TRANSLOCATION, REMINERALIZATION, AND TURNOVER OF NITROGEN IN THE ROOTS AND RHIZOMES OF SPARTINA-ALTERNIFLORA (GRAMINEAE)

We used N-15 to quantify rates of N translocation from aerial to belowground tissues, foliar leaching, and turnover and production of root and rhizome biomass in the plant-sediment system of short Spartina alterniflora areas of Great Sippewissett Marsh, Massachusetts. Decay of belowground tissues in litterbag incubations at 1- and 10-cm depths resulted in 80% remineralization of the original plant (N-15-labeled) N and 20% burial after 3 years. Translocation of N-15 from plant shoots in hydrologically controlled laboratory lysimeters maintained under field conditions was 38% of the aboveground pool while leaching of N was 10% from June to October. Most of the translocated N was not retranslocated to new aboveground growth in December but appeared to be either remineralized or buried in the sediment. Injection of N-15 into field stands of grass showed initially high incorporation into plants followed by a continuous decline over the next 7 years yielding a gross turnover time of 1.5-1.6 yr. Correcting the gross N turnover for recycling of label via translocation and uptake of remineralized label during this period, a net root and rhizome turnover time of 1.0-1.1 yr was obtained. Combining the turnover time with independent estimates of seasonal belowground biomass yielded an estimate of belowground production of 929-1,022 g C m(-2) yr(-1), similar to measurements by traditional biomass harvest, CO2 based budgets and models for comparable areas of this marsh. Integration of the production and nitrogen balance estimates for short Spartina marsh yielded translocation, 1.4 g N m(-2) yr(-1), leaching, 0.4g N m(-2) yr(-1), remineralization, 14.9-16.3 g N m(-2) yr(-1), and burial, 3.7-4.1 g N m(-2) yr(-1).