By Sally Brown, University of Washington
Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members.
Effects of fire on properties of forest soils: a review
Polyacrylamide application versus forest residue mulching for reducing post-fire runoff and soil erosion
Effectiveness of three post-fire rehabilitation treatments in the Colorado Front Range
Biosolids applications affect runoff water quality following forest fire
Composts as post-fire erosion control treatments and their effect on runoff water quality
It rained. So we can breath again.
The first 15 years or so that I lived in Seattle fire season wasn’t a thing. Times have changed. In 2020 with the fire season just beginning in CA and thankfully ending in OR and WA, we have close to 5 million acres burned. That is 3 million acres in CA, just under 1 million in OR and 0.6 million in WA. Fire destroys trees, understory, buildings, wildlife, and unfortunately people. It also destroys soils. Sticking with the good news that is 2020, this library is all about fire’s impact on soils and how biosolids and compost can fit in to limit the damage.
To understand how biosolids/ composts can help soils after a fire you first have to understand what fires do to soils. That is article #1. A summary of that article is also available in my latest column for Biocycle: https://www.biocycle.net/connections-fire-and-soil/. The damage that fire can due to soils depends on how hot the fire gets and how deep into the ground it burns. The organic matter in the soil is the flammable stuff. A fire can burn off much of the organic matter including the carbon and nitrogen. It can also transform the carbon into different forms graphite being one of those forms. Fires can essentially turn soil organic matter into pencils. You can use this graphite to write with (you’ll need to supply your own eraser) but it isn’t so good for maintaining soil structure. It can also turn some of the metal oxides into a type of cement. The end result of this is that the burned soil is typically low in active carbon and nitrogen. Soil structure is typically compromised with higher bulk density and lower pore space. It is also hydrophobic. When the rains come these burnt soils are highly susceptible to erosion and very reluctant to let any water infiltrate. That can add insult to injury when a mudslide comes down the burnt mountainside to rest on what remains of your home after the fire.
A first goal here (apart from stopping those gender reveal parties and preventing the fire in the first place) is to limit the damage once the fire has been put out. The next two articles in the library review the efficacy of traditional methods to control erosion. The first is from Spain where the authors compare polyacrylamide (PAM) and chopped up forest residue for stopping erosion post burn. The forest control plot lost 8.4 Mg ha of soil that first year. So did the PAM. The forest residue only lost about 0.8 Mg ha of soil. Forget the PAM as an option. The second study took place in Colorado. Here they tried seeding, straw mulch and contour felling (cutting the dead trees so that they fall perpendicular to the slope). Here the control plots lost 6-10 Mg ha of soil in the first years after the fire. This decreased to 1-2 tons the next year. Soil lost is typically greatest the first year after a burn. Mulching was the winner here, reducing soil loss by over 95% compared with the control. Contour felling was a mixed bag, performing well in some cases but not with big storm events.
If mulch and straw can do it, my educated guess is that biosolids and compost can do it one better. Paper #4 tests just that with biosolids compost in Colorado. This is one of a series of papers from this group on the Buffalo Creek site. They applied the compost at 40 and 80 Mg ha to the soils. Sediment loss was measured after simulated rain events. The high rate of the compost reduced sediment loss compared with the control. It also improved plant growth, which in turn will reduce erosion. My issue with this study is that the rainfall simulation took place 3 years after the fire and 2 years after the compost application. As stated earlier, most of the sediment is lost the first year. So while this study is great and the first in the field, they missed the most important window. Study #5 got right on it.
For study #5- they arranged exactly where and when the fire would occur. The authors worked with the local fire department to set up a control burn of the site. That likely meant that the fire severity was mild, but still. The biosolids and green waste compost were spread less than two months after the burn and before the rains came. The treatments included two mesh sizes of the green waste compost applied at 2.3 and 5 cm depths and two rates of the biosolids with the 5 cm depth also incorporated. While they didn’t measure soil loss per say, they did measure runoff and total suspended solids. All treatments reduced runoff by over 75% compared to the control. No differences between them. Same with total suspended solids. There were differences in nutrient flows from the plots. The biosolids compost at the higher rate lost A LOT more NH4 than the control or the other treatments. Different treatments lost different amounts of P, but all pretty much lost less than the control. These reductions in sediment loss and water volume were reflected in reduced turbidity for the water coming off of the plots as well. There is a whole section on metal loss but I stopped losing sleep over metals awhile back.
The group that did study #5, David Crohn at UC Riverside is working on another study on soils that were burnt by accident. No results or pictures so far. It is true that there is limited work on how to best use composts to stop erosion after fires. What we do know sounds like this is an answer. We don’t have the full body of research to know exactly what’s the best approach. We certainly have enough acreage out there to do a few studies/ demonstrations. I personally think that if you mulched some of the dead wood on site and mixed it with some Class B cake- you’d have a great recipe for eliminating soil loss and limited nutrient movement. Anyone want to try it?