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

Abstract

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.

Keywords: CO2 enrichment; biodiversity; productivity; soil respiration; tallgrass prairie.

Fig. 1.

(A) J_CO2, (B) ANPP, (C) Ψsoil, and (D) PPFD in relation to atmospheric CO₂ concentration on silty clay, sandy loam, and clay soil series. Large symbols represent the mean ± SE across 8 y of CO₂ treatments. Small symbols represent values for individual years. Lines denote significant regression relationships for individual soils (color) or for all soils combined (black). Insets depict means ±1 SE across years and CO₂ levels. Table 2 shows linear mixed model statistics, and SI Appendix, Table S1 shows regression parameters and statistics.

Fig. 2.

Community diversity and composition as functions of atmospheric CO₂ concentration on silty clay, sandy loam, and clay soils. (A) Plant species turnover (Bray–Curtis index) per unit change in CO₂ in relation to the difference in CO₂ between all pairwise combinations of monoliths in each soil series. Linear mixed models soil effect P = 0.0012. (B) exp(H) in relation to CO₂ concentration. Large symbols with error bars represent means ±1 SE over 8 y of CO₂ treatments. Small symbols represent data for individual years. SI Appendix, Table S1 shows regression parameters and statistics.

Fig. 3.

Rates of change in response to CO₂ enrichment in (A) dominant grass species and (B) exp(H) as a function of the rate of plant species turnover in response to CO₂ enrichment. Each datum represents the slope of the CO₂ relationship for a single year.

Fig. 4.

Structural equation models relating the CO₂ responses of 0- to 40-cm soil water potential (Ψsoil), aboveground net primary productivity (ANPP), turnover in community composition, and effective species richness, exp(H) to the CO₂ response of soil CO₂ efflux, J_CO2. The a priori model was fit separately to the individual soil series. Depicted paths indicate significant direct effects. Nonsignificant paths are omitted in the fitted models. See Table 3 for model fit statistics, Fig. 5 for visualization of total effects, and SI Appendix, Table S3 for partitioning of direct and indirect effects.

Fig. 5.

Total effects of predictors of soil CO₂ efflux on each soil series from structural equation models (Fig. 4). See SI Appendix, Table S3 for partitioning of total effects into direct and indirect components. ns, not statistically significant.