Deep Soil: Quantification, Modeling, and Significance of Subsurface Carbon and Nitrogen

Soil is the primary sink for C in forest ecosystems, but is often overlooked in ecosystem C budgets. Efforts to quantify C pools often sample soils to a depth of 0.2 m despite observations that deep soil C is neither scarce nor entirely stable. This study examined the systematic sampling depth for ecosystem C analyses in the Pacific Northwest, and compared best-fit models of C in deep soil layers with laboratory measurements. Forest floor samples and mineral soil bulk density samples were collected at regular intervals from the soil surface to depths of 2.5 m from 22 sites across the coastal Pacific Northwest Douglas-fir zone. Soil samples were screened to 4.7 mm and analyzed for C content. We found that systematic soil sampling shallower than 1.5 m significantly underestimated total soil C. On average, sampling to 2.5 m compared to 0.5 m increased total C by 156% (85.3 Mg ha-1 to 132.7 Mg ha -1), and 21% of total C within the depth range sampled was below 1.0 m. A nonlinear mixed model using an inverse polynomial curve form and predicting total C to 2.5 m given only data to 1.0 m was reliable for 20 of 22 sites; the sites that could not be accurately modeled carried the greatest C 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. Researchers seeking to quantify soil C or measure change over time should sample deep soil to create a more complete picture of soil pools and fluxes.;Nitrogen (N) is one of the primary limiting nutrients in Pacific Northwest forests, as well as many other ecosystems in the United States. Efforts to quantify total soil N and to monitor N cycling have often sampled soils to a depth of 0.2 m, occasionally to 1.0 m depth, or the bottom of the B horizon. However, tree roots often extend many meters into the soil and can produce hydraulic redistribution during seasonal drought that brings soil water (and dissolved nutrients) from deep layers up to surface horizons. This study examined the systematic sampling depth for ecosystem N analyses in the Pacific Northwest, and compared best-fit models of N in deep soil layers with observed quantities. At 22 sites across the Pacific Northwest Douglas-fir zone, forest floor and mineral soil bulk density samples were collected at depths of 0.1 m, 0.5 m, 1.0 m, 1.5 m, 2.0 m, and 2.5 m. Mineral soil was screened to 4.75 mm and analyzed for total N content. Systematic sampling shallower than 2.0 m produced significantly smaller estimates of total N. On average, only 3% of total soil N was in the litter layer, and 31% was below 1.0 m depth (almost 2700 kg N ha-1). Over 45% of soil N was below 1.0 m at three sites. A nonlinear mixed effect model using the Langmuir equation predicted total N to 2.5 m with -12.4% mean error given data to 1.0 m, and -7.6% mean error with data to 1.5 m. Shallow sampling of soil N in studies of biogeochemical cycling, forest management impacts, or ecosystem monitoring at best provides a biased estimate and at worst produces misleading conclusions. Research and monitoring efforts seeking to quantify soil N or measure fluxes should sample deep soil to create a more complete picture of soil pools and changes over time.