Public Lab Research note

Fifty Shades of Graze

by cfastie | April 25, 2016 19:59 25 Apr 19:59 | #13028 | #13028

This month Nature published an article with the following assertions:

  1. There is three times more carbon in soils than in the atmosphere.
  2. Changes in cropland management could transfer enough atmospheric carbon to soils each year to offset 10% of our annual emissions from fossil fuels.
  3. This and several other types of land management change could transfer enough atmospheric carbon to soils each year to offset up to 80% of annual emissions from fossil fuels.
  4. There is a lot of uncertainty in these numbers.

Some other good news is that a month ago Nature launched a program which allows authors of Nature articles to share links to the full text of the articles. So here it is.

Below is a figure from the article with the various land management strategies that could sequester atmospheric carbon in soils. The annual carbon emissions from fossil fuels equals 10 Pg CO2(eq) yr-1. See the article for the full caption.


An important cropland strategy for moving atmospheric carbon to soils is to keep plants growing year round. Long periods between growing seasons when soils are idle contribute to carbon loss and also erosion and phosphorus migration to waterways. Minimizing tilling is also critical for retaining carbon in soils.

For the last few years I have been planting winter rye in September and October, harvesting it in May, and planting my backyard garden in the spring without tilling the soil. The soil is now black and laced with the living and decomposing fine roots of the annual grass. Instead of importing organic matter to improve the soil, I am growing it in place.

Above: One of two exclosures (left) which prevented deer from grazing my winter rye all winter.

An emergent feature of all this fresh rye in my yard has been regular visits by the local deer herd. So last fall I fenced off parts of the winter rye to protect it from deer grazing. The exclosures were never violated by the deer, and the difference between grazed and ungrazed rye is conspicuous.

Above: The Juno pole bracket on the penultimate pole section of a Ron Thompson Gangster Carp Pole. A Canon A2200 is mounted on the Juno for nadir mapping photography.

I wanted to document the effect of deer grazing, so I got out the Ron Thompson Gangster Carp Pole that Ned Horning lent me. This carbon fiber pole, without its top two sections, is 8.1 meters tall, so I can get pretty good aerial coverage of the garden which is too close to the woods to fly a kite over.

Above: I used the Juno pole bracket on the third smallest pole section. The bracket grips the pole only near the end where the pole is reinforced with extra fiber and epoxy.

To document the deer grazing, I used a full spectrum PowerShot S110 with a dichroic filter that passed only red and near infrared light (see this note for more). The camera on a Juno bracket was at the end of five sections of carp pole (8.1 m length) and angled to take nadir photos.

Above: NDVI image made in the Photo Monitoring Fiji plugin from a mosaic of 25 stitched photos from the S110.

I stitched together 25 photos in PhotoShop and put the mosaic into Fiji to make NDVI with Ned’s Photo Monitoring plugin. The floating point image output by the plugin has the actual NDVI values for each pixel, so I used tools in Fiji to sample 14 circular plots in each exclosure and in adjacent grazed areas of winter rye.

Above: Two floating point NDVI images from the Photo Monitoring plugin with 14 circular sample plots inside and outside deer exclosure 1. Fiji computed the average NDVI in each circle and the mean and standard deviation (n=14) for each treatment.

Above: One floating point NDVI image from the Photo Monitoring plugin with 14 circular sample plots inside and outside deer Exclosure 2. I intentionally excluded the dark area in the grazed area of rye because I was responsible for disturbing the rye plants there. Fiji computed the average NDVI in each circle and the mean and standard deviation (n=14) for each treatment.

For Exclosure 1, I compared NDVI results derived from two photos taken 15 seconds apart with the same camera settings. I did not calibrate this process so comparing two separate photos introduces some uncertainty. For Exclosure 2, I compared NDVI inside and outside the exclosure from parts of a single photo which should be a more reliable approach.

Above: Means (±1 standard error) of winter rye NDVI inside ("Exclosed") and outside ("Grazed") two deer exclosures. I did not do a statistical test because I know everything I need to know about these results -- good fences make good winter rye.

I am not sure how much of the difference is due to the rye plants inside the exclosures being healthier (they are a darker green) or to more soil and dead leaf litter being exposed where the rye has been grazed shorter outside the exclosures. I think it is a combination of both, and I think both effects will fade with time. It is possible that in a month when I harvest the rye and plant the garden it will be impossible to tell the two treatments apart. If that is the case, I might not bother with the exclosures next year.

Grasses invest a lot of their resources (carbon) in roots, and in the mycorrhizal fungi which extend and enhance the root network. I imagine that when I mow down the rye and immediately slip a tomato plant into a hole in the black fibrous mat, the mycorrhizae will transfer their affection from the dying rye roots to the expanding tomato roots. This may be more important for my garden's success than the absolute amount of atmospheric carbon fixed by the winter rye. So sharing some newly fixed atmospheric carbon with the deer might not be such a bad thing.

Below is a video which describes the same story you just read about, but it includes me using a 50 year old scythe to harvest winter rye.

The Juno Pole Bracket Kit is a brand new product at the KAPtery and costs $15.00 including shipping in the US. The Ron Thompson Gangster Carp Pole is available at the Public Lab Store for $115.00 plus shipping.

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Chris, Very nice results. Interesting. I love the venison steak in the closing remarks of the video. Pat

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Awesome techniques! Is there any research on the guiding assumption that deer have an effect on the amount of carbon sequestered?

I would imagine that deer grazing on tree saplings would have a greater effect than deer grazing on grasses and bushes, which may tolerate it better, and might even benefit from a grow/graze cycle? (so that nutrients can return to the soil?)

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You betcha there's research about how mammal grazing and browsing impact carbon and nutrient cycles. A couple of my buddies from graduate school devoted their careers to this question. To defend themselves, deciduous trees and shrubs alter the chemistry of their leaves in response to heavy browsing. This changes how fast the fallen leaves decompose, the rates of soil carbon turnover, and a cascade of plant-soil interactions.

Here is the abstract of an old article by the thesis committee members of my grad school chums. This paper was part of the first author's PhD.


Here is the abstract of a younger article by my friend Knut who was a graduate student of the second author above. Knut now has his own graduate students working on the same questions in the same department. There is no end in sight.


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