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

Highlights

• No-till (NT) increases soil organic C (28%) and earthworm abundance (5 times) compared with conventional tillage (CT).

• Using rye (Secale cereale L.) instead of vetch (Vicia villosa Roth.) cover crop under NT decreases N₂O and increases yield.

• NT reduces N₂O emissions (51%) compared with CT in well-drained silty clay soils.

• Replacing CT with NT in intensive agro-ecosystems can mitigate N₂O emissions without reducing 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.

Introduction

Nitrous oxide (N₂O) is a major greenhouse gas (GHG), with a global warming potential 265 times that of carbon dioxide over 100 years (IPCC, 2014), and is the largest ozone-depleting substance emitted by human activities (Ravishankara et al., 2009). Agricultural soils are the largest source of N₂O, accounting for 45% of the total current emissions (Cayuela et al., 2017) and an estimated contribution of 59% of total emissions by 2030 (Hu et al., 2015). The majority of N₂O emissions are produced through denitrification and nitrification, which are mainly controlled by substrate availability, such as ammonium (NH₄⁺), nitrate (NO₃⁻), labile carbon (C), and oxygen concentration (Beheydt et al., 2008). As soil-crop management (e.g. tillage intensity, nitrogen (N) fertilization, irrigation, and crop residue retention) regulates these soil factors, agriculture has a diversity of means to mitigate N₂O emissions (Abalos et al., 2013; IPCC, 2014).

Conservation tillage systems (i.e. no-till and reduced tillage) have been largely promoted as suitable practices to offset GHG emissions due to their ability to sequester C in soils (Tabaglio et al., 2009). However, reports on the effect of no-till (NT) on N₂O emission have been contradictory: some studies found that N₂O fluxes were higher in NT than in conventional tillage (CT) under imperfectly drained clay-loam soils when NT increased soil compaction (Ball et al., 1999); in long-term field experiments, under well-drained soils, NT did not increase (Jantalia et al., 2008) or even decreased N₂O emissions (Omonode et al., 2011). Soil compaction increases during the transition from CT to NT (Alvarez and Steinbach, 2009; Soane et al., 2012; Fiorini et al., 2018). This tends to decrease soil porosity (Palm et al., 2014), leading to anoxic conditions that promote N₂O losses (Linn and Doran, 1984). Conversely, the increase of soil organic matter (SOM) concentration and macro aggregate water-stability in the uppermost soil layer, under NT, may decrease anaerobic conditions and improve soil gas diffusivity, resulting in lower N₂O emission than under CT (Mutegi et al., 2010; Plaza-Bonilla et al., 2014). No-till can also increase earthworm abundance, which in turn may increase N₂O emissions by increasing N mineralization (Lubbers et al., 2013), or decrease N₂O due to their burrowing activity that improves water infiltration and decreases water content of soil in the upper layers. Further studies are needed to shed light on the conditions under which NT represents a viable option to simultaneously increase SOC while mitigating N₂O emissions (van Kessel et al., 2013).

Adoption of NT is highly debated, particularly for its potential negative effects on crop yield (Pittelkow et al., 2015). However, combining NT with other practices of conservation agriculture, such as the use of cover crops (CCs), could improve crop production. Cover crops may affect N₂O emissions by regulating key soil factors (Mitchell et al., 2013), including the availability of mineral N and C sources for the soil microbial communities, soil pH, soil structure and microbial community composition (Abalos et al., 2014; Maris et al., 2018). Selecting adequate CC species may decrease soil mineral N and water content, thus reducing N₂O emissions. For instance, legume CCs, having N rich residues, decrease N fertilizer need in subsequent crops, thus potentially lowering N₂O emissions. On the other hand, non-legume CCs may offer a better option than legumes to capture excess NO₃⁻ in the soil, increasing plant biomass and improving soil structure, which in turn decrease N₂O (Barthes et al., 2006). The chemical composition of CC residues significantly affects N₂O emissions (Aulakh et al., 2001; Millar and Baggs, 2004; Garcia-Ruiz and Baggs, 2007): a low C/N ratio (e.g. legume crops) may increase N₂O emission compared with a high C/N ratio (e.g. grasses) (Toma and Hatano, 2007; Petersen et al., 2011).

The objectives of this study were to measure the effect of contrasting tillage systems (NT and CT) on N₂O emissions during soybean (Glycine max L. Merr.), and to examine how CC residues (rye vs hairy vetch) affect N₂O emissions under NT during soybean and maize (Zea mays L.) cultivation. The following hypotheses were tested: (i) N₂O emission are lower under NT with rye CC than in CT without CCs, and (ii) N₂O emissions under NT are higher with a legume (hairy vetch) CC than with a grass (rye) CC, due to the low C/N ratio of the legume residue. Nitrous oxide measurements were carried out during two field monitoring campaigns and cumulative fluxes were estimated using the DNDC model (Li et al., 1992), which was previously calibrated and validated with field data as explained below. The DNDC has been extensively used to simulate N₂O emissions, particularly under contrasting agronomic practices (e.g. Uzoma et al., 2015; Abalos et al., 2016).