Global warming will gradually increase the temperature of soils. Most models predict that increased soil temperature will allow fungi and bacteria to break down soil organic matter faster, releasing large amounts of carbon dioxide into the atmosphere. 

New research from the University of California – Irvine shows that, at least for dry soils of boreal spruce forests, rising temperatures cause fungi to dry out, reducing their rate of respiration. Instead of a feed-forward effect that increases greenhouse gas emissions,  Steven Allison and Kathleen Treseder found that warming boreal soils decreased fungal respiration.  Their work was published in Global Change Biology.

This does not remove all concerns about warming of forest soils. In areas of permafrost, where there is little microbial respiration, warming causes melting of the ice and accelerates microbial respiration.

Citation:

Allison, S. D., and K. K. Treseder. in press. Warming and drying suppress microbial activity and carbon cycling in boreal forest soils. Global Change Biology.

Sources:

• Fungi emit less carbon dioxide as climate warms – UC Irvine
• The Allison Lab, UC Irvine
• Global Change Biology

Earthworms can change the chemistry of carbon in forest soils and litter. Researchers at the Smithsonian Environmental Research Center are trying to figure out the impact of earthworms on forest soils, and how they may change soil chemistry.

Although we may think that earthworms are everywhere, they were actually introduced into North America by early European settlers, and have slowly spread out as people settled more areas.  I remember digging in Adirondack soils and finding earthworms only right around lakes and ponds, where they had been carried by fishermen.  Earthworms spread very slowly – on their own, they would only spread about 200 km in the 10,000 years since the last glaciers wiped out existing soils.  Most earthworm movement is by people carrying bait or moving soils in nursery material.

The impact of earthworms on carbon in soils is not obvious.  Earthworms consume leaf litter and carry organic matter deep into the soil.  Their work could increase carbon deep in the soil. But since they also consume leaf litter, they may strip the insulation off soils and increase temperatures, causing loss of carbon to microbial respiration.

Earthworm action has profound effects on soil processes. The Smithsonian project should help us understand their impacts on carbon storage in soils.  Since soils can store vast quantities of organic carbon, and could offset a lot of mankind’s industrial carbon dioxide emissions, understanding the role of earthworms is important.

The earthworm project is a joint effort between Purdue University, the Smithsonian, Johns Hopkins University and the University of Utrecht.

Picture: The earthworm, Lumbricus terrestris. Picture courtesy of Purdue University.

Links:

• Earthworm activity can alter forests’ carbon-carrying capabilities- Purdue University
• Earthworms, P. Bohlen 2002 – chapter from Soil Encyclopedia. Archbold Biological Station. (pdf)

Global warming is contributing to the increased frequency of forest fires in the American west. A new study by the US Forest Service shows that hot fires in western forests consume soil carbon and nitrogen, reducing the ability of soils to store carbon and reducing soil fertility.  This is a potential feed-forward effect: increasing temperatures due to carbon dioxide in the atmosphere increases fire frequency and intensity, and high intensity fires convert soil organic carbon to carbon dioxide, increasing global warming.

The new study was a fortunate outcome of an unfortunate event: the 2002 Biscuit Fire, which burned about 500,000 acres in southwest Oregon, burned research plots of an ongoing study of forest soils. That meant that there were samples of soils available before and after the fire as well as pre-and post-fire plots measuring tree and shrub growth. Bernard Bormann, the study’s lead investigator, said “Losing our experiment in fire was hard, but the opportunity to better understand fires as a dominant ecosystem process has been very exciting.”

The Biscuit Fire burned soils at more than 1300 F, converting soil organic matter into carbon dioxide, nitrogen oxides and water vapor. Soils lost 10 tons per acre of carbon and 450 to 620 pounds per acre of nitrogen. These losses were higher than estimates from previous studies.  Loss of nitrogen to this degree would take at least a century to recover, in the absence of nitrogen-fixing plants.

The loss of soil productivity means that replacement forests may grow very slowly for long periods. These new forests would not be a vigorous sink for carbon sufficient to offset carbon lost from the soil for many years. The exact balance between carbon loss from the fire, both in the standing tree crop and soils, and the rate at which replacement forests will take up carbon from the atmosphere is not known.

Sources:
Press Release from Pacific Northwest Forest Experiment Station – When it comes to forest soil, wildfires pack a 1-2 punch.
Bormann, BT, Homann, PS, Darbyshire, RL, Morrissette, BA. 2008. Intense forest wildfire sharply reduces mineral soil C and N: the first direct evidence. Can. J. For. Res. 38:2771-2783. doi 10.1139/X08-136.