Plant growth is often co-limited by nitrogen (N) and phosphorus (P). Plants might use one element to acquire another (i.e. trading N for P and P for N), which potentially explains synergistic growth responses to NP addition. We studied a 66-year-old grassland experiment in South Africa that consists of four levels of N addition with and without P addition. We investigated the response of aboveground net primary production (ANPP) to N and P addition over the last 66 years. Further, we tested whether phosphatase activity and plant P uptake depend on N availability, and vice versa, whether non-symbiotic N2 fixation and plant N uptake depend on P availability. We expected that the interaction of both elements promote processes of nutrient acquisition and contribute to synergistic plant growth effects in response to NP addition. We found synergistic N and P co-limitation of ANPP for the period from 1951 to 2017 but the response to N and P addition diminished over time. In 2017, aboveground P stocks, relative rRNA operon abundance of arbuscular mycorrhizal fungi and soil organic P storage increased with N fertilization rate when N was added with P compared to the treatment in which only N was added. Further, N addition increased phosphatase activity, which indicates that plants used N to acquire P from organic sources. In contrast, aboveground N stocks and non-symbiotic N2 fixation did not change significantly due to P addition. Taken together, our results indicate that trading N for P likely contributes to synergistic plant-growth response. Plants used added N to mobilize and take up P from organic sources, inducing stronger recycling of P and making the plant community less sensitive to external nutrient inputs. The latter could explain why indications of synergistic co-limitation diminished over time, which is usually overlooked in short-term nutrient addition experiments.

中文翻译：

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氮磷循环的相互作用促进磷的获取并解释协同植物生长反应

植物生长通常受到氮 (N) 和磷 (P) 的共同限制。植物可能使用一种元素来获取另一种元素（即用 N 换 P 和 P 换 N），这可能解释了对 NP 添加的协同生长反应。我们研究了南非一项 66 年历史的草地试验，该试验由四种水平的 N 添加和不添加 P 组成。我们调查了过去 66 年来地上净初级生产 (ANPP) 对 N 和 P 添加的响应。此外，我们测试了磷酸酶活性和植物 P 吸收是否取决于 N 可用性，反之亦然，非共生 N2 固定和植物 N 吸收是否取决于 P 可用性。我们预计这两种元素的相互作用会促进养分获取过程，并有助于响应 NP 添加的协同植物生长效应。我们发现 1951 年至 2017 年期间 ANPP 的协同 N 和 P 共同限制，但对 N 和 P 添加的响应随着时间的推移而减弱。2017 年，与仅添加 N 的处理相比，添加 N 和 P 时，地上 P 储量、丛枝菌根真菌的相对 rRNA 操纵子丰度和土壤有机 P 储量随着施氮率的增加而增加。此外，氮添加增加了磷酸酶活性，这表明植物使用氮从有机来源获取磷。相比之下，由于 P 添加，地上 N 库和非共生 N2 固定没有显着变化。总之，我们的结果表明，用 N 换 P 可能有助于植物生长的协同反应。植物使用添加的 N 来动员和吸收有机来源的 P，诱导更强的磷循环，使植物群落对外部养分输入不那么敏感。后者可以解释为什么协同共同限制的迹象随着时间的推移而减少，这在短期营养添加实验中通常被忽视。