Effects of rainfall intensity and combined application of organic and inorganic fertilizer on the dynamic process of nitrogen and phosphorus loss in typical vegetable plots in the Taihu Lake region

Although previous studies have focused on the coefficients of nitrogen and phosphorus runoff loss and the amounts of nitrogen and phosphorus lost from vegetable fields, there is a lack of research examining the dynamic process characteristics of nitrogen and phosphorus runoff loss from vegetable fields. In this study, we investigated the effects of rainfall intensity and the combined application of organic and inorganic fertilizers on the process of nitrogen and phosphorus loss from vegetabel fields in the Taihu Lake region. This study was conducted to provide scientific evidence for preventing and controlling nitrogen and phosphorus surface pollution in vegetable fields in the Taihu Lake region, where rainfall is frequent, and the amount of runoff is high. For this purpose, we established two intensities of field-simulated rainfall (55 and 110 mm∙h⁻¹) and set the following five fertilizer treatments: no nitrogen (−N), no phosphorus (−P), application of pure chemical fertilizer (N+P), combined application of organic and inorganic fertilizers (1/4MN+1/3MP), and application of pure organic fertilizer (MN+MP). The dynamic changes in the concentrations of each form of nitrogen and phosphorus in the runoff over time and the total nitrogen and phosphorus runoff loss under different rainfall intensities and fertilizer application treatments were monitored. The results revealed that compared with N+P treatment, the 1/4MN+1/3MP treatment reduced the initial runoff production time by 7.0 min and 5.3 min at 55 mm∙h⁻¹ and 110 mm∙h⁻¹ rainfall intensities. During the initial 25 min of runoff, the total nitrogen concentration of runoff among treatments fluctuated between 5.5 and 25.1 mg∙L⁻¹, whereas, during the 25−60 min of runoff, the total nitrogen concentration of different treatments fluctuated between 5.3 and 14.0 mg∙L⁻¹. The highest total nitrogen concentration in runoff was detected in N+P and MN+MP treatments at rainfall intensities of 55 and 110 mm∙h⁻¹, respectively; and 1/4MN+1/3MP treatment reduced the total nitrogen runoff loss by 21.9%−44.7%. During the 0–35-min period of runoff, the total phosphorus concentration of runoff in different treatments fluctuated between 0.31 and 3.20 mg∙L⁻¹, whereas during the 35−60 min of runoff, the total phosphorus concentration of runoff in different treatments fluctuated between 0.35 and 1.60 mg∙L⁻¹. The highest concentration of total phosphorus in runoff was detected in the MN+MP treatments at both assessed rainfall intensities. Compared with MN+MP treatment, 1/4MN+1/3MP treatment reduced the total phosphorus runoff loss by 65.4%−69.0%. In summary, the 1/4MN+1/3MP treatment applied in this study can prolong the duration of the initial loss of nitrogen and phosphorus, with initial periods of 0−25 and 0−35 min being identified as the key periods for controlling nitrogen and phosphorus loss in runoff, respectively. Moreover, the combined application of organic and inorganic fertilizers was found to effectively reduce the runoff loss of nitrogen and phosphorus.