Soil Heterotrophic Respiration in Southern Pine Plantations: Seasonality, Microbial Communities, and Models.

In order to determine the effectiveness of southern pine plantations in sequestering atmospheric carbon, we must know the amount of fixed CO₂ that is subsequently lost due to microbial activity in the soil. Furthermore, this heterotrophic proportion of total soil respiration must be quantified as it changes between different operational treatments, ecoregions, and seasons. These proportions are necessary to accurately determine net ecosystem productivity from net primary productivity, thus helping to estimate the amount of carbon accumulated by the ecosystem. The research aims to quantify heterotrophic contributions to total soil respiration in the Piedmont and Upper Coastal Plain (UCP) ecoregions under control, fertilizer, and herbicide treatments over an annual cycle. Heterotrophic respiration (R_h) is separated in the field from autotrophic respiration (R_a) by using metal root-excluding collars and CO₂ efflux measurements are made monthly over 90 days. Following final measurements, roots are excised to 30 cm below each point, dried, and weighed to account for differences in rooting mass. A soil sample of the top 10 cm at each point is taken and measured for microbial biomass. Collars are re-installed to begin the next 90-day seasonal rotation. Soil respiration is also being modeled at these sites to predict heterotrophic respiration. Preliminary results in both Piedmont and UCP regions indicate R_h of 70% during winter with little effect of fertilization but a decrease in herbicide plots, and 66 and 70% during spring with an increase in fertilized UCP plots. Microbial biomass varied among treatments, collar types, and regions in spring, but only between regions during winter.  Finally, initial modeling efforts in a generic pine plantation estimate seasonal R_h of 43 to 95%, suggesting the model may be valuable for partitioning.  These preliminary efforts suggest R_h is a substantial pathway of CO₂ return to the atmosphere in a range of pine ecosystems.