Conservation agriculture reduces climate change impact of a popcorn and wheat crop rotation

(-688 kg CO2-eq./ha), because C sequestration was superior to GHG emissions. For the con- ventional crop rotations, direct emissions contributed most to the climate change impact. For the conservation crop rotation, direct emissions had a similar magnitude as those of conven- tional crop rotations, but C sequestered contributed most to the climate change impact. When averaged over the two crop rotations, the climate change impact considering only emissions was 1% lower for wheat and 8% higher for popcorn for conservation agriculture than for conventional agriculture (Fig 2). Expressed in kg CO2-eq./ha, impacts were 2629 (con- ventional) and 2598 (conservation) for wheat, 3407 (conventional) and 3680 (conservation) for popcorn, and 966 for cover crops. For conservation popcorn, climate change impacts for other field operations and mineral fertilisers were 45% and 10% lower, respectively, than those for conventional popcorn. Trans- port of inputs and pesticides, which contributed little (ca. 1%) to impacts for all crops, were 78% and 16% higher respectively, for conservation popcorn than for conventional popcorn. The processes that contributed most were direct emissions (64% conventional and 59% con- servation), mineral fertiliser (16% conventional and 13% conservation), irrigation (10% con- ventional and 9% conservation), other field operations (9% conventional and 4% conservation), and compost 11% (only for conservation). For conservation wheat, climate change impacts for direct emissions and other field opera-

tions were similar to and 22% lower, respectively, than those for conventional wheat. Impacts associated with mineral fertilisers, pesticides, and transport of inputs did not differ for Table 3. Main direct emissions for conventional and conservation wheat and popcorn crop rotations in 2017–2019 and 2018–2020. WW = Winter Wheat; PC = Popcorn. The year indicates the harvest year. Unless indicated otherwise, direct emissions of cover crops were attributed to conservation popcorn. Unit Conventional Conservation WW_2018 PC_2019 WW_2019 PC_2020 WW_2018 PC_2019 WW_2019 PC_2020 Direct emissions To air NH3 kg/ha 18.0 24.5 19.1 22.3 18.6 27.3 20.2 26.1 NO kg/ha 4.4 4.7 4.9 4.5 4.6 5.4 5.1 5.4 N2O kg/ha 5.7 6.3 6.6 6.2 5.7 8.3 6.6 9.7 CO2 kg/ha 129 209 143 414 129 208 143 410 To water NO3 kg/ha 133 111 133 111 111 111 111 111 1 in 2019 only fava bean and phacelia seeds, in 2020 both fava bean, phacelia and sorghum seeds https://doi.org/10.1371/journal.pone.0285586.t003 Table 4. Carbon (C) balance per crop and its components for wheat and popcorn estimated by the AMG model: Organic C input from cash-crop residues (Ccrop), mineralisation of soil C (Cmin), organic C input from cover crops (Ccover), organic C input from compost (Ccomp), and humified C sequestered Cseq) for the con- ventional and conservation rotations. Values are expressed as t C/ha/year at a depth of 0–30 cm. Values in brackets for Cseq of the conventional crops correspond to the effects of a 10% increase or decrease of their yields. Conventional Conservation

Year Crop Ccrop Cmin Cseq Ccrop Ccover Ccomp Cmin Cseq 2017–2018 Wheat 0.78 -1.15 -0.37 (-0.45/-0.29) 0.78 0.00 0.00 -1.15 -0.37 2019 Popcorn 0.95 -1.06 -0.11 (-0.20/-0.02) 0.95 0.76 0.91 -1.06 1.56 Mean 0.87 -1.11 -0.24 (-0.33/-0.16) 0.87 0.38 0.46 -1.11 0.59 2018–2019 Wheat 1.25 -1.38 -0.13 (-0.25/-0.01) 1.25 0.00 0.00 -1.47 -0.22 2020 Popcorn 0.92 -1.28 -0.36 (-0.43/-0.28) 0.92 1.71 1.39 -1.35 2.67 Mean 1.09 -1.33 -0.24 (-0.34/-0.15) 1.09 0.86 0.69 -1.41 1.23 https://doi.org/10.1371/journal.pone.0285586.t004 PLOS ONE Conservation agriculture reduces climate change impact of a crop rotation PLOS ONE | https://doi.org/10.1371/journal.pone.0285586 May 23, 2023 8 / 18