Lakes in human-dominated landscapes often face high loads of nutrients that may alter ecosystem function. High N loads relative to P are especially common in agricultural watersheds, where nitrate (NO₃) in particular is elevated due to fertilizer application and runoff. While past research has focused extensively on the impact of nutrient loads on reductions in water quality, we still lack assessment of the impact of high N loading and extreme stoichiometric imbalance on ecosystem process rates, specifically measurements of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP). We used open-bottomed experimental mesocosms (limnocorrals) to establish four treatments of increasing N : P by increasing N loads while leaving P loads consistent, and measured chlorophyll concentration and metabolic rates. We observed significant differences in productivity among treatments, with higher biomass and GPP in the highest two N : P treatments and a unimodal pattern between GPP and N concentrations. Declines in GPP at high N may have been due to limitation by P or light. In contrast to other studies, we did not observe any significant differences in ER with fertilization, potentially from a lack of dissolved organic carbon loading often associated with higher nutrient concentrations in those lakes. Maximum GPP and NEP observed here was far above commonly cited thresholds for likely P limitation at a molar N : P of approximately 330. Our results highlight the potential for high productivity despite stoichiometric imbalance that is common in agricultural systems, suggesting N management may be important in reducing primary production in hypereutrophic lakes.

中文翻译：

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尽管在超富营养化实验系统中与磷存在严重的化学计量失衡，但在高氮水平下实现了最高的初级产量

人类主导景观中的湖泊通常面临着可能改变生态系统功能的大量营养物质。相对于 P 的高 N 负荷在农业流域中尤为常见，其中硝酸盐（NO ₃) 特别是由于施肥和径流而升高。虽然过去的研究广泛关注养分负荷对水质降低的影响，但我们仍然缺乏对高氮负荷和极端化学计量失衡对生态系统过程速率的影响的评估，特别是初级生产总值 (GPP)、生态系统呼吸的测量(ER) 和净生态系统生产力 (NEP)。我们使用开底实验中胚层（limnocorrals）建立了四种增加 N : P 的处理方法，方法是增加 N 负荷，同时保持 P 负荷一致，并测量叶绿素浓度和代谢率。我们观察到处理之间生产力的显着差异，在最高的两个 N:P 处理中生物量和 GPP 较高，GPP 和 N 浓度之间存在单峰模式。高 N 时 GPP 的下降可能是由于 P 或光的限制。与其他研究相比，我们没有观察到 ER 与施肥的任何显着差异，这可能是由于缺乏溶解有机碳负荷，通常与这些湖泊中较高的营养浓度有关。此处观察到的最大 GPP 和 NEP 远高于通常引用的可能 P 限制的阈值，摩尔 N : P 约为 330。尽管农业系统中常见的化学计量失衡，我们的结果强调了高生产力的潜力，这表明 N 管理可能很重要减少富营养化湖泊的初级生产。可能是由于缺乏溶解的有机碳负荷，通常与这些湖泊中较高的养分浓度有关。此处观察到的最大 GPP 和 NEP 远高于通常引用的可能 P 限制的阈值，摩尔 N : P 约为 330。尽管农业系统中常见的化学计量失衡，我们的结果强调了高生产力的潜力，这表明 N 管理可能很重要减少富营养化湖泊的初级生产。可能是由于缺乏溶解的有机碳负荷，通常与这些湖泊中较高的养分浓度有关。此处观察到的最大 GPP 和 NEP 远高于通常引用的可能 P 限制的阈值，摩尔 N : P 约为 330。尽管农业系统中常见的化学计量失衡，我们的结果强调了高生产力的潜力，这表明 N 管理可能很重要减少富营养化湖泊的初级生产。