The Relationship Between Exchangeable Base Cations and Soil Carbon and Nitrogen in Deep Forest Soils of the Pacific Northwest.

While nitrogen (N) is often the limiting nutrient in Douglas-fir forests in the Pacific Northwest, ongoing depletion of base cations can lead to other nutrient limitations. Organic acids and NO₃^- are highly mobile in soil and interact with nutrient cations on soil surfaces and in the soil solution, notably exchangeable calcium (Ca), magnesium (Mg), and potassium (K). Build-up of N through fixation from red alder can accelerate cation losses or translocation to deep soil, especially in forests prone to disturbance and repeated alder colonization. Traditional soil analysis samples soil to less than 1.0 m depth, despite evidence that substantial nutrient stores can be found in deeper horizons. This study examined soil carbon (C), N, and exchangeable base cations (Ca, K, & Mg) to a depth of 2.5 m across 22 industrially managed sites in the Pacific Northwest. On average, these soils contained 762 keq ha^-1 of Ca, 47 keq ha^-1 of K, and 496 keq ha^-1 of Mg. Substantial proportions of base cations were found in deep soil horizons; soil stocks below 1.0 m comprised 66%, 57%, and 76% of Ca, K, and Mg, respectively. These sites represent natural soil C and N gradients. As soil N increased across the sites in this study, exchangeable Ca and Mg significantly decreased, while no relationship with exchangeable K was observed. Ca and Mg also decreased significantly as soil C increased. These results suggest that as N limitations are lifted through N fixation and, consequently, as forest productivity increases, Ca and Mg may become limiting nutrients to forest productivity.