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, 7 (9), e45205

Water- And Plant-Mediated Responses of Ecosystem Carbon Fluxes to Warming and Nitrogen Addition on the Songnen Grassland in Northeast China

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Water- And Plant-Mediated Responses of Ecosystem Carbon Fluxes to Warming and Nitrogen Addition on the Songnen Grassland in Northeast China

Li Jiang et al. PLoS One.

Abstract

Background: Understanding how grasslands are affected by a long-term increase in temperature is crucial to predict the future impact of global climate change on terrestrial ecosystems. Additionally, it is not clear how the effects of global warming on grassland productivity are going to be altered by increased N deposition and N addition.

Methodology/principal findings: In-situ canopy CO(2) exchange rates were measured in a meadow steppe subjected to 4-year warming and nitrogen addition treatments. Warming treatment reduced net ecosystem CO(2) exchange (NEE) and increased ecosystem respiration (ER); but had no significant impacts on gross ecosystem productivity (GEP). N addition increased NEE, ER and GEP. However, there were no significant interactions between N addition and warming. The variation of NEE during the four experimental years was correlated with soil water content, particularly during early spring, suggesting that water availability is a primary driver of carbon fluxes in the studied semi-arid grassland.

Conclusion/significance: Ecosystem carbon fluxes in grassland ecosystems are sensitive to warming and N addition. In the studied water-limited grassland, both warming and N addition influence ecosystem carbon fluxes by affecting water availability, which is the primary driver in many arid and semiarid ecosystems. It remains unknown to what extent the long-term N addition would affect the turn-over of soil organic matter and the C sink size of this grassland.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Daily mean air temperature, annual rainfall (RF), and daily precipitation in 2006, 2007, 2008 and 2009.
Data for 2006 were obtained from a weather station located in the Chang Ling Horse Breeding Farm in Jilin Province, 3 km in distance from the study site. Data in 2007, 2008 and 2009 were obtained from an eddy tower roughly 200 m distant from the experimental plots.
Figure 2
Figure 2. Seasonal dynamics (mean ± SE; n = 6) of net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), gross ecosystem productivity (GEP) in response to warming (1.8°C) and N addition (10 g m−2 yr−1) in the Songnen grassland.
C  =  control, W  =  warming, N  =  N addition, WN  =  combined warming and N addition.
Figure 3
Figure 3. Growing season mean (n = 6) net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP).
Data are presented as mean ± SE. Different letters indicate significant differences (P<0.05) among treatments with capital and small letters indicating differences between N and warming treatments, respectively. C  =  control, W  =  warming, N  =  N addition, WN  =  combined warming and N addition.
Figure 4
Figure 4. Temporal dependence of net ecosystem CO2 exchange (NEE) on soil water content (SWC) across the four growing seasons.
Regression lines (solid) and 95% confidence level (dotted lines) are showed. All the linear regressions are statistically significant (P<0.05). Regression equations and significance levels are provided in Table 4. (A), control; (B), warming; (C), N addition; (D), combined warming and N addition.
Figure 5
Figure 5. Temporal dependence of seasonal mean net ecosystem CO2 exchange (NEE), ecosystem respiration (ER) and gross ecosystem productivity (GEP) on leaf area index (LAI) across the four treatments in May and June of 2006–2009.
Regression lines (solid) and 95% confidence level (dotted lines) are showed. All the linear regressions are statistically significant (P<0.05). Regression equations and significance levels are provided in Table 4. Control plots (A,E,I); Warming plots (B,F,J); N addition plots (C,G,K); Warming and N addition plots (D,H,L).

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Grant support

This research was supported by the National Natural Science Foundation of China (31170303), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA 05050601), the Research Fund for the Doctoral Program of Higher Education of China (20090043110007), the State Key Laboratory of Vegetation and Environmental Change (LVEC2012kf01) and the Fundamental Research Funds for the Central Universities (11NQJJ028). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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