氮素添加对内蒙古草甸草原生态系统CO_2交换的影响

Effects of Nitrogen Addition on Ecosystem CO_2 Exchange in a Meadow Steppe,Inner Mongolia

氮沉降增加将影响草原生态系统固碳,但如何影响草原生态系统CO_2交换目前为止还没有定论。同时,不同类型和剂量氮素对生态系统CO_2交换影响的差异也不明确。选取内蒙古额尔古纳草甸草原,开展了不同类型氮肥和不同剂量氮素添加条件下生态系统CO_2交换的野外测定。实验设置尿素和缓释尿素2种类型氮肥各5个剂量水平(0、5.0、10.0、20.0和50.0g N·m^(–2)·a^(–1))。结果显示,生长季初期及中期降雨量低时,氮素添加抑制生态系统CO_2交换;而生长季末期降雨量较高时促进生态系统CO_2交换。随着氮素添加水平的提高,NEE和GEP均显著增加,当氮素添加量达到10 g N·m^(–2)·a^(–1)时,NEE和GEP的响应趋于饱和。2种氮肥(尿素和缓释尿素)仅在施氮量为5 g N·m^(–2)·a^(–1)时,缓释尿素对生态系统CO_2交换的促进作用显著大于尿素,在其它添加剂量时差异不显著。研究结果表明:氮素是该草甸草原生态系统的重要限制因子,但氮沉降增加对生态系统CO_2交换的影响强烈地受降雨量与降雨季节分配的限制,不同氮肥(尿素和缓释尿素)对生态系统CO_2交换作用存在差异。

Increasing nitrogen deposition influences carbon sequestration in grassland ecosystems,but we have no consistent results on how it impacts the exchange of CO_2 at the ecosystem level.As well,the impact of different types of N fertilizers and rates of N increase are not clear yet.This study aimed to evaluate the effect of N addition on ecosystem CO_2 exchange and was performed in a meadow steppe in Erguna,Inner Mongolia.A field experiment compared two types of N fertilizers（urea and slow-release urea） with five N addition rates（0,5.0,10.0,20.0 and 50.0 g N·m^（–2）·a^（–1））.At the start and middle stage of the growing season,N addition had weak and inhibitive effects on ecosystem CO_2 exchange when precipitation was low,and significantly increased ecosystem CO_2 exchange in the late growing season when precipitation was high.Both net ecosystem CO_2 exchange and gross ecosystem photosynthesis increased significantly with N addition rate but showed a tendency of saturation when the N addition rate reached 10 g N·m^（–2）·a^（–1）.The two types of N fertilizers resulted in slightly different responses of ecosystem CO_2 exchange：slow-release urea had a stronger positive effect at 5 g N·m^（–2）·a^（–1） with no significant difference at other addition rates.Our study suggests that increasing N deposition has significant effects on carbon assimilation in this semi-arid grassland,but the direction and magnitude of effects are strongly affected by seasonal pattern and amount of precipitation.The effects of different types of N fertilizers（i.e.,urea and slow-release urea） on ecosystem CO_2 exchange may differ.