The nitrogen cycle in lodgepole pine forests, southeastern Wyoming

Abstract

Storage and flux of nitrogen were studied in several contrasting lodgepole pine (Pinus contorta spp.latifolia) forests in southeastern Wyoming. The mineral soil contained most of the N in these ecosystems (range of 315–860 g · m⁻²), with aboveground detritus (37.5–48.8g · m⁻²) and living biomass (19.5–24.0 g · m⁻²) storing much smaller amounts. About 60–70% of the total N in vegetation was aboveground, and N concentrations in plant tissues were unusually low (foliage = 0.7% N), as were N input via wet precipitation (0.25 g · m⁻² · yr⁻¹), and biological fixation of atmospheric N (<0.03 g · m⁻² · yr⁻¹, except locally in some stands at low elevations where symbiotic fixation by the leguminous herbLupinus argenteus probably exceeded 0.1 g · m⁻² · yr⁻¹).

Because of low concentrations in litterfall and limited opportunity for leaching, N accumulated in decaying leaves for 6–7 yr following leaf fall. This process represented an annual flux of about 0.5g · m⁻² to the 01 horizon. Only 20% of this flux was provided by throughfall, with the remaining 0.4g · m⁻² · yr⁻¹ apparently added from layers below. Low mineralization and small amounts of N uptake from the 02 are likely because of minimal rooting in the forest floor (as defined herein) and negligible mineral N (< 0.05 mg · L⁻¹) in 02 leachate. A critical transport process was solubilization of organic N, mostly ‘fulvic acids’. Most of the organic N from the forest floor was retained within the major tree rooting zone (0–40 cm), and mineralization of soil organic N provided NH₄ for tree uptake. Nitrate was at trace levels in soil solutions, and a long lag in nitrification was always observed under disturbed conditions. Total root nitrogen uptake was calculated to be 1.25 gN · m⁻² · yr⁻¹ with estimated root turnover of 0.37-gN · m⁻² · yr⁻¹, and the soil horizons appeared to be nearly in balance with respect to N. The high demand for mineralized N and the precipitation of fulvic acid in the mineral soil resulted in minimal deep leaching in most stands (< 0.02 g · m⁻² · yr⁻¹). These forests provide an extreme example of nitrogen behavior in dry, infertile forests.