Barre Woods Soil Warming Experiment at Harvard Forest since 2001

Two of the major results of our original soil warming study were that: (1) warming stimulated the decay of a labile soil carbon pool; and (2) it also increased the availability of inorganic nitrogen to plants. Because of the small size of the original warming plots an important question we have not been able to answer conclusively is: Has the increase in available nitrogen led to an increase in carbon storage in the vegetation? And if yes, how much? In a systems context an additional question is: What is the balance between the carbon lost from the soil and the carbon stored in the vegetation in response to soil warming? We are now addressing these important questions with a new warming experiment in the Barre Woods area of the Harvard Forest. The Barre Woods site was chosen because of its similarities to the Prospect Hill site - stand age, tree species composition, and soil type. There is no evidence of a plow horizon at the Barre Woods site, though surrounding stone walls indicate past use as pastureland. Historical records indicate the site was destroyed during the 1938 hurricane and then regrew naturally. During the summer and fall of 2001 we buried 3.4 miles of heating cable in a 30 x 30m plot. Cables were buried at a 10cm depth, spaced 20cm apart. A second 30 x 30m area was delineated to serve as the control plot. Results from the original soil warming experiment confirmed that the soil disturbance associated with the installation of heating cables has had no effect on soil temperatures and only minor and variable impacts on soil moisture. In April 2002, we began a one-year period of baseline measurements before turning on the heat in the new megaplot. These measurements included nitrogen mineralization, trace gas fluxes (CO2, CH4, N2O), tree growth (dendrometer bands), an understory species survey, canopy foliage analysis (C:N, NIR), and lysimetry. Thermistors were installed in both plots to begin tracking soil temperatures. The baseline measurements have confirmed that there are no significant differences between the two plots for key ecosystem processes prior to the initiation of the warming manipulation. We will turn on the power in April 2003 to begin the "manipulation phase" of this study. From this study, we expect to determine whether or not warming results in the movement of nitrogen from the soil to the trees and to learn how this movement affects the net carbon balance of the ecosystem.

我们最初的土壤增温研究获得了两项主要结论：(1) 增温可促进活性土壤碳库（labile soil carbon pool）的分解；(2) 增温同时提升了植物可获取的无机氮（inorganic nitrogen）有效性。由于原始增温样地规模较小，我们尚未能确定性解答一项关键问题：可利用氮的增加是否会引发植被碳储量提升？若确实如此，其提升幅度又为多少？从生态系统（ecosystem）层面来看，另有一个额外问题：响应土壤增温，土壤碳流失与植被碳储存之间的平衡关系如何？

我们目前正于哈佛大学森林（Harvard Forest）的巴尔森林（Barre Woods）区域开展一项全新的增温实验，以解答上述重要问题。选取巴尔森林样地的原因在于其与展望山（Prospect Hill）样地高度相似：林分年龄、树木物种组成及土壤类型均保持一致。尽管周边残存的石墙表明该区域曾被用作牧场，但巴尔森林样地未发现犁底层的存在。历史记录显示，该区域在1938年飓风中遭受破坏，随后自然恢复生长。

2001年夏秋两季，我们在一块30×30米的样地内铺设了总长3.4英里的加热电缆。电缆埋深为10厘米，布设间距为20厘米。同时划定了另一块30×30米的区域作为对照样地。原有土壤增温实验的结果证实，铺设加热电缆所带来的土壤扰动并未对土壤温度产生显著影响，仅对土壤湿度造成了轻微且波动的影响。

2002年4月，我们启动了为期一年的基线监测工作，待该阶段完成后才会开启新的巨型样地的加热处理。本次基线监测内容涵盖氮矿化（nitrogen mineralization）、痕量气体通量（trace gas fluxes，包括CO₂、CH₄、N₂O）、树木生长（采用测树带（dendrometer bands）法）、林下植被物种调查、冠层叶片分析（碳氮比、近红外光谱（NIR））以及渗漏测定（lysimetry）。此外，我们在两块样地中均安装了热敏电阻（thermistors），以持续追踪土壤温度变化。基线监测结果证实，在启动增温处理前，两块样地的关键生态系统过程均无显著差异。我们将于2003年4月开启电源，启动本研究的‘处理阶段’。通过本项研究，我们期望明确增温是否会引发氮元素从土壤向树木的迁移，并阐明该迁移过程对生态系统净碳平衡的影响机制。