A CO2 Concentration Gradient Facility for Testing CO2 Enrichment and Soil Effects on Grassland Ecosystem Function

Multiple constraints cause positive and negative feedbacks limiting grassland soil CO2 efflux under CO2 enrichment

Terrestrial ecosystems are increasingly enriched with resources such as atmospheric CO₂ that limit ecosystem processes. The consequences for ecosystem carbon cycling depend on the feedbacks from other limiting resources and plant community change, which remain poorly understood for soil CO₂ efflux, J_CO2, a primary carbon flux from the biosphere to the atmosphere. We applied a unique CO₂ enrichment gradient (250 to 500 µL L^-1) for eight years to grassland plant communities on soils from different landscape positions. We identified the trajectory of J_CO2 responses and feedbacks from other resources, plant diversity [effective species richness, exp(H)], and community change (plant species turnover). We found linear increases in J_CO2 on an alluvial sandy loam and a lowland clay soil, and an asymptotic increase on an upland silty clay soil. Structural equation modeling identified CO₂ as the dominant limitation on J_CO2 on the clay soil. In contrast with theory predicting limitation from a single limiting factor, the linear J_CO2 response on the sandy loam was reinforced by positive feedbacks from aboveground net primary productivity and exp(H), while the asymptotic J_CO2 response on the silty clay arose from a net negative feedback among exp(H), species turnover, and soil water potential. These findings support a multiple resource limitation view of the effects of global change drivers on grassland ecosystem carbon cycling and highlight a crucial role for positive or negative feedbacks between limiting resources and plant community structure. Incorporating these feedbacks will improve models of terrestrial carbon sequestration and ecosystem services.