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）循环的改变程度要比广为人知的碳（C）循环更大。在加拿大，最高的氮沉降发生在安大略南部，那里的森林覆盖主要是糖枫;一种已知对氮沉降增加敏感的物种。高氮沉降对安大略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英国和美国西部已被证明会影响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接的，可能的结果包括硝酸盐淋溶（酸化）增加，土壤呼吸下降和细根密度降低可能导致其他矿物营养素的缺乏。为了估计阈值沉积输入以上的不利影响开始发生，可以确定的临界负荷（CL）的营养-N。有两种方法来估计氮的临界负荷;一个经验的方法，测量的沉积量与观察到的影响，或基于过程的方法，考虑到土壤和植物的过程，使长期的慢性氮在土壤中的积累进行评估。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。这项工作的目标是进行综合评估的影响，目前和预测的N沉积硬木林，以期建立适当的临界N负荷，以保护这些生态系统。这将通过六个互补的研究项目来实现，这些研究项目调查了N对1）地衣的影响; 2）草本物种; 3）糖枫化学; 4）糖枫生长和N循环; 5）土壤地球化学循环和土壤呼吸; 6）通过土壤地球化学建模对N沉积的当前和预测影响进行综合评估。这项研究将导致6名研究生和至少4名本科生的培训，并将提供良好的科学基础，这是必要的，以帮助政策决策方面的未来N排放在加拿大。

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.