|Scott, Russell - Russ|
Submitted to: Trans American Geophysical Union
Publication Type: Abstract only
Publication Acceptance Date: 10/1/2006
Publication Date: 12/11/2006
Citation: Sun, W., Scott, R.L., Resco, V., Cable, J.M., Huxman, T.E., Williams, D.G. 2006. Responses of Soil Respiration to Precipitation in a Savannah Ecosystem: Fine Temporal Measurements of Soil CO2 Efflux. . Trans. AGU, 87(52), Fall Meet. Suppl., Abstract B33C-1202. Interpretive Summary:
Technical Abstract: Woody plant encroachment into grassland has the potential to affect net primary production, in part by changing the sensitivities of photosynthesis and respiration to precipitation. Encroachment of mesquite (Prosopis) into floodplain sacaton (Sporobolus) grassland along the San Pedro River in southeastern Arizona has altered the magnitude and seasonal pattern of net ecosystem carbon exchange and ecosystem respiration. We hypothesized that because mesquite accesses ground water in these floodplain environments, its advancement and dominance in former grassland reduces the sensitivities of photosynthesis and autotrophic respiration to inputs of growing season precipitation. The observed elevated rates of ecosystem respiration following rainfall inputs are likely to result from microbial decomposition of labile organic matter derived from the highly productive mesquite trees. We used the Keeling plot method to monitor carbon-13 composition of nocturnal ecosystem-respired CO2 (' 13CR) during the growing seasons of 2005 and 2006 at three sites spanning a gradient of mesquite invasion: C4 sacaton grassland, mixed mesquite/grass shrubland and C3 mesquite woodland. ' 13CR in the C4 grassland increased from -18.8‰ during the dry premonsoon period to -16.7‰ after the onset of summer rains, whereas ' 13CR in the mixed shrub/grass and woodland ecosystems declined from -20.9‰ to - 24‰ and from -20.8‰ to -24.7‰, respectively, following the onset of summer rains. The ' 13CR of respired CO2 was collected separately from soil, roots, leaves and surface litter to evaluate the contribution of each of these components to ecosystem respiration. Partitioning of ecosystem respiration using these isotope end-members and responses to short-term (days) changes in shallow (0-5cm) soil moisture content suggest that in former grassland now occupied by mesquite woodland, rainfall inputs primarily stimulate microbial decomposition and have little effect on autotrophic respiration. Autotrophic respiration in grassland was stimulated by rainfall inputs to a much greater degree than in woodland. As expected, ecosystem respiration responses to rainfall at the mixed shrubland site was complicated by the heterogeneous plant cover and microsite variation. Partitioning ecosystem respiration into component fluxes is necessary to model and forecast the sensitivity of net primary production to climate and land-use changes.