Abstract. In many temperate ecosystems, nitrogen (N) limits productivity, meaning anthropogenic N deposition can stimulate plant growth and subsequently carbon (C) sequestration. Phosphorus (P) and N-P co-limited grasslands are widespread, yet there is limited understanding of their responses to N deposition, which may transition more ecosystems toward P-limited or N-P co-limited states. Here, we investigate the consequences of enhanced N addition on the C-N-P pools of grasslands in different states of nutrient limitation. We explored the response of a long-term nutrient-manipulation experiment on two N-P co-limited grasslands; an acidic grassland of stronger N-limitation and a calcareous grassland of stronger P-limitation, by combining data with an integrated C-N-P cycling model (N14CP). To explore the role of P-access mechanisms in determining ecosystem state, we allowed P-access to vary, and compared the outputs to plant-soil C-N-P data. Combinations of organic P access and inorganic P availability most closely representing data were used to simulate the grasslands and quantify their temporal response to nutrient manipulation. The model suggested N additions have increased C stocks in the acidic grassland, but decreased them in the calcareous, where N provision exacerbated P-limitation and reduced biomass input to the soil. Furthermore, plant acquisition of organic P may play an important role in reducing P-limitation, as both simulated grasslands increased organic P uptake to meet P demand. We conclude that grasslands of differing limiting nutrients may respond to N deposition in contrasting ways, and stress that as N deposition shifts ecosystems toward P-limitation, a globally important carbon sink risks degradation.

中文翻译：

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磷限制草原的碳储量可能因氮沉积增加而下降：一项长期的田间操作和模型研究

摘要。在许多温带生态系统中，氮（N）限制了生产力，这意味着人为的N沉积会刺激植物的生长并随后固碳（C）。磷（P）和NP共限草地非常普遍，但是对它们对N沉积的反应了解有限，这可能会使更多的生态系统向P限或NP共限状态过渡。在这里，我们研究了氮素增加对不同养分限制状态下草原CNP池的影响。我们探索了在两个NP共限草地上进行的长期养分操纵实验的响应；通过将数据与集成的CNP循环模型（N14CP）相结合，可以使N限制更强的酸性草原和P限制更强的石灰性草原。为了探索P-访问机制在确定生态系统状态中的作用，我们允许P-访问进行变化，并将输出与植物土壤CNP数据进行比较。最接近代表数据的有机磷获取和无机磷有效性的组合被用来模拟草地并量化其对养分操纵的时间响应。该模型表明，氮的添加增加了酸性草原的碳储量，但减少了石灰性土壤的碳储量，其中钙的供给加剧了磷的限量，并减少了向土壤的生物量输入。此外，由于两个模拟草原都增加了对有机磷的吸收以满足对磷的需求，因此植物获取有机磷可能在减少磷限制中起重要作用。我们得出结论，限制养分不同的草原可能以相反的方式对氮的沉积作出反应，