Daily patterns of pine canopy activity observed in continuous soil respiration trace

Soil respiration (R_S) accounts for more than two thirds of total respiration from forest ecosystems.  Yet sparse knowledge exists on factors affecting autotrophic contributions.  Recent girdling studies show that forest canopy activity supplies half of the substrate for R_S.  High-frequency R_S measurements indicate a correlation with canopy photosynthesis in oak savanna woodlands and solar radiation patterns in sweetgum plantations.  However, R_S can be delayed 1 to 12 hours and it is important to correct for the influence of temperature.  To identify similar patters in loblolly pine, we simultaneously monitored sap flow with heat dissipation probes and R_S using the soil CO₂ gradient approach.  Respiration was measured every 30 seconds and averaged over 5 minute spans to compare daily patterns. Significant correlations were observed between R_S and plant canopy activity measured with sap flow.  However, to achieve this correlation it was necessary to offset daily traces by 6 hours during the growing season and 14 hours in the dormant season.  R_S was significantly correlated with temperature so it was adjusted to the minimum daily temperature using Q₁₀ relationships.  Significant correlations were observed between temperature-adjusted R_S and plant activity, and the correlation improved compared to that found with uncorrected R_S.  The time offset for temperature adjusted R_S was 6 hours for both growing and dormant season.  We interpret the y-intercept of this linear correlation to represent a basal soil respiration that is predominately heterotrophic oxidation of soil organic matter.  However, there is a portion of this basal rate representing root/rhizosphere respiration supplied by stored carbohydrates.  Further development of correlations between plant canopy activity and soil respiration will provide insight into the factors controlling components of soil respiration.