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Is There an Existing Healthy Threshold for Carbon Storage in the Ecosystem? Yang Gao,†,* Guirui Yu,†,* Nianpeng He,† Hongling He,† Qiufeng Wang,† and Huajun Fang† †
Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, P. R. China and water cycles in the ecosystem exist mostly through the water fluxes that connect these cycles physically.3 The C and N transport at a soil−water interface is largely controlled by the C and N inputs and flow rate. Soil mobilizes N and soluble C during intense rainfall. Then, they are transported to the deeper layers, which results in increased substrate availability for the decomposer community and improved nutrient conditions for vegetation. N availability affects the terrestrial albedo, energy and water fluxes, plant C production, respiration, and soil organic matter decay (Figure 1). N limitation occurs when the ecosystem’s C productivity is limited by N availability. N fixation reduces the extent of N limitation when C availability is increased, and it may synergistically lead to an increased C storage. Precipitation, one of the most important factors affecting soil erosion, can cause C transport through runoff and leaching. Soil erosion affects C dynamics in the ecosystem through its effect on the following: slaking or disruption of aggregates; preferential removal of C in runoff water; mineralization of soil organic matter on site; mineralization, displacement, and redistribution of soil organic C over the landscape and transportation in rivers; reaggregation of soil through the formation of organo−mineral complexes in the n the United Nations Climate Change Conference held in depositional/protected sites; and deep burial of C-enriched December 2009, China, ranking second as the leading source sediments in the depositional sites. Most soils may lose one-half of greenhouse gas emissions in the world following the U.S., to two-thirds of their soil organic C pool within 5 and 50 years advanced its reduction of CO2 emission intensity by 40−45% in the tropics and temperate regions, respectively. The new below the 2005 levels by 2020. To decrease CO2 emission and equilibrium may be attained after losing 20 Mg C.ha−1 to 50 combat climate change, China initiated six large forestation Mg C·ha−1. projects to enhance carbon (C) storage by 16−30 billion tons Based on the coupling relationships between C, N, and of C every year starting from 2006 to 2050. These projects were conducted by increasing the country’s forested area by 40 water, we discuss different conditions to understand the Cmillion ha.1 The people and the government have paid much healthy threshold in the ecosystem. If C storage by forestation attention to C sequestration for a long time. However, they is increased, the C productivity of plants will need an adequate have neglected to determine whether C sequestration has nutrient and water supply. However, if C storage exceeds the negative effects on the health of the ecosystem. For example, nutrient and water supply limit, either plant life will die or C although C sequestration as a result of China’s large-scale will be transported through soil erosion or other pathways forestation projects is a good goal, Wang and Cao2 considered because of the balanced relationships between C, N, and water C sequestration engineering to have negative effects on the in the ecosystem. Thus, there is an existing balance threshold health of the ecosystem. Therefore, we advance the concept of for C, nutrients, and water.5 The other condition is that an a C-healthy threshold in the ecosystem. Mastering the Cincrease in precipitant leads to a large amount of nutrient and C healthy thresholds for different ecosystems is important in loss by soil erosion. When the remaining C in soil cannot combating climate change and making appropriate climate maintain the normal C cycle in a terrestrial system, either the policies in the future. plants will die or the ecosystem will be in a subhealth state. We Climate change can affect forest growth because plant growth can conclude that, if the C storage in the ecosystem is not depends on the health of the ecosystem and the availability of controlled under normal fluctuation ranges and is not C, nitrogen (N), and water, all of which can be influenced by climate change as well. The effects of climate change on Received: March 21, 2012 evaporation and precipitation accelerate water cycles. The Accepted: April 3, 2012 Published: April 10, 2012 reason is that the strong coupling relationships between C, N,
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© 2012 American Chemical Society
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dx.doi.org/10.1021/es301163u | Environ. Sci. Technol. 2012, 46, 4687−4688
Environmental Science & Technology
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Figure 1. Coupling of C, N, and climate. N enters the terrestrial biosphere through biological fixation and reactive N deposition. C and N flow in parallel between vegetation, litter, and soil organic matter with respect to the stoichiometry of various organic matter pools.4
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maintained as a healthy threshold, ecological degradation will occur. The C cycle is essential in environmental assessment because it is closely related to the different environmental and socioeconomic attributes or functions, values, services, and benefits provided by forests. Forests have an important role in the global C cycle, and they are valued globally for the services they provide to society. However, extensive efforts of forestation in semiarid and waterless regions without protecting understory vegetation can also potentially increase water erosive energy, thus accelerating soil erosion. In addition, the climate effects of CO2 storage in these forests may be offset by albedo changes.2 Although forestation may initially increase forest cover and C storage, its effect on biodiversity and its ability to achieve ecological restoration will depend on the suitability of the tree species in the local environment. The above-mentioned factors suggest that maintaining a Chealthy cycle in the ecosystem should be the primary goal of C sequestration. Blind C sequestration engineering, such as forestation, may have an effect on the C-healthy cycle in the ecosystem. If the C storage exceeds the C-healthy threshold in the ecosystem, it will lead to plant or nutrient cycle being in a subhealth state. In facing the C sequestration problem, we must recognize the potentially limited C sequestration provided by large-scale forestation and the potentially large risk to ecosystem health. Therefore, suggesting other scientific, technical, and socioeconomic solutions may be a more effective way to combat climate change. Furthermore, we must design suitable forest strategies by understanding the C cycle in forest dynamics under different management regimes. In the future, mastering the C-healthy threshold will be helpful in making appropriate ecological restoration measures to combat climate change and maintain a C-healthy cycle in the terrestrial ecosystem.
AUTHOR INFORMATION
Corresponding Author
*Phone/fax: +86-10-64889040; e-mail:
[email protected] (Y.G.);
[email protected] (G.U.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Professor Geoffrey Hart (Montréal, Canada) for his help in writing this paper. This work was financially supported by National Key Basic Research Program (2010CB833504).
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REFERENCES
(1) Yin, R.; Sedjo, R.; Liu, P. The potential and challenges of sequestering carbon and generating other services in China’s forest ecosystems. Environ. Sci. Technol 2010, 44, 5687−5688. (2) Wang, Y. F.; Cao, S. X. Carbon sequestration may have negative impacts on ecosystem health. Environ. Sci. Technol. 2011, 45, 1759− 1760. (3) Hungate, B. A.; Dukes, J. S.; Shaw, M. R.; Luo, Y. Q.; Christophe,r, B. Field Nitrogen and climate change. Science 2003, 302 (5650), 1512−1513. (4) Manzoni, S.; Porporato, A. Common hydrologic and biogeochemical controls along the soil−stream continuum Hydrol. Process. 2011, 25, 1355−1360. (5) Gao, Y.; Zhong, B. L.; Yue, H.; Wu, B.; Cao, S. A degradation threshold for irreversible loss of soil productivity: a long-term case study in China. J. Appl. Ecol. 2011, 48, 1145−1154.
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