Deep soil: Quantification, modeling, and significance of subsurface nitrogen
2015
James, Jason | Knight, Erika | Gamba, Vitor | Harrison, Rob
Nitrogen (N) is one of the primary limiting nutrients in Pacific Northwest forests, as well as many other terrestrial ecosystems around the world. Efforts to quantify total soil N and to monitor N cycling have often sampled soils to a depth of 0.2m, occasionally to 1.0m depth, or the bottom of the B horizon. However, tree roots often extend many meters into the soil redistributing water to the surface during droughts and contributing to nutrient uptake. 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, O horizon and mineral soil bulk density samples were collected at depths of 0.1m, 0.5m, 1.0m, 1.5m, 2.0m, and 2.5m. Mineral soil was screened to 4.75mm and analyzed for total N content. Systematic sampling shallower than 2.0m produced significantly smaller estimates of total N. On average, only 3% of total soil N was in the O horizon, and 31% was below 1.0m depth (almost 2700kgha−1 of N). Over 45% of soil N was below 1.0m at three sites. A nonlinear mixed effect model using the Langmuir equation predicted total N to 2.5m with −12.4% mean error given data to 1.0m, and −7.6% mean error with data to 1.5m. 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.
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