THE FATE OF NITROGEN MINERALIZED FROM LEAF LITTER - INITIAL EVIDENCE FROM 15 N-LABELED LITTER

Decomposition of leaf litter includes microbial immobilization of nitrogen (N), followed by N mineralization. Th e fate of N mineralized from leaf litter is unknown. I hypothesized that N mineralized from leaf litter will be re-immobilized into other forms of organic matter, including downed wood. Th is mechanism may retain N in some forests. To test this hypothesis, oak leaves were enriched with a naturally occurring, stable isotope of N ( 15 N), collected as litter, and allowed to decompose with and without wood (10 cm long, in 3 diameter classes of < 2 cm, 2-4 cm, and 4-6 cm) in litter bags in the forest fl oor of a central hardwood forest near Morgantown, WV. As oak litter immobilized exogenous N, 15 N in litter decreased due to dilution, and then did not change signifi cantly. Nitrogen mineralization in leaf litter did not start for at least 24 months. By the 11th month of decomposition, wood N reached between 140 and 274 percent of the initial N mass, depending on wood diameter. By the 24th month, 15 N was detected in wood, supporting the hypothesis. Nitrogen-label was not detected in fresh litter above litter bags, or below litter bags in partly decomposed forest fl oor or soil. It appears that N mineralized from leaf litter may be transferred to and immobilized in small-diameter downed wood. If these results are confi rmed in a larger study, we may need to change the way we view N cycling in the leaf litter layer; N mineralized from foliar litter may not be immediately plant-available as we now assume.