Effects of controlled weed densities and soil types on soil nitrate accumulation, spruce growth, and weed growth

Abstract

Soil nitrate (NO 3 −) accumulation rates were assessed among seven weed species grown in small plots during the summer of 1997, at a northern Ontario location. Objectives were (1) to quantify soil nitrate accumulation rates at varying weed densities established on three soil types (clay, loam, and sand) and (2) to assess the effects of soil nitrate levels on weed and black spruce ( Picea mariana (Mill.) B.S.P.) seedling growth and weed growth. Controlled densities (0, 0.5, 2, 8 stems per m 2) of red raspberry ( Rubus idaeus L. var. strigosus (Michx.) Maxim.), fireweed ( Epilobium angustifolium L.), and a grass species ( Calamagrostis canadensis (Michx.) Nutt.) were planted along with spruce seedlings (1 m spacing) early in the summer of 1994. Concurrently, four more deciduous weed species, trembling aspen ( Populus tremuloides Michx.), white birch ( Betula papyrifera Marsh.), green alder ( Alnus crispa (Ait.) Pursh), and willow ( Salix humilis Marsh.) were also established at four densities (0, 0.5, 2, 4 stems per m 2). In mid-summer (July) 1997, soil samples collected from the respective weed densities were placed in polyethylene bags and the bags were incubated in the respective plots for 34 days at a depth of 12 cm. Nitrate accumulation rates (μg per 100 gm of dry soil per day), weed heights, and the effective leaf area index (LAIe, in m 2 m −2, measured with a Licor LAI-2000 Plant Canopy Analyzer) of competing vegetation were consistently higher (1.4–3.0 times) for clay soil. Nitrate levels were significantly ( p < 0.05) higher in clay soils obtained from plots planted with raspberry, grass, aspen, and willow. Carbon : nitrogen (C : N) ratios were higher for loam soils than those for clay or sand soils, and these higher ratios may have resulted in more immobilization than mineralization of soil nitrogen. Nitrate levels for clays and loams correlated significantly with the LAIe of competing vegetation ( r = 0.62–0.78) and with weed growth ( r = 0.75–0.96), but non-significantly, and often negatively, with spruce stem volume. To date, soil nitrate accumulation appears to have primarily benefited cultivated weeds and not to have improved spruce growth. These findings affirm that weeds must be controlled for seedlings to benefit nutritionally.