Prof.Dr.-Ing.WANG Xiangrong

Chief Editor

When mentioning biodiversity,we first think of plants and animals,yet the most biodiverse place on earth is the soil beneath our feet.According to statistics,every gram of soil contains thousands of species of bacteria,amounting to tens of millions,more than 1,000 species of invertebrates live in every square meter of forest soil,and soil biodiversity contributes more than 25% of global biodiversity.

Soil is the skin of the earth,a dynamic,living and complex ecosystem.The organisms living in the soil are rich and diverse,not only the common ants and earthworms,but also countless microorganisms invisible to our naked eyes.Various organisms in the soil interact with each other and are interdependent with other animals and plants in the ecosystem,forming a complex system in which ＂all things that grow live in harmony and benefit from the nourishment of nature＂.When organisms lose their life,they will be integrated into the soil and transformed into nutrients by the action of soil microorganisms,while improving the structure of the soil.The organisms in the soil can decompose not only organic matter,but also minerals,to increase soil fertility.Healthy soil can make plant roots more developed,grow stronger,and breed more diverse underground and aboveground biological habitats,thereby enriching biodiversity and enhancing ecosystem services.

Soil is the source of human food.According to the Food and Agriculture Organization of the United Nations,95% of the world's food comes directly or indirectly from the soil.Soil biodiversity ensures the healthy growth of crops,provides the nutrients needed by crops,and ensures the nutritional balance of agricultural products,which is ultimately beneficial to global food security and people's dietary health.

Soil biodiversity is also beneficial to human health.Studies have shown that children who live in a biodiverse environment in their early stages grow up to have a more diverse microbiome and are also able to develop greater resistance to disease and prevent health problems such as allergies,asthma,and autoimmune diseases.Soil is the natural element with the richest biodiversity on the earth and the most accessible environmental factor for human beings.Therefore,modern medicine encourages children to be exposed to healthy soil.There is an ancient custom in China that a wanderer should take a bag of home soil with him when he travels far away from his hometown,in case he is not acclimatized in a foreign land.This custom actually has considerable scientific basis,because the environmental microbiota of each region is different,and people who have lived in a certain place for a long time are likely to suffer from physical discomfort or even disease after changing the environment because they do not adapt to the microbiota of the new place.And microbiomes in home soils can help people better transition and adapt to new environments.Biodiverse soils are therefore essential to the health of humans and other plants and animals.

The biodiversity in cities is lower than that in rural areas and natural areas,because the soil area and soil quality in cities are far lower than the latter two,and the foundation for breeding natural ecosystems is weak.The surface of city is hardened or covered by buildings and infrastructure in a large area.The natural soil area on the surface is small,and the living space of organisms is severely squeezed;rainwater on the hardened surface cannot infiltrate into the ground,resulting in the reduction of soil water recharge,and the heat island effect causes excessive evaporation of soil water,resulting in the reduction of soil water content in urban soils,thus endangering soil biomes;urban construction has changed the soil matrix in many areas of city,a large amount of soil is mixed with gravel,muck,bricks and tiles left overfrom urban construction waste,and the soil has poor water holding capacity and low fertility;the city has a large population and has a strong interference with nature,resulting in a dense urban soil structure and poor air permeability,which affects the development of plant roots;dead branches and leaves of plants in the city are often removed as garbage,which interrupts the cycle of soil nutrients and soil organic matter cannot be replenished,resulting in a decrease in soil fertility;some urban soils are affected by pollution from garbage,sewage and pesticides,and biodiversity is destroyed.

In a high-density city where surface soil is scarce,protecting precious soil resources is fundamental to maintaining urban biodiversity.On the one hand,we can minimize the area of urban hard surface under the premise of satisfying various comprehensive and complex functions of the city,leaving more natural soil surface space for the city,planting native and suitable plants,and enriching urban habitat types and species diversity to maintain and improve the ecological environment of the city;on the other hand,we can make use of the surfaces of building roofs,terraces,elevated floors and infrastructures and cover a corresponding thickness of soil to form a soil surface,plant barrentolerant plants,and make a ＂bio-friendly＂ space with certain ecological value,thus increasing the proportion of soil surface area in the city and compensating the encroachment of urban construction on the natural surface to a certain extent.

Through Nature-based Solutions,we can also improve the physical and chemical properties of soil to a certain extent and promote the restoration and construction of urban soil biodiversity.For example,in urban green areas,leaves and branches can be retained in the forest understory to promote material circulation between the soil and plant residues;in large green areas,dead and fallen trees can also not be necessarily cleaned up,providing habitats for some animals and microorganisms;composting can be used to return dead branches and leaves to the land,increasing the organic matter content,fertility,permeability and water retention capacity of the soil,regulating soil acidity and alkalinity,improving soil structure,and promoting soil health.In addition,measures can be taken to allow more rainwater to infiltrate back into the ground to maintain soil moisture for the survival of soil organisms.Falling leaves return to roots,dead branches return to soil,and rainwater enters the ground,so that the urban soil has a relatively complete material cycle process,which will effectively improve the soil quality and make it more suitable for the survival of organisms,thereby improving the biodiversity and functions of the soil.

In areas without human interference,the soil develops and evolves in full accordance with the laws of nature,and its biodiversity is the highest.If some natural zones in cities can be preserved and left to selfcirculation and natural succession,soil and surface ecosystems with rich biodiversity can be nurtured,creating low-maintenance,sustainable urban ecological spaces.

Soil is the repository of biodiversity,and a thriving natural ecosystem starts with soil.Urban soil health is the basis for ensuring the stability of urban ecosystems and the quality of ecological services,and it is also a necessary condition for sustainable urban development.To improve urban biodiversity,it is necessary to expand the soil surface in cities and give urban soils more natural properties and a more complete material cycling process.Only by using urban land scientifically and keeping soil vitality can we restore and maintain the biodiversity of urban soil and build a healthy ecosystem for cities.

Enhancing urban biodiversity starts with paying attention to the soil under our feet.