ELEMENTAL ANALYSIS OF DEPOSITS ON THE ROOTS OF SPARTINA ALTERNIFLORA LOISEL.

Spartina alterniflora, salt marsh cordgrass, is the dominant angiosperm of a large majority of regularly flooded marshes of the Atlantic and Gulf coasts of the United States. In Louisiana, this species often occurs in two distinct zones: a more productive streamside site (adjacent to tidal creeks), and a less productive and sparsely populated inland area. Reddish-brown deposits are present on the roots of streamside Spartina and visually absent from the roots of inland plants. A study of streamside roots using scanning electron microscope and energy dispersive X-ray microanalysis demonstrated that the coatings are restricted to the outer cell wall of the epidermis and are composed primarily of iron. Roots of inland plants have minor iron deposits. Citrate-dithionite extraction of the coatings and subsequent atomic absorption spectrophotometric analysis confirmed these deposits to be iron, although some manganese was present. Approximately 50 times more iron was found on streamside roots compared to roots from inland plants. These results indicate a better developed oxidized rhizosphere associated with streamside Spartina roots than the inland and, hence, the potential for a more favorable environment in which nutrient uptake may proceed. SALT MARSH GRASSES, as indicated by their high productivity (Turner, 1976), are well adapted to anaerobic substrates. Spartina alterniflora Loisel., the dominant angiosperm of a large majority of regularly flooded salt marshes along the Atlantic and Gulf coasts of the United States, possesses a well developed aerenchyma system (Anderson, 1974) which serves as a pathway for the diffusion of atmospheric oxygen from the leaves to the roots (Teal and Kanwisher, 1966) and provides the oxygen needed by the roots for metabolic processes. The oxygen not consumed in the roots during respiration diffuses into the anoxic mud surrounding the roots and forms an oxidized rhizosphere, i.e., a relatively aerobic microzone around the root (Armstrong, 1970). The oxidized rhizosphere of wetland plants has been hypothesized to act in excluding from the plant soil toxins which may accumulate in anaerobic soils (Armstrong, 1978). Potential phytotoxins such as Fe 2, Mn 2, H2S, ethylene, and certain organic acids, may be oxidized I Received for publication 9 March 1981; revision accepted 30 May 1981. This study was supported by funds from the Louisiana Sea Grant College Program, a part of the National Sea Grant Program maintained by the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The authors would like to thank the Geology Department of Louisiana State University for the use of the analytical scanning electron microscope and Ms. Rita Strate for her typing expertise. to harmless compounds upon contact with the oxidized rhizosphere. Bartlett (1961) has demonstrated this process in rice roots where potentially toxic soluble ferrous iron is oxidized to ferric iron precipitates. Spartina alterniflora often occurs as relatively distinct height forms along a complex gradient of environmental factors landward from tidal creek banks. Along the Louisiana Gulf Coast this gradient in Spartina productivity is identified by two forms: high vigor streamside (= 1 m high) and low vigor inland (-70 cm high) (Kirby and Gosselink, 1976). Streamside habitats are approximately 10 cm higher in elevation than inland areas as a result of greater vertical accretion rates in the former (DeLaune, Patrick and Buresh, 1978). This difference in surface elevation between the two sites results in greater soil waterlogging in the inland zone compared to the streamside (Mendelssohn, McKee and Patrick, 1981). The objectives of the present study were: 1) to evaluate the extent to which the rhizosphere of Spartina alterniflora is important in precipitating iron and manganese, and 2) to determine any difference in the intensity of precipitation as a function of location (streamside vs. inland). MATERIALS AND METHODS-Field sampling-Living, i.e., turgid and structurally intact, roots of Spartina alterniflora were collected from a monospecific salt marsh located