氮素添加对内蒙古草甸草原生态系统CO2交换的影响

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CO₂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CO₂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CO₂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50.0 g 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CO₂交换; 而生长季末期降雨量较高时促进生态系统CO₂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CO₂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CO₂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CO₂交换作用存在差异。

Effects of Nitrogen Addition on Ecosystem CO2 Exchange in a Meadow Steppe,Inner Mongolia

Increasing nitrogen deposition influences carbon sequestration in grassland ecosystems, but we have no consistent results on how it impacts the exchange of CO₂ 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 CO₂ 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 CO₂ exchange when precipitation was low, and significantly increased ecosystem CO₂ exchange in the late growing season when precipitation was high. Both net ecosystem CO₂ 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 CO₂ 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 CO₂ exchange may differ.