Quantitative characterization of nutrient regimes in some boreal forest soils

Abstract

In order to assess to what extent soil nutrient properties support differentiation of field-identified soil nutrient regimes, composite samples from each forest floor and 0–30 cm of the mineral soil were collected from 116 forest stands in central British Columbia. The samples were analyzed for acidity, total carbon (tC), total nitrogen (tN), mineralizable-N (min-N), and extractable Ca, Mg, K, P, and SO4-S (eCa, eMg, eK, eP, eSO4); and the results were expressed as concentrations on a dry-mass basis. Mineralizable-N of the mineral soil showed (1) the largest amount of variation in the population of sampled soils, (2) significant differences (P < 0.01) among field-identified soil nutrient regimes, and (3) strong correlations with tC, tN, and eCa, eMg, and eK. Using mineral soil min-N as a differentiating characteristic, the following limiting values (mg kg−1) were proposed to provide an objective means of defining soil nutrient regimes: < 2 for very poor, 2–8.9 for poor, 9–27.4 for medium, 27.5–110 for rich, and > 110 for very rich. Site index (height at 50 yr breast height age) of lodge-pole pine (Pinus contorta Dougl. ex Loud) and interior spruce [Picea engelmannii Parry ex Engelmann × P. glauca (Moench) Voss] increased from very poor to very rich soils; however, the differences among site indices of contiguous soil nutrient regimes were largely insignificant. Regression analysis indicated that (1) soil moisture accounts for the largest proportion of the variation in site index, (2) models using C:N ratio of the forest floor and 0–30 cm of the mineral soil had a stronger relationship with site index (0.38 ≤ R2 ≤ 0.43, standard error < 3.0 m) than those using min-N. These results gave further evidence that min-N in the 0–30 mineral soil is a good single measure representing soil nutrient conditions, and justify the use of the existing field procedure for estimating soil nutrient regimes in montane boreal soils. Key words: Boreal soils, soil nutrient regime, quantitative classification, mineralizable-N, site index