Primary productivity in terrestrial ecosystems can be significantly altered by the predicted increases in nitrogen (N) deposition, but it is still unclear how N deposition influences the carbon (C) exchange processes especially in dryland ecosystems. In this study, a 3-year experiment with two types of fertilizers and five N addition levels was conducted in a semiarid steppe of Erguna, Inner Mongolia to assess the effects of exogenous N input on net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (ER). Our results showed that enhanced N input significantly increased NEP and GEP when the volumetric soil water content (VWC) was greater than 15% (v/v%), but had trivial and inconsistent effects when VWC was less than that threshold. Moreover, the NEP and GEP increased significantly with increasing N addition but leveled off when the N input reached 10 g N m^–2 year^–1 and the VWC was above 15%. However, the ER showed a significant increase with N addition rates, regardless of soil water content. Our main findings are as follows. First, the ecosystem C exchange can be significantly enhanced by addition of N only when soil moisture exceeds certain threshold, suggesting co-limitation of water and N in the grassland ecosystem that we studied. Second, in addition to concurrent consideration of water, temperature and N as well as their interactions also need to be accounted for. Third, the frequency of days with the VWC being higher than 15% accounted only around one third of days during the three growing seasons. Taken together, our findings imply that the expected increases of carbon sequestration should be minimal under N deposition in the Inner Mongolia grassland ecosystems.

预计氮（N）沉积量的增加会大大改变陆地生态系统的初级生产力，但目前尚不清楚氮沉积如何影响碳（C）交换过程，尤其是在旱地生态系统中。在这项研究中，在内蒙古额尔古纳的半干旱草原上进行了为期3年的试验，使用两种肥料和5种氮素添加水平，以评估外源氮素输入对净生态系统生产力（NEP），生态系统总生产力的影响（ GEP）和生态系统呼吸（ER）。我们的研究结果表明，当土壤含水量（VWC）大于15％（v / v％）时，增加的氮输入会显着增加NEP和GEP，但当VWC小于该阈值时，其影响却微不足道。而且，^–2年^–1VWC高于15％。但是，无论土壤含水量如何，ER随N的添加而显着增加。我们的主要发现如下。首先，仅当土壤水分超过一定阈值时，通过添加氮可以显着增强生态系统的碳交换，这表明我们研究的草地生态系统中水和氮的共限制。其次，除了同时考虑水，温度和氮以及它们之间的相互作用外，还需要考虑在内。第三，在三个生长季节中，VWC高于15％的天数仅占三分之一。综上所述，我们的研究结果表明，在内蒙古草原生态系统中，在氮沉降下，碳固存的预期增加应该最小。