Tag Archives: soil drainage

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.

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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.