Effects of plant diversity and heterotroph removal on total plot level C fluxes
As plant diversity increased from 1 to 16 species in control plots,i.e. in the presence of all heterotrophs, NEE increased from an average of 3.95 (±0.96) in monocultures to 6.9 (±1.36) μ mol CO2 m-2 s-1 in 16 species plots, but this trend was not statistically significant (Table S2, Fig. 2C). Mean GPP and Re roughly doubled along this gradient of plant diversity (Tables S3-S4, Figs. 2A, B). GPP increased from 8.37 (±1.06) μ mol CO2 m-2s-1 (mean ± 1 s.e.) in monocultures to 16.18 (±2.03) in the 16-species plots, and Re increased from 4.42 (±0.36) in monocultures to 9.28 (±1.88) μ mol CO2m-2 s-1 in the high diversity plots. Thus, the increase in GPP with plant diversity was offset by the increase in Re such that there was no significant effect of plant diversity on NEE.
Removal of all heterotrophs simultaneously did not affect NEE, GPP or Re (Tables S2-S4, Fig. 2). Among all heterotrophs, only foliar fungi influenced carbon fluxes when removed independently. Removal of foliar fungi increased GPP by an average of 54% at each plant diversity level (Table S3, Fig 2A). Because GPP increased with plant diversity (as noted above), the absolute change in GPP due to removal of foliar fungi was greatest in monocultures and declined with increasing plant diversity. Heterotroph removal did not significantly influence Re (Table S4, Fig. 2B). Thus, plots where foliar fungi had been removed had greater net carbon uptake: compared to the control plots, NEE increased by 45% in the subplots where foliar fungi were removed and this effect was strongest in low diversity communities (Table S3, Fig. 2C). Foliar fungi only impacted total fluxes when all other consumer groups were present; total fluxes in treatment and control plots were the same when arthropods and soil fungi were removed in addition to foliar-fungi, demonstrating that plant-foliar fungal interactions determining carbon flux rates depended the heterotroph foodweb context (Tables S2-S4; Fig. 2).