Translocation,remineralization, and turnover of nitrogen in the roots and rhizomes of Spartina alterniflora (Gramineae)

We used ¹⁵N 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 (¹⁵N-labeled) N and 20% burial after 3 years. Translocation of ¹⁵N 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 ¹⁵N into field stands of grass showed initially high incorporation into plants followed by a continuous decline over the next 7 years yielding a gross tumover time of 1.5–1.6yr. 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, 0.4 g 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.