Soil carbon and nitrogen in old-growth forests in western Oregon and Washington

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.