Soil Water Content at Harvard Forest HEM and LPH Towers 1998-2007

Water content of the top 20 cm of soil was measured using 1.5 inch diameter soil cores. Soil water content is of particular interest and importance in explaining patterns of soil respiration (including root respiration) and ecosystem respiration, 60 to 75% of which occurs below the ground surface at Harvard Forest. Soil water content in deciduous forest near the Little Prospect Hill Forest was in general found to be more variable than near the Hemlock tower. One influence contributing to this is the presence of a water table within 1 m of the soil surface near the hemlock tower, compared to a water table at unknown depth in the deeply drained soils in most parts of Little Prospect Hill. Relatively low evapotranspiration at the Hemlock tower site during the period when deciduous trees are foliated also contributes to higher water content in soil there, and a relatively thick surface organic layer. These influences tend to maintain soil respiration at higher levels in the hemlock forest during dry summers, but excessive moisture in the soil at the Hemlock site during very wet summers appears to suppress soil respiration.

本研究采用直径1.5英寸的土壤芯样（soil cores），测定了0~20 cm表层土壤的含水量。土壤含水量对于阐释土壤呼吸（含根系呼吸）与生态系统呼吸的分布模式具有关键意义：哈佛森林（Harvard Forest）中60%~75%的呼吸过程发生于地下。研究表明，小展望山森林（Little Prospect Hill Forest）周边落叶林的土壤含水量整体波动幅度显著大于铁杉塔（Hemlock tower）周边区域。造成该差异的核心影响因素之一为：铁杉塔附近地下水位距土壤表层不足1米；而小展望山森林多数区域的土壤排水性较强，地下水位深度未知。在落叶树木展叶期，铁杉塔观测点的蒸散发（evapotranspiration）强度相对偏低，这也助力该区域土壤维持较高含水量，并形成较厚的地表有机层。上述因素使得铁杉林在干旱夏季仍可保持较高的土壤呼吸水平，但在极端湿润夏季，铁杉塔观测点的土壤过湿会抑制土壤呼吸过程。