Characterization of greenhouse gas emissions from the alfalfa field in Loess Plateau during different planting years

Alfalfa is the main grass species in the farmlands returned to forests in Loess Plateau, whose planting area has been expanding. The experiment relied on the alfalfa (Medicago sativa) cultivation with different planting years (14, 12, and 5 a) in Loess Plateau. The objective of this study was to determine the effects of different planting years on soil greenhouse gas (CO₂, N₂O, and CH₄) emission of alfalfa, and we discuss the main mechanisms underlying the effects of soil temperature, soil moisture, and enzyme activities under different treatments on soil greenhouse gas emission. The results showed that all the treatments showed source effects for atmospheric CO₂ and N₂O, but sink effects for atmospheric CH₄; the flux of CO₂ peaked in June and bottomed in January, whereas the flux of N₂O peaked in July and bottomed in December, and the flux of CH₄ peaked in July and bottomed in March. According to the planting years, the amount of total CO₂ emission showed a pattern of 5 a > 14 a > 12 a; Compared with 12 and 14 alfalfa planting years, the CO₂ emission of 5 alfalfa planting years increased significantly by 16.60% and 13.01%, respectively. Total N₂O emission showed a pattern of 14 a > 12 a > 5 a, but there was no difference among the treatments. CH₄ absorption showed a pattern of 5 a > 12 a > 14 a, in opposite correlation with the increase in alfalfa planting years. The warming potential under 5 a was higher than those under 12 a and 14 a. Stepwise regression analysis results showed that the flux of CO₂ was significantly affected by soil temperature and moisture (R² = 0.870), that of N₂O was significantly affected by soil temperature (R² = 0.930), and the flux of CH₄ was significantly affected by soil temperature and urease (R² = 0.962).