Partitioning Soil Respiration to Quantify Net Ecosystem Productivity: A Regional Analysis of Fertilization and Drought in Loblolly Pine.

Forests are strong terrestrial carbon (C) sinks, but our knowledge of the impacts of management or climate variability on the C sequestered by planted pine systems is limited by our lack of information on belowground C dynamics. The challenge is
to quantify the balance between C fixation and respiration at stand and ecosystem scales. Models are commonly used to estimate net primary productivity (NPP), but require further refinement in order to account for C that passes through or resides in the soil. Specifically, net ecosystem production (NEP), requires that soil respiration (RS) be separated into heterotrophic, microbial respiration (RH) and autotrophic, root-derived respiration (RA) components (i.e., NEP = NPP - RH).  The objectives of this study were to partition RS into RH and RA at three experimental sites across the range of loblolly pine in the southeastern US.  Each site has a 2 x 2 factorial combination (n=4) of throughfall reduction (0 and 30% reduction) and fertilization (control and fertilized) treatments that were installed in the spring of 2012.  Root severing cores were used to eliminate RA and allow the direct measurement of RH. Despite the diversity of soil types, stand ages, seasons of measurement, and treatments, the proportion of soil respiration from RH was remarkably consistent. Very few statistically significant (p < 0.05) differences were observed by season, site, or treatment. Thus, a single regional estimate of 0.840 ± 0.026 emerges as the RH proportion of soil respiration, suggesting that soil C sequestration in managed southern pine forests may be smaller than some previous estimates. Most efforts to use NPP to model NEP assume that soil respiration is evenly partitioned between RH and RA. Our field measurements suggest that the RH fraction of respiration is much higher than originally thought, and that more of the photosynthetically fixed C that is allocated belowground passes through the soil and back to the atmosphere rather than being sequestered in the system.