A range of growth responses to fertilizer were observed among clones of loblolly pine (Pinus taeda L.) in field conditions. Increased leaf area is widely accepted as the long-term mechanism of fertilizer stem-volume response. However, clonal variability in short-term changes in C allocation due to fertilizer is less understood. Increased C allocation to above-ground portions of the tree may be possible due to short-term reductions in belowground C allocation. The objective of this research was to determine if short-term root exudate C allocation may be altered by fertilizer application. Our hypotheses were (1) fertilization and water stress decrease C allocation to root exudates, and (2) specific C allocation patterns vary between clones. A split plot experimental design was used in a field trial, with the whole plots being two levels of fertilization (with or without) and the split plot factor being 25 clones. Additionally, a growth chamber study with a 2 x 2 x 2 factorial experiment (clone, fertilizer, drought) was installed. Seedlings were planted in coarse, nutrient and organic matter deficient sand. Root exudates were collected for both experiments with XAD-7 resin capsules. Capsules were extracted with deionized water, and analyzed for total organic C and anthrone reactive C.
Results/Conclusions
A repeated measures ANOVA of total organic C (TOC) in the growth chamber experiment showed a statistically significant 3-way (clone by fertilizer by drought) interaction (p < 0.05). One clone increased exudation with fertilization under low irrigation, but reduced exudation under high irrigation. The other clone reduced exudation under low irrigation, but increased exudation under high irrigation. Furthermore, the clone that displayed greater stem volume growth also showed significantly less (p < 0.10) exudation in the form of anthrone reactive C (ARC). ARC represents highly labile C sources, such as simple carbohydrates. In the field study, four clones tested for root-specific exudation rates showed significant differences (p < 0.10) in TOC exudation. There was no significant fertilizer or clone by fertilizer interaction in the field trial. Overall, these results support our second hypothesis; there is significant variability in exudation between clones. However, interpretations regarding our first hypothesis appear to depend both on genotype and water relations. Some clones reduce exudation with fertilizer application, while others do not. These differences may be partially responsible for the observed range of stem volume responses to fertilizer application.
