Orchard-floor management methods include retaining specific types of weeds in a strip or placing manure and planting non-native grass on the orchard floor. The primary objectives are to suppress weeds, but also to reduce herbicide use while improving the soil’s physical, chemical, and biological functions. Recently, planting groundcover has been included among the environmental-friendly strategies of orchard-floor management [1
In Taiwan, we apply and promote groundcover planting especially in deep-rooted orchards, usually in mountainous regions. This management type can increase the soil’s organic carbon and nutrient levels, including nitrogen, exchangeable potassium, calcium, and magnesium, although the nutrition levels in the crop may not be significantly increased [2
]. The technique can modify the soil’s physical properties, including the water infiltration rate and macropore content [4
]. The changes in soil characteristics with groundcover planting may greatly affect the distribution of pesticides, especially high water-soluble pesticides such as neonicotinoid insecticides. However, we have little knowledge of this effect.
Neonicotinoid insecticides are systemic insecticides that, regardless of their route of entry, can be distributed throughout the plant and harm feeding insects [6
]. They are applied by seed treatment, foliar sprays, soil drenches, granules, and injection or irrigation systems [7
]. The sales of imidacloprid, thiamethoxam, clothianidin, acetamiprid, thiacloprid, and dinotefuran are the highest among the neonicotinoids in the United States [8
]. The global market share of neonicotinoids was greater than 25% in 2014; in 2012, thiamethoxam, imidacloprid, and clothianidin accounted for almost 85% of the total neonicotinoid sales for crop protection [9
]. In Taiwan, imidacloprid, acetamiprid, and dinotefuran are promoted to farmers to prevent and control thrips in vineyards [10
Neonicotinoids are quickly dissipated in soil [11
]. The half-life of dinotefuran is 16.5 to 21.7 days [12
], whereas with imidacloprid, only 130 days is required for 652 μg/kg to dissipate to 11 μg/kg in the field [13
]. Nevertheless, long-term accumulation and persistence in water and soil samples were reported. Levels higher than 0.1 μg/kg of imidacloprid were detected in soils not planted with imidacloprid-coated seeds for one year [11
]. Neonicotinoids are potential groundwater contaminants; the sorption coefficient (Koc) of dinotefuran is 30, whereas that of imidacloprid is 262, and the water solubility is 39,800 and 514 mg/L, respectively [14
]. Since the mid-2000s, studies raised concerns that neonicotinoids may have a negative effect on the non-target organisms, such as honeybees and bumblebees. The European Food Safety Authority (EFSA) assessed the risk of clothianidin, imidacloprid, and thiamethoxam, and concluded that the application of these compound poses a high risk to bees [16
]. Planting groundcover could reduce the environmental risk of neonicotinoids more than hydrophobic pesticides.
This study aimed to investigate the change in distribution of neonicotinoids in a groundcover-planted grape vineyard. The groundcover plants were Arachis pintoi
Krap. and Greg. and Clinopodium brownei
(Sw.) Kuntze, two intensively promoted and cultivated groundcovers in Taiwan, to enhance soil properties in the vineyard. Arachis pintoi
is a legume groundcover [3
], whereas Clinopodium brownei
(Sw.) Kuntze was recently promoted for its potential as an insect repellent with a unique peppermint smell. We hoped to obtain in-depth knowledge about the effect of groundcover planting on the dissipation of the neonicotinoids dinotefuran and imidacloprid in the vineyard, and thus provide a reference for policy-making and groundcover management in vineyards.
This study investigated the difference in neonicotinoid dissipation in a grape vineyard by planting two different groundcovers for orchard-floor management. After one day of pesticide spraying, the highest dinotefuran residue concentration was in the 0- to 15-cm soil in the CF, but was in lower levels in the MF and PF; after four days of pesticide application, the highest imidacloprid residue concentration was in the 0- to 30-cm soil in the CF. Imidacloprid was not retained in the 30- to 45-cm soils in the PF, but in the MF, residue was detected in 30- to 45-cm soil in the second and third samplings, which indicates a different distribution of neonicotinoids with different groundcover plants. The dinotefuran absorption was greater with A. pintoi than C. brownei, and the imidacloprid absorption was greater with C. brownei. A. pintoi may have a high metabolic rate toward the two neonicotinoids in the field, and can increase the SOM content in lower-layer soil, for a preferable choice as groundcover vegetation.
Pesticides can be retained in the top layer of soils throughout a growing season [24
]. In our study, the phenomena could be observed in our bare land control, showing a high amount of dinotefuran retained in the surface soil right after application, and in the imidacloprid retained almost throughout the experiment. The different distribution of the two neonicotinoids in the bare land control could be explained by the different physical properties of the two pesticides, such as Koc and water solubility [14
]. Dinotefuran has better water solubility than imidacloprid, which may explain the higher plant absorption and higher groundwater contamination potential with C. brownei
The growing of groundcover plants altered the soil physical properties and the distribution and dissipation patterns of the two pesticides tested. The change in pH may affect the degradation rates of the neonicotinoids in the soils. In a study that USEPA found acceptable (MRID# 45640118), dinotefuran has a longer half-life in pH 8.8 than pH 6.5 and 6.6 soils [25
]. In addition, the increase in soil pH led to a greater persistence of imidacloprid (pH 8.5 > pH 6.9 > pH 5.2) [26
]. The above studies showed that a decreased soil pH would accelerate the degradation rates of the two neonicotinoid insecticides.
Growing plants can reduce soil pH [27
]. The growth of the two groundcovers significantly lowered the soil pH, which suggested enhanced degradation rates by growing groundcover plants. However, this phenomenon was observed only in the top layer soils with dinotefuran application. In the 30- to 45-cm soil, the dinotefuran residue was higher in the MF than CF, whereas imidacloprid was detected in only the MF in the second and third samplings. A contradiction occurred if considering only the lower pH observed.
The soil clay and SOM contents are related to the adsorption of pesticides in soils, whereas the soil clay content is the most relevant factor for polar pesticides [28
]. Other research indicated that dissolved organic carbon competes with neonicotinoids for binding sites on soil organic carbon [30
]. Also, increased soil organic carbon by the application of organic fertilizers and manure increases the persistence of neonicotinoids in the field [31
]. However, in some research, the total degradation or biodegradation of neonicotinoids was the fastest in the soil with the highest organic carbon content [32
]. In this study, groundcover planting decreased the soil clay content, but increased the SOM content, especially in the deep layer soil in the PF.
However, a trace amount of dinotefuran could still be found after 31 days with the three treatments. Hence, the soil property changes caused by groundcover planting did not facilitate in the thorough degradation of the two neonicotinoids in this study. Groundcover planting facilitated the downward movement of the pesticides in some cases, such as imidacloprid in the MF (Figure 6
b). A trace of amount of imidacloprid was found in the 30- to 45-cm soil in the last two samplings of the MF soil, whereas in the CF and PF, imidacloprid was detected only from the surface to the 30-cm layer.
The study also suggested that the dissipation patterns of the two neonicotinoids varied depending on the groundcover grown, with C. brownei seeming to confer a greater groundwater contamination potential. This suggestion might be explained in part by the soil property differences tested in this study. For example, we observed a relatively neutral pH change and lower SOM content in the 30- to 45-cm soil with C. brownei planting, which would favor the lower degradation rates of the the pesticides, as compared with A. pintoi planting. Yet, the higher neonicotinoid leaching potential with C. brownei planting could result from the higher root density of C. brownei than A. pintoi, which was not examined in this study. More studies are needed in order to verify the relation between plant physiological traits and the field distribution patterns of the two pesticides.
Moreover, the ability and patterns of uptake, translocation, and metabolism of neonicotinoids varies in plants [33
]. A groundcover plant equipped with a high absorption and metabolic ability toward neonicotinoid could reduce the possibility of contaminated groundwater. In this study, A. pintoi
showed a higher absorption for dinotefuran, and C. brownei
showed a higher absorption for imidacloprid. However, the background levels of the two pesticides were lower with A. pintoi
planting, which suggests its higher metabolic ability for the two pesticides, although a lab test in a control environment should be conducted in order to further verify the finding.
From our observations, A. pintoi would be a preferable choice as groundcover vegetation in vineyards, because of the increased SOM content but reduced possibility of neonicotinoide contamination to the environment.