Grassland compost amendments increase plant production without changing plant communities

Compost amendment to grassland is a novel strategy proposed and demonstrated to increase ecosystem carbon (C) storage. However, the effects of compost applications on biomass production and plant diversity are not well known. We assessed impacts of a one‐time compost amendment over 4 yr on plant dynamics in two grazed grassland ecosystems in California: a coastal prairie and valley grassland. The valley grassland was dominated by exotic annual grasses and had significantly lower species diversity than the coastal prairie, which consisted of a mix of perennial and annual grasses and forbs. We observed large and persistent increases in aboveground biomass. Over the 4 study years, aboveground biomass from compost‐amended plots increased by 76% ± 21% at the valley grassland and 41% ± 21% at the coastal prairie, compared with controls. Plant N content was also greater from compost‐amended plots. There were no major shifts in species richness or abundance at either grassland site. Overall, plant communities at both grasslands were relatively resistant to the compost addition, but responses of some individual species were observed. Notably at the valley grassland, the abundance of forbs decreased slightly, while the abundance of grasses increased. The abundance of two noxious weeds did not change as a result of the compost amendment. Our results suggest that a single application of composted organic matter used to sequester C provided cobenefits to grassland plant dynamics. The compost amendments produced sustained increases in plant productivity and forage production as well as plant N content without greatly affecting the diversity of these exotic annual grasslands.