Negative pressure irrigation increases vegetable water productivity and nitrogen use efficiency by improving soil water and NO3–-N distributions

Negative pressure irrigation (NPI), which is a new subsurface irrigation technique, promotes vegetable yield, water productivity (WP), and nitrogen use efficiency (NUE). However, it is not clear how NPI improves vegetable growth, especially in terms of water supply characteristics and uniformities of soil water and nitrogen. In this study, a cucumber pot experiment that had 0 kPa (PW1), –5 kPa (PW2), –10 kPa (PW3), –15kPa (PW4), and traditional irrigation (PCK) treatments under nitrogen application (N1) and no application (N0) was conducted to reveal the water supply characteristics of NPI and its effect on vegetable growth. There are two main water supply characteristics: 1) automatically supplying irrigation water based on the consumption of soil water, and 2) keeping soil water content stable during the vegetable growth period. In addition, the relationship between vegetable growth and soil water and NO₃–-N distribution uniformities throughout the soil profile was investigated by carrying out two tomato field experiments. The treatments of one tomato experiment were NPI with –5 kPa (F1W) and furrow irrigation (F1CK). We also carried out NPI with –5 kPa (F2W), furrow irrigation (F2CK), and drip irrigation (F2D) in another tomato experiment. The results showed that cumulative water application under N1 was higher than under N0 in the PW1, PW2, and PW3 treatments in the cucumber experiment. Volumetric soil water content under the NPI system was more stable during the vegetative growth period than under traditional irrigation. The NPI system also increased yields under appropriate pressures (–10–0 kPa) compared to the PCK treatment in the cucumber experiment. The NPI in the two tomato experiments reduced fertilizer inputs and irrigation compared to furrow irrigation and drip irrigation. However, the irrigation method had no significant influence on the tomato yield in the two tomato experiments.