[This is a guest post by Thomas Ross, a 3rd year Biology student at The University of Manchester – he is currently doing a Science Media Project on the effects of grazing on soil C storage, which I am supervising. This blog post is part of his portfolio, and he has to reflect on its impact in his final submission. So don’t hesitate to leave your comment!]
The carbon stored in soil amounts to double that in the atmosphere and biomass combined and soil has the potential to sequester more. As atmospheric levels of carbon dioxide have been on the rise there has been an increase in global temperatures and climate change (here, the processes involved in soil carbon storage explained in more detail). The potential of the soil carbon reservoir to sequester this carbon from the atmosphere, and potentially ease the speed of climate change, can be influenced by our actions and the way in which we manage land. One such way is through the grazing of domestic livestock.
Grazing has the potential to modify ecosystems drastically and thus affect soil carbon storage. But how much is too much? Unfortunately, I cannot give you a definitive answer as the effects of grazing on soil carbon storage vary greatly. Some studies showing increases in soil carbon due to grazing, others decreases and some no changes at all. This causes a tricky problem when deciding how to manage livestock to ensure maximum soil carbon storage and withholding the interests of all stakeholders.
The issue is that there appear to be so many factors that affect how grazing influences soil carbon storage. Soil type appears to have a profound effect, but only at more extreme precipitation levels. For instance, high precipitation causes carbon storage to increase under grazing in sandy soils, but to decrease in clay soils. The species of grasses that make up pastures (determined by their photosynthesis process of either C4 or C3) can also affect soil carbon storage. C4 grasslands show an increase in soil carbon under heavy intensity grazing, whereas C3 grasses show a decline at high intensity (this review article summarises these findings). Then you have the problem that different species of grazers have different feeding habits. Sheep and goats graze much lower to the ground than cows but do not have the same trampling effect on the grass and soil as the heavier bovines. This all in turn affects the carbon content of the soil. Grazing can also cause erosion, especially in dryer sandy soil. The erosion removes top soils and plants, which means that the potential to store carbon in the soil is severely reduced.
Grazing by sheep in the Yorkshire Dales
Generally speaking, more carbon is stored in soils in cool temperate regions such as moorland in the UK. Natural England produced a report on grazing livestock on moorland and also alluded to the problem of predicting the effects of grazing on soil carbon storage. They found moderate evidence that soil carbon storage was not significantly affected by removing grazing, even suggesting that light grazing may increase the carbon sequestered by the soil. However, they also believe there is not enough known to make predictions; “More evidence is needed on carbon budgets in different grazing/soil combinations. As management moves towards considering ecological services in the round, then understanding the trade-offs between different services, such as carbon sequestration and livestock production will be necessary”.
When all these factors interact (as they inevitably do) it makes predicting the effects of grazing very difficult. It means there is no universal answer to the question of how grazing effects soil carbon storage and as such the management of grazing livestock must be tailored to individual regions depending on climate, soil type, grass species and grazer species.
Frantecologist 
Useful summary. The many variables do make for a complex picture, but it is clear that we do know a lot about carbon storage in grazed systems.
Might be worth looking at the mycorrhizal populations – fungi are very good at making glomalin, a stable carbon compound and precursor of humus, when they feed on plant debris. Tend to be highest where soil is rarely disturbed. Soil C can be raised by limiting exposure to the air and light – keep the surface covered with leaves and aim for actively growing roots. Holistic Management of grazing builds soil – see Allan Savory’s work.
Thank you for the comment. I just had a quick look at some of Allan Savory’s work, very interesting stuff. One question I have is (after having only briefly looked into it) does the holistic management, of very large intensive herds, result in the similar soil building effect in grasslands that have not naturally been grazed by large herds? I am assuming mycorrhizal populations differ depending on climate, soil types and regions. Would the different populations have differing effects on the amount carbon they store and how they react to grazing?
All forms of grazing hard hoof livestock are pastoral grazing…
and to summarize global science findings over the past century,
pastoral desertification is planetary cancer (planetary melanoma)…
which has reached pandemic proportions in pastoral countries…
like Australia and NZ/US/Tibet/Mongolia/ and Africa along with so many others..
What Peter Andrews in his book “Back from the Brink” ABC 2006, demonstrated at Tarwyn Park
is that by implementing zero grazing on the Bylong Creek floodplain ~
confining his horses to hill top yards and barnstaking their fodder to feed them,
floodplain ecosystems and aquifers could be rapidly revived and restored…
increasing aquifer storage more than tenfold….this causes a re-coupling of the water/carbon cycle and builds organic carbon in soils.
By re-introducing grazing on the floodplain, floodplain ecosystems collapsed rapidly
as a result of destruction of geomorphic and hydrological processes by hard hoof impacts
By removing the grazing, the floodplains ecosystems revived again..
Peter wasn’t promoting any theories ~ his demonstrations were empirical science
proven by ecological mapping and modelling using geospatial sciences and
integrating all the information in transdisciplinary watershed iGiS…and then corroborated by rangeland R&D in India, China (Loess Plateau) and NZ..
It’s all written up in UNESCO’s Encyclopedia of Life Support Systems..
in the Food and Agriculture Volume
Closed system scientists doesn’t understand these complex open system, ecological processes
because they cannot map and model geospatial systems accurately or reliably and because they work from hypothesis to theory using alpha numeric science ~these hypothetical methods are strictly invalid scientifically for complex open ecosystems…yet so many so pseudo- scientific ecologists use them… blindly.
The scientific proof of natural farming systems is probably only possible
using complex open system geosciences..and repeated field experiments by “skilled practitioners”
But you need to have spatial cognition and geospatial intelligence skills to employ these methods
and these skills cannot be gained alpha numerically…nor by academic education
Indeed, alpha numerical analytical thinking tends to destroy spatial literacy
No wonder in common parlance, PhD now means Permanent Head Damage..
The ecological, geomorphological and hydrological impacts of pastoral grazing are proven scientifically
and well recognised by UN agencies and programs blocked from delivery in ANZ..
Nevertheless, what this scientific knowledge doesn’t change
are culturally entrenched pastoral mythology and religious beliefs ….
All the Abrahamic, Islamic, Shamanist and Vedic religions generate cultural proscriptions
for pastoral grazing ~ and scientific knowledge is impotent against these beliefs
..