Benefits of sustainable management practices on mitigating greenhouse gas emissions in soybean crop (Glycine max)

Soybean in Iran is managed intensively and represents an important source of greenhouse gas (GHG). Developing an agronomic management that reduces GHG emissions while still ensuring optimum soybean yields is strongly required. Field experiments were conducted in 2014 and 2015 growing seasons in the Golestan province (North of Iran) to evaluate different combinations of GHG mitigation strategies for soybean cultivation. Treatments included: two tillage methods [conventional tillage (CT) and no-tillage (ZT)], two residue management [wheat residue removed (R−) and wheat residue left on the system (R+)] and four levels of nitrogen (N) fertilization [0, 40, 80 and 120 kg N ha⁻¹ (N1, N2, N3 and N4, respectively)]. Soil moisture and temperature, GHG fluxes, yield and agronomic efficiency of nitrogen (AEN) were measured. The CT and R+ generally caused greater CO₂ fluxes than the ZT and R−, respectively. The maximum CO₂ flux occurred in August and this was about 362.6 and 284 mg m⁻² h⁻¹ under CT-R + -N4 and ZT-R + -N4. Soil CO₂ emissions were higher in fertilized than non-fertilized treatments. Wheat residue left on the system under ZT reduced N₂O emissions than CT, especially in N1. The cumulative N₂O emissions were maximum under CT-R + -N4 and minimum under ZTR + -N1 (1.00 and 0.41 kg N₂O-N ha⁻¹, respectively). In this study, there was no significant effect on CH4 emissions. Soybean yield was similar among tillage systems and residue management, while N3 in combination with wheat residue showed the highest response of seed yield. CO₂ emissions per unit of grain yield were the lowest under no-tillage associated with wheat residue mulch and nitrogen fertilizer. The results showed that GHG emissions could be mitigated in soybean crop in Iran. In particular, wheat residues left on the soil surface under no-tillage with 80 kg N ha⁻¹ showed a reduction of GHG emissions, maintain crop yield providing environmentally-friendly option.