No enhancement of soil carbon persistence by sheep grazing in a long-term calcareous grassland experiment

Soils hold a globally important carbon pool that is generally more persistent than the carbon stored in plant biomass. However, this carbon is becoming increasingly vulnerable to disturbances such as soil warming, fire, and erosion. Managing land to increase soil carbon sequestration and persistence may therefore improve long-term soil carbon storage and contribute to climate change mitigation. It has been hypothesized that grazing by large herbivores may enhance the persistence of soil carbon by increasing the amount of soil organic matter forming more stable associations with mineral particles (mineral-associated organic matter). We compared sheep-grazed and ungrazed plots within the Gibson Grazing and Successional Experiment located in the Upper Seeds calcareous grassland in Wytham Woods, Oxfordshire, using organic matter fractionation to estimate the surface (0-5 cm) carbon stocks in the mineral-associated and particulate organic matter fractions. Counter to predictions, after 35 years sheep grazing had not increased mineral-associated organic matter carbon stocks relative to ungrazed plots. We hypothesize that this indicates the saturation of mineral surfaces in both grazed and ungrazed treatments and the inability of grazing to increase soil nitrogen stocks and decrease pH to levels conducive for mineral-associated carbon sequestration. Only one of twelve soil properties examined showed statistically detectable responses to grazing: spring-grazing increased the C:N ratio in the mineral-associated organic matter. While the number of tests performed (24) means this may be a false-positive result, if genuine it would be consistent with a more direct pathway from plant exudates to mineral-associated organic matter formation due to compensatory growth in response to spring-grazing. Overall, the results of this long-term experiment do not support the hypothesis that grazing can improve the persistence of the soil carbon pool.