Is There an Existing Healthy Threshold for Carbon Storage in the Ecosystem?

Abstract

I the United Nations Climate Change Conference held in December 2009, China, ranking second as the leading source of greenhouse gas emissions in the world following the U.S., advanced its reduction of CO2 emission intensity by 40−45% below the 2005 levels by 2020. To decrease CO2 emission and combat climate change, China initiated six large forestation projects to enhance carbon (C) storage by 16−30 billion tons of C every year starting from 2006 to 2050. These projects were conducted by increasing the country’s forested area by 40 million ha. The people and the government have paid much attention to C sequestration for a long time. However, they have neglected to determine whether C sequestration has negative effects on the health of the ecosystem. For example, although C sequestration as a result of China’s large-scale forestation projects is a good goal, Wang and Cao considered C sequestration engineering to have negative effects on the health of the ecosystem. Therefore, we advance the concept of a C-healthy threshold in the ecosystem. Mastering the Chealthy thresholds for different ecosystems is important in combating climate change and making appropriate climate policies in the future. Climate change can affect forest growth because plant growth depends on the health of the ecosystem and the availability of C, nitrogen (N), and water, all of which can be influenced by climate change as well. The effects of climate change on evaporation and precipitation accelerate water cycles. The reason is that the strong coupling relationships between C, N, and water cycles in the ecosystem exist mostly through the water fluxes that connect these cycles physically. 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 depositional/protected sites; and deep burial of C-enriched sediments in the depositional sites. Most soils may lose one-half to two-thirds of their soil organic C pool within 5 and 50 years in the tropics and temperate regions, respectively. The new equilibrium may be attained after losing 20 Mg C.ha−1 to 50 Mg C·ha−1. Based on the coupling relationships between C, N, and water, we discuss different conditions to understand the Chealthy threshold in the ecosystem. If C storage by forestation is increased, the C productivity of plants will need an adequate nutrient and water supply. However, if C storage exceeds the nutrient and water supply limit, either plant life will die or C will be transported through soil erosion or other pathways because of the balanced relationships between C, N, and water in the ecosystem. Thus, there is an existing balance threshold for C, nutrients, and water. The other condition is that an increase in precipitant leads to a large amount of nutrient and C loss by soil erosion. When the remaining C in soil cannot maintain the normal C cycle in a terrestrial system, either the plants will die or the ecosystem will be in a subhealth state. We can conclude that, if the C storage in the ecosystem is not controlled under normal fluctuation ranges and is not