Quality rice cannot be separated from a good growing environment
"The land where the rice I bought was grown clearly has no industry nearby, so how could it test positive for excessive cadmium?" "Could it be that rice has a super-strong ability to absorb cadmium?" ... It is the rice harvest season again, and the safety of rice (heavy metal safety), where one can eat "good rice," and other such topics have once again become concerns for many friends.
In fact, these problems cannot be entirely blamed on rice. According to relevant research, in the classification of different plant species by their ability to absorb and accumulate cadmium, rice itself does not have the strongest absorption capacity. In addition, different varieties of rice differ in cadmium absorption capacity; overall, indica rice has a higher cadmium absorption capacity and rice cadmium content than japonica rice.
So what intricate and complex relationships exist between good rice and the ecological environment? How can safe, good rice be grown?
The rice environment readily accumulates heavy metals
At heavy metal pollution sites or around polluted areas, the rice environment has one more pathway for heavy metal accumulation than dryland cropping environments, making it easier to accumulate heavy metals than dryland cropping. The reason is that rice grows in a paddy environment, the water contains heavy metals, and the heavy metals accumulate as the water flows into the paddy field.
According to research by the Japanese scholar Ito, when the water body contains 0.1 micrograms per liter, about 90% of the cadmium is adsorbed by the rice soil, and heavy metal accumulation begins. Normally, growing one season of rice on one mu of land requires 1500 tons of water; when the cadmium concentration in river water is 0.05-0.1 micrograms per liter, calculated at the standard of 0.01 milligrams per liter, the cadmium input per mu per season can reach 15 grams; calculated at 150 tons of soil per mu, this could theoretically raise the soil cadmium concentration to 0.1 milligrams per kilogram.
In addition, paddy cultivation differs from dryland cultivation in that rice growth requires a level field surface covered with a water layer to form a flooded environment. Because heavy metals are mainly adsorbed in the extremely fine clay fraction of the soil, during field leveling and irrigation, the extremely fine fraction forms mud that accumulates on the surface layer, causing the heavy cadmium content in the top 3 cm of paddy soil after land preparation and before sowing to be one time or more higher than that of the underlying soil layer. Therefore, contaminated paddy fields at this time plant a "hidden danger" for the rice's heavy metal absorption.
When exogenous cadmium meets acidified soil
China's farmland has thousands of years of cultivation history. Over the past nearly thousand years, our ancestors continuously applied pond mud, animal-based fertilizers, and green manure in crop rotation. Although a considerable amount of external material was added each year to build fertility, before industrial pollution these materials contained very few harmful heavy metals, and the soil became more and more fertile while becoming safer and safer. During the "Sixth Five-Year Plan" period, 4095 soil points were surveyed, and China's soil cadmium was very low, only 0.097 milligrams per kilogram.
However, with large-scale mining, the widespread use of fossil energy, and the extremely rapid development of the metal smelting industry, environmental protection measures did not simultaneously control the emission of harmful heavy metals into the environment, causing large amounts of heavy metals to enter the soil within a few decades.
Correspondingly, during this period, the application of large amounts of chemical fertilizers acidified the soil. Heavy metals entering the soil would originally be continuously fixed and "aged" by clay minerals and iron-manganese oxides in the soil, but under acidic conditions this aging process slows down or does not occur, and the activity of heavy metals remains very high. This further created conditions for rice to absorb cadmium and for the formation of cadmium rice.
In addition, the application of large amounts of chemical fertilizers reduced the soil's ability to absorb heavy metals due to a lack of high-molecular-weight organic matter, and on the other hand also led to soil compaction, making it difficult for roots to grow downward, with roots growing almost entirely in the surface soil layer.
Research shows that when the cadmium concentration in the surface soil is 5 milligrams per kilogram, the cadmium content in rice grains is 0.264 milligrams per kilogram, exceeding the national grain hygiene standard (0.2 milligrams per kilogram); when the surface soil is uncontaminated, even when the cadmium concentration in the deep soil is 10 milligrams per kilogram, the cadmium content in rice grains is only 0.032 milligrams per kilogram, below the national grain hygiene standard.
At the same time, due to frequent acid rain, the soil has accumulated large amounts of sulfur, which increases the availability of heavy metals in the soil. Although under reducing conditions (soil lacking oxygen and also lacking other oxidizing substances), it is favorable for making heavy metals "ineffective" in the form of sulfides, once the soil dries out, usually only 4 days for a paddy field, cadmium can dissolve out from cadmium sulfide, bringing the cadmium content in the soil back to the level before flooding, and under acidified conditions its availability is very high, leading to rapid cadmium absorption.
The rice growing environment is "sensitive" to heavy metals
Rice grows in a flooded environment, where the soil redox potential can be as low as -100 millivolts; under such conditions, heavy metals such as cadmium and lead can be converted into metal sulfides with very low solubility and thus not absorbed by rice.
Many researchers, when conducting flooded rice cultivation, have found that in contaminated farmland under long-term flooding, cadmium and lead in rice do not easily exceed standards. Bingham et al. in 1976 studied the effect of cadmium on the economic yield of rice under flooded and drained conditions. Under flooded reducing conditions, when the cadmium content in the soil was 320 milligrams per kilogram, there was no effect on rice yield; while under drained oxidizing conditions, a cadmium concentration of 17 milligrams per kilogram in the soil caused rice yield reduction, showing the effect of long-term flooding.
Flooding management therefore became an important measure in Japan for controlling cadmium accumulation in rice. In 2007, flooding management was implemented on nearly 600,000 mu of farmland. Its effect was summarized as follows: without flooding, rice cadmium content can reach 0.58 milligrams per kilogram, while under effective water management, rice cadmium content can reach 0.08 milligrams per kilogram.
To promote nutrient transformation under long-term flooding and control tillering, in farming practice rice undergoes a mid-season drainage and drying process. In addition, due to weather factors, water is often lacking in the later period, and the soil transforms from a reduced state to an oxidized state. Cadmium sulfide and the like formed under reducing conditions dissolve rapidly. Originally, zinc and iron in the soil could competitively inhibit cadmium absorption through root uptake.
Research shows that during the dissolution of sulfides, the dissolution of zinc sulfide and the like lags half a step behind, causing the cadmium-zinc ratio and cadmium-iron ratio in the soil solution to increase. Cadmium uptake by the roots becomes "unimpeded" and is easily absorbed, and cadmium absorbed by the roots during this period readily enters the grains directly, causing a rapid increase in the cadmium content of rice.
The flooded environment has a very good effect on controlling cadmium and lead, but if mercury and arsenic contamination is present, the reducing environment strengthens the rice's absorption of arsenic and mercury. This is because under flooded conditions arsenic exists as arsenious acid, and mercury under reducing conditions is easily reduced to methylmercury. These two types of substances become more toxic and more easily absorbed, so under contaminated conditions rice readily accumulates these two elements (methylmercury and arsenic).
The safety of rice is highly related to the ecological environment
Heavy metals entering rice do not come through soil as the only pathway. In 2008, someone studied the effects of atmospheric pollution at different distances from a highway - 10 m, 40 m, 100 m, 200 m, 300 m, and 450 m - on the heavy metal content in rice, and used lead isotopes as the research object, comparing two situations: exposure and non-exposure to highway pollution.
The results found that lead, cadmium, and zinc in leaves, zinc in stems, and lead and cadmium in grains differed greatly between the two exposure situations. About 46% of lead and 41% of cadmium in grains could be attributed to foliar absorption from the atmosphere, while for chromium, zinc, and copper, atmospheric sources made no obvious contribution to these three heavy metals in the grains. It can be seen that atmospheric pollution also has a significant impact on rice safety, and it also shows that the safety of crops growing on both sides of busy highways needs more attention.
In recent years, concern about cadmium rice has led us to discover that the safety of rice is highly related to its ecological environment. Over several years we have also measured many rices with very low heavy metal content, and analysis found that most of these rices were produced in beautiful rural villages. On the one hand, rural areas have fewer heavy metal sources; on the other hand, the proportion of organic fertilizer in fertilizer application in rural areas is relatively high, the safety of organic fertilizer is also higher, and the soil is less prone to compaction, and so on.
What is gratifying is that on China's vast land, many beautiful rural villages are still distributed, and these villages are still silently producing safe, healthy, and nutritious good rice for us. As people who take rice as our staple food, it is time for us to cherish and protect these beautiful rural villages and countryside.