Carbon isotope discrimination and the factors affecting it in a summer maize field under different tillage systems

Based on two years of field experiments, under different soil tillage methods and straw management practices, which included conventional tillage (CT), subsoiling (SS), rotary tillage (RT), and no-tillage (NT), combined with either straw return (S) or straw removal (0), we characterized the dynamic changes in Δ¹³C among three height layers [upper (U, 240 cm above the ground), middle (M, 120 cm above the ground), and lower (L, 30 cm above the ground)] of the summer maize canopy. The Δ¹³C, the factors affecting it, and the relationships between Δ¹³C and soil water content (SWC), the leaf area index (LAI), canopy microclimate, and the CO₂ concentration were elucidated. The results indicated that the Δ¹³C of summer maize at the pre-filling stage was greater than that at the post-filling stage. Δ¹³C also varied at different heights, with the order of the Δ¹³C values being L > U > M. Among the different tillage methods, the Δ¹³C values were ordered SS_S > CT_S > RT_S > NT_S. SS_S and NT_S significantly increased the LAI; air temperature and relative humidity tended to gradually decrease with the increase in height of summer maize. Correlation analyses of the various influencing factors and Δ¹³C showed that SWC, LAI, air temperature, and CO₂ concentration were all positively correlated with Δ¹³C, in which LAI and air temperature were significantly or extremely significantly positively correlated with Δ¹³C. In addition, we show that Δ¹³C can be used as a prediction index for summer maize yield, providing a theoretical basis for future yield research that may save precious time in summer maize breeding efforts.