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Carbon (C) sequestration was quantified in two tree-based intercropping and in conventional agricultural systems in southern Ontario, Canada. In the intercropping systems, 13-year-old hybrid poplar (Populus deltoides · Populus nigra clone DN-177) and Norway spruce (Picea abies L.) were intercropped with barley (Hordeum vulgare L. cv. OAC Kippen). In the conventional agricultural system, barley was grown as a sole crop. Above-and below-ground carbon in trees, soil C, soil respiration and C leaching from each system were determined in situ. These data coupled with complementary data obtained from the literature were compiled and used to construct C cycle models, C pools and fluxes for each system. The total mean aboveand below-ground C sequestered in permanent tree components was 15.1 and 6.4 t C ha À1 for poplar and spruce trees, respectively at 111 stems ha À1 . Soil C pools were 78.5, 66 and 65 t C ha À1 in poplar, spruce intercropping and in barley sole cropping systems, respectively. Soil respiration rates were 3.7, 4.5 and 2.8 t C ha À1 y À1 in poplar, spruce intercropping and barley sole cropping systems, respectively. Carbon leaching within the intercropping systems was greater below the tree row compared to the middle of the alley, but average values differed little from the sole cropping system. Total C pools (including an assumed barley C pool of 3.4 and 2.9 t C ha À1 within the sole cropping and the intercropping systems respectively) were 96.5, 75.3, and 68.5 t C ha À1 within poplar, spruce intercropping and in barley sole cropping systems, respectively. Estimated net C fluxes for the poplar and spruce intercropping systems and for the barley sole cropping system in 2002 were +13.2, +1.1, and À2.9 t C ha À1 y À1 , respectively. These results suggest that intercropping systems have a greater potential in reducing the atmospheric carbon dioxide concentration compared to sole cropping systems.

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Alongside the steep reductions needed in fossil fuel emissions, natural climate solutions (NCS) represent readily deployable options that can contribute to Canada’s goals for emission reductions. We estimate the mitigation potential of 24 NCS related to the protection, management, and restoration of natural systems that can also deliver numerous co-benefits, such as enhanced soil productivity, clean air and water, and biodiversity conservation. NCS can provide up to 78.2 (41.0 to 115.1) Tg CO2e/year (95% CI) of mitigation annually in 2030 and 394.4 (173.2 to 612.4) Tg CO2e cumulatively between 2021 and 2030, with 34% available at ≤CAD 50/Mg CO2e. Avoided conversion of grassland, avoided peatland disturbance, cover crops, and improved forest management offer the largest mitigation opportunities. The mitigation identified here represents an important potential contribution to the Paris Agreement, such that NCS combined with existing mitigation plans could help Canada to meet or exceed its climate goals.

Land use affects the carbon sequestration potential of soils across landscapes. Tree-based intercropping (TBI) systems where annual crops are grown between established tree rows are expected to exhibit spatial heterogeneity in the soil organic carbon (SOC) content due to differences in carbon input and decomposition rates of litter from trees and herbaceous plants. This study aimed to quantify variability in the SOC of TBI systems, compare the SOC content of TBI and nearby conventional agroecosystems, and determine if SOC was related to soil fertility. The TBI research sites were established 4 years (St. Paulin and St. Edouard, Quebec, Canada), 8 years (St. Remi, Quebec, Canada) and 21 years (Guelph, Ontario, Canada) before soil samples were collected for this study. The SOC content was greater within 0.75 m of the tree row than in the intercropped space at the St. Edouard and St. Remi sites. At the Guelph site, the SOC was spatially heterogeneous in plots with Norway spruce (Picea abies L.) but not hybrid poplar (Populus deltoides 9 P. nigra clone DN-177), probably due to litterfall distribution. Formerly a tree plantation, the TBI system at St. Remi contained 77% more SOC than a nearby conventional agroecosystem, while there was 12% more SOC in the TBI system than the conventional agroecosystem at Guelph. There was no difference in the SOC content of 4-year old TBI sites and nearby conventional agroecosystems. However, an increase in SOC at all TBI sites was positively related to the plant-available N concentrations, indicating the benefit of temperate TBI systems for soil fertility.