Forest floor Sampling: Forest floor samples were taken, with a 5-cm diameter core sampler, at five points above the sampling face of the soil pit. These samples were composited by pit. The samples included relatively undecomposed aboveground litter whose tissue type was recognizable, as well as decomposed humified material whose tissue type was not recognizable. The forest floor was distinguished from mineral soil by its low content of mineral material (i.e. sand, rocks, and clay). Forest floor samples include woody debris <1-cm in diameter on the surface and all woody debris, irrespective of size, within the forest floor that is not visible from the surface. Sample procedures for treatment in the field, returning to the lab, and greenhouse drying were the same as for the mineral soil samples.
Mineral Soil Sampling: On the perimeter of each plot, one or more 1-meter wide by 1-meter deep soil pits were dug, sampled and described. Pits were located to best represent the stand in terms of slope, aspect, vegetation density and cover. At each pit, three mineral soil layers were sampled (0- to 20-cm, 20- to 50-cm, and 50- to 100-cm). Depth strata, as opposed to horizon, sampling was chosen because it is more repeatable and comparable to other studies.
To ensure a representative sample, mineral soil samples were obtained by collecting material in three swaths, 5- to 10-mm deep, across the face within each layer. For pit faces that were too rocky to make swaths, samples were collected as part of Bulk Density Sampling.
Soil samples were kept in the shade for 4-10 days, brought back to the lab and placed in a cooler (6?C) until they were laid out to air-dry in a greenhouse (within 3 days). The samples were stirred every other day until dry (1-2 weeks) and were then bagged, weighed, and stored until processed.
Bulk density Sampling: Bulk density was determined for each layer with a core sampler for non-rocky soils or by excavating a known volume of soil for rocky soils. For non-rocky soils, a 5-cm diameter x 5-cm deep soil core bulk density sampler with sampling ring inserts (AMS, USA) was used in most cases. For soils that were too loose for the bulk density sampler to be effective (e.g., the top 35 cm at Pringle Falls), a 5-cm diameter tube was inserted approximately 10 cm into the profile face. Inserting a measuring tape in the open end of the corer confirmed this depth. If the depth was uneven (as with sandy, loose soil) an average was taken. Multiplying the area of the corer by the depth of fill attained a core volume. With both samplers, cores were taken at three locations in each layer.
For rocky pit faces, a cube was cut (approximately 20-cm x 10-cm x layer depth), soil excavated, and dimensions of the hole measured. The volumes of any large rocks protruding into the hole were estimated and subtracted from the volume of the hole to obtain a total sample volume, Vt. The material removed from the hole was sieved and weighed in the field to yield material <20-mm and 20- to 75-mm per layer. Thoroughly mixed subsamples of each fraction were obtained, weighed and brought back to the lab. The <20-mm material was used to determine bulk density and was further subsampled and processed for C and N analysis. The oven-dried mass of the 20- to 75-mm material was necessary to determine the soil volume fraction of the total sample volume, Ss.
Forest floor Processing: Forest floor samples were oven dried (70 degrees C), weighed, and blended in a kitchen blender (Braun AG, Frankfurt, West Germany) to break up material and ensure homogenous subsampling. A tablespoon of this material was subsampled and more finely ground with an analytical mill (IKA-A 10, Staufen, Germany) to <850 micro-m (<20-mesh). The Central Analytical Laboratory, Oregon State University, Corvallis, analyzed these samples for C and N content using a LECO CNS 2000 analyzer. Samples were randomized for analysis. Quality control samples and replicates represented 20% of the run. Quality control samples (10%) consisted of reference material of known C and N concentration obtained from the EPA Environmental Research Laboratory, Corvallis, Oregon. Replicates (10%) were randomly chosen samples. Total C and N values were reported as g kg-1 at 60 degrees C. Mass per sampling area was multiplied by C and N contents to yield kg C or N per sampling area.
Forest floor Processing: Each air-dried sample was sieved and hand-sorted into the following components: <2-mm C-bearing soil fraction, 2- to 4-mm C-bearing soil fraction, >4-mm C-bearing soil fraction, >2-mm rock (non-C bearing), and >2-mm buried wood, roots, charcoal. The C-bearing soil fractions were defined as soil that could not be broken up with a rubber stopper on a sieve. The C-bearing fraction >2-mm were hardened soil aggregates or soft, weathered rocks, which were assumed to be nutrient-rich and should be included in estimating C stores. The >4-mm C-bearing was typically between 4- to 10-mm in size. Each component was weighed. Buried wood, roots, and charcoal accounted for <3% of the sample mass, and they were disregarded. C-bearing fractions >2-mm were only analyzed for C and N if they were greater than 10%, by weight, of the total sample. Otherwise, the weight of any >2-mm C-bearing fraction was incorporated into the rock mass used to estimate soil volume./
Subsamples (50-100 g) of <2-mm, 2- to 4-mm, and >4-mm C-bearing fractions were obtained with a sample splitter (SoilTest Riffles, CL-280 series). These subsamples were ground to 850-?m (<20-mesh) using a 20-cm disc pulverizer (BICO Inc., Burbank, California) and analyzed for total C and N concentration using a LECO CNS 2000 analyzer by the Central Analytical Laboratory, Oregon State University, Corvallis. Samples were randomized for analysis in one of two groups: high C mineral soil or low C mineral soil. Assignment into a group was based on expectation of either a high or a low C concentration. Typically, deeper layers are expected to contain low C concentrations while surface layers are expected to contain high C concentrations. The groups were run in four batches of up to 120 samples in each. Quality control samples and replicates represented 20% of each batch. Quality control samples (10%) consisted of reference material of known C and N concentration obtained from the EPA Environmental Research Laboratory, Corvallis, Oregon. Replicates (10%) were randomly chosen samples from the current and previous batches. Total C and N concentrations were obtained from the lab as g kg-1 at 60?C. A dry weight ratio of 60 degrees C to 105 degrees C was determined on the bulk density samples and applied to these lab values for conversion to a 105 degrees C basis.
Bulk density Processing: As with the mineral soil processing, each air-dried bulk density sample was sieved and hand-sorted into the same components: <2-mm C-bearing soil fraction, 2- to 4- mm C-bearing soil fraction, >4-mm C-bearing soil fraction, >2-mm rock (non-C bearing), and >2-mm buried wood, roots, charcoal. The mineral components were oven-dried (5-7 days) and weighed at both 60 and 105 degrees C. This provided a soil moisture conversion factor since the soils at the Central Analytical Laboratory were analyzed at 60 degrees C and values in this text reported on 105 degrees C basis.
Buried wood, roots, and charcoal:Since the volume contribution of buried wood, roots, and charcoal was insignificant, it was omitted from the calculations.
