Deep Soil: Accurately Quantifying and Modeling Soil Carbon and Nitrogen In Subsurface Layers.

Soil is the primary sink for C in forest ecosystems. Nonetheless, soil C is overlooked in ecosystem C budgets and underreported in the literature. N is one of the principle limiting nutrients for forests in the Pacific Northwest. Due to its mobility, considerable amounts of N may be found in deep soil layers. This study examined the systematic sampling depth for ecosystem C and N analyses in the Pacific Northwest, and compared best-fit models of C and N in deep soil layers with laboratory measurements. We gathered forest floor samples and mineral soil bulk density samples at regular intervals down each profile to 2.5 or 3.0 m from 22 sites across the coastal Pacific Northwest Douglas-fir zone. Soil samples were screened to 4.7 mm and analyzed for C and N content. We found that systematic soil sampling shallower than 1.0 m significantly underestimated total soil C and N. On average, 21% of total C and 34% of total N was below 1.0 m. Models predicting total C and N to 2.5 m given only data to 1.0 m were reliable for 20 of 22 sites; the sites that could not be accurately modeled carried the greatest C and N at depth and contained noncrystalline minerals. Shallow soil sampling at best provides a biased estimate and at worst leads to misleading conclusions regarding soil C and N. Researchers seeking to quantify soil nutrient pools or measure change over time should sample deep soil to create a more complete picture of soil organic matter and nutrient dynamics.