Tag Archives: soil organic matter

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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A forest on glass beads

Defying all expectations,  a forest has grown on the beach. Just east of Philadelphia is  the New Jersey Pine Barrens- a vast wilderness almost completely ignored outside of a small group of park rangers, volunteer fire fighters and hardy locals whose families have eked out a subsistence living here for generations. And yet is is considered the largest undisturbed wilderness in the northeastern corridor with a incredibly unique ecology and a set of rare endemic species.

The Pine Barrens region spans most of southeastern New Jersey from the Atlantic coast inland across the mid-Atlantic coastal Plain. The coastal plain is essentially a huge wedge of sand that has accumulated over 5 million years of sedimentary deposition from the Applalachian mountains and southerly flowing rivers such as the Hudson. A long history of sedimentary accumulation has created a flat, unvarying topography. 18 feet of height divide the highest point in the Pine Barrens “uplands” region and the lowest point in the “lowlands”, and yet this seemingly homogeneous landscape has produced broad environmental gradients over incredibly short distances.

The reason for these gradients seems to relate to the unique nature of the sandy, nutrient depauperate soil. Sand can act as either a sieve or a water trap depending on its spatial position. At higher elevations,  rain drains freely through the sandy soils, leaching away any nutrients that accumulate and producing highly acid conditions that few plants can survive on. The uplands forests are dominated by drought-resistant, fire-tolerant pitch pine trees with a smattering of oaks. A few scrubby, low-nutrient requiring members of the Ericaceae family, such as huckleberry and highbush blueberry, dominate the understory. At low elevations, rain accumulates and has nowhere to go. The water table is generally high, producing soils that are saturated year-round. In the most saturated places, one finds peat bogs and white cedar swamps more reminiscent of the deep south than the mid-Atlantic. Swamps grade into tall, shady oak-dominated forests dotted with an occasional pine in the drier lowlands.

The stark contrast between uplands and lowlands vegetation is not just a product of  the soil conditions. The uplands and lowlands communities produce feedbacks on the environment that maintain the land in precisely the same condition for generations, such that nothing new can manage to gain a competitive edge.

In the uplands,  pine trees exude organic acids into the soils, maintaining their soils in a state of poor nutrient quality that nothing else can survive on. Every few years pines drop their needles, but not before sucking nearly all the nutrients out and back into the branches, ensuring that few nutrients are added back to the soil. The low nutrient-quality of this leaf litter slows microbial decomposition, causing years of litter to accumulate on the surface. This litter serves as kindle that enhances the spread of forest fires the pine trees require to sprout. These forest fires keep the less fire-resistant oaks at bay.

In the lowlands, wet conditions prevent forest fires from scorching the landscape with regular ferocity seen in the uplands. Oak trees are able to gain a firmer footing here, and once established, produce a shady understory that pine saplings cannot survive in. The oak trees drop their leaves annually, adding more nutrients to the ground and producing a soil richer in organic matter than enhances the development of a herbaceous understory, which helps crowd out pine saplings.

I stood over a pit we had just dug in the ground, staring down into what resembled a layer cake of dark chocolate, vanilla and red velvet. Distinct stratification in the soil profile  generally indicates a long history of mobilization processes. Organic matter leached down through the chocolately topsoil will sometimes produce a white, organic-free layer beneath, known to soil scientists as an E-horizon. Deeper still, weathering products from the underlying bedrock will accumulate in the subsoil and form complexes with the organic matter that has been transported down. The reddish layer I was seeing in the deep soil was the result of iron accumulation and subsequent oxidization by microbes in need of an energy source. In fact, the entire soil profile, from brown to white to red, is very typical of a class of soils known as Spodosols that dominate the New England and Canadian boreal forests, where low temperatures cause decomposition and other soil-forming processes to occur slowly, resulting in a surface buildup of organic matter and eventually the formation of distinct, colorful stratified layers. What, then, was such a soil doing in the Pine Barrens, a much warmer climate than New England, and a region with few soil nutrients and barely any organic matter inputs?

It turns out that the Pine Barrens soils which so closely resemble Spodosols may in fact be a relic from a much earlier time and different climate. At the height of the Last Glacial Maximum approximately 18,000 years ago, a large continental ice sheet known as the Wisconsan Glacier ended a mere 40 miles north of the Pine Barrens.  The New Jersey climate probably resembled those seen in the high Canadian boreal today, and there is little doubt that the soils that formed were some version of Spodosols. It is entirely possible that, given the state of extreme stasis that the Pine Barrens have existed in since the beginning of the Holocene, not much has occurred to alter the soils from their former state.  A forest that grows today on glass beads has thrived because of its ability to maintain stasis. Peeling back the layers of that forest reveals this stasis to be true, but only for a fleeting moment in the geologic record.

Isotopes?

Isotopes?

A term thrown in so many different fields of science but never really successfully explained outside the realm of the super nerdy. They’re pretty simple, really- essentially just flavors of the same atom with different numbers of neutrons. More neutrons, heavier isotope. Too many neutrons, and your isotope becomes unstable, radioactively decaying over time to a different version of that element or perhaps another element entirely. With that brief introduction I’d like to explain one stable isotope system that is particularly interesting to me because it allows scientists to take a piece of the earth and reconstruct ancient environments.

C12 and C13 are stable isotopes of carbon- they both occur naturally in the environment and do not undergo any natural physical transformations over time. However, because of a small difference in their molecular weights due to the “extra” neutron in the C13 isotope, these two isotopes are processed quite differently in the environment.

Being slightly heavier means that C13 is a bit more difficult for biological systems to process. Most biological processes are adapted to using the lighter isotope, which is far more abundant.  When air diffuses into plant leaves via stomates, the tiny pores that suck up carbon dioxide for photosynthesis, CO2 is tightly bound by an enzyme before it can diffuse out again. In most cases this enzyme is Rubisco (see post: enzymes in the environment) . It turns out that Rubisco preferentially binds to C12, causing C13-enriched air to be released back into the atmosphere. Since most plants take up CO2 via Rubisco (this is known as the C3 photosynthetic pathway), most plant tissue on Earth is depleted in C13 relative to the atmosphere.

However, when a plant keeps its stomata open to take up CO2, a problem emerges- transpiration. Water loss occurs primarily through these same pores that plants must keep open if they want to feed themselves. In hot, arid environments, this puts your normal C3 plants in a sticky situation. They must open their stomata to eat, but risk losing dangerous amounts of water when they do so.

Millions of years ago, a group of plants evolved a rather elegant solution to this problem, known as the C4 photosynthetic pathway. They co-opted an enzyme already present in mitochondria for cellular respiration and gave it Rubisco’s job. This enzyme, known as PEP-carboxylase (I’ll call it PEPC here for simplicity), has a much higher affinity for CO2 than Rubisco- in fact, it binds CO2 so tightly that leaf stomata only need to be open for a fraction of the time they would otherwise. The high affinity of PEPC for CO2 also means that it doesn’t “distinguish” C12 from C13- it grabs whatever CO2 molecule is closest and binds tightly.

What does it matter that two classes of plants fractionate C13 differently? Scientists now have the tools to analyze the molecular composition of plant tissue and can determine a plant’s specific C13/C12 ratio.  C3 and C4 plants have distinct C13/C12 ratios and are easy to distinguish once isotopic analysis has been performed. For living plants, this would not be a terribly illuminating exercise- there are other anatomical and taxonomical ways to distinguish C3 and C4 plants that would be much more straightforward and less expensive.

But what about dead plants? Soil organic matter is composed principally of decomposed plant material, but even the most knowledgeable soil scientists aren’t able to look at a soil and say exactly what plants produced it. If we could, however, the soil would tell us numerous things. Accumulating over hundreds to tens of thousands of years, the soil profile from bedrock to the surface essentially represents a continuum of accumulated material that represents different floral and faunal assemblages, climate regimes and major environmental disturbances.

However, a complex series of transformation processes take place as plant material is decomposed and moved down the soil profile, some of which lead to C13 accumulation while others lead to C13 depletion. Carbon compounds are sorbed to surfaces, eaten by microbes, recycled, taken up by plants, leached, oxidized, and protected, to name a few. Given the inherent complexity of these systems, how can scientists can’t always sample down a soil pit and accurately describe species assemblages at different times using carbon isotopes alone.

A more fruitful path has involved obtaining environmental  samples that have undergone relatively little decomposition, such as cores of sediment from the bottom of a lake, or a core of peat from an inundated field. The plant material within samples that have been buried or otherwise protected from decomposition will be relatively similar, at least at the molecular level, to the original plant tissue, and can thus provide meaningful information about a past environment. For example, a sudden switch from C3 to C4 dominated plant material could indicate a transition from a cooler, wetter climate to a warmer, drier one. Stable carbon isotopes have proved incredibly valuable in tracing the spread of human agriculture, which can often lead to rather dramatic changes in the isotopic signature of a sample.

Still don’t think isotopes are interesting? If you have a friend whose fidelity to vegetarianism is in question, sending a sample of their hair to an isotope lab should resolve the situation. Chances are, if you’re a vegetarian your C13 levels will be relatively high, indicating a more plant-rich diet (a disproportionate number of the world’s major crops are C4 plants).

Carbon sequestration in soils: sources, sinks and pitfalls

When we talk about “sequestering” carbon in the soil to mitigate anthropogenic climate change, what are we really saying as scientists, and what is the public getting from it?  And perhaps even more importantly, what implicit assumptions are we as scientists making that may affect the validity of the widely held belief that increased C storage in soils is a good thing, and can offset our CO2 emissions?

In a recent soil carbon series in the European Journal of Soil Science, one review paper attempts to address these issues. “Carbon sequestration” has become a popular idea among policy makers and scientists alike. As anyone who keeps up on ecosystem science literature will know, C sequestration has also become an explicit motivation for numerous studies of soil and ecosystem properties.

Current best estimates claim that worldwide, soils store 684-720 Pg of C within the upper 30 cm, and 1462-1548 Pg to a depth of 1m. (For anyone not familiar with these terms, 1Pg = 1×10ˆ15 gram). The upper 30% of the soil profile to 1m clearly stores a disproportionate amount of C; this ecologists know to be the zone where continuous inputs from roots and organic matter replenish C. To put these numbers in an ecosystem context, the amount of C stored globally in soils to 30cm is twice the amount  of C stored as CO2 in the atmosphere and three times the amount of C stored in above ground vegetation. Clearly, soils represent a huge pool in global C budgets and should be managed with the knowledge that changes may represent a powerful feedback on the global C cycle. This vast storage potential is what has popularized the idea of sequestering even more C in soils and has spurred a myriad of different research approaches, from ecosystem-scale reforestation efforts to molecular studies of C-mineral binding interactions.

Given the amount of hope that has already been invested in this powerful idea, it is important for scientists interested in soil C to keep several caveats in mind when planning and conducting their research, and when communicating that research to a broader audience at a science or policy conference.

Caveat #1) Carbon sequestration does not necessarily mean climate change mitigation.

Increasing the amount of C stored in a particular ecosystem’s soil does not by default decrease the amount of CO2 released to the atmosphere. This is partially an issue of scale. If policy makers choose to set aside a certain amount of pasture land for reforestation (with the assumption that this will increase the net C storage in these systems), it is often the case that new land will have to be cleared somewhere else to compensate for the lost agricultural production.  The clearest way around this problem is to be selective in land use changes: land that is more fertile should be exploited for farming, and land that is less fertile should be reforested to boost C storage. However, an increasing global population poses obvious constraints to this line of reasoning. Ultimately, the fact remains that in the upcoming decades, we will need to boost our agricultural yields to meet global demand, and furthermore, climate change is predicted to decrease the fertility of some of the world’s most productive regions.

Moreover, allowing land to go back to its “natural” state does not necessarily lead to a net accumulation of C. A forest respires far more CO2 per acre than a wheat field. Due to the high water demands of forests relative to crops, it can sometimes be the case that trees dry up otherwise inundated subsurface soils, and in doing so create a soil environment that accelerates the decomposition of C.  One must have a refined understanding of the sources and sinks of CO2 in any particular ecosystem to claim that it will or will not “sequester” C.

Finally, management changes leading to increased C storage may increase or decrease the flux of other greenhouse gases, most notably N2O and methane. Given that these gases have 298 and 25 times the global warming potential of CO2, respectively, they are not a trivial consideration. Forests often hold less nitrogen than pastures (due to the different C:N demands of woody vs. non-woody tissue), and consequently release more N2O through the microbial process known as denitrification.

Caveat #2) The amount of carbon that can be locked up in soil in finite

Our understanding of how C is actually “stabilized” in soil on a molecular scale is still evolving. Scientists are using powerful technologies such as x-ray crystallography to understand in detail the chemical interactions between soil minerals and carbon-rich compounds that cause C to be tightly bound and inaccessible to microbes. As a general rule, the more mineral surface area available for C binding, the more tightly C is bound. In subsurface soils lower levels of C mean higher (mineral surface area: C) ratios and thus tighter binding of the C that is present.

However, mineral stabilization has its limits. Laboratory experiments are finding that as C is added to soils, “steady states” are sequentially reached. Careful manipulation of the  chemical and thermodynamic parameters of a soil may bump that soil from a lower steady state to a higher one, but whether such techniques can be applied on a whole-ecosystem scale remains to be seen.

To repeat, caveat #2 is that the amount of carbon that can be locked up in soil is finite, which leads to caveat #3.
At present, the amount of CO2 we put in the atmosphere does not seem to be.

Powlson et al. 2011. Soil carbon sequestration to mitigate climate change: a critical re-examination to identify the true and the false. European Journal of Soil Science 62: 42-55.

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.