Savannas, grasslands and other rangelands exhibit persistently high production and large soil carbon stocks in excess of 100 Mg/ha to 30 cm depth despite ongoing losses of nitrogen (N) and carbon (C) to herbivory and fire. Recent reviews suggest that N-fixation by grass root-associated microbes may account for N acquisition by plants that balance these losses. Further, recent theoretical models suggest that episodic or “pulsed” herbivory characteristic of may explain maintenance of dense soil carbon stocks. Measurements were made of aboveground production, community biomass relative growth rate, and nitrogen fixation in grass roots in a 20-year fence experiment in the Serengeti ecosystem. These results, combined with measurements of soil texture and dominant plant lignin and cellulose informed new herbivore-plant-soil dynamic models used to predict changes in soil organic carbon (SOC). Predictions of the models were tested in a case study of varying implementation of short duration high density grazing in 13 pastoralist community grazing practices in northern Kenya.
Results show that N-fixation by dominant perennial grasses is substantial and accounts for 12-25 kg N annual inputs. Episodic grazing by wildlife stimulated aboveground production by up to 60%. Together, these processes enhance soil carbon sequestration by 30% over ungrazed conditions and reverse otherwise continued declines in soil carbon over continuous livestock grazing at current livestock densities. Restoring N-fixing perennial grasses by implementing short duration high density grazing in the semi-arid northern Kenya pastoralist system led to annual sequestration of 0.3 Mg/ha soil organic carbon over 10 years despite two droughts and a locust plague. These processes and their outcome supported a successful payment-for-ecosystem services project and suggest that restoration of natural nutrient acquisition and migratory grazing could help restore millions of hectares of degraded rangeland worldwide.