Static and Dynamic Aspects of Nitrogen Cycling in the Salt Marsh Graminoid Spartina Alterniflora

Abstract

Processes of nitrogen cycling including root uptake, translocation, leaching, detrital incorporation, and overwinter storage were determined for a medium—height form of Spartina alterniflora in Georgia, USA. Mean concentration of total N in live above—and belowground plant tissues was 0.60% of dry mass. Aboveground concentrations (1.05%) were more than twice the belowground N concentrations. The seasonal patterns of nitrogen concentrations were similar for aerial and subaerial Spartina tissues with maxima observed between December and May. Concentrations were highest in short, young stems and decreased as plants matured, senesced, and died. Seasonal patterns of nitrogen accumulation were out of phase for aerial and subaerial tissues. Maximum accumulation was in midsummer for aboveground plant parts and in late winter for belowground tissue. During winter ≤83% of total plant N was in roots and rhizomes. During spring, there was a large net transfer of N from belowground storage to aerial tissues; °70% of total nitrogen was in aboveground plant parts by May. A nitrogen budget for Spartina alterniflora was constructed by combining organic productivity rates with nitrogen concentrations in plant tissues. Total uptake of N by roots was 34.8 g°m—2 yr—1. Of this, 43% was lost by death or leaching from aboveground plant parts, while the rest was lost by death of roots and rhizomes. Total transfer of N from below— to aboveground tissues was 33.0 g°m—2°yr—146% of which was new nitrogen taken up from the soil. Of the N transferred aboveground, 14.4 g was lost to detritus upon culm death, 0.7g was leached from the living culm, and 17.9 g was translocated from senescing leaves to rhizomes for storage or immediate transfer to activity growing shoots. A large percentage of the total flux into and out of leaves occurred during the period of active growth. The high degree of N conservation in Spartina, low leachability, and retraction from senescent tissue are consistent with the idea that primary productivity in salt marshes is nitrogen limited.