Within the past eight years, growing evidence for a large autotrophic contribution to total soil respiration has motivated the ecological community to work towards a better understanding of respiratory partitioning on a global basis and associated environmental influences. The goal of this study was to evaluate contribution of Quercus and its associate mychorrizal fungi to total soil respiration and to better understand the partitioning of autotrophic and heterotrophic respiration at Black Rock Forest. Trees on twelve plots (75m x75m) in a randomized block design grouped by slope position (high, medium and low) were girdled according to four treatments: girdling all the oaks on the plot (O), girdling half of the oak trees on a plot (O 50), girdling all non-oaks on a plot (NO), and a control (C). In addition to the 12 established plots, one circular plot (50m in diameter) was created specifically for this experiment where all trees were girdled (ALL). Soil respiration was measured from May through October on all plots prior to and following the girdling treatment.
A conservative estimate of the total autotrophic contribution is 66% as revealed from a fully girdled plot. Rapid declines in both the fully girdled and oak girdled plots observed within the first month of the treatment, 66% and 62% respectively, support a fast turnover between recently fixed photosynthates and their release back to the atmosphere via autotrophic respiration. Responses from the NO and O 50 treatments were similar to the control plots. Due the non proportional decline in respiration rate along a basal area gradient, the contribution of Quercus to total soil respiration was unable to be decoupled from overall soil respiration. These findings are the first to show that the girdling method may be used in an oak-dominated forest to partition the contribution of autotrophic and heterotrophic activity to total soil respiration. They also indicate that both biotic and abiotic controls on photosynthesis may have large implications for the forest carbon budget.