Soil improvement: the action principle and influence mechanism of fumigant (Mianlong) on soil

Dazomet is a fumigant, which will release methyl isothiocyanate MITC when it is decomposed in soil.

Other by-products include formaldehyde, monomethylamine and hydrogen sulfide. Carbon disulfide may also be released in acid soils (US EPA, 2008).

Methyl isothiocyanate (MITC) is the decomposition product of Mianlong. The parent pesticide was applied to the soil before planting and sowing, and rapidly decomposed into MITC in the form of mole to mole in the presence of water. MITC is a multi-purpose fumigant, which is used to resist the activity of plant pathogenic nematodes, weeds, oomycetes and a variety of plant pathogenic fungi through a non selective enzyme inhibition mechanism. MITC can also be generated in soil by applying granular product dazomet (trade name LongXin;). Its half-life ranges from hours to days, depending on environmental conditions. MITC is unstable and may be decomposed into methylamine through thiocarbamate in water. A faster hydrolysis rate is obtained at a lower pH level. The adsorption of MITC is weak, and because of its volatility and water solubility, it can be divided into vapor phase and water phase. Therefore, it comes into contact with the soil through leaching and diffusion. Compared with other soil fumigants, MITC showed only moderate diffusion mobility. In particular, adding organic amendments to the soil surface will accelerate degradation. In soil, the degradation rate of MITC was about 6 times higher when modified with 5% compost chicken manure. Adding compost or manure will add more organic substances to the soil, which can generate new microbial populations and enhance the degradation capacity of MITC.

Compared with other fumigants, the use of Mianlong has many advantages in agriculture, economy and environment, such as increasing crop yield, reducing production costs, improving management flexibility and reducing the impact on the environment.

With the gradual expansion of the planting area of high value-added cash crops in China, the phenomenon of soil borne diseases has become increasingly rampant.

This also puts forward requirements for the development of crops in China from extensive field planting to intensive and refined planting mode to a certain extent. Therefore, it also brings broad space for the development of fumigants. At present, fumigants (methyl bromide, Mianlong, etc.) have been well known in some large provinces where high value-added cash crops (strawberries, vegetables, medicinal herbs, etc.) are grown, and the soil disinfection industry has made great progress. The purpose of this study is to evaluate the effect of various fumigants on soil borne diseases, at the same time, to clarify the impact of fumigants on nitrogen nitrification and mineralization, and then to explore the specific impact mechanism of Mianlong fumigants on various aspects of soil nitrogen transformation, hoping to provide a theoretical basis for reasonable fertilization in fumigated protected areas.

The main conclusions are as follows:

1) After fumigation, the soil microbial biomass carbon and nitrogen decreased significantly, while the corresponding soil soluble organic nitrogen increased significantly, which indicated that the killed microbial cells in the soil released from the microbiota to become soluble organic matter after fumigation. 7 days after fumigation, the content of soluble amino acids in the soil of each treatment was significantly higher than that of the control.

2) According to the indoor fumigation test results of fumigants, fumigation treatment can promote soil mineralization in a short term, lead to the accumulation of ammonium nitrogen content, and inhibit nitrogen nitrification. Fumigation treatment reduced the activities of glutaminase and asparaginase to a certain extent. Even if the fumigation was removed, some soil enzyme activities could not be recovered in a certain period.

3) After urea fertilization, adding a certain amount of Mianlong had a significant impact on soil nitrogen runoff loss. After conventional application of urea, in addition to a large amount of ammonium nitrogen and nitrate nitrogen, there is also a large amount of nitrite nitrogen in the soil, accounting for 15.7%~18.2% of the total inorganic nitrogen. The content of nitrite nitrogen in soil fumigated with Mianlong is extremely low, and it can make the nitrogen in soil mainly exist in the form of ammonium nitrogen that can be adsorbed by soil and retain for a long time, reducing the pollution of water environment caused by nitrogen fertilizer application in farmland.

4) Using the soil nitrification kinetic model, the nitrification kinetic parameters after soil fumigation in different regions were obtained by solving the parameters, so as to clarify the difference of inhibition of soil nitrogen nitrification by different fumigants. The indoor test results showed that fumigation significantly reduced the transformation rate of ammonium nitrogen to nitrate nitrogen in soil, and played a certain role in nitrification inhibition. The difference of inhibition effect of fumigation on nitrification is mainly related to soil pH value, organic matter content and soil viscosity.

5) After fumigation, there was a positive correlation between the content of ammonium nitrogen and mineral nitrogen in soil and the concentration of Mianlong in short term. At a certain dosage of Mianlong, the more nitrogen fertilizer was applied in the soil, the more significant the nitrification inhibition was.

Note: Mianlong is provided by Nantong Shizhuang Chemical Co., Ltd. and arranged by Zhao Hongming