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Abstract
A changing precipitation regime and increasing nitrogen deposition are likely to have profound impacts on arid and semiarid ecosystem C cycling, which is often constrained by the timing and availability of water and nitrogen. However, little is known about the effects of altered precipitation and nitrogen addition on grassland ecosystem C exchange. We conducted a 3-year field experiment to assess the responses of vegetation composition, ecosystem productivity, and ecosystem C exchange to manipulative water and nitrogen addition in a meadow steppe. Nitrogen addition significantly stimulated aboveground biomass and net ecosystem CO2 exchange (NEE), which suggests that nitrogen availability is a primary limiting factor for ecosystem C cycling in the meadow steppe. Water addition had no significant impacts on either ecosystem C exchange or plant biomass, but ecosystem C fluxes showed a strong correlation with early growing season precipitation, rather than whole growing season precipitation, across the 3 experimental years. After we incorporated water addition into the calculation of precipitation regimes, we found that monthly average ecosystem C fluxes correlated more strongly with precipitation frequency than with precipitation amount. These results highlight the importance of precipitation distribution in regulating ecosystem C cycling. Overall, ecosystem C fluxes in the studied ecosystem are highly sensitive to nitrogen deposition, but less sensitive to increased precipitation.
Highlights
Ecosystem C cycling is an important ecological process in grasslands, which are often constrained by the availability of key resources, such as water and nitrogen [1, 2]
Compared to the control (CK) plots, the nitrogen addition (N) and water addition (W) treatments showed no effects on soil pH in 2013 and 2014 (Table 1)
We found that water addition had no impact on net ecosystem CO2 exchange (NEE) in the studied meadow steppe
Summary
Ecosystem C cycling is an important ecological process in grasslands, which are often constrained by the availability of key resources, such as water and nitrogen [1, 2]. According to the IPCC report [3], alterations in global precipitation regimes are likely to happen in mid-latitude regions, with increased annual precipitation and more frequent extreme rainfall events. Worldwide nitrogen deposition, which has been studied extensively, is likely to increase due to the accelerating industrialization and use of nitrogen fertilizer [4,5,6]. The predicted alteration in the precipitation regime and enhancement in nitrogen deposition may have a profound. Effects of W and N Addition in Songnen Steppe
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