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

whereas the annual rate of SOC mineralisation depends on the soil (i.e. clay and lime contents, pH, C:N ratio of soil organic matter), soil tillage (type and depth), and meteorological condi- tions (i.e. mean annual air temperature and water balance). The model runs on a yearly basis at the field level to estimate SOC stock at a depth of 0–30 cm. To estimate the amount of crop biomass left on the field after harvest, crop yield is con- verted into above-ground biomass using a crop-specific harvest index. Below-ground biomass is estimated using an allometric relationship with aerial biomass [35]. Total (above- and below-ground) biomass minus crop yield equals the amount of biomass that provides fresh organic C inputs. In the comparison of the conservation and the conventional agriculture crop rotation field data for crop yields were available for conservation agriculture but not for con- ventional agriculture. We assumed that crop yields of the latter were identical to those of the former. This is not necessarily true, we therefore conducted a sensitivity analysis assessing the effects of a 10% increase or decrease of the yield of conventional crops on carbon storage and subsequent environmental impacts. For the comparison of three scenarios, changes in the SOC stock over 100 years were esti- mated using the SIMEOS-AMG tool [36, 37], which is a decision-support tool based on the AMG model designed to facilitate simulation of long-term changes in SOC. Model description and model inputs and outputs for AMG and SIMEOS-AMG are presented in the S1 and S2 Tables in S1 File.

3. Results 3.1 Characteristics of crop rotations Emissions of NH3, NO, and N2O were higher for the conservation popcorn crops (including cover crop) than for the conventional ones by 11%, 15%, and 32%, respectively, in 2019 and by 17%, 22%, and 58%, respectively, in 2020, whereas CO2 emissions were similar (Table 3). NH3 and NO emissions were higher for conservation wheat crops than for conventional wheat crops by 3% and 4%, respectively, in 2019 and for both gases by 6% in 2019. NO3 leaching was 17% lower for conservation wheat crops than for conventional wheat crops (Table 3). According to the AMG model, conventional wheat and popcorn sequestered -0.24 t C/ha for both the 2017–2019 and the 2018–2020 crop rotation (Table 4). A 10% decrease or increase of assumed yields of the crops in the conventional rotation resulted in a moderate (-0.33/-0.16 and -0.34/-0.15) variation of these values (Table 4). C sequestration of conservation wheat was similar to that of conventional wheat, while conservation popcorn, due to the cover crop and compost application, sequestered 1.6 (2019) and 2.7 (2020) t C/ha. Mean annual C sequestra- tion was 0.6 and 1.2 t C/ha for the 2017–2019 and 2018–2020 conservation crop rotations, respectively. The difference in annual C sequestration between the two systems was thus 0.83 and 1.47 t C/ha, with C input from compost of 0.46 and 0.69 t/ha, for 2017–2019 and 2018– 2020, respectively. For conservation popcorn, percentages of C input from crop residues, cover crop, and compost were 36%, 29%, and 35%, respectively, in 2019 and 23%, 43%, and

34%, respectively, in 2020. 3.2 Climate change impact of crop rotations The conservation crop rotation had higher GHG emissions than the conventional crop rota- tion (15% higher in 2017–2019; 25% in 2018–2020) and also sequestered much more C (-347% as much in 2017–2019; -608% in 2018–2020) (Fig 1). Consequently, conventional rotations had positive climate change impacts (7440 kg CO2-eq./ha in 2017–2019 and 8027 kg CO2-eq./ ha in 2018–2020), as well as the conservation rotation in 2017–2019 (2422 kg CO2-eq./ha). On the other hand, the conservation rotation in 2018–2020 had a negative climate change impact 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 7 / 18