Water quality changes seasonal variations in root respiration, xylem CO₂, and sap pH in citrus orchards

Effects of treated wastewater (TWW) and fresh water (FW) on autotrophic belowground respiration and respiratory coefficients (Q₁₀) in summer and winter were determined in a commercial citrus orchard. Efflux of CO₂ from soil and the often-ignored CO₂ transported in xylem sap were quantified; the latter derived from sap flux, CO₂ concentration ([CO₂]), pH, and temperature. Xylem [CO₂] scaled with xylem sap flux, pH and temperature. TWW and summer increased xylem sap pH (by 12% and 19%), soil CO₂ efflux (32% and 65%), and root respired CO₂ (10–15% and 55%) in comparison to FW and winter, respectively. About twice as much CO₂ from the below ground autotrophic portion of respiration moved in xylem sap as compared to that diffused from the roots into the soil, with seasonal variations of about ±10%. Maximum temperature-dependent respiratory coefficients (Q₁₀) were 4.7 for autotrophic root respiration and 3.8 for bulk soil CO₂ efflux, and values varied with water quality and season. Total below ground respiration exceeds that previously reported and is a large part of the tree's carbon balance. Increased respiratory losses caused by poor quality water may explain reduced orchard root growth and overall productivity.