Yield and soil carbon sequestration in grazed pastures sown with two or five forage species

Increasing plant species richness is often associated with an increase in productivity and associated ecosystem services such as soil C sequestration. Here we report on a 9-yr experiment to evaluate the forage production and C sequestration potential of grazed pastures sown to either a two-species cool-season grass-legume mixture or a five-species mixture of grasses, legumes, and a non-legume forb. We hypothesized that forage production and soil C sequestration would be greater in the five-species compared with the two-species mixture. We also evaluated the effects of rainfall and temperature on the difference in forage production between mixtures. When averaged across years, the five-species mixture produced 31% more forage biomass than the two-species mixture, and the difference between mixtures tended to be greater in wet than in dry summers (R² = 0.42, P = 0.06). There was no relationship between growing season temperature and differences in biomass production. Soil C accumulation to 100 cm depth in the five-species mixture was 1.80 vs. 0.50 Mg ha^-1 yr^-1 for the two-species mixture (P = 0.15) with significant increases in the five-species mixture occurring at the 10 to 20 cm (P = 0.06) and 20 to 30 cm (P = 0.03) depths. Increased productivity in the five-species mixture persisted throughout the experiment despite changes in species composition that resulted in similar composition between the mixtures by the last 2 yr of the study. Increased soil C possibly had a positive feedback effect on forage production permitting the five-species mixture to out-yield the two-species mixture even after differences in species composition had largely disappeared.