In a recent presentation, titled Managed Sheep Grazing Can Improve Soil Quality and Carbon Sequestration at Solar Photovoltaic Sites, researchers from Temple University investigated the effects of periodic sheep grazing on soil properties. Micro and macro nutrients, carbon storage, and soil grain size distribution at six commercial solar PV sites were compared to undisturbed control sites.
The authors point to existing studies which show biomass production of forage species under shade (e.g Pang et al, 2019 Agroforest Syst (2019) 93:11–24) Data from the: Horticulture and Agroforestry Research Center (HARC), New Franklin, Missouri. One of the more interesting findings of this research describes the relationship between levels of shading and the biomass production of various plants, shown in the chart below. The analysis found that the ‘meaningful forage productivity’ of most of their chosen ruminant appropriate species significantly increased under 45% shade, while most plants under 80% shade saw a marked decrease in biomass volume.
The results align with prior research from the University of Oregon, which found that solar panels altered the microclimate variables of mean air temperature, relative humidity, wind speed, wind direction, and soil moisture. This improved water efficiency, allowing for much greater biomass growth, drove a 90% increase in grasses for sheep and cows.
The extensive list of plants examined is a definite positive, as the rate of installation for ground-mount solar power accelerates. Efforts are currently being made by the U.S. Department of Agriculture’s National Institute of Food and Agriculture and University of Illinois to determine which row crops, foraging crops, and specialty crops are best to couple with solar panels in agrivoltaic settings.
In this project’s case, land at the facility has been managed by the Minnesota Native Landscapes’ Conservation Grazing Program. Their approach to regenerative land management relies on the planned impact of livestock grazing. This approach gives holistic consideration to the type of soil and plants, as well as the animals which will be grazing, with an eye towards rehabilitating the soil over time. The group works with sheep, bison, cattle, and goats in their land conservation programs.
Across the six sites, researchers allowed 500-700 sheep to graze the sites two to three times per year.
The researchers’ early estimates suggest that an acre of grazed native plants may improve soil at a rate of one ton of sequestered carbon per year, and that carbon and other nutrients may accumulate for 12 to 15 years before the soil is fully saturated.
The total exchange capacity of the soil (a soil fertility measurement based on how many cations can be retained on soil surfaces) showed a marked improvement at four of the six sites, and an overall increase in the estimated nitrogen release, ammonia, and nitrate was found at all but one of the sites. A similar increase was seen in other nutrients as well, including Mg, Na, K, P, Ca, and S.
This data points in the same direction as an analysis of Upper Midwest solar power facilities integrating natural prairie grasses, which sequester roughly half a ton of carbon per acre without livestock. That study demonstrated a 65% greater carbon retention compared to a purely agricultural scenario, and 35% improvement over a solar-turfgrass scenario.
This article was amended on Feb. 8, 2022 to note the separate study on biomass.
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