Optimizing nitrogen and irrigation application for drip irrigated sweet potato with plastic film mulching in eastern China

Excessive agricultural nitrogen application can lead to low nitrogen use efficiency, but the use of drip irrigation with plastic film mulching has been shown to improve crop yield, quality, and nitrogen use efficiency. Field experiments were conducted in 2020 and 2021 at the Special Potato Experimental Station, China Agricultural University, in Rizhao City, Shandong Province, China. The aim was to investigate the effects of different soil wetted percentages and nitrogen (N) application rates on soil nitrate-N (NO3-N) distribution and the growth of sweet potatoes (Ipomoea batatas L.). Three irrigation levels (no irrigation, P0; soil wetted percentage 30 %, P1; and soil wetted percentage 60 %, P2) and three N-application rates (90 kg hm−2, N1; 180 kg hm−2, N2; and 270 kg hm−2, N3) were designed. Various parameters such as soil water and nitrate-N content, dry matter, tuber grade, yield, quality, and nitrogen use efficiency were measured. The results showed that the soil water content was lower in the treatment with N-application rate of 180 kg hm−2 compared to the treatments with N-application rates of 90 kg hm−2 and 270 kg hm−2. Increasing nitrogen application rates led to an increase in soil nitrogen content and residue at a depth of 0–40 cm. The soil nitrate-N residue in the treatment with N-application rate of 180 kg hm−2 was significantly lower than that in the treatment with N-application rate of 270 kg hm−2 treatment. Sweet potato growth, tuber dry matter, number and weight of large and medium tubers, and yield initially increased and then decreased with increasing nitrogen application rates. In the soil wetted percentage of 60 % treatment, the tuber yield increased in the treatment with N-application rate of 180 kg hm−2 compared to the treatments with N-application rates of 90 kg hm−2 and 270 kg hm−2. A moderate nitrogen application rate improved tuber quality, including the contents of crude protein, starch, and soluble sugar. Nitrogen use efficiency, sweet potato nitrogen uptake amount, and nitrogen use efficiency initially increased and then decreased with increasing nitrogen application rates in soil wetted percentage of 60 % treatment. Irrigation increased nitrogen uptake efficiency and nitrogen use efficiency. In the treatment with N-application rate of 180 kg hm−2, nitrogen use efficiency in the soil wetted percentages of 30 % and 60 % treatments were higher than that in the treatment without irrigation. In conclusion, the optimal combination of 180 kg/hm2 and soil wetted percentage of 60 % was found to be suitable for sweet potato cultivation in the study area, considering factors such as soil nitrate-N residue, sweet potato yield and quality, and nitrogen use efficiency.