Nitrogen (N) deposition has been critical to the increase in the terrestrial carbon (C) sink in recent decades. Clarifying the C-N cycling responses in rangelands to N enrichment is essential for quantitatively understanding and predicting how N enrichment influences terrestrial C sinks. However, the N enrichment effects on rangeland C-N cycling under livestock grazing, rather than grazing exclusion, have rarely been investigated. Here, we conducted a 4-year in situ field experiment to examine the effects of multilevel N addition (NH₄NO₃: 0, 10, 20, 40, 80 and 160 kg N ha⁻¹ yr⁻¹) on soil N characteristics and ecosystem C fluxes in an alpine pasture on the Tibetan Plateau. We found that there were no significant differences in the soil organic C, soil total N, or soil C:N ratio across the six N addition levels. Over the four years, N addition generally increased the average soil inorganic N, with an increase ranging from 6% for the addition of 10 kg N ha⁻¹ yr⁻¹ to 292% for the addition of 160 kg N ha⁻¹ yr⁻¹. Such an increase in soil inorganic N was dominated by soil NO₃⁻-N enrichment rather than NH₄⁺-N variations, resulting in a decreased soil NH₄⁺/NO₃⁻ ratio. Unlike the significant response of soil inorganic N to N addition, the net ecosystem CO₂ exchange, gross ecosystem productivity and ecosystem respiration averaged over the growing season were not significantly changed by N addition in the third and fourth years, possibly due to the regulation by livestock grazing. In the last experimental year, N addition significantly enhanced belowground biomass but had no significant effect on aboveground biomass, leading to an increase in the root:shoot ratio. Our study highlights that N addition altered C-N cycling in alpine pastoral ecosystems and provides experimental evidence for quantitatively parameterizing the C process responses to N variations in terrestrial ecosystem models.

氮（N）沉积对于近几十年来陆地碳（C）汇的增加至关重要。明确牧场中CN循环对N富集的响应对于定量了解和预测N富集如何影响陆地C汇至关重要。然而，很少研究在放牧而不是放牧的情况下氮富集对牧场CN循环的影响。在这里，我们进行了为期4年的原位田间试验，以检验多水平添加N（NH ₄ NO ₃：0、10、20、40、80和160 kg N ha ^-1  yr ^-1的影响青藏高原高寒草场土壤氮素特征和生态系统碳通量的变化）。我们发现，在六个氮添加水平上，土壤有机碳，土壤总氮或土壤碳氮比没有显着差异。超过四年，N除了通常增加了平均土壤无机N，随着范围从6％添加10千克氮公顷^-1 年^-1至292％用于添加160公斤氮公顷^-1 年^{- 1}。土壤无机氮的增加主要由土壤NO ₃ ^-- N的富集而不是NH ₄ ⁺ -N的变化决定，导致土壤NH ₄ ⁺ / NO _3-减少^。比率。与土壤无机氮对氮的显着响应不同，在第三和第四年，氮的添加并没有显着改变生长季平均的生态系统净CO ₂交换量，生态系统总生产力和呼吸系统平均水平，这可能是由于放牧的牲畜。在最后一个实验年中，氮的添加显着提高了地下生物量，但对地下生物量没有显着影响，导致根：枝比率增加。我们的研究突出表明，氮的添加改变了高山牧草生态系统中的CN循环，并为定量表征C过程对陆地生态系统模型中N变化的响应提供了实验证据。