Doubled organic matter levels increase soil respiration from intensively managed loblolly pine plantations

Background/Question/Methods

Soil organic matter (SOM) can be a useful means of indexing site productivity in intensively managed forest plantations. Post-harvest management of forest floor and slash can alter SOM levels, but the ensuing ecosystem effects remain poorly understood. Because SOM management is critical to long-term forest productivity, a study was designed to clarify the effects of manipulated SOM levels on a loblolly pine (Pinus taeda L.) plantation in Pamlico County, North Carolina.

We measured soil respiration rates across four of six treatments, which are the product of three organic matter levels (removed ["R_"]: 0 Mg biomass ha-1; control ["C_"]: 16 Mg biomass ha-1; and doubled ["D_"]: 32 Mg biomass ha-1) and 2 levels of incorporation of that material into soil with a combination plow (mixed ["_M"]; unmixed ["_U"]) in 2006. We targeted four of these treatments with a multiplexed infrared gas analyzer (Licor 8100A): CU (control, unmixed), RU (removed, unmixed), DM (doubled, mixed), and DU (doubled, unmixed). Continuous measurements of soil respiration, temperatures, and moistures over ~36-hour intervals were obtained once per month; spatial variability within each of the four treatments was also quantified. 

Results/Conclusions

Wintertime (Nov-Jan) and springtime (Feb-May) measurements suggest that one-time doubling of organic matter (unmixed) leads to substantially greater soil respiration fluxes. Over a 6-month interval (Nov. 2011-May 2012), average fluxes from DU, DM, and RU plots were 64, 19, and 18% greater than CU (control) plots.  We estimate annual CO2 fluxes range between 700 (CU; control) and 1100 (DU) g CO2-C/m2/year. Soil temperatures at 5 cm averaged 11oC, but showed an expected seasonal trend, ranging from ~13oC in November to ~6oC in January to ~20oC in May, with little variation between treatments. Volumetric soil moistures showed less of a seasonal trend.  

While there has been no evidence of differences between treatments in aboveground productivity for the loblolly pine, these results--together with prior work here (e.g., Zerpa et al. 2010 [Forest Ecology and Management 259: 1480-1489]; Sucre et al. 2011)--suggest that the one-time, post-harvest doubling of organic matter in 2006 continues to stimulate microbial and root activity as indexed by soil respiration. Continued monitoring will help clarify treatment x season interactions, refine annual estimates of soil respiration, and refine management practices seeking to maximize long-term site productivity.