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

with a spring-toothed harrow, to a depth of 20 cm on the future popcorn row with one set of teeth, and to a depth of 8 cm with a second set of teeth. Some of the soil of the future inter row was moved to the future row to form a ridge. In the same operation, winter fava bean (98.6%) and phacelia (1.4%) was sown to establish a cover crop in the future inter-row. During winter, the soil of the ridge settled, which created a seedbed for the popcorn. In April 2019, herbicide application killed the cover crop. Subsequently, popcorn was sown on the ridges using a machine that flattened the cover crop and simultaneously applied fertiliser (diammonium phosphate, 58 kg/ha), insecticide (cypermethrin, 92 g/ha), and molluscicide (methaldehyde, 79 g/ha) on the row. Popcorn was sown on ridges because this favours germination due to higher soil temperature. After harvesting the popcorn in October 2019, winter wheat was directly sown and mineral fertiliser was applied. For the 2018–2020 rotation, the operations were iden- tical, but two cover crops were grown: a sorghum cover crop was sown at the beginning of July, and chopped and left in the field in mid-September before sowing the fava bean-phacelia cover crop. For the conventional agriculture rotation farmer practices reflected dominant practices for popcorn and winter wheat rotations in the Gers department according to expert judgment (S. Hypolite, Agro d’Oc, pers. comm.) (Table 1). Popcorn and wheat yields were assumed to be the same as those in the conservation agriculture rotation based on expert judgement. Both

crops received only mineral fertilisers. For popcorn, the soil was ploughed in autumn and superficially tilled twice in spring. After the popcorn harvest, stubble was tilled at a depth of 10 cm before sowing the winter wheat. No cover crops were grown. All other farming practices were the same as in the conservation agriculture system. Wheat yielded 5.9 and 9.5 t/ha in 2018 and 2019 respectively, whereas popcorn yielded 6.5 and 6.2 t/ha in 2019 and 2020, respectively (Table 1). The fava bean/phacelia cover crop pro- duced 3.1 and 3.9 t dry matter/ha in 2019 and 2020, respectively, whereas the sorghum cover crop produced 3 t dry matter/ha in 2020. Conservation popcorn received 6.5 and 9.9 t/ha of compost in 2019 and 2020, respectively, corresponding to 22% and 30% of total N applied, respectively. Compared to conventional popcorn, conservation popcorn (including the cover crop) used 26% (2019) and 37% (2020) more fertiliser N, 6% less (2019) and 5% more (2020) diesel, and 33% (2019) and 32% (2020) more pesticide active ingredient. Both conservation wheat crops (2018 and 2019) used 20% less diesel than the conventional ones. Yields and amounts of cash-crop seed, irrigation water, mineral fertiliser, and pesticide applied were identical for conventional and conservation wheat crops. 2.1.1.2 Comparison of three scenarios to assess long-term outcomes. In the scenario-model- ling approach, a conventional scenario (Conv) and two conservation scenarios (Cons_CC and Cons_full) were compared (Table 2). The Conv scenario corresponded to the conventional

popcorn and wheat rotation described previously. The two conservation scenarios were based on Nataïs’ conservation agriculture rotation. The Cons_CC scenario used sorghum and fava bean + phacelia as cover crops (as in the 2018–2020 cropping system), whereas the Cons_full scenario used these cover crops as well as 6.5 t/ha of compost of green waste (as in the 2017– 2019 cropping system). For all scenarios, yields were set at 6.8 t/ha for winter wheat and 6.3 t/ ha for popcorn, based on expert judgement for long-term average yield level of these crops in the region. 2.1.2 Life cycle inventory. Life cycle inventories (LCIs) were calculated using the MEAN- S-InOut web application v3.0, which is a customized agricultural LCA tool that generates LCIs of agricultural production systems [21]. It contains forms to guide data entry, a reference data set for the pollutant emissions and resource use of main inputs of agri-food systems, analytical models to estimate direct pollutant emissions and resource use, and an export function that 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 5 / 18