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Tree-based intercropping (TBI) systems, combining agricultural alley crops with rows of hardwood trees, are largely absent in Canada. We tested the hypothesis that the roots of 5-8 years old hybrid poplars, growing in two TBI systems in southern Québec, would play a ''safety-net'' role of capturing nutrients leaching below the rooting zone of alley crops. TBI research plots at each site were trenched to a depth of 1 m on each side of an alley. Control plots were left with tree roots intact. In each treatment at each site, leachate at 70 cm soil depth was repeatedly sampled over two growing seasons using porous cup tension lysimeters, and analyzed for nutrient concentrations. Daily water percolation rates were estimated with the forest hydrology model ForHyM. Average nutrient concentrations for all days between consecutive sampling dates were multiplied by water percolation rates, yielding daily nutrient leaching loss estimates for each sampling step. We estimated that tree roots in the TBI system established on clay loam soil decreased subsoil NO 3 -leaching by 227 kg N ha -1 and 30 kg N ha -1 over two consecutive years, and decreased dissolved organic N (DON) leaching by 156 kg N ha -1 year -1 in the second year of the study. NH 4 ? leaching losses at the same site were higher when roots were present, but were 1-2 orders of magnitude lower than NO 3 -or DON leaching. At the sandy textured site, the safety net role of poplar roots with respect to N leaching was not as effective, perhaps because N leaching rates exceeded root N uptake by a wider margin than at the clay loam site. At the sandy textured site, significant and substantial reductions of sodium leaching were observed where tree roots were present. At both sites, tree roots reduced DON concentrations and the ratio of DON to inorganic N, perhaps by promoting microbial acquisition of DON through rhizodeposition. This study demonstrated a potential safety-net role by poplar roots in 5-8 yearold TBI systems in cold temperate regions.

Increasing awareness of global climate change has pressured agricultural producers to reduce greenhouse gas (GHG) emissions while at the same time encouraging them to maintain food production needed for an increasing population. Tree-based intercropping (TBI) systems are believed to be useful in climate change mitigation, especially in temperate regions, due to their potential to reduce GHG emissions from agricultural practices. The purpose of this paper is therefore to review some of the research conducted on GHG mitigation in TBI in southern Ontario and Quebec, Canada. Research conducted at the University of Guelph Agroforestry Research Station (GARS) indicated that TBI systems had the potential to lower N2O emissions by 1.2 kg ha-1 y-1 compared to a conventional agricultural field cropping system. Trees can assimilate residual nitrate (NO3-) left from nitrogen (N) fertilizer applications, thereby leaving less NO3- available for denitrification and subsequently reducing N2O losses. Carbon sequestration is also enhanced in TBI systems as carbon (C) is stored in both above and below ground tree components. Soil Organic Carbon (SOC) is higher in systems incorporating trees because tree litter decomposes slowly, therefore reducing CO2 loss to the atmosphere. The C sequestration potential of TBI systems and the possibility to include fast-growing tree species for bioenergy production in TBI systems make it a valid solution to mitigate climate change in temperate regions. The opportunity of C trading credits to offset the costs of implementing a TBI system and provide additional income to farmers could facilitate the adoption of TBI amidst agricultural producers in temperate regions.

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