In order to control salinization, they actually raise shrimps on the Loess Plateau?

Salinization refers to the process of accumulation of salt and alkali components in the surface layer of the land. Due to the high concentration of soil solution, crops are difficult to grow in saline-alkali land. In my country's Huanghuaihai region, the western plains of Northeast China, the Hetao Plain and the inland arid areas of Northwest China, a total of 1.48 billion mu of contiguous saline-alkali land has been unusable for a long time, and the production and life of the people living here have been in difficulties.

Image source: Tuchong Creative

In order to control salinization, people have made great efforts in various ways. Some regions focus on soil improvement, that is, using the normal soil dug out from deep to cover the saline-alkali soil layer. Some regions try to cultivate salt-alkali tolerant crops. The "sea rice" cultivated by Academician Yuan Longping's team is a successful case in this direction.

There is also a rather creative idea that - since a large amount of salt-alkali is accumulated on the surface of the saline-alkali land, wouldn't it be possible to dissolve the salt-alkali by digging ponds in the saline-alkali land and filling them with fresh water, thus forming a saltwater environment? Wouldn't it also be possible to achieve the purpose of utilizing the saline-alkali land by breeding seafood in such ponds?

You know what, this idea is indeed feasible. In Jingtai County, Gansu Province, my country, attempts to use transformed saline-alkali land to dig fish ponds to cultivate whiteleg shrimp have been carried out for many years.

Lack of water is annoying, but too much water is also unbearable

Historically, Jingtai County has not been an area prone to frequent saline-alkali disasters. Before 1974, Jingtai County, which is adjacent to the Tengger Desert, suffered mainly from land drying up and desertification. Although it is close to the Yellow River, the main arable land is on the Jingtai River, which is more than 700 meters above the Yellow River. The Yellow River water flows but cannot be used for irrigation. Therefore, there is a local folk song that goes "Water flows in the low place, people on the river are worried, the wind and sand are endless, and nine out of ten crops fail."

In 1974, the first phase of the Jingtai River Electric Irrigation Project was completed. Through the relay of 13 electric water pumps, the Yellow River water was raised 472 meters, and finally 300,000 mu of wasteland in the low part of Jingtai River could be irrigated. Ten years later, the second phase of the Jingtai River Project with a higher irrigation height started. Not only did it transform another 500,000 mu of fertile land in Jingtai River, but it also supplied water to Minqin County in the Gobi Desert through the Tengger Desert. With sufficient agricultural water, Jingtai River has changed from its old appearance of yellow sand to the Jiangnan of the Great Wall. The people of Jingtai believe that the days to come will be sweeter than honey.

However, the good times did not last long, and the situation did not continue to improve as people expected. Although the Jingtai River is very high, there are always low places. Although the irrigation of the Yellow River water has solved the problem of agricultural water use, after the irrigation water penetrates deep into the soil, it will always gather in the low places on the river in the form of groundwater. In this process, they also dissolve and carry a large amount of minerals and inorganic salts in the soil. Since the mid-1980s, the groundwater levels in six low-lying towns such as Wufo Township and Caowotan Township on the Jingtai River have continued to rise, and the minerals in the water have continued to move to the surface with the evaporation of the scorching sun, eventually resulting in a saline-alkali area of ​​more than 6,700 hectares. The salinization of cultivated land in some towns has reached 2/3. The people of Jingtai, who have been poor for several lifetimes due to lack of water, have returned to poverty again due to salinization caused by too much water.

What to do to control salinization? Raise shrimp!

In order to solve the salinization problem in Jingtai County, the local government has tried a variety of measures. Since salinization is caused by water, the fundamental solution is to reduce the groundwater level in the salinized area. For example, in the irrigation area of ​​Jingtai River, more scientific drip irrigation is used instead of flood irrigation, and drainage channels and underground pipes are dug in low-lying towns to drain excess water. The symptomatic solution is to improve the surface soil, that is, to cover the salinized farmland with a layer of non-salinized soil to "raise the field and create land."

In the process of practice, people suddenly realized that if the saline-alkali farmland was converted into fish ponds, it would have the effect of treating both the symptoms and the root causes: the excavated soil is deep soil that has not been salinized, which can be used to "raise fields and create land" for the surrounding farmland, and the process of pumping water from the ground to fill the pond can also consume excess groundwater, and the natural infiltration of water in the pond can also bring the salt in the surface soil back to the ground, and breeding aquatic products in fish ponds is also a good way to increase income for local residents. This is the original intention of the policy of "digging ponds to reduce precipitation, raising fields to create land, giving equal importance to fisheries and agriculture, and restoring the ecology" proposed by Jingtai County in 2015.

However, we must note that although the groundwater poured into the pond after Jingtai County dug a pond in the saline-alkali land also has a certain salinity, and the salt and alkali in the pond soil will also dissolve into the water, the salt water obtained from this is still different from seawater. The main components in seawater are relatively constant, and rarely fluctuate violently, but saline-alkali land (and its underground saline-alkali water) are just the opposite. They have the characteristics of "three mores and one high", namely high pH, ​​high carbonate alkalinity, high ion coefficient and a variety of water quality types. The quality of saline-alkali water in different regions varies greatly. Our domestic saline-alkali water has at least bicarbonate type, sulfate type and chloride type, which can be subdivided into at least a dozen types of water quality. In some water quality, some ions are dozens of times higher than seawater, and some ions are much less than seawater. It is obviously impossible to directly use such water bodies to cultivate marine life. In addition to the necessary water purification, it is also necessary to add sea crystals and other ingredients to further increase the salinity. The ultimate purpose of Jingtai County in digging ponds and engaging in aquaculture in saline-alkali land is to control salinity. Isn’t it contrary to the goal of controlling salinity to cultivate marine shrimps here?

This is actually directly related to the super adaptability of the local farmed marine shrimp, Penaeus vannamei. This type of shrimp, originally distributed in the coastal waters of southern South America, has a wide range of adaptability to salinity. Wild Penaeus vannamei often move in the "soda zone" where rivers flow into the sea, and even directly enter freshwater rivers to forage. The salinity of the fish ponds excavated from saline-alkali land is slightly higher than that of freshwater, which may be a challenge for pure freshwater fish and is not suitable for pure marine life. However, from the perspective of Penaeus vannamei, it is an ideal environment. Breeding Penaeus vannamei without increasing the total amount of saline-alkali substances in the soil and without large-scale water quality improvement is naturally the best choice for Jingtai County.

In fact, the use of saline-alkali land for aquaculture is not limited to Jingtai County, Gansu. Internationally, the first country to use saline-alkali land for aquaculture was the former Soviet Union. Exactly 100 years ago, the Soviets began to inject underground brine into saline-alkali land to culture marine fish, shrimp, crabs and shellfish. Eastern European countries, the United States, India and Israel have also made similar attempts.

To control salinization, we must adapt to local conditions.

In my country, there are also precedents for attempts at aquaculture in saline-alkali land, and the largest scale of these is the transformation of saline-alkali land in the eastern coastal areas. The causes of the saline-alkali land here are completely different from those in Jingtai County. The main reason for its salinity is seawater. Whether it is the silt land reclamation in Shandong and Jiangsu, or the seawater intrusion in Hebei and Tianjin, the culprit of salinity is seawater, and the saline-alkali substances in the soil are mainly chlorides left over from seawater. Such saline-alkali land or underground brine can be very close to real seawater as long as the salinity is adjusted, and it is also very simple to add new seawater here. The cost of changing water during the breeding process is very low, and most of the farming here is real seafood farming. However, because these areas are close to the sea, their uniqueness of "raising seafood in saline-alkali land" has been largely overlooked.

The saline-alkali areas in the Songnen Plain in Northeast China were mainly caused by the rise of the Songliao watershed in the late Pleistocene. The Nenjiang River lost its access to the sea, and a large amount of accumulated water and saline-alkali substances in the water gathered in the Songnen Plain. After the water flowed away, the salt and alkali remained. The low plains here can still flush the salt on the surface of the soil back to the ground, so most of the aquaculture here is freshwater, and its purpose is to eventually solve the problem of salinization.

In the work of controlling salinization, Jingtai County has taken its own path. Like Academician Yuan Longping's sea rice plan, it has provided valuable Chinese wisdom for the global problem of salinization control. However, salinization in China and even around the world is caused by many different and complex reasons, and there is no universal experience for their utilization and transformation. Perhaps the success story of Jingtai County does not lie in the innovation of "salt-alkali land can be used to cultivate shrimps" itself. Their management ideas that are adapted to local conditions and scientifically coordinated are more meaningful inspirations for the management of other saline-alkali areas.

The article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.

Author: Wandering science writer

Reviewer: Jiang Zhongcheng, Researcher, Institute of Karst Geology, Chinese Academy of Geological Sciences