Eighteen Years of Mob-Grazing Did Not Increase Long-Term Soil Carbon or Mycorrhizal Fungi in a Semi-Arid Grassland.

Mob-grazing is the practice of grazing high densities of livestock for short periods where every plant is either eaten or trampled followed by long rest periods. For eighteen years, replicated field plots have been manipulated with an annual mob-grazing treatment or no grazing. During 2015, the seasonal dynamics of soil properties, root biomass and mycorrhizal fungi were measured in the grazed and ungrazed plots underneath a C₄ grass, Bouteloua gracilis, a C₃ grass, Pascopyrum smithi and a forb, Artemisia frigida. Grazing decreased soil moisture and soil aggregate stability but increased soil compaction. In August, the grazing treatment decreased root biomass but increased the particulate organic matter biomass. Mycorrhizal hyphal density increased across the season but it was not affected by grazing. Soil organic matter measured through loss on ignition was higher in grazed plots compared to ungrazed plots. However, when soil bulk density was taken into account and the measurements were scaled up to Mg C ha^-1 (at 19 cm depth), then there were no significant differences in soil organic C in grazed and ungrazed plots. There is evidence that the quality of organic C changed through grazing because δ¹⁵N was significantly higher in the grazed plots.  Also, there was a significant date by grazing interaction for δ¹³C. Our study shows that 18 years of annual mob-grazing did not increase Mg C ha^-1. To better understand the mechanisms for this finding, it is important to measure soil respiration across the season and also analyze seasonal soil C dynamics at different depths.