Determination of optimum solum thickness of sloping cropland for maize plantation in an Entisol based on water use strategy and plant traits

Shallow purple soils (Entisols) were widely distributed in hilly areas of southwest China and were easily subjected to drought due to the weak soil water retention capacity, which largely reduced the crop productivity. However, the impact of solum thickness on ecological traits and water use strategies of crops remain poorly understood under the shallow Entisol environment. In this study, water use patterns and physiological adaptation of maize with different solum thicknesses were investigated to ascertain the optimum solum thickness based on an in-situ experiment. Isotopic (2H and 18O) tracing technique was used to investigate the water sources for maize growth in sloping croplands with solum of 20 cm (C1), 40 cm (C2), 60 cm (C3), 80 cm (C4), 100 cm (C5) thickness. The morphological and physiological characteristics were also discussed under different soil moisture regimes. The results showed the soil water storage capacity significantly increased with solum thickness. The isotopes of xylem water in shallow solum (≦40 cm) were more enriched, indicating more contributions from the bottom soil water, especially under dry regimes (62.3% for C1, 42.8% for C2, at Jul. 09). The water sources utilized by maize in the treatments with solum thickness greater than 60 cm were relatively evenly distributed throughout the soil profile (mean 19.3%, 19.6% and 17.6%, respectively, in the 0–10 cm, 10–20 cm, 20–40 cm soil layer), and the water niche breadth increased with the solum thickness. Maize in shallow solum (≦40 cm) had lower biomass and higher specific leaf area (SLA) to reduce transpiration rate and improved water use efficiency (WUEi) (highest for C1, P<0.05, mean 14.8 μmol·mmol−1 under dry regimes), accompanying by lower photosynthetic rate, especially during the grain mature stage. The thicker solum (>60 cm) had significantly higher yields than in shallower solum (P<0.05), indicating that higher soil water storage in the thicker Entisol solum provided the most important base for cereal production. These findings suggested that maize on shallow croplands tended to use deep soil water and reduce water loss by restraining transpiration during drought conditions. However, the adaptation strategy through maintaining high SLA, low leaf area and height only ensured limited crop yields. Solum thickness is a critical index for affecting water use strategies and growth of maize, we recommended that greater than 60 cm is the optimum solum thickness for the maize plantation in the hilly croplands of the purple soil regions.