Soil Respiration in a Subtropical Savanna Parkland: Response to Water Additions

Soil respiration (SR) is controlled by abiotic parameters (temperature, water availability) interacting with biotic characteristics of the vegetation (quantity and quality of litter inputs, root respiration) and the soil microbial community. Because SR is a major flux in the C cycle, it is important to understand how vegetation change may interact with predicted climate changes to alter SR and ecosystem C storage. We quantified the SR response to increased soil water availability in a subtropical savanna parkland in the Rio Grande Plains of southern Texas. Diel SR was measured monthly from July 1996 to August 1997 in control and irrigated plots located in grasslands and in three contrasting woody plant communities known to have developed on grassland during the past 100 yr. Irrigation increased SR in all plant community types. Soil respiration in woody communities was higher (12.7 g CO2 m-2 d-1 averaged across months and treatments) and more responsive (+103%) to increased water availability than grasslands (9.8 g CO2 m-2 d-1 and +48%, respectively). This SR pattern is probably the result of woody communities having greater soil microbial biomass, soil C pools, and root biomass than adjacent grasslands. Irrigation increased the sensitivity of SR to temperature (Q10 = 1.6 and 2.6 for control and irrigated plots, respectively), but Q10 values were similar in woody communities and grasslands. Results suggest SR is water limited, that sensitivity of SR to soil moisture availability increases with increasing woody plant abundance, and that shifts from grass to shrub dominance may have little impact on SR response to the changes in temperature projected to accompany global warming.