Nutrient dynamics in sugar maple forests and assessment of long-term sensitivity to nitrogen deposition

在全球范围内，人类对氮(N)循环的改变程度超过了更广为人知的碳(C)循环。在加拿大，最高的氮沉降发生在安大略省南部，那里的森林覆盖主要是糖枫；已知对氮沉降增强敏感的物种。高氮沉降对安大略省南部其他生态系统成分（地衣、草本植物）的潜在影响尚不清楚，但目前的氮沉降水平在已被证明影响英国和美国西部植物和地衣物种的范围内。然而，氮富集对森林的影响并不是直截了当地的，可能的结果包括硝酸盐浸出（酸化）增强、土壤呼吸作用下降和细根密度减少，这些都可能导致其他矿质营养素的缺乏。为了估计开始产生不利影响的阈值沉积输入，可以确定营养氮的临界负荷（CL）。估计N的临界负荷有两种方法；一种将测量到的沉积与观测到的影响联系起来的经验方法，或者一种基于过程的方法，它考虑了土壤和植物过程，从而能够评估土壤中长期氮积累。这些模型对政策制定者很有吸引力，因为它们提供了对“生效时间”的估计，因此更容易与排放目标联系起来。本研究的目的是综合评估当前和预测的氮沉降对阔叶林的影响，以期建立适当的临界氮负荷来保护阔叶林生态系统。这将通过六个互补的研究项目来实现，研究N对地衣的影响；2)草本种；3)糖枫化学；4)糖枫生长与氮循环；5)生物地球化学循环和土壤呼吸；6)通过生物地球化学模拟综合评价氮沉降的现状和预测影响。这项研究将培养6名研究生和至少4名本科生，并将为帮助制定有关加拿大未来氮排放的政策提供必要的科学依据。

Globally, humans have altered the nitrogen (N) cycle to a greater degree than the much more publicized carbon (C) cycle. In Canada, the highest N deposition occurs in southern Ontario, where forest cover is dominated by sugar maple; a species known to be sensitive to enhanced N deposition. Potential impacts of high N deposition on other ecosystem components (lichens, herbs) in southern Ontario are unknown, but current N deposition levels are within the range that has been shown to affect plant and lichen species in the UK and western USA. The effect of N enrichment on forests, however, is not straight forward, and possible outcomes including enhanced nitrate leaching (acidification), declines in soil respiration and a reduction in fine root density may cause deficiencies in other mineral nutrients. In order to estimate the threshold deposition input above which adverse impacts begin to occur, the critical load (CL) for nutrient-N can be determined. There are two approaches for estimating the critical load of N; an empirical approach that links measured deposition with an observed impact, or a process-based approach, which takes into account soil and plant processes, enabling an assessment of long-term chronic N accumulation in soils. These models are appealing to policy makers as they provide an estimate of the 'time to effect', and are thus more readily linked to emission targets. The objective of this work is to undertake an integrated assessment of the impacts of current and predicted N deposition on hardwood forests, with a view to establishing appropriate critical N loads to protect these ecosystems. This will be achieved through six complimentary research projects that investigate N impacts on 1) lichens; 2) herbaceous species; 3) sugar maple chemistry; 4) sugar maple growth and N cycling; 5) biogeochemical cycling and soil respiration and 6) an integrated assessment of the current and predicted impacts of N deposition through biogeochemical modeling. This study will result in the training of six graduate students and at least 4 undergraduate students and will provide the sound scientific basis that is necessary to aid policy decisions with respect to future N emissions in Canada.