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

Soil cores were extracted between 0800 and 1700 hours standard time, using 1.5-inch diameter steel pipe which was driven 20 cm into the soil. From 1998 through 2000, cores were collected along six of 12 transects with compass directions from the base the Hemlock flux tower of 0 to 330 degrees, by 30 degree increments, with the exact location of each sampling point at a randomly selected distance between 30 and 50 m from the tower. The exact soil core locations were at randomly chosen points between 1 and 2 meters away from the locations of soil respiration collars (see data set HF148). The number 1 in the measurement location indicates that it was on the 0 degree transect, 2 indicates a 30 degree transect, and so on by 30 degree increments up to 12, indicating the 330 degree transect. Beginning in 2001, each core at both the Hemlock and LPH flux tower sites was collected at a randomly determined point between 1 and 2 m from each of the collars that were used in soil respiration measurements (for locations of these, see data set HF148). Six cores were usually collected at the Hemlock site, and 12 to 16 at the Little Prospect Hill site. To reduce loss of soil from the bottom of the corer during extraction, the corer was twisted at least a full revolution prior to pulling from the soil. Cores were immediately capped at both ends, and transported to the lab, where they were either cut into sections on the day of collection, or stored 1-2 days in a refrigerator at 5 degrees C. Cores were removed from corers by pushing from the bottom end of the core, in mineral soil, and then measured. There was almost always some compression of the core, and to correct for this, it was assumed that the degree of compression of the core was inversely proportional to depth in the soil, so that 40% of the compression occurred in the top 5 cm, 30% in the layer 5-10 cm below the surface, 20% in the layer from 10 to 15 cm, and 10% in the layer 15-20 cm below the soil surface. This assumption was made because of the higher air content of surface layers of soil, and while not precise, the correction resulted in a better approximation of the true moisture content in surface layers. Each of the four slices of each core was weighed immediately after cutting and placed in a jar where it was dried at 105 degrees C for three days and reweighed. In addition to the gravimetric soil moisture, approximate bulk density and volumetric water content in each layer were calculated on the assumption that it represented a 5-cm thickness of the soil prior to compression during coring.