Are there tradeoffs in plant dynamics in grasslands managed for carbon sequestration?

Background/Question/Methods

Grasslands have the potential to provide valuable ecosystem services. Management interventions that aim to increase one type of service may have secondary effects on others. Grasslands occupy a large geographic area and are thought to have considerable capacity to increase soil carbon (C) storage. As a result, there has been appreciable interest in managing grasslands for C sequestration. Compost amendments and keyline plowing are two techniques that have been proposed for soil C sequestration in grasslands. However, these practices also alter nutrient dynamics, water flows, and soil structure, which may in turn affect plant dynamics. The impact of these practices on net primary production (NPP), plant community structure, and forage nutritional quality are not well known, but critical for understanding potential tradeoffs among C sequestration, fostering biodiversity, and economic sustainability of managed grasslands. We used replicated field experiments in two bioclimatic regions of California (valley and coastal grasslands) to investigate the separate and combined effects of compost amendments and keyline plowing on plant growth, community composition, and plant nitrogen (N) content.

Results/Conclusions

The plant community at the valley grassland was dominated by exotic annual grasses and was less species rich than that of the coastal grassland, which consisted of a mix of perennial and annual grasses and forbs. Overall, plant communities at both grasslands were relatively resistant to these management events, but responses of some individual species were observed. Notably, we detected a short-term increase in the abundance of a noxious forb (Carthamus lanatus) at the coastal grassland and a decrease in abundance of a noxious annual grass (Taeniatherum caput-medusae) at the valley grassland with the combined compost and keyline plow treatment. We observed significant increases in aboveground biomass and plant N content following compost amendment at both sites, which persisted for four years. Our results suggest that a single application of the keyline plow produced short-term (< 1 y) declines in aboveground biomass and changes to noxious weed abundance, and that a single application of compost produced sustained increases in aboveground biomass and forage nutritional quality without affecting the floristic communities of these annual grasslands. The lack of major shifts in vegetation or prolonged responses of noxious species suggests that there are few trade-offs between C sequestration and plant dynamics in these grasslands.