Synergistic effects of tillage depth and nitrogen application rate on soil aggregate structure and water-nitrogen use efficiency of potato in the arid region

Objectives The effects of different tillage depths and nitrogen application rate combinations on soil aggregate structure and water-nitrogen use efficiency were studied for the efficient potatoe production in the arid region of southern Ningxia, China.

Methods A three-year (2021−2023) field experiment was conducted consecutively. The treatments consisted of four tillage methods (plowing to a depth of 20 cm, subsoiling to depths of 30 cm, 40 cm, and 50 cm) and four N application rates (0, 90, 180, and 270 kg/hm²), with plowing to a depth of 20 cm without nitrogen application serving as the control (CK). Measurements of soil aggregates, soil moisture content, and potato yield were taken at 0, 60, 90, and 150 days after potato planting, and water use efficiency and nitrogen use efficiency were calculated.

Results Compared with CK, subsoiling to depths of 30−50 cm combined with N application rates of 90−180 kg/hm² increased the content of >0.250.25 mm aggregates in the 0−60 cm soil layer over three years, with an increase of 3.9%−21.3% compared to the control. Under the same tillage depth, an N application rate of 180 kg/hm² significantly increased the content of >0.25 mm soil aggregates in the 0−60 cm soil layer during normal (2021) and drought years (2022), as well as in the 0−20 cm and 40−60 cm layers during an extreme drought year (2023). However, in the 20−40 cm layer during the extreme drought year, the optimal N rate for promoting >0.25 mm aggregates was 90 kg/hm². With the same N application rate, in the normal year, the content of >0.25 mm aggregates increased with the increase of tillage depth. In the drought year, subsoiling to 50 cm was most effective in the 0−40 cm layer, while subsoiling to 30 cm performed best in the 40−60 cm layer. During the extreme drought year, subsoiling to 40 cm consistently yielded the highest >0.25 mm aggregate content across the 0−60 cm soil profile. Increasing tillage depth enhanced soil water storage during key potato growth stages, whereas higher N application rates reduced it. Under conditions without nitrogen application, the depth of deep loosening that results in the highest water storage was 50 cm in normal and extreme drought years, and 30 cm in drought years, representing an increase of 10.5% to 31.1% compared with 20 cm of conventional tillage. Subsoiling at 30−40 cm combined with N 180 kg/hm² significantly increased potato yield−subsoiling at 30 cm was best in normal years, and subsoiling at 40 cm in drought years, improving yield by 59.2%−90.2% over the control. Subsoiling at 40 cm enhanced marketable tuber rates; the optimal N rate was 90 kg/hm² in a normal year and 180 kg/hm² in a drought year, with increases of 5.1%−21.4%. Regression analysis showed that subsoiling at depths of 36.1−37.7 cm with N application rates of 145.5−152.2 kg/hm² maximized potato yield. Subsoiling at depths of 30−40 cm combined with an N application rate of 180 kg/hm² significantly increased water use efficiency and growing-season precipitation use efficiency by 59.2% and 91.6%, while subsoiling at depths of 30−40 cm with an N application rate of 90 kg/hm² was more beneficial for improving nitrogen agronomic efficiency (3.8−13.0 times higher) and nitrogen partial factor productivity (19.0%−24.1% increase). Correlation analysis revealed that >0.25 mm soil aggregate content was positively correlated with potato yield, commercial potato percentage, and water-nitrogen use efficiency.

Conclusions The combination of suitable tillage depth and nitrogen application rate improves soil structure, enhances water retention, and boosts potato yield and water-nitrogen use efficiency. The effect is significantly influenced by rainfall types. Subsoiling at depths of 30−40 cm with an N application rate of 145−180 kg/hm² is recommended in normal and drought years to achieve high yield and efficient resource use for rainfed potato production in southern Ningxia.