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

2.1.1.1 Comparison to assess short-term effects. For the conservation agriculture crop rota- tion field data (i.e. farmer practices and yields) was collected on the Nataïs commercial farm (43˚52’ North, 0˚89’ East) in the Gers administrative department (south-western France) dur- ing two rotations: 2017–2019 and 2018–2020 (Table 1). Conservation agriculture results for each crop were calculated by averaging results for five wheat or popcorn fields. With slopes of 4–12%, the Nataïs farm’s soils are prone to erosion, even though they contain 25–30% clay. The study site has a temperate oceanic climate accord- ing to the Ko¨ppen-Geiger climate classification [20], with mean monthly precipitation of 54 mm and mean annual air temperature of 15.1˚C from 2018–2020. The 2018–2020 period was relatively warm, and the 30-year mean air temperature was 13.5˚C. Soil texture was mainly clay-limestone. The initial soil organic matter content (0–30 cm) at the experimental sites was 1.8–2.1%, and the initial SOC stock was 10.3–12.0 g/kg. In the conservation agriculture rotation, popcorn received compost of green waste along with mineral fertiliser, while winter wheat received only mineral fertiliser. Green waste con- sisted mainly of grass clippings, leaves and branch cuttings from private and public gardens. Popcorn was sown according to a technique called “green-tillage” as described hereafter. For the 2017–2019 rotation, winter wheat was sown in October 2017. In July 2018, after the wheat harvest, stubble was tilled twice, at 5 and 10 cm. In mid-September 2018, the soil was tilled

Table 1. Yields and inputs 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, inputs of cover crops were attributed to conservation popcorn. For conservation agriculture yields and inputs were based on field data, for conventional agriculture they were based on expert judgement. Unit Conventional Conservation WW_2018 PC_2019 WW_2019 PC_2020 WW_2018 PC_2019 WW_2019 PC_2020 Yield, cash crop t/ha 5.9 6.5 9.5 6.2 5.9 6.5 9.5 6.2 Input/Process Seeds, cash crop kg/ha 127 15 122 16 127 15 122 16 Seeds, cover crop 1 kg/ha - - - - - 120 - 154 Fava bean/phacelia cover crop dry matter t/ha - - - - - 3.1 - 3.9 Sorghum cover crop dry matter t/ha - - - - - - - 3 Compost of green waste t/ha - - - - - 6.5 - 9.9 N compost of green waste kg/ha - - - - - 52 - 79 N mineral fertiliser kg/ha 179 190 202 190 179 188 202 182 N total kg/ha 179 190 202 190 179 240 202 261 P2O5 total kg/ha 24 50 24 50 24 53 24 66 K2O total kg/ha 0 50 0 50 0 89 0 122 Diesel kg/ha 40 73 40 75 32 68 32 78 Irrigation water m3/ha - 1 659 - 2 396 - 1 659 - 2 396 Pesticide active ingredient kg/ha 5.2 2.8 6.6 2.8 5.2 3.7 6.6 3.7 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.t001

Table 2. Scenarios of conventional and conservation popcorn and wheat crop rotations. Scenario Agricultural system Cover crops Compost applied (t/ha) N applied (kg N/(ha) Conv Conventional Absent 0 190 Cons_CC Conservation sorghum and fava bean + phacelia 0 190 Cons_full Conservation sorghum and fava bean + phacelia 6.5 220 https://doi.org/10.1371/journal.pone.0285586.t002 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 4 / 18