Soil organic carbon (SOC, kg C m-2) is an important component in evaluating global C stores. The nitrogen (TN, kg N m-2) cycle is closely linked to C and understanding its role is also important. Contents and distributions of SOC and TN in soil profiles, to 1-meter depth, were estimated from 79 soils pits, in old-growth forests, in 7 physiographic provinces in western Oregon and Washington. Soils were sampled in four layers, forest floor, 0- to 20-cm, 20- to 50-cm, and 50- to 100-cm, and analyzed on a LECO CN Analyzer. Material 2-mm. Forest floor SOC ranged from 0 to 14 kg C m-2 (mean = 2.7) and forest floor TN ranged from 0 to 0.4 kg N m-2 (mean = 0.07). The SOC of mineral soil ranged from 1.0 to 18 kg C m-2 (mean = 6.6) for 0- to 20-cm depth and 2.2 to 57 kg C m-2 (mean = 17) for 0- to 100-cm depth. The TN of mineral soil ranged from 0.04 to 1.0 kg N m-2 (mean = 0.31) for 0- to 20-cm depth and 0.12 to 3 kg N m-2 (mean = 1.0) for 0- to 100-cm depth. Up to 66% of SOC and TN measured was found below 20-cm, illustrating how failing to sample at depth can grossly underestimate SOC. As much as 44% of SOC and TN measured was found in C-bearing material >2-mm, material for which many methods neglect to account. Longitudinal differences in SOC and TN contents were evident between Coastal, Cascade, and Eastside Cascade sites, implying effects from site and climatic factors. Regression analysis was used to quantify relationships of SOC and TN to site and climatic factors. Response variables included forest floor, forest floor plus 0- to 20-cm, 0- to 20-cm, and 0- to 100-cm layers. Moisture and soil texture played important roles in most cases examined. The results of this study, and of other studies assessing the effects of site and climatic characteristics on the factors controlling soil organic matter accumulation, suggest the relationships are regionally specific.
Bernard T. Bormann, Mark E. Harmon, Peter S. Homann, Suzanne M. Remillard
Experimental design; ; Seven study sites were selected in western Oregon and Washington based on physiographic provinces outlined by Franklin and Dyrness (1988). The sites chosen for this study vary widely both physically and climatically. These sites were located from the Pacific Ocean coast to the eastern slopes of the Cascade Range and were assumed to represent steady-state ecosystems. Therefore, C input to the soil from plant and animal residues in the form of detritus is balanced by oxidation of SOM, in which C is released as CO2. The sites are associated with the Andrews Long Term Ecological Research (LTER) program. Within each site, 1 to 8 stands were sampled to estimate SOC and TN and their vertical distribution to 1-meter depth in mineral soil. This was accomplished by excavating and sampling 1 to 3 soil pits on the perimeter of each stand. In total, 79 soil pits were sampled to estimate forest floor and mineral SOC and TN contents.; | Field methods; ; <p><b>Forest floor Sampling:</b>
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.</p>
<p><b>Mineral Soil Sampling:</b>
On the perimeter of each plot, one or more 1-meter wide by 1-meter deep soil pits were du
The results of this study contribute to a total ecosytem carbon budget estimation for old-growth forests in the Pacific Northwest. These estimations serve as potential bounds for ecosystem carbon in these forests as net primary productivity is assumed to be in a steady state.