Cities generate a lot of waste. What we think of as biodegradable or compostable material is known as organic waste. Leftover food scraps, fallen trees, and cow poop are all examples of organic waste. When organic waste breaks down in landfills, it releases a greenhouse gas called methane. Methane traps heat in the atmosphere and contributes to global warming. To decrease methane emissions, urban areas like Chicago, IL, are trying to divert their organic waste away from landfills.
One way scientists have proposed to divert organic waste is to use it as fertilizer on urban farms. Organic waste is a good potential fertilizer because it contains nutrients like nitrogen and carbon that plants need to grow. Researchers suggest that using organic waste on urban farms could allow cities to reuse waste materials and cut back on chemical or mineral fertilizers.
Soil microbes like bacteria and fungi break down organic materials for energy, making nutrients available for plants and other organisms in the process. Scientists refer to the community of different microbes in the soil as the soil microbiome. Since all microbes require energy, the soil microbiome is shaped by the different energy sources in the soil. For example, some microbes like to get their energy from sugar in plant material while others prefer to use the nitrogen found in animal manure.
Researchers at the University of California, Davis, and the University of Wisconsin-Madison wanted to know if adding organic waste mixtures to the soil could change the soil microbiome and improve crop growth compared to conventional mineral fertilizers. They produced 4 mixtures of organic waste collected in San Francisco, CA. One was a liquid mixture of expired supermarket produce, one was a compost of food scraps, one was a mixture of poop from plant-eating animals at a nearby zoo, and one was a compost of yard clippings like dead plants. The researchers also included a control mixture containing the main ingredient in mineral fertilizers, a nitrogen compound called urea.
The researchers hypothesized that adding organic waste to the soil would increase the number and diversity of soil microbes compared to the mineral fertilizer. They tested their hypothesis by growing tomato plants in a greenhouse experiment. They applied the organic waste mixtures to soil from a local urban farm and put the soil into pots with tomato seedlings.
The researchers grew the tomato plants for 75 days, then harvested them and sampled the soil microbiome. They used a process that identifies microbes based on their DNA, called 16S rRNA sequencing. They measured the amount of carbon inside bacterial and fungal cells, called microbial biomass carbon, to estimate the number of microbes in each soil sample. They also measured how the microbes moved nutrients through the soil by analyzing the enzymes they released.
The team found that soils treated with liquid food waste and animal poop had 127% and 120% more microbes than the soils treated with urea. They found the identities of the microbes also changed. For example, 3 of the 4 soils with organic waste mixtures contained more carbon-cycling microbes and fewer Planctomyceota bacteria than the soil with urea. They concluded that the organic waste mixtures maintained or even improved the soil microbiome.
However, the researchers were surprised to find that the organic waste mixtures didn’t change the overall microbial diversity or the amount of microbes involved in nitrogen cycling in the soils. The team suggested that microbial diversity could still increase if they fertilized the soils for a longer time.
Next, the team examined the tomato plants. They determined plant quality by measuring plant height and weight, and the number and size of tomato fruits. They also analyzed the taste of the tomatoes by measuring the soluble sugars in the fruit. They found that plants treated with organic waste were 15% to 75% smaller and had 15% to 65% lower fruit yield than those treated with urea. However, the tomatoes were all the same size, and some plants grown with organic waste had better-tasting tomatoes. The researchers agreed that organic waste mixtures could enhance certain crop qualities like fruit taste, but further research is needed.
The team concluded that organic waste mixtures can support a healthy microbiome in urban farming soils. They proposed farmers could use organic fertilizers as alternatives to mineral fertilizers like urea. They suggested that repurposing local waste can help bring more fresh food to urban communities.
