Production-related emissions and hotspots
Generally, field-level nitrous oxide (N2O) emissions and SOC changes were the largest determinants of observed differences in carbon footprints. In most cases, when SOC change was excluded, Canadian (particularly PP and Saskatchewan) crops had lower footprints compared to the same crops produced in other countries. Only Australian rapeseed was lower because of low field-level N2O emissions due to aridity and lack of irrigation, despite high tillage rates (Fig. 1). With SOC change included, Canadian crops always had the lowest emissions because their soils currently sequester carbon due to low- or no-till practices, as well as carbon inputs from manure and crop residues 11. The SOC sequestration estimates for Saskatchewan were higher than the PPs and national average because some regions in Canada have lower sequestration or carbon emissions, due to differences in tillage, carbon inputs, and perennial-to-annual cropping conversion 16–18. All other countries considered had net SOC losses 12–15. Australia had the lowest rates of SOC loss due to a decreasing trend in the amount of forest land converted to cropland, and an increase in adoption of conservation and no-till practices 13. The next lowest country was the U.S., which had net sequestration from cropland remaining cropland due to conservation tillage and C inputs (although with a declining trend), but with larger SOC losses, mainly from conversion of forest and grassland 15. France had higher SOC losses due to land conversion from grassland, though increased adoption of conservation tillage practices has slowed this trend over recent years 12. Germany had the highest SOC losses, also driven primarily by conversion of grasslands to agricultural land, though similar increases in adoption of conservation tillage practices as in France has decreased losses recently 14.
For rapeseed (Fig. 1), upstream production of nutrients (27–33%) and application-associated N2O emissions (49–59%) were the main contributors to the carbon footprint of Saskatchewan, PP and average Canadian production. For Saskatchewan, all N2O emissions came from a combination of synthetic nitrogen fertilizer (55%) and crop residues (45%). For the Canadian average, less than 1% of the N2O emissions came from mineralization losses due to soil carbon change, reflecting small regional soil carbon losses in the Eastern provinces and British Columbia. Emissions from fertilizer production were predominantly due to production of ammonia for N fertilizers. Carbon dioxide from combustion of diesel for field activities contributed 6% of the total impacts of Canadian rapeseed. Canadian average rapeseed production had higher emissions than Saskatchewan and PP due to higher fertilizer inputs and associated N2O emissions. When SOC changes were included in the overall carbon footprint, Saskatchewan rapeseed had the lowest emissions of all regions studied (0.37 kg CO2e kg− 1).
Australian and Canadian rapeseed crops had similar overall impacts when soil carbon was included (0.53 and 0.55 kg CO2e kg− 1), while Australia had lower impacts when soil carbon was excluded. Though Australian soils were found to have net emissions of CO2 compared to net sequestration in Saskatchewan and Canadian soils, these losses only increased the carbon footprint of Australian rapeseed by 10% when they were included. The highest contributor to the carbon footprint for Australian rapeseed was fuel use for field activities (30%), followed by nutrient inputs (26%). Field-level N2O emissions were low compared to all other regions, contributing only 11% of the total carbon footprint. German and French rapeseed had the highest impacts, both with (1.38 and 1.17 kg CO2e kg− 1) and without (0.99 and 0.94 kg CO2e kg− 1) SOC change. Field-level N2O emissions were the highest contributors to the carbon footprint of German and French rapeseed (55–58%). These emissions were due to N inputs from synthetic N fertilizer (34–51%), N mineralization from SOC loss (20–44%), crop residues (14–22%), and manure (6–8%). When soil carbon was included, it added an additional 24–39% to the carbon footprint. Upstream production of nutrients was also an important contributor (25–27%).
For wheat (Fig. 1), nutrient inputs (22–31%) and associated field-level N2O emissions (47–51%) were the main contributors to the emissions for Saskatchewan, PP and average Canadian production. Approximately 65% of the N2O emissions came from synthetic fertilizer, and 35% from crop residues. For the Canadian average, < 1% came from mineralization due to soil carbon losses. Field activities contributed 7–15% of the carbon footprints. When included, SOC change reduced the overall carbon footprint by 20–40%. Saskatchewan wheat production had the lowest impacts of all regions both with and without soil carbon (0.36 and 0.21 kg CO2e kg− 1, respectively), followed by PP and the Canadian average (1–5% higher without SOC, and 24–41% higher with SOC). All other regions had much higher impacts than Saskatchewan due to the higher life cycle impacts of production and net carbon emissions from soils. Among the other regions, Australia and France (0.59 kg CO2e kg− 1) had the lowest impacts, followed by the US (0.60 kg CO2e kg− 1) and Germany (0.65 kg CO2e kg− 1).
For German and French wheat, field-level N2O emissions made the highest contribution to the carbon footprint (47–48%). Of these N2O emissions, 32–50% were from synthetic N fertilizers, 21–47% from mineralization due to SOC loss, 13–23% from crop residues, and 6–8% from manure N. This was followed by SOC change (contributing an additional 20–36% when included) and upstream production of fertilizers and manure (19–20%). For the US, field-level N2O emissions contributed 27% to the carbon footprint, of which 45% came from synthetic N fertilizers, 34% came from crop residues, 11% from soil carbon losses, and 9% from manure. Upstream fertilizer and manure production contributed 24% of the carbon footprint, and fuel use for field activities contributed 18%. SOC change contributed only an additional 10% to the carbon footprint of U.S. wheat. For Australia, fuel use for field activities contributed 17% to the carbon footprint, fertilizer and manure production contributed 16%, and field-level N2O emissions 15%. When included, SOC loss contributed an additional 5% to the carbon footprint. The rest of the impacts for Australian wheat (52%) came from other inputs and activities, including seed, CO2 emissions from lime and urea, and post-harvest energy use.
Field peas (Fig. 1) produced in Saskatchewan had the lowest carbon footprint (0.26 kg CO2e kg− 1 without soil carbon), followed closely by the Prairie average (0.27 kg CO2e kg− 1), and the Canadian average (0.29 kg CO2e kg− 1). The carbon footprints of French, American, and German peas were 0.42, 0.59, and 0.64 kg CO2e kg− 1, respectively. The highest contributor to the carbon footprint of Canadian pea production was field-level N2O emissions (66–69%). Approximately 95% of the N2O emissions were due to N inputs from crop residues, and 5% from N fertilizers, consistent with the high levels of crop residues and low synthetic fertilizer and manure application rates for peas. Field activities contributed 15–17% of the life cycle impacts of Canadian, PP and Saskatchewan peas. Overall, fertility contributed 8–9%, including the upstream impacts of fertilizer and manure production, as well as a credit for biological N fixation. When included, SOC change decreased the carbon footprint of Saskatchewan, PP and Canadian peas by 55–76%. Saskatchewan peas still had the lowest carbon footprint (0.07 kg CO2e kg− 1), followed by PP and the Canadian average (0.10 and 0.13 kg CO2 kg− 1). Because all other countries already had higher impacts of production and much higher impacts from soil carbon changes, their combined impacts compared to Saskatchewan range from 9 times higher in France (0.64 kg CO2e kg− 1) to 15 times higher in Germany (1.05 kg CO2e kg− 1).
The highest contributor to the carbon footprint of French peas was field-level N2O emissions (48%), 55% of which were due to N mineralization from soil carbon loss, 29% from crop residues, and 16% from manure inputs. There were no synthetic N fertilizers applied to French peas. After N2O emissions, field activities were the next highest contributor to the carbon footprint of French peas (18%). Overall fertility contributed 12%, and SOC change increased the carbon footprint by an additional 51%. German peas had the highest carbon footprint among regions due to higher SOC loss, field-level N2O emissions, and fertilizer inputs. Field-level N2O emissions contributed 56% of the life cycle GHG emissions. Seventy-one percent of these emissions came from soil carbon losses, 13% from manure, 9% from crop residues and 7% from synthetic N fertilizer. Fertility contributed 15% of the carbon footprint, field activities 13%, and SOC change added an additional 64%. Pea production in the U.S. had the second highest carbon footprint, due to high levels of field activities and relatively high field-level N2O emissions (lower than Germany). Field-level N2O emissions were the highest contributors to the carbon footprint of US peas (43%). These emissions came from crop residues (69%), soil carbon losses (12%), synthetic N fertilizer (10%), and manure (10%). Fertility contributed 17% of the carbon footprint of U.S. peas, field activities 16%, and SOC change added an additional 17%.
See SI 1 for detailed LCI and LCIA results.
When soil carbon was included, crops produced in Saskatchewan then shipped overseas to Australia, France, and Germany still had lower carbon footprints than crops produced in each destination country, with the exception of Australian rapeseed. In the most extreme cases, differences in production emissions were sufficient to offset shipping the crops from Canada to Europe an additional 14 times before breaking even (for German peas and canola), equivalent to circumnavigating the globe more than three times (Figs. 2 and 3). With soil carbon included, transportation accounted for 14–77% of the impacts of shipping Saskatchewan crops overseas. This proportion varied with the relative carbon intensities of production for each crop, as well as the different transportation distances to each country. Saskatchewan peas had the lowest carbon footprint of all the crop types, therefore transportation made the highest proportional contribution, ranging from 47% of the impacts for transport to Germany, to 77% to Australia. In contrast, Saskatchewan wheat had higher impacts of production than peas, so transportation contributed a lower percentage to the total carbon footprint (22% to France and Germany, and 50% to Australia). Rapeseed had the highest impacts of production of the three crops produced in Saskatchewan, and thus transportation only made up 14–37% of the total carbon footprint to market in Europe/Australia.
With soil carbon excluded, transportation accounted for 18–44%, 14–38%, and 9–27% of the impacts of shipping Saskatchewan peas, wheat, and rapeseed to Europe/Australia. For Saskatchewan crops, the exclusion of soil carbon meant higher overall impacts of production, since Saskatchewan soils had net carbon sequestration. Therefore, transportation overseas made a relatively smaller contribution. However, when calculating the break-even distances without SOC change, production-related emission differences between countries were lower and hence transportation of Canadian crops was proportionately more important (Fig. 2). Without SOC change, Saskatchewan rapeseed and peas shipped to Europe still had lower emissions than crops grown in France or Germany, with the exception of French peas, but the opposite was true for wheat. For German rapeseed and peas, the offset was still sufficient to ship the crops produced in Saskatchewan to Germany an additional four times before breaking even, almost one circumnavigation of the globe (Fig. 3). This shows that despite the higher importance of transportation when SOC was excluded, the differences in production emissions can still be large enough to more than offset the transportation emissions, but not in all cases.