Conditions Under Which Nitrogen Can Limit Steady-State Net Primary Production in a General Class of Ecosystem Models
Conditions Under Which Nitrogen Can Limit Steady-State Net Primary Production in a General Class of Ecosystem Models
复制标题
在一般类型的生态系统模型中氮可以限制稳态净初级生产的条件
作者：
D. Menge
人类活动正在极大地改变全球氮的可获得性。生态系统吸收额外N的程度--以及随之而来的额外二氧化碳--取决于净初级生产力(NPP)是否受到N的限制，因此了解N在什么条件下可以限制NPP是很重要的。在这里，我使用一个一般的动力学模型来表明，在稳定状态下的氮素限制--例如在古老的森林中--取决于生物可控和不可控氮输入和损失之间的平衡。只有当不可控的输入(例如，大气沉积)超过可控的损失(例如，植物有效N的淋失)时，才有可能进行稳定的氮限制，这与当不可控的损失(例如，植物不可用的土壤N的淋失)超过可控的输入(生物固氮)时是相同的。这些基本结果对许多模型细节是稳健的，例如植物不可用土壤N池的数量以及N固定器的数量和类型。来自古老热带(夏威夷)和温带(俄勒冈、华盛顿、智利)森林的经验数据支持该模型的见解。实际上，这意味着任何N固定剂--共生或非共生--都可以克服生态系统N的限制，因此理解N的限制需要了解对所有N固定剂的控制。此外，将植物有效氮的损失与非生物输入进行比较，可以快速诊断生态系统是否可以受到氮的限制，尽管这一结果的适用性仅限于具有稳定的氮循环的生态系统，如基本上没有干扰的老森林。
Human activity is drastically altering global nitrogen (N) availability. The extent to which ecosystems absorb additional N—and with it, additional CO2—depends on whether net primary production (NPP) is N-limited, so it is important to understand conditions under which N can limit NPP. Here I use a general dynamical model to show that N limitation at steady-state—such as in old-growth forests—depends on the balance of biotically controllable versus uncontrollable N inputs and losses. Steady-state N limitation is only possible when uncontrollable inputs (for example, atmospheric deposition) exceed controllable losses (for example, leaching of plant-available soil N), which is the same as when uncontrollable losses (for example, leaching of plant-unavailable soil N) exceed controllable inputs (biological N fixation). These basic results are robust to many model details, such as the number of plant-unavailable soil N pools and the number and type of N fixers. Empirical data from old-growth tropical (Hawai’i) and temperate (Oregon, Washington, Chile) forests support the model insights. Practically, this means that any N fixer—symbiotic or not—could overcome ecosystem N limitation, so understanding N limitation requires understanding controls on all N fixers. Further, comparing losses of plant-available N to abiotic inputs could offer a rapid diagnosis of whether ecosystems can be N-limited, although the applicability of this result is constrained to ecosystems with a steady-state N cycle such as old-growth forests largely devoid of disturbance.