Organic matter, carbon, and nitrogen relationships of regreened forest soils in an industrially impacted landscape

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.