An agronomic assessment of greenhouse gas emissions from major cereal crops

Agricultural greenhouse gas (GHG) emissions contribute approximately 12% to total global anthropogenic GHG emissions. Cereals (rice, wheat, and maize) are the largest source of human calories, and it is estimated that world cereal production must increase by 1.3% annually to 2025 to meet growing demand. Sustainable intensification of cereal production systems will require maintaining high yields while reducing environmental costs. We conducted a meta-analysis (57 published studies consisting of 62 study sites and 328 observations) to test the hypothesis that the global warming potential (GWP) of CH ₄ and N ₂O emissions from rice, wheat, and maize, when expressed per ton of grain (yield-scaled GWP), is similar, and that the lowest value for each cereal is achieved at near optimal yields. Results show that the GWP of CH ₄ and N ₂O emissions from rice (3757 kg CO ₂ eq ha ^-1 season ^-1) was higher than wheat (662 kg CO ₂ eq ha ^-1 season ^-1) and maize (1399 kg CO ₂ eq ha ^-1 season ^-1). The yield-scaled GWP of rice was about four times higher (657 kg CO ₂ eq Mg ^-1) than wheat (166 kg CO ₂ eq Mg ^-1) and maize (185 kg CO ₂ eq Mg ^-1). Across cereals, the lowest yield-scaled GWP values were achieved at 92% of maximal yield and were about twice as high for rice (279 kg CO ₂ eq Mg ^-1) than wheat (102 kg CO ₂ eq Mg ^-1) or maize (140 kg CO ₂ eq Mg ^-1), suggesting greater mitigation opportunities for rice systems. In rice, wheat and maize, 0.68%, 1.21%, and 1.06% of N applied was emitted as N ₂O, respectively. In rice systems, there was no correlation between CH ₄ emissions and N rate. In addition, when evaluating issues related to food security and environmental sustainability, other factors including cultural significance, the provisioning of ecosystem services, and human health and well-being must also be considered.