A body of emerging research shows the promise of charcoal soil amendments (“biochars”) in restoring fertility in degraded agricultural and forest soils. “Sustainable biochars” derived from locally produced waste biomass and produced near the application site are of particular interest. We tested the effects of surface applications of wood-derived biochars (applied at 7.5 t·ha⁻¹) on soil physiochemical properties (N, P, K, pH, soil moisture content, organic matter content, and bulk density) in three land-use types: agriculture (Camellia sinensis monoculture), agroforestry (C. sinensis with shade trees), and secondary forest (Dipterocarpus dominated) assessed over seven months. We found significant positive effects of biochar on soil physiochemical properties in all land-use types, with the strongest responses in the most degraded tea monoculture sites. Although biochar had no significant effect on soil N and K, it improved soil P—the primary nutrient most commonly limiting in tropical soils. Biochar also enhanced soil moisture and organic matter content, reduced bulk density, and increased soil pH in monoculture sites. Our results support the general hypothesis that biochar can improve the fertility of degraded soils in agricultural and forest systems in Bangladesh and suggest that biochar additions may be of great benefit to the most degraded soils.

Surface albedo is one of the most important processes governing climate forcing in the boreal forest and is directly affected by management activities such as harvesting and natural disturbances such as forest fires. Empirical data on the effects of these disturbances on boreal forest albedo are sparse. We conducted ground-based measurements of surface albedo from a series of instrument towers over 4 years in a replicated chronosequence of mixedwood boreal forest sites differing in stand age (to 19 years since disturbance) in both post-harvest and post-fire stands. We investigated the effects of stand age, canopy height, tree species composition, and ground vegetation cover on surface albedo through stand development. Our results indicate that winter and spring albedo values were 63 % and 24 % higher, respectively, in post-harvest stands than in post-fire stands. Summer and fall albedo values were similar between disturbance types, with summer albedo showing a transient peak at ∼10 years stand age. The proportion of deciduous broadleaf species showed a strong positive relationship with seasonal averages of albedo in both post-harvest and post-fire stands. Given that stand composition in mixedwood boreal forests generally shows a gradual replacement of deciduous trees by conifers, our results suggest that successional changes in species composition are likely a key driver of age-related patterns in albedo. Our findings also suggest the efficacy of increasing the proportion of deciduous broadleaf species as a silvicultural option for climate-friendly management of the boreal forest.

Scientists unequivocally agree that winter air temperature (T_A) in northern high latitudes will increase sharply with anthropogenic climate change, and that such increases are already pervasive. However, contrasting hypotheses and results exist regarding the magnitude and even direction of changes in winter soil temperature (T_S). Here we use field and satellite data to examine the ‘cold soil in a warm world’ hypothesis for the first time in the boreal forest using a proxy year approach. In a proxy warm year with a mean annual temperature similar to that predicted for ~2080, average winter T_S was reduced relative to the baseline year by 0.43 to 1.22 ^°C in open to forested sites. Similarly, average minimum and maximum winter T_S declined, and the number of freeze-thaw events increased in the proxy warm year, corresponding to a reduction in the number of snow-covered days relative to the baseline year. Our findings indicate that early soil freezing as a result of delayed snowfall and reduced snow insulation from cold winter air are the main drivers of reduced winter active-layer T_S (at ~2-cm depth) under warming conditions in boreal forest, and we also show that these drivers interact strongly with forest stand structure.

The net effect of forest disturbances, such as fires and harvesting, on soil greenhouse gas fluxes is determined by their impacts on both biological and physical factors, as well as the temporal dynamics of these effects post-disturbance. Although harvesting and fire may have distinct effects on soil carbon (C) dynamics, the temporal patterns in soil CO₂ and CH₄ fluxes and the potential differences between types of disturbances, remain poorly characterized in boreal forests. In this study, we measured soil CO₂ and CH₄ fluxes using a cavity ring-down spectroscopy system in snow-free seasons over two years in post-harvest and post-fire chronosequence sites within a mixedwood boreal forest in northwestern Ontario, Canada. Soil CO₂ efflux showed a post-disturbance peak, with differing dynamics depending on the disturbance type: post-harvest stands exhibited a nearly tenfold increase (from ~1 to ~11 μmol CO₂^.m^-2.s^-1) from 1 to 9-10 years post-disturbance, followed by a steep decline; post-fire stands showed a more gradual increase, peaking at ~6-7.2 μmol CO₂^.m^-2.s^-1 after ~12-15 years. The youngest post-harvest stands were net sources of CH₄,whereas post-fire stands were never net CH₄ sources. In both disturbance types, the strength of the CH₄ sink increased with stand age, approaching ~2.4 nmol^.m^-2.s^-1 by 15 years post-disturbance. Volumetric water content, bulk density, litter depth, and pH were significant predictors of CO₂ fluxes; for CH₄ fluxes, litter depth, pH, and the interaction of VWC and soil temperature were significant predictors in both disturbance types, with EC also showing a relationship in post-harvest stands. Our findings indicate that while soil CH₄ oxidation rapidly recovers following disturbance, both post-harvest and post-fire stands show a multi-decade release of soil CO₂ that is too large to be offset by C gains over this period.

Methane (CH₄), a potent greenhouse gas (GHG) with high global warming potential, significantly contributes to urban GHG emissions. Organic mulching, commonly practiced in urban forestry, may promote CH₄ emissions via anaerobic decomposition; yet its impact on the urban carbon budget has largely been unexamined. Biochar has shown promise in mitigating CH₄ emissions in agricultural soils, but its effectiveness in urban mulched systems remains unknown. This study employed a mesocosm experiment to investigate the effects of organic mulches (woodchips and bark) and biochar amendments (50 t/ha), applied either on the surface (top-dressed) or incorporated (mixed), on fluxes of CH₄, CO₂, and H₂O. Fluxes were measured using an off-axis integrated cavity output spectroscopy analyzer. Results indicate that mulched soils emitted CH₄ at 1.0-1.5 nmol m⁻^2.s⁻¹, whereas biochar amendments promoted CH₄ uptake, in the case of both woodchips (-1.65 ± 1.03 nmol m⁻^2.s⁻¹) and bark mulch (-0.49 ± 0.16 nmol m⁻^2.s⁻¹) by the second year. Mixed treatments showed greater CH₄ uptake; for instance, incorporating biochar into bark mulch led to a mean CH₄ uptake (-2.02 ± 1.02 nmol m⁻^2.s⁻¹), nearly fivefold greater than controls. While mulch additions reduced water loss and increased soil organic carbon-factors contributing to CH₄ emissions-biochar amendments increased CO₂ emissions by 26.7%-121.1%. Biochar-mediated CH₄ uptake correlated with substrate pH, bulk density, and C:N ratio, suggesting enhanced microbial activity and increased CO₂ release. Overall, findings indicate that biochar, combined with organic mulching, can serve as an effective GHG mitigation strategy, informing climate-smart soil management in urban landscapes.