Restoring Soil Biocrusts by using “Soil Nutrient and Organic Matter Thresholds”

Beneath our feet in grasslands and deserts lies a Lilliputian-sized community of algae, cyanobacteria, lichens, bryophytes, and assorted bacteria, fungi, archaea, and bacteriophages that colonize the soil surface, which are known as biological soil crusts, according to the United States Geological Survey definition.

These tiny masses of living material are known to support the environment, according to scientific literature. Biocrusts provide multiple benefits: stabilizing soils by acting like glue to prevent desert soil from blowing away and causing dust storms; increasing soil fertility by absorbing carbon and nitrogen, which improves the quality of desert soil and supports plants, wildlife, and agriculture; improving soil water retention during monsoon seasons, which is important for the desert ecosystem; erosion control by reducing sediment loss and helping control erosion; improving air quality and climate regulation by fixing atmospheric carbon dioxide; and supporting a complex food webbecause they contain many different types of life, including cyanobacteria, green algae, fungi, protists, bacteria, lichens, and bryophytes. They may also influence and reduce allelopathy from invasive species, according to some studies.

But while biocrusts provide these critical and important ecological services, they are easily disturbed and slow to recover without help.

Biocrusts have gained increasing importance in scientific inquiry. They are estimated to reduce global atmospheric dust emissions, which contribute to global warming, by approximately 60%, according to a May 2022 research briefing, "Global cycling and climate effects of aeolian dust controlled by biological soil crusts," published in the journal Nature Geoscience.

Until 2070, biocrust land coverage is expected to be severely reduced by climate change and intensified land use, according to the Nature research briefing.

Biocrusts -- and the potential to restore them -- recently took center stage during a recent web discussion by the Dust Alliance for North America, a partnership of scientists and practitioners, that took place September 13. And while small-scale restoration methods have added biocrust inoculant to cloth, which is tacked to the ground, the process had not been proven implementable on thousand-acre or million-acre scale.

But soil-fertility analysis I have done of areas where native biocrusts are present and where they are missing, found key components of nitrogen, phosphorus and organic matter that are crucial at minimum threshold amounts for the biocrust system -- and the overall native plant ecosystem -- to meet self-sustaining restoration levels.

The biocrust ecosystem needs a minimum amount of nitrogen, phosphorus and organic matter in the top 5 cm of soil in order to survive and function, according to my tests of biocrust-positive and biocrust-negative soils.

I first began studying the need for added mulch and improved soil-nutrient levels in grasslands in 1993, when I undertook a project to replant a 100-mile stretch of the Tuscarora gas pipeline north of Reno with native grasses. The area was located in the sagebrush and cheatgrass-infested desert in 6-8 inches of annual rainfall.

In areas where we did not add the organic fertilizers and mulch when sowing native seeds, the seeds would sprout and grow, but they then shriveled up and died of starvation when 5 cm tall. Photos at www.ecoseeds.com/good.example,html.

But soil tests taken from the top 5 cm determined the local soil nutrient and organic matter threshold necessary for seedling survival of each of the five native grass species we were planting. Combined with sowing the seeds densely enough so that the native seedlings that produce allelochemicals would suppress the cheatgrass seeds from sprouting, we were able to produce 100% native cover in six months. Thirty years later, the pipeline planting is still 95% native cover. Pictures at www.ecoseeds.com/greatbasin.html.

We did not, however, seek to identify any biocrusts nor any nutrient levels needed to encourage them at the pipeline site.

Fast-forward to 2016. For the last eight years, I have been working to restore a 14-acre serpentine grassland at Kite Hill in Woodside, California by expanding on the same "species threshold" methods I used for the pipeline project: determining the nutrient thresholds for individual plant species and, additionally, the thresholds of allelochemicals that would allow or suppress the native seed bank in the soil.

Adding organic matter and nutrients at thresholds necessary for seedling survival had some dramatic effects. In one area that I call "the barrens," a serpentine slope with 0% vegetation cover used as a bike path, sprouted a thriving native meadow from dormant seeds that is 99% weed-free. No seeds were sown.

Soil tests showed adding 1 mm of organic fertilizers and 3 mm of mulch were enough to coax the seedbank from the soil.

But there was another surprise: biocrusts were also evident throughout the treated area where none was evident before. This begged the question, "what soil nutrients, and at what levels, are needed for biocrusts to thrive -- and perhaps to expand?"

The next step, then, was to begin comparing soil nutrient levels where there were biocrusts and where they didn't thrive. In a 10-acre location of the preserve, once the we got the weeds managed and the native plants started growing back, biocrusts were growing back. However, since the recovery was spotty, we tested the biocrust-positive and the biocrust-negative areas to find the nutrient relationships between the two.

In paired soil tests, I sampled the soil from a biocrust-positive site, and a biocrust-negative site located one meter away from each other. A quart sample was removed from each site from the top 5 cm of soil. The biocrust was included in the sample from the biocrust-positive site. Waypoint Lab in Anaheim, California tested for the PPM of nitrogen and phosphorus and the percentage of organic matter in each sample. The lab used the A-01 test “data only in a bar graph format,” and a separate organic matter test. The total cost for the paired tests is $96.

The nitrogen biocrust-positive threshold showed 25% more nitrogen than in the biocrust-negative test. The phosphorus biocrust-positive threshold was double the biocrust-negative level; and the organic matter biocrust-positive sample was 50% more than the biocrust-negative level in the top 5 cm of soil.

I used this first paired test as a guide for setting up test plots. The value of paired soil testing should be viewed only to get an idea of the relationships between these three nutrient levels. Testing fertilized and mulched biocrust plots in different amounts, could further define the nutrient parameters -- the "sweet spot" to potentially grow back biocrusts in one year. Those tests are currently in progress at Kite Hill. Theoretically, fertilizers and mulch mix alone might be able to restore the biocrusts rapidly. Adding very small amounts of biocrust inoculum could be tested as well to increase biocrust recolonization.

Rainfall comparisons for biocrust nutrient thresholds

The Kite Hill location receives 20 inches of average rainfall annually. Additional paired soil tests from samples taken in Central California based on total annual rainfall also indicated that necessary nutrient levels have a sliding scale of thresholds based on rainfall. While these samples are small and preliminary, there may be a correlation between rainfall levels and thresholds for biocrust-positive soils elsewhere. These findings could be particularly important in low-rainfall areas where large tracts of desert are hoped to be restored.

The more annual rainfall, the higher the threshold appeared to be for the nitrogen, phosphorus and organic matter.

Table 1. Annual rainfall in Central California -- in relation to the nitrogen, phosphorus and organic matter for biocrust thresholds. Rainfall is in inches, PPM for nitrogen and phosphorus, and percentage for organic matter in the top 5 cm of soil.

While much more research is necessary, I was encouraged by these findings. But one should not consider the results of any one location as a recipe for all sites. Each location, whether coastal, desert or montane, must be sampled and considered as to its soil type, rainfall, vegetation and perhaps its biocrust composition. But paired soil samples can help reveal what the nutrient parameters might be, for restoration of the local biocrusts.

Mixing the organic fertilizers with the mulch for nutrient and organic matter thresholds needed for the local biocrusts, and hydro-mulching an area with that mix, could be useful to try in small-scale test plots in the future. Caution might be used, however, regarding the use of tacifiers such as psyllium powder or guar gum, which might entrap insects and small animals.

Copyright © 2024 by Craig Carlton Dremann, The Reveg Edge, P.O. Box 361, Redwood City, CA 94064 – Email craig@ecoseeds.com – Phone 650-325-7333