Context Soil organic matter (SOM) is largely composed of carbon (C) and nitrogen (N), the proportions of which often change with soil depth. The relationships between SOM, C, and N in forest soils can be greatly altered in degraded landscapes and understanding these relationships is integral for successful forest restoration planning.Aims The current study investigated SOM, C, and N relationships in highly degraded forest soils by depth following regreening (one-time application of soil amendments and afforestation). Additionally, the use of standard C:OM ratios (which are commonly used to estimate soil C) were assessed.Methods The SOM, C, and N were measured at five different depths, at nine sites, ranging in time since regreening treatment applications across one of the world's largest regreening programmes in the City of Greater Sudbury, Canada.Key results The C:OM and C:N ratios decreased with soil depth while N:OM increased. The C and N were significantly correlated with SOM at all depths (excluding the L horizon). The C:OM ratio was lower than standard values and did not change between 16 and 41 years since the application of 10 Mg ha-1 of dolomitic limestone.Conclusions Despite massive soil degradation, SOM, C, and N relationships over soil depth at the regreening sites are consistent with unimpacted forest soils. Applying commonly used C:OM ratios drastically overestimated soil C pools, especially at lower depths.Implications Even in the most degraded landscapes, restoration can improve soil properties. Standard C:OM ratios should be used with caution. Soil organic matter (SOM), carbon (C), and nitrogen (N) management is critical for successful forest restoration. Our study investigated SOM, C, and N in forest soils following the restoration of one of the world's most industrially degraded landscapes. We found that despite immense degradation, 20-years post-restoration SOM, C, and N relationships, and how they change with soil depth, closely resembled those of unimpacted forested sites, suggesting that regardless the level of degradation restoring soil structure and function is possible.

Starting in the mid 1970's, researchers, industry leaders, and residents collaborated on one of the world's largest regreening programs in the industrial region of Sudbury, Canada. The Sudbury Regreening Recipe included the application of crushed dolomitic limestone, nitrogen-phosphorus-potassium fertilizers, grass-legume seed mixtures across 8200 ha, and subsequent tree planting across 25 000 ha of acidic metal-contaminated land. The current study evaluated shifts in understory vegetation diversity and soil geochemistry on a chronosequence of sites treated the same way between 1982 and 2012. Fifty-six plant species were identified across the 24 sites, only four of which were planted in the initial remediation effort. Key factors influencing plant community composition and diversity were related to shifts in soil properties over time: bulk density, LFH layer depth, and mineral soil horizon pH. Plant communities differed with stand age and rocky sites had significantly different plant communities and lower canopy cover than less rocky sites. Mineral soil horizon pH increased with age, reflecting the movement of applied dolomite in soil. Despite high concentrations of total copper and nickel in soil, plant succession patterns were generally similar to those in naturally recovering forests demonstrating the overall success of the restoration program.

We took advantage of the northern forest near Sudbury, Ontario, Canada where logging and metal-mining pollution had degraded the landscape prior to it being restored and reforested to investigate how soil organic matter is stabilized with stand age in highly 'eroded' sites with minimal residual soil and in 'stable' sites with residual soil. Soil organic matter in sandy, acidic soils of the northern forest is thought to be stabilized in organomineral particles (<20 m in size); however, forest disturbance and soil erosion might help to promote lessdecomposed plant detritus being stabilized within aggregate structures (53 mu m in size). We use a combination of physical fractionation (size) and chemical analysis (carbon to nitrogen ratio, natural abundance of stable isotopes of carbon and nitrogen) of the fractions. The composition of bulk soils with young trees (15-yr old) was 25% microaggregates (53 to 250 mu m in size) and 47% in a smaller silt + clay fraction (<53-m in size). Bulk soil with older trees (28- and 30- to 32-yr old) had equal proportions of macroaggregates (>250 mu m in size), microaggregates, and silt + clay fraction. The C density was much greater and less decomposed for macroaggregates than for the silt + clay fraction. Untreated sites, with trees but not restored with added lime, had a smaller proportion of macroaggregates in bulk soil. Even though aggregate structures typically are not associated with sandy, acidic forest soils, the results for these Regosols in the northern forest of the Greater Sudbury region show that stabilized soil C involves formation of macroaggregates along with microaggregates, both of which help to stabilize particulate organic matter.

Increasing forest cover by regreening mining and smelting degraded landscapes provides an opportunity for global carbon (C) sequestration, however, the reported effects of regreening on soil C processes are mixed. One of the world's largest regreening programs is in the City of Greater Sudbury, Canada and has been ongoing since 1978. Prior to regreening, soils in the City of Greater Sudbury area were highly eroded, acidic, rich in metals, and poor in nutrients. This study used a chronosequence approach to investigate how forest soil C pools and fluxes have changed with stand age in highly "eroded" sites with minimal soil cover (n = 6) and "stable" sites covered by soil (n = 6). Encouragingly, the relationship between stand age and soil C processes (litterfall, litter decomposition, soil respiration, fine root growth) at both stable and eroded sites were comparable to observations reported for jack pine (Pinus banksiana Lamb.) and red pine (Pinus resinosa Ait.) plantations that have not been subject to over a century of industrial impacts. There was a strong "home-field advantage" for local decomposers, where litter decomposition rates were higher using a site-specific pine litter compared with a common pine litter. Higher soil respiration at eroded sites was linked to higher soil temperature, likely because of a more open tree canopy. Forest floor C pools increased with stand age while mineral soil C and aggregate C concentrations decreased with stand age. This loss of soil C is small relative to the substantial increases in aboveground tree and forest floor C pools, leading to a sizeable increase in total ecosystem C pools following regreening.

Peatlands are unique habitats that function as a carbon (C) sink and an archive of atmospheric metal deposition. Sphagnum mosses are key components of peatlands but can be adversely impacted by air pollution potentially affecting rates of C and metal accumulation in peat. In this study we evaluate how the loss of Sphagnum in peatlands close to a copper (Cu) and nickel (Ni) smelter in Sudbury, Ontario affected C accumulation and metal profiles. The depth of accumulated peat formed during the 100+ year period of smelter activities also increased with distance from the smelter. Concurrently, peat bulk density decreased with distance from the smelter, which resulted in relatively similar average rates of apparent C accumulation (32-46 g/m2/yr). These rates are within the range of published values despite the historically high pollution loadings. Surface peat close to the smelters was greatly enriched in Cu and Ni, and Cu profiles in dated peat cores generally coincide with known pollution histories much better than Ni that increased well before the beginning of smelter activities likely a result of post-deposition mobility in peat cores.

Mining and smelting degraded landscapes are characterised by heavily eroded, acidic soils that are contaminated with toxic metals and depleted of essential nutrients. Increasing forest cover through regreening of degraded landscapes has been highlighted to support carbon (C) mitigation measures and protect biodiversity. One of the worlds largest regreening programs in the City of Greater Sudbury, Ontario has been ongoing since 1978 and involves the liming and fertilization of selected areas followed by planting of primarily coniferous trees. In this study, we assessed how aboveground biomass (AGB) and aboveground nutrient (calcium (Ca), magnesium (Mg), nitrogen (N), phosphorous (P), potassium (K), and C) pools changed using a space-for-time approach. We established a series of sites ranging from 15 to 42 years since treatment. To determine the potential effects of erosion on AGB and AGB nutrient pools, each site was categorized as "stable" (<10% bedrock cover) or "eroded" (greater than 30% bedrock cover). Both AGB and AGB nutrient pools increased with time since regreening at rates similar to conifer plantations grown in undisturbed regions. Individual tree growth and nutrient accumulation did not differ between stable and eroded sites; however, stable sites had a higher stem density leading to overall higher per hectare AGB and AGB nutrient pools. Future N limitation of the regreening forests does not appear to be a concern as aboveground N pools were six times larger than applied N. Conversely, aboveground P concentrations decreased with time since tree planting and the 40-year-old study sites had aboveground P concentrations below values for "healthy" trees. This study shows that the regreening efforts have led to a massive addition of 1,144,588 Mg of AGB (550,547 Mg C) onto the landscape, and capable of sustaining healthy tree growth up to 40-years post regreening. However, as the regreening stands age, nutrient limitation may impact future tree growth and warrants further study.

Peatlands account for 15 to 30% of the world’s soil carbon (C) stock and are important controls over global nitrogen (N) cycles. However, C and N concentrations are known to vary among peatlands contributing to the uncertainty of global C inventories, but there are few global studies that relate peatland classification to peat chemistry. We analyzed 436 peat cores sampled in 24 countries across six continents and measured C, N, and organic matter (OM) content at three depths down to 70 cm. Sites were distinguished between northern (387) and tropical (49) peatlands and assigned to one of six distinct broadly recognized peatland categories that vary primarily along a pH gradient. Peat C and N concentrations, OM content, and C:N ratios differed significantly among peatland categories, but few differences in chemistry with depth were found within each category. Across all peatlands C and N concentrations in the 10–20 cm layer, were 440 ± 85.1 g kg -1 and 13.9 ± 7.4 g kg -1 , with an average C:N ratio of 30.1 ± 20.8. Among peatland categories, median C concentrations were highest in bogs, poor fens and tropical swamps (446–532 g kg -1 ) and lowest in intermediate and extremely rich fens (375–414 g kg -1 ). The C:OM ratio in peat was similar across most peatland categories, except in deeper samples from ombrotrophic tropical peat swamps that were higher than other peatlands categories. Peat N concentrations and C:N ratios varied approximately two-fold among peatland categories and N concentrations tended to be higher (and C:N lower) in intermediate fens compared with other peatland types. This study reports on a unique data set and demonstrates that differences in peat C and OM concentrations among broadly classified peatland categories are predictable, which can aid future studies that use land cover assessments to refine global peatland C and N stocks.

Mining and smelting degraded landscapes are characterised by heavily eroded, acidic soils that are contaminated with toxic metals and depleted of essential nutrients. Restoring forests on these landscapes has been highlighted to support carbon (C) mitigation measures and protect biodiversity. Understanding how tree growth and aboveground nutrient accumulation changes following restoration will be essential to planning future forest restoration projects. In this study, we assessed aboveground biomass (AGB) and aboveground nutrient (calcium (Ca), magnesium (Mg), nitrogen (N), phosphorous (P), potassium (K), and C) pools across as series of sites ranging in age from 15 to 42 years old; to determine the effects of erosion on AGB and AGB nutrient pools each site was categorized as 'stable' (less than 10% bedrock cover) or 'eroded' (greater than 30% bedrock cover). Both AGB and AGB nutrient pools increased with time since restoration at rates similar to coniferous plantations grown in areas unimpacted by centuries of mining and smelting practices. Individual tree growth and nutrient accumulation did not differ between stable and eroded sites; however, stable sites had a higher stem density leading to overall higher AGB and AGB nutrient pools. Future N limitation of the regreening forests does not appear to be a concern as aboveground N pools are six times larger than applied N, indicating additional N is entering into the system whether through residual soil organic matter or the establishment of N-fixing species. Conversely, aboveground P concentrations are decreasing with time since tree planting and the 40-year-old study sites have aboveground P concentrations below values for 'healthy' trees. This study shows that the regreening efforts have led to a massive addition of 1, 144, 588 Mg of AGB (550, 547 Mg C) onto the landscape, and capable of sustaining healthy tree growth up to 40-years post restoration. However, as the regreening stands age, nutrient limitation may impact future tree growth. Future studies should continue to investigate nutrient cycling within these remediated forests particularly nutrients of concern such as P.

Soil mineral surface area is regarded as a key uncertainty in the estimation of base cation weathering rates, yet is rarely measured. Acidification studies rely heavily on pedotransfer functions (PTFs) that use widely available soil data to estimate mineral surface area. This study examined the relationship between soil properties andmineral surface area in soils (n = 25) from Kitimat, British Columbia, an area that is receiving elevated sulphur (S) deposition due to recent modernization of an aluminum (Al) smelter. Mineral surface area was measured on bulk soil samples using BET (Brunaeur, Emmett and Teller) gas-adsorption. Previously published particle size-based PTFswere a poor predictor of surface area in Kitimat soils (R-2 between 0.42 and 0.66). Instead, mineral surface areawas best predicted using a regionally-specific PTF (R-2 = 0.81), which used particle size as well as the concentration of kaolinite, the most abundant clay mineral in the region. Surface area values estimated using the regionally-specific PTF were applied to the PROFILE model to calculate weathering rates for critical load estimates. These estimates predicted that none of the sites received S deposition in exceedance of their critical load for acidity. However, as surface area is largely related to kaolinite content (a mineral that does not largely contribute toweathering rates), the applicability of using surface area functions forweathering rates is questionable. Further, the texture-based PTF developed for Kitimat did not provide accurate estimates of measured surface area for other soils in Canada, particularly at surface area values exceeding 2.5 m(2) g(-1). (C) 2019 Elsevier B.V. All rights reserved.