Switchgrass Growth in Southern Loblolly Pine Forests Alters Soil Organic C and N Fractions Under Bedded Rows of Pine Trees.

Managing forests for biomass-for-biofuel production may influence soil organic matter (SOM) dynamics and have important feedbacks on soil carbon (C) storage and therefore the global C cycle.  Fractionating SOM pools can help differentiate finer-scale influences of forest management practices on SOM storage and turnover potential.  In this study, we aimed to determine if switchgrass (Panicum virgatum L.) production in a southern loblolly pine (Pinus taeda L.) forest has altered SOM stability.  We hypothesized that switchgrass would increase hydrolyzable fractions of SOM compared to pines due to inputs of labile SOM via switchgrass root exudates and turnover.  To evaluate switchgrass intercropping effects on SOM dynamics, a study consisting of seven treatments replicated four times (n =28) on approximately 0.8 ha treatment plots was established in the summer of 2008 on a recently harvested 25-year-old loblolly pine plantation near Dover, NC.  Of the seven treatments in the study, three were included in this analysis: 1) pine with biomass left in place; 2) pine with biomass removed; and 3) pine intercropped with switchgrass with biomass removed.  Soil samples were collected from the 0-5, 5-15, 15-30, and 30-45cm depths in May 2011, two years following switchgrass establishment.  Chemical, biochemical, and physical protection of SOM were tested using acid hydrolysis, density fractionation, and aggregate fractionation methods, respectively.  We found no changes in SOM fractions observed under switchgrass in the intercropped treatment.  Interestingly, non-hydrolyzable SOC and SON fractions decreased by 13% and 5%, respectively, in the 0-5 cm depth in bedded rows of pine trees when switchgrass was located in the interbed.  Reduced non-hydrolyzable SOM led to an increase in proportion of total SOM pools as hydrolyzable C and N.  These results suggest that intercropping of switchgrass in these forests influence C and N dynamics in adjacent beds, possibly due to altered rooting dynamics of pines.  If these trends continue and manifest themselves at other soil depths, this could have important implications for turnover and long-term soil C storage and possibly pine productivity in this system.