Effect of tillage method change on soil greenhouse gas emission and yield during winter-wheat growing season

Under long-term rotary tillage, soil bulk density, carbon decomposition and nutrient in sub-surface soil in the shallow plow layer significantly increase, but wheat growth and soil carbon sequestration become limited. However, subsoiling and deep plowing can break the bottom of the plow layer and reduce soil bulk density, which are conducive for good growth of plant root and absorption of nutrients to ensure high crop yield. The objectives of this study were to analyze changes in greenhouse gases emission and wheat yield after 16 years (2001-2016) of rotary tillage (X) treatment and conversion into other tillage treatments, including rotary tillage-deep plowing (XF) and rotary tillage-subsoiling (XS) treatments in 2016, and to determine the best rational tillage strategy. Carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄)emission fluxes in the three tillage treatments were sampled and measured using static chamber-gas chromatography. Soil temperature at the 0 cm depth, soil gravimetric moisture content, soil bulk density at different depths and other related factors were monitored during wheat growth period and winter wheat yield analyzed after harvest. The experimental results showed prominent high fluxes of CO₂ and N₂O one week after the three tillage treatments and during harvest, with minimum emission fluxes of CO₂and N₂O during winter period. Compared with XF treatment, X and XS treatments significantly increased CO₂ emission fluxes from the start of the three tillage treatments to the end of October. Compared with X and XF treatments, N₂O fluxes under XS treatment were significant high after tillage treatment, fertilization and irrigation. CH₄ fluxes fluctuated from November 2016 to February 2017, and became more stable from March 2017. From January 2017 to the harvest season, soil uptake of CH₄ under XS treatment was higher than those under XF and X treatments. The fields under the three tillage treatments during winter wheat growth were the sources of CO₂ and N₂O. The cumulative fluxes of the three tillage treatments served as CH₄ sink. In winter wheat fields, cumulative CO₂ emission was in order of XS > X > XF, with total CO₂ emissions of 5 241 kg·hm^-2, 5 160 kg·hm^-2 and 4 840 kg·hm^-2, respectively. Cumulative N₂O emission was in order of XS > XF > X, with total N₂O emissions of 4.38 kg·hm^-2, 2.39 kg·hm^-2 and 2.26 kg·hm^-2, respectively. Cumulative CH₄ sink was in order of XS > XF > X, with total CH₄ absorptions of 6.14 kg·hm^-2, 5.64 kg·hm^-2 and 3.70 kg·hm^-2, respectively. The contribution of cumulative greenhouse gases to CO₂-equivalents was expressed as XS > X > XF, which were 6.23 t·hm^-2, 5.66 t·hm^-2 and 5.32 t·hm^-2, respectively. Using deep plowing and subsoiling, soil organic matter decreased in the 0-10 cm soil depth, but increased in the 10-20 cm soil depth. Soil organic carbon was the main source of CO₂. Reduction in soil organic matter led to reduction in CO₂. Winter wheat grain yield under X treatment was higher than that under XS and XF treatments. Considering the changes in soil physical properties, greenhouse gas emission and wheat yield, XF treatment was the most suitable tillage practice. However, more and longer research work was needed to determine an ideal tillage treatment to ensure future ecological benefits and grain yield.