How to eliminate cadmium in rice? Treating both internal and external factors is key

With the launch of China's “Collaborative Innovation Action for Comprehensive Prevention and Control of Heavy Metal Pollution in Rice in Southern China,” the cadmium rice issue has once again become a hot topic. The public's understanding of cadmium rice mostly stays at the harm cadmium does to the human body, while there are some misconceptions regarding “how cadmium in rice is produced and whether there is any way to eliminate it.” Some people simply assume that cadmium rice occurs because rice paddies are irrigated with cadmium-containing industrial wastewater, and some even think that cadmium-producing rice paddies can be dealt with simply by replacing the old soil with new soil using the foreign-soil method. The author attempts, through this article, to provide some answers to the relevant questions about cadmium rice that are as objective and comprehensive as possible.

1 Cadmium is the foremost heavy metal pollutant in farmland

In geochemistry, cadmium is a dispersed element, difficult to form independent ore deposits, and at the same time its content in ores is very low and not easy to recover. Therefore, during mining it is easily discharged into the environment along with tailings, and especially under conditions where acidic mine water is present, cadmium readily enters rivers and farmland along with acidic mine water. The melting point of cadmium is 321 degrees C, while its boiling point is 767 degrees C, so in the absence of effective cadmium removal equipment, nonferrous metal smelting, steel plants, garbage incineration plants and thermal power plants can all easily release cadmium into the atmosphere, thereby causing relatively widespread soil cadmium pollution that is centered on the plant and most severe downwind.

Soil acidity and alkalinity (pH) have an extremely important influence on the adsorption capacity of cadmium and other heavy metals in soil organic matter, iron-manganese oxides and clay minerals. When soil acidifies, these substances have a very weak capacity to adsorb cadmium. Compared with other heavy metals such as zinc, copper, lead and mercury, the portion of cadmium in soil that is naturally available for plant uptake (water-soluble and exchangeable) is somewhat higher than that of other heavy metals, so cadmium migrates relatively easily in the soil-plant system. Although the background value of cadmium in China's soil is only 0.097mg/kg, exogenous cadmium in soil is not easily fixed by soil colloids, especially under acidic soil conditions, so the total cadmium content in many cultivated soils is actually not high, yet its availability to plants is very high.

2 Why cadmium rice is easily produced

Many people believe that cadmium rice occurs because rice is the crop that absorbs cadmium most strongly. Actually this is not the case. Some scientists divide plant tolerance to cadmium into three categories, and rice's tolerance is only moderate. The reason rice readily absorbs cadmium is that its growing environment and the chemical change processes of cadmium in soil play a key role.

Cadmium in soil is mainly adsorbed on the clay particle fraction. During paddy cultivation, because plowing causes the clay particle fraction to settle easily on the soil surface, the cadmium content in the top 1 to 3 cm is very high. In polluted areas, there is cadmium deposition from atmospheric deposition, and irrigation water also brings in cadmium, which likewise deposits on the surface of paddy soil. Rice is a fibrous-root crop, and most of its roots are within the top 5 cm. The overlapping distribution of cadmium content and roots creates the conditions for rice to absorb more cadmium. Although under flooded conditions an iron plaque forms on the surface of rice roots, which helps prevent cadmium uptake by the roots. But rice is an ammonium-loving crop, and the uptake of ammonium nitrogen by the roots acidifies the root surface, thereby helping cadmium uptake.

Some scientists using isotope labeling found that about 70% of the cadmium in rice comes from uptake by the roots during the reproductive period of rice, with the rest coming from cadmium accumulated in stems and leaves. This is because under flooded conditions, cadmium forms cadmium sulfide with the negative divalent sulfide ion formed under reducing conditions, which is very insoluble and very difficult for roots to absorb. But in the later stage of rice, for the convenience of harvesting, farmers intentionally drain the water from the field surface, and cadmium sulfide is oxidized by oxygen infiltrating into the soil surface into cadmium ions and sulfate ions, making cadmium easy for the roots to absorb. This chemical process can usually be completed within 4 days after the water is drained. So the surface-layer distribution of roots and cadmium, plus the lack of water, causes large amounts of cadmium uptake by rice.

Temperature and variety have a certain influence on cadmium uptake by rice. Research results from Japan and China's Taiwan region show that indica rice generally has a stronger cadmium enrichment capacity than japonica rice. The mainland has also done a large amount of analysis on different varieties, but possibly because there are too many varieties or because hybrid rice has not been well traced and distinguished, the cadmium uptake capacities of different types of rice are contradictory and no clear distinction has been made. Usually when the temperature is high, plant leaf transpiration is large, and cadmium ions in the soil easily enter the roots along with water. But in southern China the early rice season has a lot of rain, and the field surface is not short of water, so even at high temperatures, the cadmium in early rice is generally lower than in late rice.

3 Cadmium in rice is not closely related to cadmium in soil

The main sources of cadmium in farmland are industrial wastewater, atmospheric deposition, livestock and poultry manure, chemical fertilizers and irrigation water. But people usually think that cadmium exceeding standards in rice is because the soil has been polluted by cadmium, and that the more cadmium in the soil, the more cadmium in the rice. This result can be obtained in pot experiments under various controlled conditions, and scientists have worked hard to find this relationship so that the heavy metal content in rice can be inferred from the heavy metal content in soil.

But under field production conditions, the cadmium content of rice and the cadmium content of soil do not have this kind of correlative relationship. Even the cadmium content of rice plants at the head and tail of the water flow in one paddy field can differ. On the one hand, it is very difficult for irrigation water to contain not even a trace of heavy metals; even if the concentration is as low as a few hundredths of one percent, the incoming heavy metals are also easily adsorbed by the soil near the water inlet, making its heavy metal content slightly higher. On the other hand, before harvesting, in order to facilitate harvesting, irrigation is stopped in the field, and at harvest time the soil at the water inlet is relatively dry while the soil at the water outlet is relatively wet. These two factors together generally cause the rice at the water inlet to contain a little more heavy metals. Under polluted conditions, if the concentration of cadmium in the irrigation water is higher, this difference becomes even more obvious.

Under field production conditions, investigations in Japan, Thailand, South Korea, mainland China and China's Taiwan region have all found that cadmium in rice is not closely related to cadmium in soil, so when Japan formulated cadmium standards for farmland it used the cadmium content in brown rice instead.

4 How to “eliminate” cadmium rice

The various characteristics of cadmium make soil vulnerable to widespread pollution, and once polluted, cadmium easily enters rice. Therefore source control must be strengthened, including control of atmospheric pollution and industrial wastewater discharge, use of phosphate fertilizer with low cadmium content, and reasonable and appropriate use of livestock and poultry manure, to reduce or even eliminate pollution of paddy soil by exogenous cadmium. This is the key to eliminating cadmium rice caused by exogenous pollution.

Soil acidification is a major cause of cadmium exceeding standards in rice. China has a large population and little land, a high multiple cropping index and large fertilizer application rates, causing soil acidification, and together with acid rain from the atmosphere and acidic mine water in sulfur-containing mining areas, this leads to serious soil acidity, especially in acid rain areas and around sulfur-containing mining areas. Therefore improving soil with alkaline substances such as lime is an important measure for eliminating cadmium rice. Of course the lime requirement needs to be scientifically calculated, and the adjustment of soil pH must reach above 6.5.

Water management in paddy fields is another important measure for controlling cadmium in rice. The ideal condition is flooding throughout the whole process, which helps reduce cadmium in rice to the minimum. But flooding throughout the whole process is unrealistic in field production. First, it consumes a large amount of water; conventional rice growth management requires mid-season drainage to control rice tillering and change the soil environment, releasing nutrients through the mid-season drainage process. In Japan, for soil with low to moderate cadmium pollution, keeping a 2 to 3 cm water layer on the field surface for three weeks around heading of rice prevents the soil surface from being in an oxidized state, thereby greatly reducing the cadmium content of rice. For paddy fields with very soft soil, such water management will bring difficulties for mechanical cultivation, but keeping a 2 to 3 cm water layer for 10 days after heading of rice can also achieve a relatively good cadmium-reduction effect.