Long-term trends of changes in pine and oak nitrogen metabolism (foliar and sapwood) in response to chronic nitrogen amendments at the harvard forest, MA
Increase in N input from human activities has led to harmful effects to terrestrial ecosystems. The present study was conducted at the Harvard Forest Long-Term Ecological Research (LTER-http://harvardforest.fas.harvard.edu) site (Petersham, MA), where long-term fertilization with ammonium nitrate (NH4NO3) has been ongoing since 1989. Previous reports from this experiment found that chronic N additions resulted in species-specific changes in the foliar biochemistry and metabolism that accompanied alterations in soil N cycling rates. In addition, these treatments affected soil respiration and microbial biomass, along with bacterial community structure in the hardwood soils. The current report complements the earlier studies and evaluates the long-term effects of N treatments on soil solution chemistry and species-specific foliar and sapwood biochemistry of pine and oak trees at this site. Common stress-related nitrogen metabolites like polyamines (PAs), free amino acids (AAs) and inorganic elements were analyzed for control, low N (LN, 50 kg NH4NO3 ha-1 yr-1) and high N (HN, 150 kg NH4NO3 ha-1 yr-1) treatments using previously published HPLC and ICP methods.
In the pine stands, partitioning of excess nitrogen into foliar PAs and AAs increased with both N treatments until 2002. By 2005, several of these effects on N metabolites disappeared for HN, and by 2008 they were mostly observed for LN plot. A significant decline in foliar Ca and P was observed mostly with HN for a few years until 2005. However, sapwood data actually showed an increase in Ca, Mg, and Mn and no change in PAs in the HN plot for 2008 while AAs data revealed trends that were generally similar to foliage for 2008. Concomitant with these changes, mortality data revealed a large number of dead trees in HN pine plots by 2002; the mortality rate started to decline by 2005. Oak trees in the hardwood plot did not exhibit any major changes in PAs, AAs, nutrients and mortality rate with LN treatment indicating that oak trees were able to tolerate the yearly doses of 50 kg NH4NO3 ha-1 yr-1. On the other hand, HN trees suffered from physiological and nutritional stress along with increased mortality in 2008. In this case also, foliar data were supported by the sapwood data. Overall, both low and high N applications resulted in greater physiological stress to the pine trees than the oaks. The correspondence of these metabolites with other measures of forest functions suggests that the metabolite analyses are useful for long-term monitoring.