Introduction

Population growth, land use and urbanization are currently increasing rapidly. This is accompanied by a high volume of food and green waste such as kitchen and garden waste in cities. In many countries, this waste is not recycled but is burned or ends up in landfills, releasing significant amounts of greenhouse gases and polluting groundwater as well as the environment. Waste contains many nutrients that are released into the atmosphere. At the same time, many soils suffer from nutrient deficiencies, leading to loss of soil organic matter, soil degradation and desertification in many countries. 

Furthermore, the impacts of climate change are worsening on a global scale. This not only happens in dry areas, but also poses a major threat to temperate zones such as Central Europe. All this can lead to three main problems worldwide, namely soil loss, erosion,  and salinization. In doing so, they endanger the health of the soil and the livelihoods of local farmers as well as the livelihoods of the rapidly growing world population. In such a situation, to produce food for 8 billion people at a time of rapid soil loss, endangering soil health and improving soil quality is a major challenge. 

In addition, the Paris Agreement aims to limit global warming to well below 2°C while in the same year the UN's 17 Sustainable Development Goals were adopted to achieve a better and more sustainable future for all. In order to achieve these goals and avoid disruptions, the concept of circular economy should be pursued in agriculture to promote sustainability. In addition, the question of eliminating the pollutant potential of green and food waste should also be addressed here. The accumulation of green and food waste in the soil can cause it to slowly break down that can contaminate soil, air and groundwater. To prevent this, green and food waste can be converted into valuable products, such as a natural soil amendment. This happens primarily under the concept of the circular economy.

Recycling green waste; an action to decrease and remove greenhouse gases for climate protection

More than 80% of green and food waste such as kitchen, garden and agricultural waste is not recycled. They usually end up in landfills or are burned in open fields. The accumulation and burning of the waste releases large amounts of CO2, as well as other greenhouse gases such as methane, into the atmosphere, causing the Earth's global temperature to rise. Returning nutrients to the soil through recycling urban waste is an opportunity to eliminate landfills and reduce global greenhouse gas emissions toward the Paris Agreement goal. It also promotes soil health and supports the soil's ability to remove greenhouse gases. 

Soil is the second largest carbon storage on earth after the oceans. The healthier the soil is, the more carbon it contains. Depending on how soils are managed, they can absorb thousands of tonnes of CO2 per year, making a massive contribution to climate protection and climate change mitigation. Increasing resilience, food security and boosting productivity are important functions of healthy soils that can be promoted through nutrient recycling.

Green waste to natural soil amendment; promoting concept of circular economy in practice

Converting green and food waste into resources with fewer pollutants requires an efficient solution. Following the concept of circular economy green and food waste can be transformed from a problem into a resource and can become part of the solution. In addition, converting green waste into valuable products returns necessary nutrients to the soil. This allows nitrogen and carbon cycles to be closed to improve soil health and fertility. It also increases the soil's ability to remove atmospheric CO2, making soil a carbon sink rather than a carbon emitter.

To reduce the environmental impact of waste, green and food waste in cities can be collected with the support of the city council. This waste can be processed and recycled into a natural soil amendment. Natural soil amendments contain organic matter and provide sufficient nutrients that support the growth and reproduction of microorganisms. These products are mostly porous structures, they act as a house where fungi and bacteria as well as other microorganisms can colonize. In addition, because of these sponge characteristics, they bind nutrients and increase the water storage capacity of the soil. This structure is a boost for soil health. 

This combination contains millions of microorganisms and is free from harmful substances. These products keep the soil alive and create the conditions for the increase of organic matter (humus) in the soil. This ensures a sustainable transformation of waste into humus-rich soil and more sustainable food production.

Bringing back nutrients to sustainably improve soil health  

Natural soil amendments make the soil healthier. They contain a high level of organic matter, which is a key factor in improving the chemical, physical, and biological properties of the soil. They mostly contain the necessary nutrients. This leads to high soil fertility and balanced soil functionality. They also contribute to climate protection by increasing carbon sequestration in the soil. Above all, the high proportion of organic matter makes them a suitable habitat for many organisms living in the soil. This promotes an increase in soil biodiversity. So the soil is kept alive by protecting the biodiversity in the soil.

Circularity for soil health; making food production more resilient in the rapidly changing climate 

Necessary nutrients returned to the soil through the circular economy promote soil health. This is very important for the production of food. Plants grown in healthier soils are much stronger and produce higher yields. Better and healthier soil makes plants more resilient to long dry periods that occur regularly due to climate change. These soils contain a lot of organic matter (humus), which stores water and serves as a nutrient reservoir. This helps the plant survive periods of drought and get enough water to continue growing. 

In better soils, the risk of soil degradation and erosion is low, which counteracts high crop productivity. In arid and semi-arid areas, better soils help increase water availability for plants, resulting in higher crop yields. Soil salinity is one of the factors closely linked to soil degradation such as soil erosion. The increase in water-soluble salt content in the soil, called soil salinity, can significantly affect soil quality and vegetation cover. Therefore, soil salinity increases the soil's susceptibility to easy erosion by wind and water. In such a case, massive soil degradation can occur, destroying soil structure, depleting nutrients and microbes, and ultimately leading to desertification in arid and semi-arid regions. In better soils, such a situation would be massively reduced or even prevented by high organic matter content. It has high nitrogen and carbon content that decreases the salinity of the soil and supports vegetation growth. 

Additionally, more water is recycled as the water-holding capacity of better soils increases. This in turn reduces soil compaction and improves soil structure. Therefore they don't need to be watered as often. Overall, natural soil amendment products are a positive result of increasing yields in terms of, for example, biomass as well as the photosynthesis potential of trees. This allows farmers to generate more income by selling the excess biomass as a feed or food source. This is the positive outcome of the circular economy as it converts waste into a useful resource. It promotes soil health, which in turn increases and protects biodiversity above and below ground. Last but not least, circularity promoting soil health removes or breaks down pollutants from the air, soil, and water, which is a necessity in large cities.

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