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.
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.