1, the factors that affect pesticide residues in the soil
1) The effect of the nature of chemical pesticides: chemical properties of the pesticide itself, such as volatility, solubility, chemical stability, dosage forms, etc. The organochlorine pesticide has low volatility, but its vapor pressure has a certain relationship with the residual in the soil. Moreover, the speed of evaporation is related to the concentration of pesticides, the relative humidity of the atmosphere, the speed of the air above the soil surface, and the temperature in the soil. Generally, the greater the concentration, the greater the humidity, the higher the moisture content, and the greater the velocity, the stronger the volatilization effect. .
2) Effect of soil properties: Pesticides have a longer residence time in soils with high viscous and organic matter content. The main reason is that the soil is a clay mineral-organic compound colloid, and its adsorption properties can form a stable insoluble binding residue.
Soil pH is more sensitive to organic phosphorus pesticides than organochlorine pesticides. This is mainly due to the influence of pH on the decomposition rate of soil pesticides and the main way of decomposition is chemical decomposition or microbial degradation.
Pesticides mainly disappear through chemical degradation, bacterial decomposition, and volatilization. These processes are all affected by temperature. At low temperatures, these processes slow down and the degradation of pesticides slows down.
The effect of soil moisture on pesticide residues is mainly because water is a polar molecule that competes with pesticides for adsorption sites and is strongly adsorbed by colloids. In dry soils, there are fewer water molecules that compete with pesticides for adsorption sites.
3) Residues of pesticides in the soil: Chemical pesticides that enter the soil are susceptible to various chemical, physical, and biological processes, and react or degrade in many ways, but only with different types of pesticides. The degree is different. Therefore, the residence time of pesticides in soil is different, and the time for pesticides to remain in soil is often expressed in terms of two concepts: half-life and residual period. The so-called half-life is the time required for the pesticide applied to the soil to reduce its concentration by half due to degradation. Residue refers to the amount of pesticides in the soil due to degradation and other reasons, and the amount of pesticides remaining in the soil, the unit is mg/kg.
Many scholars have determined the persistence of pesticides in soil. Most results suggest that the organochlorine pesticides have the longest remaining period in soil, usually for several years; followed by benzodiazepines, substituted ureas, and phenoxyacetic acid. Herbicides usually last from months to one year. Residues of organophosphorus and carbamate insecticides and general bactericides are usually only a few days or weeks. Soils rarely accumulate, but there are also few. The residual period of organophosphorus pesticides in soil is longer and can reach several months.
2. Residue accumulation of chemical pesticides in soil
Once pesticides enter the soil ecosystem, residues are unavoidable. Although the amount of residues varies from time to time, the number of residues is not equal to that of residual pesticides. Only when pesticide residues in the soil have accumulated to a certain extent, The self-purification effect of soil produces dislocation, misadjustment, and endangers agricultural environmental organisms, including the safety of target organisms and non-target environmental organisms of pesticides, and indirectly endangers the health of humans and animals. Generally speaking, the residual accumulation and poisoning of soil chemical pesticides mainly manifest in two aspects: the harm caused by the transfer of residual pesticides; and the direct poisoning of residual pesticides to target organisms.
1) The transfer of residual pesticides is mainly related to food. According to reports from the United States, there are three main routes for the transfer of residual pesticides in organisms:
Article 1: Soil → Terrestrial Plant → Herbivore
Article 2: Soil → Invertebrates in Soil → Vertebrates → Predators
Article 3: Soil → Water System (plankton) → Fish and aquatic organisms → Fish-eating animals
In general, water-soluble pesticides are easily leached with precipitation, irrigation water, percolation, and enter the groundwater in the longitudinal direction of the soil body, or are lost by surface runoff, drainage and irrigation water, migrate laterally, and diffuse to the surrounding water sources (body) to form a pair. Harmful contamination of autotrophic and heterotrophic organisms in the aquatic environment. Fat-soluble or systemic-adsorptive pesticides are easily adsorbed by the soil and have poor mobility. They are absorbed by the crop roots or transported, distributed, and accumulated in the crops of the season. They may even constitute secondary phytotoxicity and damage to the crops of the later seasons. Contamination causes chronic hazards in terrestrial environments of autotrophic and heterotrophic organisms and high-ranking food chain organisms. Among them, the residual pesticide accumulation may be mostly in the past. Some people have studied the pesticide content (DDE, dieldrin) and other organochlorine pesticides of various kinds of bird chest muscles and found that the fish-based heron in vivo The residue is much more than the residues of terrestrial animal-based hawks, while the eagles, which mainly eat terrestrial animals, have much more residues than herbivorous birds.
Pesticides that enter animals are decomposed and excreted in internal organs such as the liver. However, if pesticides that are difficult to decompose continue to be ingested by animals, they cannot be decomposed and excreted and accumulated in the body. In particular, fat-soluble pesticides such as DDT and dieldrin may remain in the body for a long period of time because they dissolve in body fat, causing animals to survive. The body is polluted. Pesticides accumulated in animals are also transferred to eggs and milk, causing contamination of various animal products. Humans feed on certain parts of plants and animals. Because animals and plants are contaminated, food contamination is bound to occur.
It can be seen that the problem of pesticide pollution becomes more serious due to the transfer of residual pesticides and the effect of bio-concentration.
2) Direct poisoning of residual pesticides on target organisms
Residues of pesticides in the soil have different degrees of influence on microorganisms, protozoa, and other arthropods, annelids, and mollusks in the soil. Flemming E. et al. (1994) found that three insecticides, dimethoate, anti-indoles, and Fenpropimorph, have negative effects on the natural population of soil protozoa. Fenpropimorp has an adverse effect even at the lowest use concentration, and application of dimethoate for 10 days significantly reduces the respiration of soil microorganisms. Wang Zhenzhong (1996) researched the effect of organophosphorus pesticide wastewater irrigation on soil animal community and found that: the type and quantity of soil animals decreased with the deepening of the impact of pesticides, and the types and quantities of animals in the experimental areas with serious pesticide pollution were significant. Below the lightly polluted area and the control area, some species disappeared completely. There are also tests that show that pesticide pollution has an impact on the metabolism of soil animals as well as the number and hatchability of eggs.
In addition, residual pesticides in the soil also have a significant impact on plant growth and development. It has been found that soil contaminated by TC significantly inhibits the germination of wheat seeds. When the concentration is 2 mg/L, the germination inhibition rate reaches 30%. It is also pointed out in experiments that pesticides may cause plant physiology changes when they enter the plant body. Causes plants to be more sensitive to host or predator attacks, such as the use of herbicides have increased the pests and diseases of corn. In addition, pesticides have also been reported to inhibit or promote the growth of crops or other plants, either earlier or later.