Effects of restoration on carbon storage in smelterimpacted industrial barrens

Abstract

Landscape carbon (C) storage is a key component of climate change mitigation. Globally, industrial barrens cover large areas and their restoration can facilitate C storage in otherwise under-utilized sites, while concomitantly enhancing numerous other ecosystem services. I assessed how restoration of smelter impacted barren land enhanced C storage by studying a site in Sudbury, Ontario near a former Ni and Cu metal smelter that ceased operation in 1972. The site was treated by aerial liming, fertilizing, and grass and legume seeding in 1994-1997, followed by jack pine (Pinus banksiana) planting in the upland areas in 1997-2001. Forty-five 0.1 ha size plots were selected across restored and untreated adjoining areas, 32 in exposed upland industrial barrens and 13 in sheltered lowland valleys. The focus of my study was on upland industrial barrens, which exhibited severe site conditions and little natural regrowth. I measured the amount of C in coarse woody debris, fine woody debris, herbs, mineral soil, organic soil (LFH layers), shrubs, and trees in each plot. Measures of wetness index, plant species richness, soil metal concentrations, soil pH, distance from smelter, and elevation were then used to assess factors affecting total ecosystem C storage. In lowland valleys where no active tree planting occurred (only natural regeneration) the treatments with lime, fertilizer, and grass and legume seed showed a 38% increase in C storage (101.1 ± 5.5 Mg C ha-1 (mean ± S.E.)) compared to untreated lowland plots (73.3 ± 5.9 Mg C ha1 ). In upland areas where growing conditions were more severe (i.e., thin soils, low moisture), tree C increased from 0.5 ± 0.4 Mg C ha-1 in areas of natural regeneration to 19.3 ± 1.4 Mg C ha-1 following liming, fertilizing, seeding, and tree-planting. There was no significant difference in total C storage in untreated reference plots (36.1 ± 8.4 Mg C ha-1) compared to limed, fertilized, seeded, and tree-planted plots (58.2 ± 4.4 Mg C ha-1), likely due to variable site conditions across the landscape. Wetness index, plant species richness, and soil bioavailable metal concentrations were the best predictors of C storage in upland industrial barrens, with the best model explaining 64% of variance in C storage. Overall, mineral soil remained the largest C pool in both the uplands (53%) and the lowlands (40%). The forests in my study were not mature, so C storage is expected to continue to increase in the future. My findings demonstrate that soil amendments and tree planting can increase tree C storage in industrial barrens, but site characteristics, particularly wetness, are key to the rate of total C accumulation. C storage in less disturbed lowland valleys also benefitted from restoration. Well-designed restoration efforts that optimize C storage in globally extensive industrial barrens can therefore sequester C and may in turn assist in climate change mitigation.