By leaving garden waste alone, we could store 600,000 tons of CO2 per year

Danes shuttle nearly a million tons of garden waste from their gardens every year. But we would be doing the climate a great service if we just left it alone, according to the calculations of University of Copenhagen professor Per Gundersen

Reports and Proceedings

UNIVERSITY OF COPENHAGEN - FACULTY OF SCIENCE

Danes shuttle nearly a million tons of garden waste from their gardens every year. But we would be doing the climate a great service if we just left it alone, according to the calculations of University of Copenhagen professor Per Gundersen. At the same time, leaves and other garden waste are a boon for backyard biodiversity and soil.

Come fall, the sound of a leaf blower blasting leaves into piles, or the sight of a queue of garden waste-packed trailers at the recycling center is all too common, as trees shed their golden brown and yellow robes.

For the most part, Danes are happy to "tidy up" their gardens. Ministry of Environment figures report that Danes disposed of 983,000 tonnes of garden waste in 2019, all of which is transported, sorted and processed by municipal waste management systems. The largest branches and trunks are burned for bioenergy, while small branches, leaves and grass clippings are composted.

But perhaps we should leave more of our garden alone. According to Per Gundersen, a professor of forest ecology at the Department of Geosciences and Natural Resource Management at the University of Copenhagen, there are large CO2 savings to be made. Professor Gundersen calculated the implications for Denmark’s national climate account if Danes simply allowed garden waste to decompose in their gardens instead of burning it.

"If everyone went all-in and learned how to manage their garden’s branches and twigs, for example, my calculations suggest that we could store about 600,000 tons of CO2 annually," he says.

The calculations are based on a simple model that includes the different decomposition processes for leaves, twigs and branches.

When Gundersen deducts the utility that branches and trunks used for bioenergy in the current system provide, the CO2 storage potential of garden waste could meet about half of one percent of Denmark’s 70 percent reduction target for 2030.

"When garden waste is burned or composted by municipal waste schemes, CO2 is returned to the atmosphere very quickly. By keeping waste in the garden, the decomposition process is significantly slower. In practice, this means that one builds up a larger and larger storage of CO2 in the garden in the form of twigs, dead branches and leaves that are left to decompose," says Professor Gundersen.

Autumn leaves are good for the garden and biodiversity

Leaves are flying about and filling up yards at this time of the year. But instead of raking them up and hauling them off to a recycling center, there are other great reasons to leave them in the yard.

"Leaves help nourish the garden as they feed an entire ecosystem of decomposers. From fungi, bacteria and tiny, invisible soil creatures to earthworms and roly poly bugs, which help break down and metabolize organic material so that nutrients are released into the soil. These decomposers then act as important food source for larger animals like hedgehogs and birds," explains Per Gundersen.    

As microorganisms, worms and other decomposers wage war on the leaves, a large part of the biomass ends up as CO2 that is slowly released into the air over several years. A small portion of the gnawed leaves becomes soil humus, a common name for the complicated organic material that takes a very long time to decompose and is of great importance for plant health.

"A high humus content in topsoil makes for a dark soil that has a good structure and retains water and nutrients well, which is important if you want garden plants to thrive," says Per Gundersen.

Facts

• According to figures from the Danish Ministry of Environment, Danes disposed of 983,000 tons of garden waste in 2019. This figure includes garden waste from businesses and parks.
• The garden waste is extracted CO2 from the atmosphere. Converted, this amount corresponds to about 1.2 million tons of bound CO2.
• If Danes kept their garden waste in their yards and deducted the amount of garden waste used for bioenergy, about half of one percent of Denmark’s 70 percent reduction target for 2030 could be met.
• Leaves consist largely of the gas CO2. Minerals taken up into the soil account for less than 5% of a leaf.
• Leaves are small CO2 stores that absorb CO2 from the air and release some of it while decomposing.
• Leaves take 3-6 months to decompose, until half are left (their half-life), while twigs and branches take 2-5 years. Tree trunks decay over 10-20 years.
• Find inspiration for managing 'yard waste' on the Facebook page 'Klimahaven'

We must dare to let go of control

But what to make of all those leaves suddenly filling up the garden? Per Gundersen suggests raking them into planting beds where vegetables are grown, or over to areas where you want to avoid weeds. They can also be gathered in a part of the garden that you may be willing to let grow wild and manage itself.

"Funnily enough, while the wind tends to blow leaves around the garden, they often remain in beds or around bushes where there is less wind, which is a good thing," he says.

By spring, a large portion of the leaves will have already disintegrated and returned into the invisible garden cycle. Leaves from linden and ash trees are especially good at disappearing quickly from gardens. Oak and fruit tree leaves are slower to break down, making them ideal for soil insulation around bulbous plants or vegetables, for example.

"Our gardens can contribute to both the climate and biodiversity crisis by harnessing more garden waste. I also think that it will lead to a little less gardening in the long run. And then it's more fun, because there will be a greater abundance of life around the garden. We just need to dare to let go of control and make more room for nature by allowing for twigs and a few dead branches, for example" concludes Per Gundersen.

WVU researchers dig into study of manure as organic fertilizer

Grant and Award Announcement

WEST VIRGINIA UNIVERSITY

 

IMAGE: A MANURE SPREADER DRIVES OVER A PLOT OF LAND FOR A PREVIOUS RESEARCH PROJECT CONDUCTED BY ELIZABETH ROWEN, A SERVICE ASSISTANT PROFESSOR OF ENTOMOLOGY IN THE WEST VIRGINIA UNIVERSITY DAVIS COLLEGE OF AGRICULTURE, NATURAL RESOURCES AND DESIGN. view more 

CREDIT: SUBMITTED PHOTO

These West Virginia University researchers are the No. 1 experts in No. 2. 

Led by Elizabeth Rowen, a team of faculty from the Davis College of Agriculture, Natural Resources and Design will study the use of manure as an organic fertilizer, thanks to a $750,000 grant from the United States Department of Agriculture.

More specifically, they are digging into the most effective ways to reduce insects and pathogens that threaten crops.

The U.S. is the world’s largest consumer of beef — much of which is grain-fed — and spends $233 billion annually to import wheat, corn and soybeans for organic livestock feed. However, domestic field crop producers might have an opportunity to meet those needs if the barriers to organic production, like pests and nutrient sources, were lowered.

The key may be cow manure, which helps build nutrient content and organic matter content of the soil and bolsters soil health.

“In organic production, manure is very common because it is one of the only sources of nutrients that you can have,” said Rowen, assistant professor of entomology. “You can’t use chemical sources of nutrients to grow things like organic grain to feed your cows. You have to produce them with manure or other types of composts.”

Manure not only makes an effective fertilizer, meeting plant nutritional needs, but it’s also a common one. It’s readily available in Pennsylvania and West Virginia, where the livestock and dairy industries are prevalent.

Agricultural managers can apply manure in dry-stacked, liquid or composted form, but it also may bolster weeds, insects and soil pathogens, all of which will damage organic crops. Rowen and her colleagues are studying which form is most likely to carry or combat these pests.

“We have put together a team that's going to look at the soils, the weeds, the microbiome of these plants,” Rowen said. “We’re looking at how the microbiome from the manure transfers to the soil, and how it then affects insects. Because there's good evidence that applying manure can make plants more resistant to insect pests.”

Researchers don’t know why, but possible factors include the microbial community in the soil or the micronutrients from the applied manure fertilizer.

The right soil conditions will also encourage beneficial insects like spiders and ground beetles, which are important predators in agricultural systems. Organic matter in the soil feeds and stabilizes predator populations so they can prey on destructive insects.

“We know that organic systems tend to have fewer insect pests even though they’re not getting sprayed with insecticides,” Rowen said. “But is there something about manure that’s making these plants more resilient? We’re going to be investigating in the greenhouse, trying to pick apart that system. I’m really excited about it.”

Cost effectiveness, too, plays a role in what fertilizers a farmer may choose, so the research team will be looking at ways to make an organic farm more profitable.

Researchers at the WVU Davis College of Agriculture, Natural Resources and Design are utilizing plots at the WVU Agronomy Farm to research effective ways to use manure as organic fertilizer

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Researchers at the WVU Davis College of Agriculture, Natural Resources and Design are utilizing plots at the WVU Agronomy Farm to research effective ways to use manure as organic fertilizer

“The U.S. is importing organic grain, and it’s got a higher economic value than conventional grain,” Rowen said. “That’s one of the reasons you would transition to growing your own organic grain, because the alternative is expensive.”

She said team member Ana Claudia Sant’Ana, assistant professor of resource economics and management, will be looking at how fertilizer management affects profitability. Other members of the research team include Ember Morrissey, Jim Kotcon, Eugenia Peña-Yewtukhiw and Rakesh Chandran.

Currently, Rowen and her colleagues are preparing for the three-year study at the WVU Agronomy Farm where they’ll plant four different crops. In the spring, they’ll apply either liquid, raw (dry-stacked) or composted manure to the soil before planting corn, soy and wheat. The fourth will be a control that doesn’t receive any fertilizer. Researchers will also have plots that are certified organic and others that will transition to organic. The latter will represent farmers who want to become organic and need an appropriate fertilizer.

“We want to be able to present the options,” she said. “Especially for people who are trying to decide what to invest in. Things like composting their manure and using liquid manure, those are investments in terms of equipment and time and such. If there’s really an advantage for their pest management strategies, it's valuable to have that information.”

Rowen hopes the results will support decision-making in the future. In a nation with growing interest in organic milk and beef, she sees the potential to increase organic feed production and help farmers transition to more profitable organic production.

“We drink a lot of milk and we eat a lot of beef,” she said. “We’re not getting rid of cows anytime soon.”