Combining no-till with rye (Secale cereale L.) cover crop mitigates nitrous oxide emissions without decreasing yield

Abstract

No-till (NT) often increases soil carbon (C) sequestration compared with conventional tillage (CT), yet its net effect on N2O emissions is controversial. Cover crops (CCs) adoption is promoted in NT systems because CCs growth curbs nitrate losses via leaching. However, incorporating CC residues into the soil may have positive or negative effects on N2O emissions depending on CC species and agro-ecosystem management. A better understanding of how tillage practices and CC species affect N2O emissions is therefore needed for the development of productive agroecosystems that contribute to climate change mitigation. The objectives of this three-year (2015–2017) field experiment on a Udertic Haplustalf soil in the Po Valley were to compare N2O emissions and crop yield of soybean under NT and CT, and to examine how contrasting residues from two CCs (rye, Secale cereale L. vs hairy vetch, Vicia villosa Roth) affect N2O emissions in NT soybean and maize. We hypothesized that N2O emissions would be lower with NT than with CT and with rye residues than with vetch ones. Nitrous oxide was continuously sampled using automatic chambers during three periods (emergence, N-fixation and maturity) over the soybean-cropping season in 2015 and during the entire cropping maize season in 2017. The DNDC model was calibrated (2015 data) and validated (2017 data), and then used to estimate the annual cumulative N2O emissions in different treatments. Overall, N2O emissions in NT were 40–55% lower than in CT, for both in situ measurements (Period I) and modelled estimations. These differences could be ascribed to the higher water-filled pore space (WFPS) and soil nitrate availability in CT than in NT. No-till also increased SOC content (28%; 0–5 cm) and earthworm abundance (5 times) compared with CT. Within NT systems, N2O emissions were 20–36% lower with rye CC than with vetch CC (P < 0.05), which was a consequence of the lower availability of soil mineral N under rye than under vetch due to the high C/N ratio of rye residues. Yield of soybean and maize under NT was higher with rye CC than with vetch CC. The combination of NT and rye CC that led to the lowest N2O emissions and highest yields should be recommended in the Po Valley region.