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Earthworms facilitate net carbon sequestration in soil

Professor Shenglei Fu's group from South China Botanical Garden, Chinese Academy of Sciences (SCBG), together with colleagues from University of Georgia and US Environmental Protection Agency etc., reveal that earthworms facilitate net carbon sequestration in soil, although the humble animals are often found stimulating CO2 emissions. This finding contradicts recent claims from a meta-analysis paper published in Nature Climate Change that earthworm presence in soils leads to a 33% increase in CO2 emissions. The study is published in Nature Communications this week (DOI: 10.1038/ncomms3576), and has been selected for highlighting. The paper is featured in the press release entitled “Earth Sciences: Global Worming - Earthworms mediate soil carbon sink”. This study is also reported by science news magazines such as New Scientist, Live Science, Carbon Brief and Wenhui Bao (Shanghai).  

Biotic interactions between soil invertebrates and microbiota play essential roles in regulating land-atmosphere exchanges of carbon and carbon-cycle feedbacks on climate change. A timely and debatable topic is how earthworms interact with microbiota and, thus, affect carbon sequestration. Earthworms were often found to stimulate CO2 emission, especially in short-term experiment, but also were reported to enhance carbon stabilization in soil aggregates in some longer-term experiments. However, more experimental data support the view that earthworms reduce carbon sequestration due to the fact that CO2 emission is easier to detect than carbon stabilization.  In this study, Zhang and Fu et al hypothesize that neither an increase in CO2 emission nor in stabilized carbon would entirely reflect the earthworms’ contribution to net carbon sequestration; that is, the impacts of earthworms on the two coupled processes of carbon mineralization and carbon stabilization should be studied simultaneously.

The results suggest that, firstly, although earthworms accelerate carbon mineralization, the total amount of CO2 that can potentially escape from the soil with earthworms differs little from soil containing no earthworms because the capacities of carbon mineralization of earthworms and soil microbiota are similar. Most previously studies did not note this and, thus, were likely to conclude that earthworms decrease carbon sequestration only because CO2 emission was often enhanced by earthworms. Secondly, given that an increase in carbon mineralization (Cmin) and carbon stabilization (Csta) may be a natural consequence of an increased pool of activated carbon, the pool size of the activated carbon (Cact) and its allocation pattern into carbon mineralization and carbon stabilization then determine the net carbon sequestration. Our study reveal that the presence of earthworms is more likely to create a carbon sink as the carbon stabilized by earthworms outweighs that converted to CO2 during carbon mineralization. Thus, we introduce the new concept of sequestration quotient (SQ, Csta/Cact) to quantify the unequal processes. The patterns of CO2 emission and net carbon sequestration are predictable by comparing SQ values between treatments with and without earthworms. The global contribution of earthworms to carbon cycling may be clearer when the ranges of SQ values have been determined at different time-scales in a range of systems with variable environmental characteristics (e.g., climate, plant diversity, root carbon input, soil texture and food web structure). In addition, the concept of SQ could also be helpful to estimate the contributions of other factors/treatments (not only earthworms) to net soil carbon sequestration.

 

 

Photo by Weixin Zhang. Earthworm on forest floor. The processes and magnitude that earthworms impact net carbon sequestration are debatable.

 

Links:

Original linkhttp://www.nature.com/ncomms/2013/131015/ncomms3576/full/ncomms3576.html

Press release: http://www.nature.com/ncomms/press_releases/ncomms1013.html

New Scientist: http://www.newscientist.com/article/dn24404-worm-turns-from-zero-to-climate-change-hero.html#.Ul3n8I2BSUQ

Live Science: http://www.livescience.com/40413-how-earthworms-offset-their-carbon-emissions.html

Carbon Brief: http://www.carbonbrief.org/blog/2013/10/return-of-the-worm-do-they-really-increase-greenhouse-gas-emissions-from-soil/

Wenhui Bao: http://wenhui.news365.com.cn/jkw/201310/t20131016_1645015.html

 
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