Tag Archives: soil respiration

Don’t treat soil like dirt!

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Soil is a precious resource, yet most of us pay little attention to the stuff under our feet.  It is the medium in which we grow our food and the foundation on which we build our cities. Soils filter our water, detoxify our pollutants, decompose our waste and hold vast reserves of the nutrients required for life. Soils are also fragile, taking thousands to million of years to develop but destroyed in minutes by human development.

For the past three years, myself and my fellow soil enthusiast Aurora have spent our Saturdays in December showing kids how awesome soil and the microbes that inhabit it actually are. We’ve developed a number of soil and microbiology activities to teach kids of all ages what soil actually is, who lives in it, and why we should value it. The take-home message? Don’t treat soil like dirt! Human beings (and nearly ever other species on earth) depend on soil for our very survival.

Here are some highlights from the last two weeks of the workshop:

Shahada_microscope
Checking out some protozoa under the microscope!
Making a hypothesis before conducting an experiment!
Making a hypothesis before conducting an experiment!
MAKING SOIL! This is always a favorite. Want to make soil at home with your kids? Check out my homebrew recipe below...
MAKING SOIL! This is always a favorite. Want to make soil at home with your kids? Check out my homebrew recipe below…
Probably the coolest thing I've ever made in photoshop.
Probably the coolest thing I’ve ever made in photoshop.

We are even participating in an international, crowd-sourced science experiment known as the Tea Bag Index experiment to measure rates of decomposition in different soil types! This is a fun and easy experiment you can do in your backyard. All you need is a few teabags and a scale.  Decomposition, the breakdown of once-living organic matter and conversion into soil organic matter, is an important step in the global carbon cycle that is driven primarily by soil microorganisms. Ultimately, decomposed carbon is respired back to the atmosphere as carbon dioxide. Scientists are currently trying to understand how global climate change will affect decomposition and the microbial “respiration” of CO2 from soils. Projects such as the Tea Bag Index experiment provide scientists with valuable data that can be used to inform predictions about changes to the global carbon cycle. For more information on the Tea Bag Index experiment check out the website:

http://decolab.org/tbi/concept.html

Or click here to access the protocol and get involved directly!

http://decolab.org/tbi/protocol.html

Most importantly, our workshop strives to underscore the importance of soils in our everyday lives.  Kids (and parents) often come unsure of what exactly soil is or why it should matter to them, and often enjoy the experience so much that they return week after week.

Live in the Philly area and got kids? Check us out, every Saturday for the rest of the month!

And since I can’t seem to stop geeking out about this stuff, here are some more cool resources to check out on soil science education:

USDA NRCS Healthy Soil Fact Sheets

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Tree girdling reveals that photosynthates drives soil activity late in the growing season

The role of trees in nourishing the soil has long been understood by ecologists, but the extent to which trees control belowground processes remains unclear. This is due in large part to the difficulties associated with measuring root activity and the dynamic interactions between roots, mycorrhizal fungi, and the soil environment.

To develop a better understanding of how photosynthates, the sugars produced by photosynthesis, influence patterns of growth and metabolic activity in the soil, a group of ecologists in Sweden decided to conduct a large scale girdling experiment. Tree girlding involves stripping tree bark all the way to the depth of the tree’s xylem, the long tubes that transport water from the soil to the leaves using negative pressure and the difference in water potential between the soil and the atmosphere. Xylem is located well inside the living tissue of a tree, but most importantly for a girdling experiment, it is located directly inside the phloem. Phloem is the piping that runs down the tree and transports photosynthates produced in the leaves via gravity into the rest of the plant. Thus by stripping a tree down to its xylem, the researchers are removing phloem and cutting off the supply of photosynthates to the roots and the soil. This allows them to study the effect of photosynthate removal on the soil without influencing roots, water transport or any other soil parameters.

In this large-scale experiment conducted in a boreal Scots pine forest in northern Sweden, six 900m2 plots were chosen and rouglhy 120 trees per plot were girdled.  In each plot, half of the trees were girdled early in June and half in late August. Early and late girdling were used to detect whether phenological (seasonal) differences affected root photosynthate production.

The results of this experiment were striking. In the early-girdled plots, soil respiration declined by 27% compared to control plots within the first five days of girdling. (Remember, “soil respiration” refers to the amount of CO2 released by the soil. It is a proxy for total soil metabolic activity, including the respiration of microbes, fungi, nematodes and other small soil invertebrates, and even roots themselves! Thus more respiration = healthier, more metabolically active soil.) By the end of the growing season, roughly 50% less respiration had occurred in the early-girdled plots. The occurrence of ectomychorizal fungi, an essential nutrient-acquiring symbiote for most plants, was also dramatically reduced. The late-girdled plots responded even faster, with respiration declining almost 40% within the first 5 days of girdling. Interestingly, the researchers found less response to girdling toward the edge of the girdled plots, indicating that soil organisms here were acquiring some photosynthates from neighboring, ungirdled trees.

The more rapid declines of soil respiration in the late girdling plots fit with our current understanding of how trees use their photosynthates. Early in the growing season, and especially in conifer forests, trees allocate most new photosynthates towards shoot, bud and needle production. Simultaneously, trees begin tapping their root carbon stores to enhance above ground growth. Later in the growing season more photosynthates are sent belowground to nourish the soil community. By this point root carbon stores are also diminished.  Girdling trees later in the growing season cuts off carbon supply to the soil at precisely the moment when it is being ramped up.

The flux of photosynthates to the roots has a big impact on belowground processes, with a clear seasonal component. In moving forward in our understanding of whole ecosystem carbon balances, understanding that trees are conduit, connecting the earth to the atmosphere and transforming both in the process, will, I believe, be an essential paradigm to adopt.

 

Reference:

Ho¨gberg P, Nordgren A, Buchmann N et al. (2001) Large-scale forest girdling shows that photosynthesis drives soil respiration. Nature, 411, 789–792.

 

 

 

Earthworms play key role in regulating carbon storage in tropical ecosystems

A principle frontier in our understanding of global carbon budgets is tropical forests, on which research is historically scarce. At temperate and high latitudes, a warmer climate is predicted to increase the rate of decomposition and soil carbon turnover, resulting in a positive feedback on atmospheric carbon as CO2 is released from soils at increasing rates. A better understanding of the mechanisms regulating tropical carbon storage is needed in order to develop a holistic picture of global carbon cycling and feedbacks due to climate change.

Earthworms are important regulators of many ecological properties of soils. Their burrowing activity increases soil pore space and contributes to soil structure and drainage. Most importantly, earthworms can digest a huge quantity of dead and partially decomposed plant material. This digestion causes chemical transformations that ultimately produce nutrient-rich soil organic matter, or SOM. SOM helps ensure soil fertility, and contributes to numerous physical and chemical soil properties such as soil structure, porosity, water retention, and the capacity of soils to buffer pH changes. SOM’s aggregate structure causes it to have high water stability. This is an essential property in tropical forests, which have the highest rainfall levels of any biome on Earth.

SOM produced by earthworms is also rich in both carbon and nitrogen. A detailed biochemical and molecular analysis of earthworm casts suggests that these creatures may in fact play a key role in controlling tropical carbon storage.

Casts are clumps of digested organic matter excreted by earthworms that aggregate into large and distinctive structures. Researchers working in the rain forest neighboring the Dong Cao village in Northeast Vietnam studied the effect of cast production by Amynthas Khami on soil C storge. A. Khami is a species of tropical earthworm that can grow up to 50 cm long and produce tower-like casts. The researchers first used a “simulated rainfall” experiment to determine the relative stability of casts versus control soils. They then measured total carbon content, lignin and mineral-bound SOM content of casts and control soils.

An earthworm cast produced by A. Khami, a large tropical species found in Northeast Vietnam.

The study found striking differences in the chemical composition of earthworm casts versus control soils that ubiquitously indicate higher carbon storage in casts. Casts are more structurally stable and can withstand at least twice as long a rainfall event as control soils without compromising their structural integrity. They are enriched in carbon compared with controls, and particularly in carbon compounds such as lignin that have a high “carbon storage” potential. Lignin, a primary constituent of woody plant tissue, is a complex and heterogeneous molecule that is both carbon-rich and difficult for microbes to decompose. Earthworms probably excrete high quantities of lignin after obtaining the more digestible carbon sources from the roots and leaves that they eat. Finally, high levels of mineral associated-SOM were found in casts. Soil minerals bind to organic matter through electrostatic interactions, and in doing so make it unavailable for decomposers.

Though it well known that earthworm digestion initially speeds up decomposition, this new study suggests that casts may in fact contribute to long-term carbon stabilization. In tropical soils, which tend to cycle carbon quite rapidly, this mechanism should not go unappreciated. Future tropical land-use decisions may want to account for the welfare of this often-unappreciated soil organism.

Hong et al. 2011. How do earthworms influence organic matter quantity and quality in tropical soils? Soil Biology and Biochemistry 43: 223-230.