Characterization of nutrient regimes in some continental subalpine boreal forest soils

Abstract

To determine whether field-identified soil nutrient regimes (SNRs) can be characterized and segregated by direct soil nutrient measures, we collected samples of forest floor and mineral soil (0 to 30 cm) from a wide range of forest sites throughout the Engelmann Spruce — Subalpine Fir (ESSF) zone of British Columbia. The samples were analyzed for acidity, total C, total N, mineralizable N, and extractable Ca, Mg, K, P, and SO4-S. The study sites were stratified according to an a priori field classification (SNRs) and an a posteriori classification derived from quantitative classification (groups) using all direct measures of nutrients as discriminating variables. The two classification methods had 72% agreement. Regardless of the classification, nitrogen-related variables (C:N ratio, total N, and mineralizable N) in the mineral soil segregated best among SNRs or groups indicating the presence of a steep, N-driven regional soil nutrient gradient. Multiple regression models using SNRs, groups, or direct measures of nutrients together with surrogates of climate (elevation, latitude, and/or longitude) as predictors had the similar accountability for the variation in subalpine fir and Engelmann spruce site index (0.41 ≤ R2 ≤ 0.65). The similarity in the accountability for site index justifies the use of the a priori classification in estimating site quality. Comparison of mineralizable-N values for field-identified SNRs between different climatic regions showed similarities between boreal climates and discrepancies between boreal and cool mesothermal climates. The study gave further evidence that indices of plant-available nitrogen in the upper mineral soil provide useful measures for field-identified SNRs, but indicated that it may be necessary to expand the existing five-class a priori classification to accommodate differences in regional soil nutrient gradients. Key words: Classification, Engelmann spruce, subalpine fir, nitrogen, site index, soil nutrient regime