INTERACTION BETWEEN CARBON AND NUTRIENT CYCLES IN DEEP SOILS OF THE PACIFIC NORTHWEST

The vertical distribution of soil exchangeable cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) results from the integration of multiple processes: weathering of primary minerals, atmospheric input, leaching, and biological cycling. While weathering and atmospheric input affect the location of cation inputs to the soil system, leaching and biological cycling translocate cations in opposing directions within the profile. Cations interact with soil organic matter in many ways, most notably cation bridging between negatively charged functional groups and either clay minerals or other organic structures. Little research has been conducted on deep soil relative to surface soil, and thus attempts to verify hypotheses about the drivers of exchangeable cation vertical distributions have not been possible on a broad scale. This study excavated soils down to 2.5 or 3 m at 22 sites across the coastal Pacific Northwest ranging from northern Washington to southern Oregon. Samples were analyzed for soil carbon (C), nitrogen (N), and exchangeable cations. PERMANOVA was used to evaluate the effect of soil chemical (C, N, and pH), environmental (climate, parent material, elevation, slope, aspect), physical (texture) and spatial (horizon, depth) gradients on the distribution of exchangeable cations. The majority of exchangeable cation stocks are located in deeper soil horizons. 66% of Ca²⁺, 76% of Mg²⁺, 57% of K⁺, and 63% of Na⁺ stocks were below 1.0 m, on average, while 25% of soil C stocks were below 1.0 m. Climate was the dominant control over cation distribution, with wet, cool sites containing less exchangeable Ca²⁺ and Mg²⁺. There was also significantly lower Ca²⁺ and Mg²⁺ in both soil profiles with greater total C and in soil horizons with more C content among master horizons of the same type. This observation supports the hypothesis that coordinating complexation between divalent cations and dissolved organic matter increases Ca and Mg solubility and contributes to leaching loss from soil.