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

Tuber yield

Potato tuber yield increased gradually under 0 to 150 kg ha⁻¹ of applied nitrogen (Fig. 1). Compared with the yield in N0, the yields in N60, N120 and N150 were greater by 16.1%, 21.5% and 67.9%, respectively, in 2017 and 18.2%, 27.4% and 44.9%, respectively, in 2018. However, at nitrogen rates of more than 150 kg ha⁻¹, yield did not significantly differ. Furthermore, the fitted parabolic equation of each dataset from 2017 and 2018 showed maximum tuber yields of 19.7 and 20.4 t ha⁻¹, respectively, where the nitrogen rates were 191 and 227 kg ha⁻¹, respectively.

Potato nitrogen uptake

Nitrogen uptake in potato differed among all the treatments (Table 1). In both 2017 and 2018, the highest nitrogen uptake was attained in the N240 treatment, and the lowest uptake was attained in the N0 treatment. Compared with that in N0, the uptake levels measured in the N60, N120, N150, N180, N210 and N240 treatments were greater by 61.2%, 100%, 115%, 144%, 142% and 237%, respectively, in 2017. Meanwhile, the uptake levels measured in the N60, N120, N150, N180, N210 and N240 treatments were greater than that in the N0 treatment by 76.4%, 98.5%, 142%, 198%, 190% and 284%, respectively, for 2018. There were no significant differences among the N120, N150, N180 and N210 treatments in 2017. There were also no significant differences among the N150, N180 and N210 treatments in 2018.

Apparent nitrogen balance

The apparent nitrogen balance in potato production in red soil increased with increasing nitrogen application rates (Fig. 2). For both 2017 and 2018, all treatments with nitrogen application exhibited a surplus of nitrogen, while the N0 treatment exhibited a deficiency of nitrogen (18.6 and 18.0 kg ha⁻¹ in 2017 and 2018, respectively). Furthermore, the surplus was highest in the N240 treatment. Compared with that in the N60 treatment, the nitrogen surplus levels in the N120, N150, N180, N210 and N240 treatments were 176%, 266%, 348%, 450% and 490% greater in 2017 and 198%, 277%, 347%, 459% and 505% greater in 2018, respectively.

Nitrate nitrogen content and stock at different soil depths

Nitrogen application increased the nitrate nitrogen content at all three depths of soil samples (Fig. 3). Generally, as nitrogen fertilizer rates increased, the nitrate nitrogen content increased in both 2017 and 2018 samples. For the same treatments among the three depths of soil in 2017, there were no significant differences in nitrate nitrogen contents. The result for 2018 was similar to that for 2017, with the exceptions of the N210 and N240 treatments, both of which showed higher nitrate nitrogen contents at 20–40 cm than at 0–20 and 20–40 cm. The nitrate nitrogen contents in N210 and N240 were higher than the contents in the other treatments among the three soil depths. Compared with the content in the N0 treatment, the nitrate nitrogen contents at 0–20 cm in the N210 and N240 treatments were 185% and 193% greater in 2017 and 218% and 268% greater in 2018, respectively. The nitrate nitrogen contents at 20–40 cm in the N210 and N240 treatments were 186% and 188% greater in 2017 and 339% and 367% greater in 2018, respectively. The nitrate nitrogen contents at 40–60 cm in the N210 and N240 treatments were 209% and 271% higher in 2017 and 212% and 318% higher in 2018, respectively.

There was less consistency in significant differences in nitrate nitrogen content among treatments with less than 210 kg ha⁻¹ nitrogen fertilizer applied. In the 2017 samples, no significant differences were observed among N0, N60, N120, N150 and N180, with the exception of the nitrate nitrogen content in N180 being higher than those in N0, N60, N120, and N150 at the 0–20-cm soil depth. In the 2018 samples, the nitrate nitrogen contents in N180 and N150 were higher than those in N0, N60, and N120 at the 0–20- and 20–40-cm soil depths. In contrast, the nitrate nitrogen content in the N180 treatment at the 40–60-cm soil depth was not lower than that in the N240 and N210 treatments. Additionally, no significant differences were observed at the 40–60-cm depth among the N0, N60, and N120 treatments, but the nitrate nitrogen contents of the N150 and N180 treatments were higher than the content of the N0 treatment by 57.3% and 111% in 2017 and by 147% and 234% in 2018, respectively.

The nitrate nitrogen stock results were similar to the nitrate nitrogen content results. Table 2 shows that there were no significant differences in nitrate nitrogen stock among the three depths of soil in 2017. In 2018, the nitrate nitrogen stocks at 20–40 cm were higher than those at 0–20 and 20–40 cm. Compared with those in 2017, the nitrate nitrogen stocks at 0–60 cm (sum of measurements from the three depths) in 2018 were greater due to the application of nitrogen fertilizer. The nitrate nitrogen stock was higher in the nitrogen application treatments among the three soil depths than in the no-nitrogen fertilizer treatment (Table 2). A general trend of higher levels was also observed in the N210 and N240 treatments compared with the other treatments. The nitrate nitrogen stock at 0–60 cm was greater in the N210 and N240 treatments than in the N180 treatment by 31.6% and 41.6% in 2017, respectively, and 30.5% and 51.6% in 2018, respectively. For the three soil depths in 2017, there were no significant differences among the N0, N60, N120 and N150 treatments, but their nitrogen stocks were lower than that of the N180 treatment (except for that at the 40–60 cm depth). The results in 2018 revealed no significant differences among the N0, N60 and N120 treatments, but their nitrogen stocks were all lower than those in the N150 and N180 treatments.

The relationship between the apparent nitrogen balance and nitrate nitrogen stock

The nitrate nitrogen stock at the 0–20-, 20–40- and 40–60-cm soil depths increased exponentially with the apparent nitrogen balance (Fig. 4). The relationship at each of the three depths showed a gradual rate of increase in the nitrate nitrogen stock as the values calculated for the apparent nitrogen balance increased.

The relationship between the apparent nitrogen balance and nitrate nitrogen stock at the 0–60-cm soil depth was fitted by double linear equations in both 2017 and 2018 (P < 0.01, Fig. 5). The rates of increase in nitrate nitrogen stocks in 2017 and 2018 were 0.34 and 0.50 kg ha⁻¹, respectively, as the apparent nitrogen balance increased up to 100 and 94.3 kg ha⁻¹, respectively. However, beyond those values of the apparent nitrogen balance, the slopes of the equations were steeper, indicating faster rates of increase for 2017 and 2018 at 1.73 and 1.92 kg ha⁻¹, respectively.

Source: Ecology - nature.com