calculating carbon storage for 2,100' sq. rain garden

Hi folks,

Last year, we converted 2,100' sq. of turfgrass into four rain gardens filled with 34 species of native Illinois sedges and forbs for a total of 2,300+ plants. It took three years and was the final part of a three-year LEED Platinum 4.1 remodel. Our site was one of the featured gardens on a tour back in May and someone asked "do you know how much carbon you are storing in the soil?" and I only had "uhhhh, I dunno" as an answer. So I thought there should be a way to figure this out and my initial Googling revealed there are different methods. I'm posting here to see what the community has to say as far as the most accurate way to figure this out -- just so I can get a rough estimate to provide an answer when I am asked again. I figure we _have_ to be storing more now than we were with 100% turfgrass for 20 years, but I don't know how much. Thanks.

Soil Carbon Storage with BioChar
Area = 2,100sqft
Depth = 1ft (2ft at 50% biochar)
Volume = 2,100 cubic feet
Density = 10lbs/cubic feet

Total Weight = Volume * Density
Total Weight = 2,1000ft3 * 10lbs/ft3
Total Weight = 21,000lbs

I would have been nice if that was your answer in your reconstructed peatland/wetland/rain garden, but its not.


But I have seen studies that say that sedge captures the same amount of carbon as grasses but 20% more of it is stored in the roots/underground. Seeing as how regular soils have about 5% carbon/biochar vs the 50% that I gave in my best case answer I would just divide that by 10. To get around 2,100lbs.

I have to imagine that varying root mass/root depth and the density of the plants in each rain garden would be valid inputs.

I had a similar issue when I wanted to quantify the carbon stored in my new woodland. Logically it seemed that trees would consist of a greater mass than thin turf, but it was not that easy. I suggest work it out in a couple of ways and average the answer. Short of digging up and burning your soil and plants, I don't think there is an exact answer.

I'd love some pictures of your rain garden! I'm contemplating some bogland as a soakaway when we do our new extension, so all ideas are welcome.

Source: Nature paper
This paper may be of use? You could perhaps use the graphs and your number of grass species to work out a now and future level of soil carbon?

Here's our site. All of this is irrigated by a 1,350-gallon rainwater harvesting system under the deck. In addition to the 2,300+ rain garden plants are 12+ fruiting shrubs, which I gather should be storing carbon, too.

The rain gardens capture and store up to 11,200 gallons and are designed to withstand a "100-year rain event" (8.57"/24 hrs), thanks to a 4+"/hr infiltration rate. That was possible because we required manual excavation to minimize soil compaction (the kiss of death for infiltration), so no heavy machinery. Took the entire summer of 2022. This is why the front rain garden is a 9"-deep square, flat-bottomed pit (9"-deep at sidewalk and 20" deep at the front porch due to slope) bisected by a raised wooden boardwalk. All impervious surfaces (driveway, walkways, patio, and garage terrace) direct runoff to the rain gardens. This eliminates water running off from our site into the municipal combined stormwater/sewer system.

Mike Baker wrote:Here's our site.

Truly stunning - thank you for sharing!

It would be interesting to know what figures you do come up with for the carbon storage too.

I recommend sending off a sample of your soil to a lab to get the percentage of carbon in your soil. I estimate that the number will be 5%, and so your numbers will probably look something like this:

Estimated Soil Carbon Percentage = 5%
Depth = 2ft
Surface Area = 2,100sqft
Density = 10lbs/cubic feet
Above Ground Weight = 20% of Below Ground Weight
Carbon Growth rate = doubles every 25yrs, until it maxes out when the soil carbon reaches 15%-18% from it's currently point of 5%

Soil Carbon Volume = Surface Area * 2ft depth * 5% = 210 cubic feet

Soil Carbon Weight = Volume * Density
Soil Carbon Weight = 210ft3 * 10lbs/ft3
Soil Carbon Weight = 2100lbs

Total Weight = Soil Carbon Weight + Above Ground Weight
Total Weight = 2100 + (20% * 2100)
Total Weight = 2100 + 420
Total Weight = 2,520lbs (The CO2-equivalent is 10x aka 25,200lbs)
(and in 25yrs it will be 2 times that)