Effects of land reclamation on organic matter decomposition at smelter impacted hill slopes

Mining, smelting and forest harvest led to severe environmental disturbances across Sudbury region, resulting in extensive forest cover loss, soil erosion and contamination with acids and metals. Regreening efforts began in 1978, by liming, fertilizing and tree planting. However little work has been done to assess the influence of reclamation practices on soil processes and functioning in these industrially damaged landscapes. In particular there is a lack of such information for hillslopes and other challenging sites. In view of this, the present study examined the effects of land reclamation on key ecosystem processes such as litter decomposition and soil respiration (CO2) and CH4 and N2O fluxes and the ecological controls that regulate these processes. The study was conducted along two parallel transects, one within a barren area and the other within a regreened area at a steep hillslope within the Kelly lake watershed, a study site a few kms of the Copper Cliff smelter in Sudbury. Traditional litter bag decomposition studies are time consuming, expensive, and lack standardization of litter quality. I therefore used a new Tea Bag Index methodology to measure early stage decomposition with two contrasting standardized substrates (Green and Rooibos tea). Tea bag Index parameters were used to quantify the rate at which standardized litter is broken down (decomposition rate index (k)) and to measure the undecomposed residual substrate in the soil (stabilization factor (S)) which has a potential for sequestration. The results revealed that decomposition of Green tea (labile substrate) was twice as fast as Rooibos tea (recalcitrant substrate) in both the reclaimed and barren areas. I found that at reclaimed areas the average Green tea mass loss rates (1.37 ± 0.01 (SE) gm), CO2 production rates (98.84 ± 5.46 (SE) mg m-2 h-1), CH4 consumption rates (- 0.0104 ± 0.00 (SE) µg m-2 h-1 ) were statistically higher than at barren areas, where the average Green tea mass loss were (1.25 ± 0.01 (SE) gm), CO2 production rates were (33.79 ± 2.26 (SE) mg m-2 h-1 ) and CH4 flux rates were (0.0014 ± 0.001 (SE) µg m-2 h-1) respectively. Average Rooibos tea mass loss on the other hand showed no significant differences occurred between reclaimed areas (0.69 ± 0.02 (SE) gm) and at barren areas (0.64 ± 0.02 (SE) gm). N2O flux rates also did not differ significantly between reclaimed (-0.01048 ± 0.0006 (SE) µg m-2 h-1) and barren areas and (-0.00073 ± 0.000452 (SE) µg m-2 h-1). However, when the combined substrate measure of decomposition rate index (k) was compared, there was no significant differences between the reclaimed area soils (0.012 ± 0.001 (SE) g·g-1·day-1) and the still barren untreated soils (0.013 ± 0.001 (SE) g·g-1·day-1). Only the Stabilization factor (S) proved to significantly different with a higher value for the barren soil (0.16 ± 0.009 (SE) g g-1 ) than the reclaimed soil (0.09 ± 0.006 (SE) g g-1) suggesting that the barren areas had slower decomposition resulting in a greater potential for accumulation of undecomposed organic matter in the soil. All soils tested proved to be sources of carbon dioxide. The reclaimed areas appeared to be small sinks for methane and nitrous oxide, whereas at barren areas results were less consistent and the soils appeared as likely to emit or consume methane and nitrous oxide fluxes My main finding was that land reclamation enhanced decomposition of labile substrates CO2 production rates and CH4 consumption rates but decomposition rate index (k) and soil respiration rates (CO2) and CH4, and N2O fluxes were still very low compared to other less disturbed ecosystems. Stepwise multiple regression analysis revealed that decomposition of Green tea (labile substrate) was most likely influenced by microclimatic factors (i.e. moisture) and that decomposition and soil respiration (CO2 production rates) decreased with increasing temperature at barren areas. Topography (i.e. elevation(m)) had little or no effect on carbon turnover rates across the study sites. In conclusion, land reclamation influenced key ecosystem processes such as decomposition of labile organic matter and soil respiration, but even after 37 years post-treatment, Sudbury soils in these hillslope sites were shown to be still under severe environmental stress.

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