ENHANCEMENT OF REVEGETATION ON CONSTRUCTION FILL BY FERTILIZER AND TOPSOIL APPLICATION - EFFECT ON MYCORRHIZAL INFECTION

The influence of topsoil and fertilizer application on denuded road co nstruction sites was evaluated to assess its contribution to improveme nt of vegetation re-establishment. The study sites were within a mixed hardwood and conifer forest on crushed, unweathered subsurface materi al with low fertility and low biological activity. Topsoils were remov ed from the site, stockpiled and reapplied to the site after construct ion. The effect of topsoil amendment on plant growth, soil fertility, mycorrhizal infection and an index of microbial biomass were measured in field and greenhouse experiments. Plant growth on the topsoil amend ed field plots were greatly increased relative to treatments with fert ilizer but no topsoil. Three years after establishment, dry weight pro duction on the plots without topsoil treatment was about 40 per cent o f the plots treated with topsoil. Greenhouse experiments were designed to compare fresh . dried and stockpiled topsoil. These experiments in dicated that storage of the harvested topsoil for five months in a sto ckpile had minor effects on plant growth, soil fertility, mycorrhizal infection and microbial biomass. Topsoil volumes had to exceed 20 per cent of the total soil volume to achieve statistically significant ben efits and higher ratios showed greater benefit. The percentage of myco rrhizal infection was greatest in topsoil treatments without fertilize r. The addition of fertilizer increased growth but reduced the percent age of roots forming mycorrhizae. When the total weight of infected ro ots was calculated, however, infection was found to be greatest with a moderate level of fertilizer (equivalent to approximately 27 kg N ha- 1 and 39 kg P ha-1), and was less in both higher fertilizer treatments and in unfertilized treatments. Topsoil amendment increased microbial biomass nitrogen but fertilizer treatment did not.