Impact of soil thickness on productivity and nitrate leaching from sloping cropland in the upper Yangtze River Basin

Abstract

Shallow soils are widely distributed in mountainous and hilly areas due to severe soil erosion. However, the impacts of soil thickness on soil productivity, water and nutrient retention in shallow cropland soils have not been well documented. This study was conducted to investigate how soil thickness affects soil productivity and nitrate leaching from sloping croplands in the upper Yangtze River Basin, China. Free-drained lysimeters on sloping cropland with soil thicknesses of 20 (ST1), 40 (ST2), 60 (ST3), 80 (ST4) and 100 (ST5) cm were used to monitor soil moisture, surface runoff and nitrate leaching. The results showed that total crop yields during an entire winter wheat-summer maize rotation year (October 2017-September 2018) for ST1 to ST5 were 2.38 ± 0.07, 3.22 ± 0.01, 6.43 ± 0.61, 8.21 ± 0.56 and 8.58 ± 0.29 Mg ha−1, respectively. The annual cumulative total nitrogen (N) loss loadings via surface runoff and leaching for ST1 to ST5 were 21.09 ± 1.54, 13.08 ± 0.79, 5.61 ± 0.36, 3.49 ± 0.27 and 1.96 ± 0.22 kg N ha−1, respectively. The annual cumulative nitrate leaching loadings for ST1 to ST5 were 18.41 ± 1.07, 11.27 ± 0.56, 4.93 ± 0.45, 3.05 ± 0.32 and 1.66 ± 0.12 kg N ha−1, respectively, which accounted for more than 84 % of the cumulative total N loss through hydrological processes. This finding indicates that leaching dominates the hydrological N loss in sloping cropland. Moreover, significant differences were observed in yield-scaled nitrate leaching losses among ST1 (7.74 ± 0.62), ST2 (3.49 ± 0.18) and ST3 (0.78 ± 0.13 kg N Mg−1) (P < 0.05), while no significant differences were found among ST3 (0.78±0.13), ST4 (0.37±0.02) and ST5 (0.19±0.01 kg N Mg−1). This finding implies that if the soil thickness is greater than 60 cm, then it may be possible to maintain crop yields and mitigate nitrate leaching losses on sloping croplands. Therefore, a soil thickness of 60 cm is recommended as a threshold soil layer for basic water and nutrient retention as well as land reclamation and restoration of degraded cropland suffering from severe soil erosion. Soil thickness is a critical index for evaluating soil functions for water and nutrient retention, crop productivity improvement and agricultural non-point source pollution control.