Team:CAU China/Background

Background

Since ancient times, agriculture is the foundation of the national economy, the cornerstone of national stability and security, and arable land, as the basic element of agricultural production, is the material basis for human survival and development, and its quality and quantity are directly related to economic construction, social development and the improvement of people's lives. China is the world's largest country with a large population, and the relative shortage of resources, especially the shortage of cultivated land, is an important factor restricting China's survival and development. In the 1960s, China experienced a famine, and many people died as a result. Now China has become the largest country in grain, the problem of more people and less land still exists. Saline soil is one of the main types of low- and middle-yielding soils. According to incomplete statistics from UNESCO and the Farming Organization, the world's saline land area is 954.38 million hectares, accounting for 20 per cent of the world's total arable land, while the number of people without nutrition and hunger is still on the rise globally, reaching 821 million hungry people by 2018, according to the 2018 State of World Food Security and Nutrition Report. Currently, 27 people per hectare of rice paddies can be provided, and as the population increases year by year, 43 people per hectare will have to provide rations by 2050 to meet their needs. At present, at least 30 million hectares of arable land are needed to feed the 821 million hungry, and to prepare for future population growth trends will require more arable land, improving saline-alkali land is a good way. China has 99.13 million hectares of saline land, of which 280 million mu of saline land can be exploited, this 18.66 million hectares of cultivated land can provide half of China's population rations.In addition to affecting grain yield, soil salinization on the non-cultivated environment harm can not be underestimated: with the accumulation of a large amount of salt in the soil, the soil will appear the deterioration of physical characteristics, which makes the soil good gas microbial vitality decreased, water release slowed, soil capillary effect increased, further aggravated surface soil salinization, so that the soil environment is more serious. Moreover, under the influence of natural or man-made factors, the trend of soil salinization only increases, in order to ensure the food security of all mankind, to give the future space for development, saline land management is the step we must step up.
China's saline-alkali land is widely distributed, including about 210 million mu of salty soil in the eastern coastal seashore and sea-coated, about 20 million mu of salted soil in the Huanghuaihai Plain about the northeast plain, including tens of millions of mu of semi-desert inland salt soil such as Inner Mongolia's river cover irrigation area, Ningxia Yinchuan irrigation area, Gansu Hexi Corridor, Xinjiang Quanggar Basin, etc.
Saline soil, also known as saline soil, is the general term for salinized and alkaline soil. Salt soil refers to soil with a salt content greater than 0.6%, while alkali soil refers to soil with more exchanging sodium ions and 15% alkalinity in the soil colloid, and alkali soil because the soil colloid absorbs sodium ions in the soil solution while constantly exchanging calcium and magnesium ions, thus increasing the percentage of exchangeable sodium in the colloid.

Saline land can be divided according to the source of salt in cultivated soil, divided into primary salinization (cultivated land, improved measures, but not completely desalinated) and secondary salinized soil (the soil has been desalinated, but for some reason, re-salting) The alkalinity of the soil alkalinization layer (ESP) of the soil is less than 5%, the alkalinity of the soil alkalinization layer (ESP) of the weak alkalinized soil is 5% to 10%, and the alkalinization of the soil alkalinization layer (ESP) is 10% -15%, strong alkalinized soil: Alkalinity of soil alkalinization layer (ESP) 15 (20) % to 25 (30%)%.
There are many hazards of saline soil
1, salt content, endangering seed germination and crop growth
Salt in the soil, so that the normal ion balance in the crop is destroyed, so that some elements in the crop too much or too little causes nutritional disorders, affecting the normal growth and development of the crop.
2, soil texture sticky weight, poor permeability performance
Saline soil is generally located in low-lying areas, soil long-term by the above-ground water and groundwater immersion, soil clay particles continue to deposit, so that the soil gradually become sticky, structural differences. Due to poor ventilation and water conditions, low ground temperature, weak microbial activity, slow nutrient decomposition, plant long-term nutrient supply is insufficient.
3, effective nutrients have a fixed effect
The carbonate contained in saline soil has adsorption fixation effect on trace element zinc, and produces zinc carbonate that is insoluble in water. Zinc carbonate can only be slowly decomposed and released when the temperature rises. In addition, carbonates and effective phosphorus in the soil react to calcium phosphate, which is insoluble in water, reducing the effectiveness of fertilizer.
4, not drought-resistant, not flood-resistant
When the climate is dry, the evaporation of saline-alkali land is strong, so the soil tillage layer water consumption is more, especially the lower part of the saline soil water is not easy to replenish upward, coupled with soil clay heavy, poor physical and chemical properties, farming is very laborious, easy to pick up large strips, rough, not protected, not drought-resistant, not protected seedlings. At the same time, due to the low level of saline soil, summer rain, high water table, soil permeable and poor, ground drainage and soil drainage are not good, easy to cause waterlogging.

The treatment of saline soil has always been what Chinese scientists are doing. In the 1970s, the predecessors of China Agricultural University went to Qu Zhou, a small county town in Hebei province, to help them improve saline-alkali land through fresh water flushing. From the 1970s to the present, China's saline-alkali land treatment has reached a bottleneck period.
There are probably a few ways to treat saline soil:
1, engineering measures: engineering measures are by laying under the cultivation layer dark tube or layer. To separate the interference of deep soil and groundwater sources, freshwater irrigation is used to improve saline-alkali ecosystem.
2, agricultural measures: mainly include deep-cultivated fine, fresh water irrigation, salt water ice irrigation and other measures, only to meet short-term planting needs.
3, chemical measures: the use of exogenous additives and soil colloids to have a chemical reaction to improve saline-alkali land.
4. Biological measures: In recent years, biological measures have developed rapidly, mainly through the metabolism and growth activities of plants or microorganisms, to absorb, transform or transfer salts in the soil, and to improve soil quality. Usually includes plant restoration and microbial recovery, mainly plant restoration.
4.1 Plant restoration: Planting salt-resistant, salt-resistant crops on salinized plots. Increase the evapotranspiration effect, reduce the water table, prevent soil return to salt to slow down the phenomenon of secondary salinization of the land caused by the rise of the water table. Some researchers in Australia and the United States have urged plant restoration measures to treat saline-alkali lands, i.e. to repair the old salinized land by screening salt-tolerant crop varieties and improving crop salt resistance. Saline land can also increase farmers' income by planting some saline-resistant economic plants, such as rape, chrysanthemum, dates, reeds, ice grass, etc., and reduce the losses caused by salinization of the land. It can be seen that plant repair measures are more economical and efficient than engineering measures. The use of salt-tolerant seedlings or cash crops in salty wastelands has enormous economic and ecological benefits for developing countries. Plant restoration has the advantages of high economic and ecological benefits, energy-saving fresh water, long-term improvement and large application area.
4.2 Microbial repair: In recent years, microbial repair has been widely used in saline restoration. First, microbial agents can significantly increase microbiome in soil. To enhance the life activities of soil microorganisms, mainly to promote the activity of bacteria and line bacteria, and thus improve the physical and chemical properties of soil, secondly, microbial metabolism activities also produce a large number of organic acids, can neutralize the alkalinity of saline soil, to improve the effect of saline soil, and finally, microbial agents can significantly improve the soil structure and physical properties of saline-alkali cultivation layer, break soil formation, promote soil surface, reduce soil load, improve soil moisture content and soil temperature. Microbial repair time is generally relatively long, the technology is more suitable for long-term idle land with low economic output requirements. In addition, China's microbial improvement of saline land started late, is currently only in the initial stage, microbial fertilizer production is not standardized. The effect of fertilization is unstable, which seriously reduces the effect of microbial fertilizer.

Many saline soils are distributed in places without fresh water, such as coastal saline soil, Northeast, Lanzhou and other inland saline soil, which requires a large amount of fresh water engineering treatment of saline is restrictive, while most of the time the use of fresh water to treat saline soil can not control the regeneration of saline land. After interviewing a large number of professionals, CAU-iGEM chose synthetic biology to modify the dead grass spore bacteria to produce γ-PGA, a substance that helps saline land return to normal. Through the understanding of γ-PGA, we realize that its water retention and moisturizing ability is strong, is conducive to improving the soil capillary structure to prevent secondary salinization, and non-toxic harmless to human body, can be biodegradable, environmentally friendly, while the structure of the base can provide hydrogen ions for alkali soil, and adsorption of salt soil metal ions. The original intention of the team project is to reduce the surface salt content of saline soil, resist the infiltration of foreign salt water, prevent secondary salinization and make the product can be well interacted with plants, so the role of γ-PGA just coincides with our team's goal, so choose it as our action material to deal with saline soil.
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