There is no good rice without a good environment.

"there is obviously no industry around the piece of land where the rice I bought is grown. How can the cadmium exceed the standard when tested?" "is it true that rice has a strong ability to absorb cadmium?" …… When it comes to the rice harvest season, the safety of rice (heavy metal safety), where to eat "good rice" and so on has become a topic of concern to many friends.

In fact, rice is not entirely to blame for these problems. According to the relevant research, in the division of cadmium absorption and accumulation capacity of different kinds of plants, the absorption capacity of rice itself is not the strongest. In addition, different varieties of rice have their own differences in cadmium absorption capacity. on the whole, the cadmium absorption capacity and cadmium content of indica rice are higher than those of japonica rice.

So what is the complicated relationship between good rice and ecological environment? How can we grow safe and good rice?

Rice environment is easy to accumulate heavy metals.

In the heavy metal pollution sites or around the polluted areas, the rice environment has one more way to accumulate heavy metals than the dry farming environment, and it is easier to accumulate heavy metals than dry farming. The reason is that rice grows in the water farming environment, and the water contains heavy metals, which accumulate as the water flows into the paddy field.

According to Japanese scholar Ito, when the water contains 0.1 micrograms / liter, about 90% of cadmium will be absorbed by paddy soil and begin to accumulate heavy metals. In general, it takes 1500 tons of water to grow rice per mu, and the concentration of cadmium in the river is 0.050.1mg / L. If calculated according to the standard 0.01mg / L, the cadmium per mu can reach 15g per season. Theoretically, the concentration of soil cadmium can be increased to 0.1mg / kg.

In addition, water farming is different from dry farming, rice growth requires a flat field surface and covered with water layer to form a flooded environment, because heavy metals are mainly adsorbed in the very fine clay part of the soil, and in the process of leveling and irrigation, the very fine part of the field forms mud to accumulate on the surface, resulting in the content of heavy cadmium in the surface 3 cm of the paddy soil before sowing will be twice or more than that of the underlying soil layer after soil preparation. Therefore, the polluted rice fields lay a "hidden danger" to the absorption of heavy metals in rice at this time.

When exogenous cadmium encounters acidified soil

China's farmland has been cultivated for thousands of years. For nearly a thousand years, our ancestors have been using pond mud, animal fertilizer and green manure for rotation. Although there are a lot of substances added to fertilize each year, before industrial pollution, these substances contain very few harmful heavy metals, and the soil is getting more and more fertile, but the soil is getting safer and safer. In the survey of 4095 soil sites during the sixth five-year Plan period, the amount of cadmium in Chinese soil is very low, only 0.097 mg / kg.

However, with the extensive mining of mines, the wide application of fossil energy and the rapid development of metal smelting industry, environmental protection measures do not synchronously control the discharge of harmful heavy metals into the environment, resulting in a large number of heavy metals into the soil for decades.

Correspondingly, during this period, the application of a large amount of chemical fertilizer made the soil acidified, and the entry of heavy metals into the soil would be continuously fixed and "aging" by clay minerals and iron and manganese oxides in the soil, but under acidic conditions, the aging process became slower or did not exist, and the activity of heavy metals was always very high, which further created conditions for rice to absorb cadmium and the formation of cadmium rice.

In addition, the application of a large number of chemical fertilizers reduces the ability of soil to absorb heavy metals due to the lack of macromolecular organic matter, on the other hand, it also leads to soil hardening, the root system is difficult to grow downward, and the root system almost grows on the surface of the soil.

The results showed that when the concentration of cadmium in topsoil was 5 mg / kg, the content of cadmium in rice grain was 0.264 mg / kg, which exceeded the national food hygiene standard (0.2mg / kg). Even when the concentration of cadmium in deep soil was 10 mg / kg, the content of cadmium in rice grain was only 0.032 mg / kg, which was lower than the national food hygiene standard.

At the same time, due to the frequent acid rain, the soil accumulated a large amount of sulfur, which led to the improvement of the availability of heavy metals in the soil. Although under the condition of reduction (lack of oxygen in the soil and lack of other oxidizing substances), it is beneficial to "invalidate" heavy metals in the form of sulfides, but once the soil is dry, it usually takes only 4 days in the paddy field. Cadmium can be dissolved from cadmium sulfide, making the content of cadmium in the soil reach the level before flooding, which is highly effective in acidified environment, resulting in rapid absorption of cadmium.

Rice growth environment is "sensitive" to heavy metals.

Rice grows in a flooded environment, and the redox potential in the soil can be as low as-100 millivolts. In this environment, heavy metals such as cadmium and lead can be converted into metal sulfides with low solubility without being absorbed by rice.

When planting rice under flooding, many researchers have found that the cadmium and lead in rice are not easy to exceed the standard when the polluted farmland is flooded for a long time. Bingham et al studied the effect of cadmium on the economic yield of rice under flooded and dry conditions in 1976. In the reduced state of flooding, the content of cadmium in the soil is 320 mg / kg, which has no effect on the yield of rice, while under the condition of dry oxidation, the concentration of cadmium in the soil is 17 mg / kg, which will reduce the yield of rice. The effect of long-term flooding can be seen.

Therefore, flooding management has become an important measure to control the accumulation of cadmium in rice in Japan. The flooding management of nearly 600000 mu of cultivated land was implemented in 2007. The effect is that the content of cadmium in rice can reach 0.58mg / kg without flooding, while the content of cadmium in rice can reach 0.08mg / kg under effective water management.

In order to promote the transformation of nutrients under water for a long time and control tillering, rice has a process of medium-dry drainage in farming. In addition, due to the frequent lack of water in the later stage, the soil changes from reduced state to oxidized state. Cadmium sulfide formed in the reduction environment is rapidly dissolved, zinc and iron in the soil can inhibit cadmium absorption through root absorption competition.

Some studies have shown that in the process of sulphide dissolution, the dissolution of zinc sulfide is slow by half a beat, resulting in an increase in the ratio of cadmium to zinc and the ratio of cadmium to iron in soil solution, and the absorption of cadmium by roots is easy to be absorbed because it becomes "unimpeded". In this period, the cadmium absorbed by the roots is easy to enter the grain directly, which leads to the rapid increase of cadmium content in rice.

The flooded environment has a good effect on the control of cadmium and lead, but if the pollution of mercury and arsenic exists, the reduction environment strengthens the absorption of arsenic and mercury in rice, this is because under the flooded condition, arsenic exists as arsenite. Mercury is easily reduced to methylmercury under reduction conditions, and these two kinds of substances become more toxic and easier to be absorbed, so under polluted conditions. Rice is easy to accumulate these two elements (methylmercury and arsenic).

The safety of rice is highly related to the ecological environment.

Soil is not the only way for heavy metals to enter rice. In 2008, some people studied the effects of air pollution at different distances of 10 meters, 40 meters, 100 meters, 200 meters, 300 meters and 450 meters from the highway on the content of heavy metals in rice, and took lead isotopes as the research object. compare exposure and non-exposure to highway pollution.

The results showed that lead, cadmium, zinc in leaves, zinc in stems and lead and cadmium in grains were very different under the two exposures. About 46% of lead and 41% of cadmium in grains could be attributed to the absorption of leaves from the atmosphere, while for chromium, zinc and copper, the atmospheric source had no significant contribution to the three heavy metals in grains. It can be seen that air pollution also has a significant impact on rice safety, and it also shows that more attention should be paid to the safety of crops growing on both sides of busy highways.

In recent years, the attention to cadmium rice makes us find that the safety of rice is highly related to its ecological environment. Over the past few years, we have also determined a lot of rice with very low heavy metal content, and the analysis shows that most of these rice are produced in beautiful villages. On the one hand, the source of heavy metals in rural areas is less, on the other hand, the proportion of organic fertilizer in fertilizer application is relatively high, the safety of organic fertilizer is also high, and the soil is not easy to consolidate and so on.

It is gratifying that there are still many beautiful villages in the vast territory of China, which are still silently producing good rice for us with safety, health and nutrition. As we take rice as the staple food, it is time to cherish and protect these beautiful rural areas.