Nitrogen stable isotopes indicate differences in nitrogen cycling between two contrasting Jamaican montane forests

The aim of this study is to enhance our knowledge of nitrogen (N) cycling and N acquisition in tropical montane forests through analysis of stable N isotopes (delta N-15). Leaves from eight common tree species, leaf litter, soils from three depths and roots were sampled from two contrasting montane forest types in Jamaica (mull ridge and mor ridge) and were analysed for delta N-15. All foliar delta N-15 values were negative and varied among the tree species but were significantly more negative in the mor ridge forest (by about 2 aEuro degrees). delta N-15 of soils and roots were also more negative in mor ridge forests by about 3 aEuro degrees. Foliar delta N-15 values were closer to that of soil ammonium than soil nitrate suggesting that trees in these forests may have a preference for ammonium; this may explain the high losses of nitrate from similar tropical montane forests. There was no correlation between the rankings of foliar delta N-15 in the two forest types suggesting a changing uptake ratio of different N forms between forest types. These results indicate that N is found at low concentrations in this ecosystem and that there is a tighter N cycle in the mor ridge forest, confirmed by reduced nitrogen availability and lower rates of nitrification. Overall, soil or root delta N-15 values are more useful in assessing ecosystem N cycling patterns as different tree species showed differences in foliar delta N-15 between the two forest types.