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The case for carbon storage

Alberta grasslands study to help develop policies

Good-news stories for beef producers are beginning to flow out from a massive dataset collected during a three-year carbon benchmarking study done to evaluate the effects of long-term grazing on native grasslands of Alberta.

Some of the findings won’t surprise beef producers who see the positive effects first hand, but this is the first time they have been quantified across Alberta, says Dr. Edward Bork, Mattheis Chair, Rangeland Ecology and Management, University of Alberta. This is also just the start of a much broader effort to define and quantify the benefits society receives from Alberta’s native grasslands. The long-term goal is to encourage development of policies and markets that reward grassland managers for maintaining and improving those benefits. A group of Bork’s colleagues has proposed a similar project on tame forage lands.

The benchmarking study involves an unprecedented 114 native grassland sites where the provincial government has maintained a monitoring system of permanently fenced-off grassland exclosures on public grazing lands. Some were established as recently as 20 years ago and others have been in place for 70 years. Most are now around 50 years old. The sites are located across six natural subregions. From driest to wettest they are the dry mixed grass, mixed grass, central parkland, foothills fescue, montane, and upper foothills.

Public rangelands are typically grazed on rotational plans under moderate stocking rates to maintain range health and allow for other uses based on information collected by local rangeland agrologists.

Soil, forage and litter from inside and nearby outside of each enclosure were sampled to assess the impact of grazing on carbon storage above and below ground, plant diversity, biomass and soil health.

Grazing and 
carbon storage

When you are out there managing native grassland, know that there is a huge pool of carbon beneath your feet, Bork says.

Research by others since 2000 indicates that temperate grasslands cover eight per cent of the world’s surface and hold more than 300 gigatonnes of organic carbon, 97 per cent of which is in the soil. Of Alberta’s nine million hectares of grazing land, more than 6.5 million hectares are native grassland. The benchmarking study estimates that prairie and parkland native grasslands are currently holding approximately 140 million tonnes of organic carbon in the soil, litter and vegetation.

As of yet, no scientific consensus has been reached on how soil carbon responds to grazing. Increases, decreases and no change have been reported, but most of those studies have looked at only one or two sites.

Bork says his group wasn’t surprised to find that grazing reduced carbon stored in surface vegetation and litter — after all, cattle consume biomass to produce beef. The encouraging finding was stable and even minor increases in root mass and soil organic matter under grazed conditions.

“In five of the six regions, we observed a subtle bump in total soil organic carbon in the top foot of soil, suggesting grazing may be helping maintain higher carbon levels, at least in the mesic (moist) soil zones,” he explains. “After 18 months in the field we found that grazing may be responsible for helping stimulate more rapid degradation of litter. While some say this is no good because the carbon released is just respiring into the atmosphere, what remains uncertain is perhaps the litter is being partly incorporated into the soil.”

Further analyses of the benchmarking data, will evaluate how factors, such as changes in plant species composition, soil moisture and temperature, and the complexity of soil microbe communities might influence where, when and how grazing changes carbon accumulation.

“When we looked at below-ground carbon in root biomass, we found more roots under grazing in the wetter regions, a pattern paralleled by shoot biomass as well. This suggests that ongoing grazing under those conditions may be stimulating plant growth. Regardless of the mechanism, these results reinforce the notion that maintaining some grazing is critical for promoting energy flow and turnover of roots in the ecosystem, at least under moderate stocking rates.”

Preliminary trends on greenhouse gas emissions from a subset of the benchmarking sites shows lower carbon dioxide emissions from grazed soils, particularly from early June until mid-August.

Another preliminary finding indicates a trend toward increased uptake of atmospheric methane in grazed soils. This is in line with other Alberta studies that have recently reported positive greenhouse gas uptake of carbon dioxide and methane in grazed soils.

A related study is comparing this methane-scrubbing action under continuous grazing and high-intensity-low-frequency (mob, pulse) grazing scenarios. Results so far show methane uptake is improved when plots are intensively defoliated and then given a long rest period.

What is carbon 
storage worth?

Additional soil, litter and vegetation samples were collected from native grasslands paired with adjacent croplands. Maps from Alberta Biodiversity Monitoring Institute showing land areas for each land-use change were referenced to determine the value of total carbon stored in native grasslands and croplands today and to peg the value of carbon lost because of the shift from native grassland to cropping through the years since settlement.

The Alberta Climate Change Emissions Management Corporation currently values carbon dioxide equivalents at $15 per tonne and that will double early next year. This is basically a tax paid by large emitters in Alberta that goes into a fund to pay for practices that have been proven to capture greenhouse gases to offset emissions.

At $15 per tonne, the approximately 140 million tonnes of carbon currently retained in prairie and parkland native grassland in Alberta because they are not cropped is nearly $8 billion.

Approximately 30 per cent of the carbon mass has been lost across prairie areas. The loss is even more dramatic in the parkland, where nearly 50 per cent of the carbon mass has been lost. All of the carbon no longer in the soil is now in the atmosphere potentially contributing to global warming, Bork adds.

The value of carbon lost due to cultivation across more arid grasslands of southeastern Alberta is $4.2 billion. Across the parkland, because the area is larger and contains a lot more cropland than the prairie areas, the loss is $11.3 billion.

“These are mind-blowing numbers,” Bork says. This is a particularly big deal in Alberta because the carbon offset market will pay approximately $1 per acre per year to farmers who practise minimum tillage to help restore carbon. Ironically, he adds, while native grasslands continue to store the most carbon per acre, there isn’t a market incentive to maintain it, putting that carbon at further risk of loss due to land-use conversion.

Other great news stories

Grazing improved plant diversity across many subregions. The largest increases were in the moderate-precipitation regions of the parkland and foothills fescue areas. The greatest overall plant diversity, with or without grazing, was in the three wettest environments of southwestern Alberta.

“This is important because more diversity leads to more habitat for other organisms and also serves as a form of insurance to mitigate fluctuations in growing conditions (temperature and moisture) over time,” Bork explains.

The perception that grazing might be leading to an increase in introduced plant species was found to be true, but only in regions that receive more than 350 millimetres of annual growing season precipitation, most notably the montane and upper foothills areas of the southwest. Otherwise, whether grazed or not, introduced species made up about 10 per cent of the plant community, suggesting that grazing isn’t necessarily the universal vector for increases in introduced species.

Some studies have implied that introduced species are harmful. Others say species that are productive, such as quality forage plants, may enhance overall productivity to the benefit of grazing cattle and wildlife.

“In our study, introduced species appear to have had a role in boosting productivity of grazed range in the three wetter study regions. This indicates that grazing may be important in helping stimulate and maintain plant productivity over time. While some people have suggested that cattle should be removed from public rangeland, the counter argument could be made that grazing is critical to maintaining ongoing productivity,” Bork says.

The percentage of woody cover was five per cent or less in all but the two wettest regions, where it was close to 20 per cent inside the enclosures. Grazing significantly helped to keep shrub encroachment in check in the montane and upper foothills regions, reducing it from near 20 per cent to 12.5 per cent and 7.5 per cent respectively.

The findings on soil health indicate that native grasslands are superior to annual crops and tame pasture in all three measures: porosity, aggregation (sizes of soil particles) and maximum water availability as measured by a soil’s ability to deliver water to plants under increasing drought stress in the lab.

“Ultimately, native grassland is doing much more than providing forage or storing carbon for that matter. These ecosystems are healthier overall than cropland and tame pasture, although more work needs to be done to more fully understand the differences,” Bork concludes.

Other studies related to grazing and native grasslands include examining: how differing grazing systems influence response to drought and how microbial communities might come into play; interviewing producers on land management to understand how various practices might affect biodiversity; and residual feed intake and grazing behaviour in a cow-calf system.

A bee abundance and biodiversity study has already identified 140 species of bees collected from native grasslands and found a positive relationship between bee abundance, range health and forage quality.

These studies are tied to the Rangeland Research Institute established by the University of Alberta in 2012 to draw attention to the importance of rangeland research and streamline leading-edge research and knowledge transfer. The institute’s primary facilities include the university’s Kinsella Ranch in the parkland region near Edmonton and the Mattheis Ranch (donated in 2010 by Edwin and Ruth Mattheis) in the dry mixed grass prairie region near Brooks. The university is also working closely with Alberta Environment to maintain a research presence at the former Ag Canada research substations at Onefour in the far southeast, and near Stavely in the foothills south of Calgary.

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