Estimating the in situ carbon balance of a Bromus inermis grassland using a leaf-level ecophysiological approach

Knowledge of the CO2 budgets of cool season grasslands are necessary for construction and interpretation of the global carbon cycle. In order to estimate the net CO2 flux of a Bromus inermis grassland, the effects of environmental and morphological/physiological variables on net CO2 flux were assessed on aboveground leaf tissue and the effects of air temperature on the CO2 efflux of belowground root tissue and soil microbial activity. Soil CO2 efflux and the light response of CO2 assimilation were measured on B. inermis leaves growing in situ over the course of 15 months in a grassland in northeastern Kansas. Photosynthetic parameters were modeled using a non-rectangular hyperbola to yield the maximum rate of gross photosynthesis (Amax), quantum yield of CO2 uptake, and the rate of dark respiration (Rd). The effects of air temperature, vapor pressure deficit (vpd), CO2 concentration, leaf chlorophyll content, leaf midday water potential, and leaf mass per area on the photosynthetic parameters of the plants were analyzed for upper and lower canopy and late-season leaves separately using linear regression. Amax was most impacted by midday water potential and chlorophyll content, whereas Rd was most affected by air temperature and vpd. Multiple linear regression of air temperature and vpd on the photosynthetic parameters and an exponential regression of air temperature on soil efflux were used in conjunction with an on-site weather station and collections of aboveground biomass to estimate the hourly carbon budget of the grassland community on days when field measurements were not made. The results suggest that the brome grassland was a net carbon sink over the course of the study; estimates of annual carbon uptake of the five plots used ranged from 80-462 g C m-2yr-1. Normalized differential vegetation index (NDVI) imaged over the grassland at 3200 m above ground level were not correlated strongly with biomass, leaf chlorophyll content or Amax, however, the model estimates of daily net carbon exchange correlated well with NDVI during the period of measurement (R2 > 0.7). The present study suggests that B. inermis may represent an efficient species for carbon management in areas subjected to moderate drought and that remote sensing holds promise as a method of ecosystem carbon flux monitoring.