The role of grass-legume communities in revegetation of a subalpine mine site in British Columbia
Creators: name: Yamanaka, Koji
Publisher: name: University of British Columbia
Year: 2010

description: This study describes an investigation of the potential for pioneer grass-legume communities to stabilize and ameliorate geologically-fresh soil leading to the establishment of a self-sustaining, progressive plant succession on a surface-mined subalpine site. The study area is located 2,000 m above sea level in the Canadian Rocky Mountains. Field surveys at the site indicated extremely limited invasion of reclaimed areas (3-7 years old) by native species from the adjacent subalpine forest. Soils on revegetated sites were generally warmer and drier than soils of the associated forest and have less than half the content of fine soil fragments (&lt;2 mm). Field studies revealed chronological trends in grass-legume communities at four sites revegetated during 1974-1978 including: species composition, legumes (Trifolium repens L., T. hybridum L. and Medicago sativa L.) performing increasingly poorly on the older sites; biomass changes, a shoot to root ratio (S/R) decreasing from 2.3 to 0.2 as the communities aged; and litter accumulation which continued even on the oldest site. Fertilizer (13-16-10) operationally applied at 150 -391 kg/ha enhanced the growth of Dactylis glomerata L. and litter degradation, and acidified the soil. Nitrogen fertilization was also associated with two clear inverse relationships identified between D. glomerata and Festuca rubra L. biomass, and between soil pH and phosphorus levels. In greenhouse tests grasses were revealed to be more efficient soil nitrogen consumers than were legumes and nitrogen fixation decreased significantly (P&lt;0.01) and linearly with increasing grass seeding rates. In the presence of grasses, nitrogen fixation was positively correlated with aboveground legume biomass at all nitrogen fertilizer levels tested. The results further revealed that operational seeding and fertilizer rates at this site may not optimize plant productivity and the ability of legumes to fix nitrogen symbiotically. Field trials based upon the experimentaly derived combination (17.5 : 30 : 50 kg/ha grass seeding rate : legume seeding rate : nitrogen fertilizer rate) would be desirable to evaluate these data on the site. Other potential practical implications from this study are: (1) The need for improved legume establishment, involving legume seed germination, species and variety selection, and selection of Rhizobium strains. (2) Improved control of the operational fertilizer application. (3) Alteration of grass and legume species composition of the present seed mix. (4) Selective placement of initial material (overburden or spoil) handling. A modification of the acetylene reduction assay, "the open system" technique, was developed for evaluation of legume nitrogen fixation of mine spoils. Although the unit developed is limited to detection of the presence or absence of ethylene, calibration with the closed system of ethylene levels obtained by the open system appeared feasible. Further refinement of the system for quantitative use would increase its usefulness in nitrogen fixation studies of legumes on mine spoils, nitrogen fixing woody plants in forests, and legumes in grassland sods.