A restatement of the natural science evidence base concerning grassland management, grazing livestock and soil carbon storage
Matthew W. Jordon1, Jean-Charles Buffet1, Jennifer A. J. Dungait2,3, Marcelo V. Galdos4, Tara Garnett5, Michael R. F. Lee6, John Lynch7, Elin Röös8, Timothy D. Searchinger9, Pete Smith10 and H. Charles J. Godfray1,11
This useful meta-analysis of the evidence concerning grassland and livestock management, manure use and soil carbon storage provides practical guidance for farmers and growers and policy makers. It supports my challenge to the ambitious claims being made by some interested parties for infinite soil carbon sequestration, and so saving the world.
With 50% of UK land in pasture, mostly grazed by ruminants, the importance of storage of carbon in the soil is not in question. Nor is the importance of appropriate soil organic matter levels for proper soil functioning, nor the depletion of soil carbon through long term and excessive cropping and cultivations, nor the emissions from ruminants. What is in question and hotly contested, is the effect of different management strategies to secure ongoing carbon sequestration and the potential to address climate change.
Please read the paper to get a full understanding of the evidence, here are a few relevant extracts, which my detractors may claim are selective and biased:
· Changing from tillage crops to no-till cropping will reduce Soil Organic Carbon (SOC) losses but is unlikely to result in meaningful increase in SOC.
· Introducing leys into an arable rotation will increase SOC compared to an all-arable cropping system.
· Rotational grazing will maintain or increase SOC compared to set stocking.
· Initial annual increase in SOC resulting from any change in management decreases over time.
· SOC sequestration and potential level is determined by soil type (clay/sand fraction) and the degree of soil degradation.
· The claims made by advocates that Holistic Planned Grazing or mob grazing can fully mitigate anthropo-genic climate change are not backed up by evidence and the claims made “are implausible”.
· “It is implausible that perpetually high rates of soil carbon sequestration will occur” due to eventually reaching a carbon saturation point.
· A diversity of plant species will have a positive effect on SOC.
· Application of manure to carbon saturated permanent pastures will have little benefit to SOC, however application to arable land has the potential to increase soil carbon stocks by as much as 20 or 30%.
The policy implications discussed are important. These observations and practices are consistent with good organic farming practice which, as reported by FiBL in arable rotations https://www.fibl.org/fileadmin/documents/shop/1349-soil-and-climate.pdf , sequesters more carbon than conventional ploughed systems and similar to conventional no-till systems.
The paper notes the inconsistency in monitoring procedures between the various research projects assessed. My experience is that reliable information for long term monitoring or comparison of different treatments requires the sampling soils to a depth of at least half a meter, a standardised frequency and sampling route is used and that the same laboratory is used throughout any trial.
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