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 Δ(13)C 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 Δ(13)C, the factors affecting it, and the relationships between Δ(13)C and soil water content (SWC), the leaf area index (LAI), canopy microclimate, and the CO(2) concentration were elucidated. The results indicated that the Δ(13)C of summer maize at the pre-filling stage was greater than that at the post-filling stage. Δ(13)C also varied at different heights, with the order of the Δ(13)C values being L > U > M. Among the different tillage methods, the Δ(13)C 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 Δ(13)C showed that SWC, LAI, air temperature, and CO(2) concentration were all positively correlated with Δ(13)C, in which LAI and air temperature were significantly or extremely significantly positively correlated with Δ(13)C. In addition, we show that Δ(13)C 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.