I. Experimental Design of Integrated Water–Carbon–Nitrogen Regulation

In protected vegetable production, traditional furrow irrigation not only wastes water and fertilizer but also increases air humidity, which in turn raises the incidence of pests and diseases. Compared with traditional furrow irrigation, drip irrigation can more evenly and effectively supply water and nutrients within the crop root zone, thereby reducing water and fertilizer inputs and lowering the incidence of plant diseases. This ultimately allows for reduced fertilizer and pesticide use.
Furthermore, in response to the problem of low water and nutrient retention caused by an imbalanced soil carbon-to-nitrogen ratio (carbon deficiency) in protected soils, a “carbon regulation of nitrogen” approach can effectively save fertilizer. To this end, the research team established a long-term fixed-position trial of integrated water–carbon–nitrogen regulation at the Wuqing experimental base of the Tianjin Academy of Agricultural Sciences, using tomatoes as the test crop. The trial design is shown in Table 1. Compared with traditional furrow irrigation, drip irrigation can reduce nitrogen fertilizer input by 1,000 kg/ha annually (Table 1).

Table 1: Experimental Design of Integrated Water–Carbon–Nitrogen Regulation

II. Integrated Water–Carbon–Nitrogen Regulation Improves Tomato Fruit Quality While Maintaining Yield

Compared with traditional furrow irrigation, drip irrigation had no significant effect on most fruit quality indicators (vitamin C content, soluble sugars, titratable acidity, soluble solids, nitrate, nitrite, and flesh firmness; Figures 1A, B), but significantly reduced lycopene content (Figure 1C), which is a naturally occurring antioxidant pigment in ripe tomatoes. In addition, although drip irrigation significantly reduced water and nitrogen inputs (Table 1), it did not affect fruit yield (Figure 1D).

Figure 1. Effects of different water–carbon–nitrogen treatments on nitrate (A), nitrite (B), lycopene content (C), and fruit yield (D) in tomatoes.
(Note: *P < 0.05, **P < 0.01; different letters indicate significant differences at the 0.05 level.)

Compared with the untreated control, adding straw and biochar to the soil had no significant effect on most fruit quality indicators (vitamin C, soluble sugars, titratable acidity, soluble solids, and flesh firmness), but significantly reduced nitrate and nitrite contents (Figures 1A, B). Biochar application showed a more significant reduction in nitrite content in fruit (Figure 1B).

Under drip irrigation, adding straw to the soil had no significant effect on lycopene content, whereas under furrow irrigation, straw addition significantly reduced lycopene content, indicating an interaction effect between irrigation method and straw treatment. Furthermore, carbon regulation treatments in the soil did not affect fruit yield (Figure 1D).

III. Advantages of Integrated Water–Carbon–Nitrogen Regulation

1.Integrated water–carbon–nitrogen regulation can significantly reduce water and fertilizer inputs, with nitrogen fertilizer application reduced by 1,000 kg N/ha/year and irrigation water reduced by 600 mm/year.
2.Integrated water–carbon–nitrogen regulation can significantly reduce nitrate and nitrite contents in tomato fruits, and this effect is mainly achieved through regulating soil carbon content.
3.Although integrated water–carbon–nitrogen regulation significantly reduces water and nitrogen inputs, it does not cause yield reduction, thus achieving stable yield.

IV. Potential Issues of Integrated Water–Carbon–Nitrogen Regulation

Although integrated water–carbon–nitrogen regulation does not affect most fruit quality indicators, it is detrimental to the accumulation of lycopene in tomato fruits, and this negative effect is mainly caused by drip irrigation.