What is the impact of fire on desert soils of Nevada?

The cold desert lands of Northern Nevada are plagued with wildfire annually. These can start from natural causes like lightning strikes, or human activities like camping fire. The fires in the semi-arid rangelands cause immediate and long-term changes to the ecosystem.

Studies show fire can cause permanent changes to soil physical, chemical, and microbial communities. In forest environments, there can be natural burns, or prescribed burns for management, with different results. Although some studies have been done looking at cheatgrass invasion and ecosystem restoration after burning, not a lot has been done looking at the impact of fire on soil biochemistry in Nevada desert lands.That’s what my research focuses on.

One of the major challenges associated with wildfire in northern Nevada rangelands is the invasion of species known as cheatgrass. Cheatgrass is a non-native annual plant that can grow sporadically after fire. It’s more competitive than native plant species, and steals nutrients and moisture from native plants. This inhibits the growth of desirable native plants species like sagebrush.

sagebrush plants in desert
This photo depicts spacing between sagebrush plants in Northern Nevada. The soil under the bushes tends to have higher average fertility. The soil between sagebrush has lower fertility. Credit: Christina T. Igono

Sagebrush bushes naturally space out over the desert. We refer to this as fertile islands. In northern Nevada, a common type of sagebrush is the Wyoming variety. This sagebrush grows in bunches with spaces between individual plants.

Cold desert rangelands dominated by Wyoming sagebrush are often plagued with cheatgrass invasion after fire. And more cheatgrass can lead to the spread of more fire because it changes the natural “fire buffers” in a natural sagebrush system that has natural spacing.

The soil under sagebrush is widely different from that in the spaces. Soil organic matter, minerals, moisture, microbial communities, and nutrients are greater under the shrub canopies. This makes sense, as there is the root mass of the sagebrush as well as more shade from the actual plant. That attracts microbial and other soil-dwelling organisms that are all part of the natural ecosystem. The interspaces between shrubs tend to be less fertile, or even sterile.

Prescribed burns

Natural resource managers often use “prescribed burns” in prairie and forest environments. These burns are low-heat but burn off dry plants – lessening the chance of a bigger fire from lightning strikes or human error. However, from a research standpoint, the fire temperature of our prescribed burn measured was around 800-1000°C, which is as high as a natural wildfire in a sagebrush ecosystem.  We are looking at prescribed burns to mimic natural wildfire and how it affects soil biochemistry in sagebrush ecosystems.  The effects of fire on soil biochemistry are based primarily on the severity and frequency of the fires.

In our northern Nevada climate, fire has caused a lot of damage. Some areas have sterile soil – killing off microbial life. This interrupts the natural cycle of recycling organic matter and improving soil health.

males tending to controlled burns in desert
To study the effects of fire on desert soils, the research team created areas for controlled burns. There, they did prescribed burns, then studied the effects on the soil. Credit: Christina T. Igono

Immediate effects of prescribed burns

Our study considers the immediate effects of fire on soil nutrients one and three weeks after burning. There are varying results. Fire indeed causes immediate changes to soil nutrients. Soil conditions prior to burning are an important factor in after-burn conditions. The amount of organic matter in the soil as well as plant litter also have an impact. Most importantly, the severity of the fire has a big impact.

Fire even changes some of the chemical composition in soils. We found an immediate increase in phosphorus concentration and organic matter one and three weeks after burning. This was in samples taken both under and between sagebrush shrubs. This is not particularly surprising as phosphorus is known to be higher in alkaline soil and tends to increase after fire.

female scientist in lab holding tool
The author, Christina Igono, in her lab analyzing samples. Credit: Christina Igono.

Our expectations in total carbon and nitrogen content positively correlates with the organic matter content after burning. The soil organic matter was higher under the shrub canopies than the spaces between shrubs. It increased one and three weeks after burning. The immediate increase in organic matter was caused by part of the research process. Because we wanted to promote a consistent, uniform burn, we added dry straw to the top of the soil. Thus, this increased the organic content immediately after the burns.

As with many scientific studies, we found some conflicting results about the inorganic nitrogen levels.  There was an increase in ammonium and a decrease in nitrate after burning. Nitrate was lost via volatilization.

Although not a lot of studies have been done on the effect of fire on sagebrush ecosystems, we were able to see small increases in fertility in a very short time after burning. Our next studies will research the long-term effects of fire on soil chemistry and fertility. We are also observing fire effects on soil microbial communities, micronutrients, and greenhouse gas fluxes.

Answered by Christina T. Igono, University of Nevada Reno

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