering) interest in island biogeography with a current interest in climate change and carbon storage.

If you have been reading my blog long enough then you already know my love of islands. They are just so darn useful. In the past, I have focused on oceanic islands, but lake islands are also really neat. These types of islands typically form when lower lying land areas fill with water, effectively cutting off higher areas from the mainland and making them into islands. As such, these islands usually already contain forest as opposed to an oceanic island that emerges from the ocean and must be colonized. A new study, published in journal Science, looks at a fire-driven boreal forest chronosequence on forested lake islands in northern Sweden. Such a chronosequence allows the study of soil carbon sequestration over time scales of centuries to millennia.

This new study looks at roots and their associated fungi (mycorrhizae) as sources of this stored carbon. I’m not going to spend the space to describe mycorrhize, but will, instead, send you over to my Free Market Fungi post for more information, if you need it. It is known that 16 percent of the global carbon stock is sequestered in soils. To date, most carbon studies of this type focus on aboveground leaf litter as the fundamental determinants of this carbon accumulation. But a large portion of photosynthetically fixed carbon is actually directed belowground to the roots and, subsequently, the mycorrhizae. Now, let’s add fire. It has been shown that when a forest doesn't burn, the soil and ecosystem carbon accumulate unabated, and in a linear fashion. Add this information together and it becomes a big deal when it comes to correctly allocating carbon, calculating the long term sequestration rates, and predicting how forests will respond to climate change and other environmental shifts.

The study sites were in two adjacent lakes, Lake Hornavan and Lake Uddjaure, in northern Sweden. The islands in these lakes were formed after the most recent glaciation and come in a variety of sizes. In terms of fire, larger islands burn more frequently because they are larger targets for lightning strikes. As a result, several of the large islands in these lakes have burned in the last century, whereas some of the small islands haven’t burned in at least 5000 years. This lack of fire leads to very thick humus layers (or organic layers towards the top of the soil column) on smaller islands, up to 1 meter thick!

The researchers divided islands into three size classes of 10 islands each: large (over 1 ha), medium (0.1-1.0 ha), and small (less than 0.1 ha). They took soil samples from these islands to look at the organic soil profiles and found that large islands accumulated 6.2 kg of C per square meter belowground with a mean time since fire of 585 years, medium islands accumulated 11.2 kg of C per square meter with a mean time since fire of 2180 years, and small islands 22.5 kg of C per square meter with a mean time since fire of 3250 years. Then they looked at the carbon dynamics across this chronosequence by analyzing bomb 14C. This allowed them to determine the age since fixation of soil carbon. Then they fitted a mathematical model to measurements of carbon mass and age distribution across the soil profiles for six of the islands (3 large, 3 small). This model revealed that the distribution of carbon mass and age could only be predicted when they included carbon from roots. This root-derived carbon accumulation was found to be larger on small islands (70 percent, that's a LOT!) than large islands (47 percent). They were able to explain the entire carbon sequestration difference on small islands from these root-derived inputs. The model also showed that small islands store a major proportion of their soil carbon in the deeper soil layers, those over 100 years old. However, below 20 cm depth, the root-derived carbon inputs were shown to be low and to decompose slowly. So the root-derived carbon input into the upper layers probably contributes to the long-term buildup of humus that is seen on these islands. But, as usual, that's not the end of the story.

We know that fungi play very important roles in forest ecosystems, both as decomposers and in root-assoicated carbon transport and respiration. So the researchers also profiled the relative abundance of major groups of fungi by depth in the soil profiles. They found that the upper soil layers are dominated by free-living saprotrophs (fungi that obtain their nutrition heterotrophically from non-living organic materials), and greater depths were dominated by mycorrhizal and other root-associated fungi. Their model suggests that these mycorrhizae live at the spots where the largest difference in carbon sequestration between the island size classes exists, which also tends to be the areas of highest root mass. When they ran tests for fungal biomass throughout each soil profile they found greater mycelial (the vegetative part of a fungus, consisting of a mass of branching, threadlike hyphae) production on large islands, but less mycelial necromass (dead stuff) on small islands. This suggests that the large production is counterbalanced by faster decomposition of mycelial remains. “Correspondingly, the 14C model indicated faster decomposition of root-derived [carbon] on large islands, despite inputs being conservatively constrained to be equal across all islands.”

I found these conclusions to be interesting because of the amount of soil carbon from roots and mycorrhizal fungi, especially on small islands. And although they saw less carbon accumulation on large islands, these islands have a greater root density and so should have more carbon allocation to roots and the associated fungi. Did you catch the contradiction? Well, in response to increased carbon dioxide, there will be an increase of carbon inputs to the roots which will accelerate the turnover of soil organic matter. This counteracts carbon accumulation and enhances nitrogen cycling through the microbial pools, an effect these researchers observed when they tested the C:N-ratios in the humus of large islands. This is much lower on small islands, possibly because of impared mycorrhizal nitrogen mobilization and the accumulation of nitrogen in fungal remains. This leads to progressive nutrient limitations, then leads to changes plant productivity, leading to changes in community composition, which leads to changes in total belowground carbon allocation, that leads to changes in fungi.

Definately starting to grasp the importance of the belowground dirty stuff. There’s a whole lot of carbon down there that we need to start looking at, accounting for, and seeing where it goes. We know that changes in the environment such as climate change, soil fertilization, fire suppression, and forest management make big differences to the aboveground stuff. It only makes sense that the belowground stuff is impacted as well.

ResearchBlogging.orgClemmensen, K., Bahr, A., Ovaskainen, O., Dahlberg, A., Ekblad, A., Wallander, H., Stenlid, J., Finlay, R., Wardle, D., & Lindahl, B. (2013). Roots and Associated Fungi Drive Long-Term Carbon Sequestration in Boreal Forest Science, 339 (6127), 1615-1618 DOI: 10.1126/science.1231923

If you would like some follow-up reading I suggest:

Treseder, K. K. (2013-03-29) Fungal Carbon Sequestration. Science, 339(6127), 1528-1529. (DOI: 10.1126/science.1236338

Also check out the write-up in Nature "Fungi and roots store a surprisingly large share of the world's carbon"

(image via Forest Keepers)

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