The microbial community in soils is one of the most important processes that drives productivity in terrestrial ecosystems. Yet little research has been done to better understand how soil microbes interact with plants, wildlife, and insects both above and below the soil surface. The research in this article provides some insights into the relationships between grazing, plant productivity, and soil biology.
The researchers gathered samples of Poa pratensis (a native perennial grass) from inside and outside a 40-year, 2-ha exclosure located in the northern range of Yellowstone National Park. This area supports winter grazing of elk, bison, and pronghorn. According to the paper, the sampling and analysis was designed to conduct “a multifactorial plant growth experiment with plant grazing history (grazed/ungrazed), soil microbial community (grazed/ungrazed), and fungicide (benomyl) as treatments. The fungicide treatment was included [to] determine how the fungal community influenced the results.” Sampling also included soils to a depth of 15 cm, again inside and outside of the grazing exclosure.
Plants were separated out and grown into pots; these pots “were inoculated with soil communities from grazed or ungrazed P. pratensis grassland” and then grown in a greenhouse under naturally simulated light and water conditions. After 5 weeks of growth shoots and roots were harvested from the different treatments and weighed, allowing for the calculation of above ground and below ground biomass. Additionally, the percent of root length colonized by arbuscular mycorrhizal (AM) fungi was calculated, along with the spore abundance, species richness, and diversity of AM fungi.
The authors summarize the results of these measurements:
Whole plants, both grazed and ungrazed combined, grew 34% more with the microbial community of the grazed grassland compared to that of the fenced grassland. Plant biomass was less in fungicide treated pots; probably a result of removing AM fungal symbionts. But more interesting, the microbe effect did not occur among plants treated with fungicide, indicating that the facilitating influence of the grazed microbial community on plant growth was a result of the fungal component of the microbial community.
In regards to the measurements of AM fungi, the authors note:
AM colonization of roots was greater for plants grown with the grazed soil community compared to that for plants potted with the fenced microbial community. By comparison, the mean colonization of roots in benomyl-treated pots was negligible, indicating that our fungicide treatment severely reduced plant-mycorrhizal associations…Spore abundance, species richness and [diversity] were greater in grazed compared to fenced soil. In addition, AMF spore composition was significantly different in grazed and fenced soil…Acaulospora spinosa was twice as frequent and three-fold more abundant in grazed than in fenced soil. Similar patterns were observed for Glomus citricola and for one unidentified species of both Glomus and Gigaspora.
This paper provides compelling evidence that herbivory via grazing ungulates can dramatically affect the species composition and diversity of symbiotic fungi in the soil, hence leading to the improved productivity of certain grass species. As the authors note, there may be an equally dramatic effect associated with soil bacteria, but variables associated with bacteria were not measured. It is possible that the fungicide treatment also had a detrimental effect on soil bacteria.
This paper provides some insight as to why grazing can often improve the health and productivity of land. The actual mechanisms at work are still poorly understood, and questions remain: What is it about grazing animals that promote the colonization of AM fungi on perennial grass roots? Is it related to biophysical characteristics, like hoof action, dunging, and urinating? Or is it a physiological plant response induced by the defoliation associated with grazing? Are soil fungi also dependent on combinations of bacteria and other microorganisms associated with specific species like bison and pronghorn?
The challenge for Holistic Management is to assimilate research such as this into a robust management framework. The improved management of ecosystem community dynamics is perhaps the greatest challenge facing us in the years to come. Our ability to face this challenge largely depends on our capacity to understand and apply the lessons of research and practice in the field. Working with researchers, papers such as this can provide a basis upon which actual monitoring tools can be developed and applied in a real world management context.
Frank, Douglas & Gehring, Catherine & Machut, Leonard & Phillips, Mark. (2004). Soil community composition and the regulation of grazed temperate grassland. Oecologia. 137. 603-9. 10.1007/s00442-003-1385-2.
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