*3.3. Dissipation of Fluopyram in Soil*

To investigate the uptake/translocation of pesticide from soil to rotational crops, it is necessary to understand the residual characteristics in the soil by examining pesticide application during the cultivation of the preceding crops. According to the OECD guidelines [14], a suspension solution of fluopyram should be prepared at the maximum label rate (dilution factor 4000) and applied to bare soil for the maximum number of applications (three times). However, the guidelines recommend a single application of the pesticide suspension at the maximum seasonal rate, rather than multiple applications for practical purposes. Therefore, in this study, fluopyram was applied to soil at a treatment level of 0.06 g a.i./m2. The OECD guidelines suggested plant-back intervals (PBIs) of 30 and 60 days. However, considering the domestic cropping system with a short uncultivated period, the PBI in this study was set at 30 days. The residual amount of fluopyram in soil was calculated in dry weight by correcting for moisture content. The moisture content of soil samples was in the range of 16.2–28.4%. The average residues at DAP 0 were 0.94 and 0.96 mg/kg in trials A and B, respectively, which decreased to 0.80 and 0.69 mg/kg at DAP 48, representing 84% and 72% of the initial residual amount, respectively. The DT50 values of fluopyram in soil were 231 (95% CI, 139–347) days and 87 (95% CI, 63–173) days in trials A and B, respectively. These were 2.4–14.6 times longer than 15.8–24.8 days (watermelon) and 36 days (tomato and bell pepper) in soil during fruit vegetable cultivation [7,21]. Generally, pesticides degrade and dissipate in soil owing to biological factors, such as plants and microorganisms, and non-biological factors, including rainfall, irrigation, hydrolysis, and

photolysis [2,22]. However, our results showed that scallion cultivation gave no significant reduction in the fluopyram residue in the soil.

#### *3.4. Uptake of Fluopyram by Scallions from Soil*

The residual levels of fluopyram in scallions are shown in Table 4. Fluopyram residues at DAP 20 were 0.35 mg/kg and 0.22 mg/kg in trials A and B, respectively. As the plants reached maturity at DAP 48, residual levels decreased to 0.22 mg/kg and 0.15 mg/kg in trials A and B, which represented 61% and 69.5% of residues at DAP 20, respectively. The dissipation constants calculated using regression analysis were 0.014 (trial A) and 0.016 (trial B), respectively, and there was no significant difference between them (Figure 2A).

**Table 4.** Residues of fluopyram in field soil and scallions.


<sup>1</sup> Mean ± standard deviation of triplicate samples.

**Figure 2.** Time-dependent residual patterns of fluopyram in scallions. ((**A**) including dilution effect; and (**B**) excluding dilution effect).

Pesticide residue in crops decreases due to reduction factors such as respiration, volatilization, metabolism, and weight increase through growth [23,24]. In contrast to previous research indicating an increase in residual amounts of fluopyram in tomato and pepper leaves during the cultivation period [7], this study found that decreased fluopyram residual levels in scallions were primarily due to the dilution effect resulting from the 9-fold increase in weight over the 24-day growth period (Figure 1). The residual amount (mg/individual) was calculated to determine the dissipation characteristics, while excluding the dilution effect. As shown in Figure 2B, there was a 5.6 to 6.4-fold increase in individual residues during the 24-day growth period. These results indicated that fluopyram uptake by the roots was greater than dissipation by metabolism, respiration, and other processes. The water solubility and the octanol/water partition coefficient (log Kow) of pesticides are important indicators of their mobility in soil and transportation in plants, since pesticides dissolve in the soil water, also known as the bioavailable portion of soil, and are taken up by plant roots and translocated to the aboveground tissues through

xylem [4]. Fluopyram is classified as readily soluble, with a water solubility of 16 mg/kg and hydrophobic with log Kow of 3.3 [10]. Considering its physico-chemical properties, fluopyram has a higher tendency to accumulate in roots than to be transported from roots to shoots. However, the gradual increase in individual residue levels over time can be attributed to the stability of fluopyram, which does not degrade and instead accumulates in the plant tissue. Therefore, fluopyram has the potential to accumulate through uptake and translocation.
