The response of potato tuber yield, nitrogen uptake, soil nitrate nitrogen to different nitrogen rates in red soil.

Nutrient-deficient red soil found in the southern region of China is increasingly being used for potato crops to meet the demand for this staple food. The application of nitrogen fertilizer is necessary to support the production of higher tuber yields; however, the links between nitrate nitrogen and the nitrogen balance in red soil are unknown. A field experiment was conducted in Jiangxi Province in 2017 and 2018 to determine the effects of different nitrogen application rates, 0 kg ha-1 (N0), 60 kg ha-1 (N60), 120 kg ha-1 (N120), 150 kg ha-1 (N150), 180 kg ha-1 (N180), 210 kg ha-1 (N210), and 240 kg ha-1 (N240, the highest rate used by local farmers), on potatoes growing in red soil. Data on tuber yield, crop nitrogen uptake, and the apparent nitrogen balance from the different treatments were collected when potatoes were harvested. Additionally, the content and stock of nitrate nitrogen at different soil depths were also measured. Nitrogen fertilization increased tuber yield but not significantly at application rates higher than 150 kg ha-1. We estimated that the threshold rates of nitrogen fertilizer application were 191 kg ha-1 in 2017 and 227 kg ha-1 in 2018, where the respective tuber yields were 19.7 and 20.4 t ha-1. Nitrogen uptake in potato in all nitrogen fertilization treatments was greater than that in N0 by 61.2-237% and 76.4-284% in 2017 and 2018, respectively. The apparent nitrogen surplus (the amount of nitrogen remaining from any nitrogen input minus nitrogen uptake) increased with increasing nitrogen application rates. The nitrate nitrogen stock at a soil depth of 0-60 cm was higher in the 210 and 240 kg ha-1 nitrogen rate treatments than in the other treatments. Moreover, double linear equations indicated that greater levels of nitrogen surplus increased the nitrate nitrogen content and stock in soils at 0-60 cm depths. Therefore, we estimate that the highest tuber yields of potato can be attained when 191-227 kg ha-1 nitrogen fertilizer is applied to red soil. Thus, the risk of nitrate nitrogen leaching from red soil increases exponentially when the apparent nitrogen balance rises above 94.3-100 kg ha-1.