Carbon and nitrogen storage and decomposition in decaying taproots at the Calhoun Experimental Forest, SC

Background/Question/Methods

The above- and belowground ecosystem carbon and nitrogen budgets have been monitored at the Calhoun Experimental Forest in South Carolina since 1957, when the loblolly pine spacing experiment was initiated in an old cotton field. Previous budgets did not incorporate coarse woody detritus (CWD) as a carbon or nutrient reservoir, because CWD did not accumulate until the forest approached 30 years of age. This study quantified carbon and nitrogen stocks of belowground CWD in the Calhoun forest and estimated mean residence time (MRT) of three root CWD decay classes, complementing an earlier analysis of aboveground CWD. That analysis demonstrated decreasing density, decreasing %C due to mineral soil incorporation, and N immobilization as log decomposition progressed. Thirteen dead loblolly taproots of varying states of decay were excavated by hand and backhoe, were then separated into four depth increments and two density fractions--a low-density, decomposed material vs. a relatively undecomposed, dense core--and were then ground and analyzed for C and N content. This study provides an improved biogeochemical understanding of C and N cycling through above- and belowground detrital wood.  

Results/Conclusions

Preliminary results show that total root mass and volume were well-predicted by log diameter for the least decomposed roots (Class I roots; mass R² =0.88, p=0.04; volume R²=0.90, p=0.03), but there was no relationship of volume or mass to log diameter for the more decomposed roots (Class II & III). Percent C (p=0.02) and N (p<0.001) both increased along the decomposition trajectory from Class I to Class III, while the C:N ratio (p<0.001) and the mass of C (p=0.03) and N (p=0.06) all tended to decrease. The density of the resinous cores did not change with extent of decomposition, however the proportion of root mass represented by the dense cores decreased (p<0.001) as decay penetrated more deeply. Percent C (p<0.001) and N (p=0.03) of the cores increased with decomposition. Low-density, decomposed root material increased in %N (p=0.01), decreased in %C (p=0.02) and C:N (p=0.002), with no change in masses of C or N, with extent of decay.  The density and C and N percentages of the root cores did not change with root depth, but the light fraction of root mass increased with root depth (p=0.04). Overall, the density, C, and N changes along the decomposition trajectory were very similar to the patterns observed aboveground. Work comparing above- to belowground CWD decomposition rates and C and N stocks is ongoing.