Chronic Nitrogen Amendment Experiment at Harvard Forest since 1988

The purpose of this study is to increase our understanding of ecosystem nitrogen dynamics in response to elevated nitrogen inputs. With atmospheric nitrogen deposition in the Northeastern United States currently at 10 to 20 times above historic background levels, it is possible that excessive nitrogen inputs could saturate the retention capacity of a forest ecosystem. Potential effects of nitrogen saturation include increased nitrate leaching and simultaneous base cation losses, soil acidification, altered fluxes of trace gases and forest decline. Two adjacent stands were chosen for the study: an even-aged red pine (Pinus resinosa Ait.) stand planted in 1926 and a 50-year-old mixed hardwood stand that had regenerated naturally after clearcutting in approximately 1945. The hardwood stand is dominated by black and red oak (Quercus velutina Lam.; Q. rubra L.) with significant amounts of black birch (Betula lenta L.), red maple (Acer rubrum L.) and american beech (Fagus grandifolia Ehrh.). The dominant soil types are stony- to sandy-loams formed from glacial till, and are classified as Typic Dystrochrepts of the Canton or Montauk series. Four treated plots were established within each stand: control, low N, low N plus sulfur (N+S) and high N. Each plot measures 30 x 30 meters (0.09 ha) and is divided into thirty-six 5 x 5 m subplots.

本研究旨在加深对生态系统氮动态响应外源氮输入的认知。当前美国东北部的大气氮沉降量已达历史背景值的10至20倍，过量氮输入可能使森林生态系统的氮固持能力达到饱和。氮饱和的潜在效应包括硝酸盐淋溶加剧、盐基阳离子同步流失、土壤酸化、痕量气体通量改变以及森林衰退。本研究选取两处相邻林分开展试验：其一为1926年栽植的同龄红松（Pinus resinosa Ait.）林分；其二为约1945年皆伐后自然更新的50年生阔叶混交林。该阔叶混交林以黑栎（Quercus velutina Lam.）和红栎（Q. rubra L.）为优势种，伴生有大量黑桦（Betula lenta L.）、红枫（Acer rubrum L.）与美洲山毛榉（Fagus grandifolia Ehrh.）。区域内优势土壤类型为由冰碛物发育而成的石质至砂质壤土，归类为Canton或Montauk系列的典型干润始成土（Typic Dystrochrepts）。每个林分内设4个处理样地，分别为对照、低氮、低氮加硫（N+S）以及高氮处理。每个样地面积为30×30米（0.09公顷），并被划分为36个5×5米的亚样地。