ASSESSMENT OF CARBON ALLOCATION AND BIOMASS PRODUCTION IN A NATURAL STAND OF THE SALT-MARSH PLANT SPARTINA-ANGLICA USING C-13

The proportional allocation of photosynthetically fixed carbon to the root and shoot system of salt marsh plants is an important element in the carbon cycle of tidal salt marshes. The commonly applied field met hods giving insight on this point are based on successive harvesting o f biomass. These methods, however, lack accuracy and do not yield reli able data over short time intervals. In the present study, the stable carbon isotope C-13 was used as a tracer of carbon flow in Spartina an glica. Shoot clusters of this halophyte were incubated with (CO2)-C-13 (ca 1 h) in a salt marsh in the SW Netherlands. Four days after the i ncubations, shoots and roots/rhizomes were sampled to determine enrich ment with the heavy isotope. Although S. anglica is a clonal plant, on ly a minor part of the excess C-13 incorporated in plant tissues due t o the incubation procedure was found outside the incubation plots. The data revealed the dynamic nature of carbon allocation in S. anglica, leading to seasonal changes in the ratio of carbon allocated to above- ground versus below-ground tissues. The ratio was highest in August (6 .19) and lowest in September (1.60), but more carbon was always invest ed in above-ground plant parts than in below-ground plant parts. These findings strongly suggest that, on an annual basis, above-ground biom ass production outweighs below-ground biomass production in this S. an glica population. Root/rhizome production over short-term periods (10 to 12 d) was calculated by combining the data on the ratios of C-13 al location with measurements of above-ground biomass increments over the se periods. On an annual basis, above-ground and below-ground biomass production was estimated to be 1130 and 556 g dry wt m(-2), respective ly. A survey of the Literature shows that the ratio of shoot to root/r hizome production of Spartina spp. vegetation is highly variable, rang ing from a clear dominance of shoot production (as is suggested by our data) to the completely opposite situation with root/rhizome producti on amply exceeding shoot production. This may imply considerable diffe rences in the functioning of Spartina-dominated marsh systems, e.g. wi th respect to the dominance of aerobic or anaerobic mineralization pro cesses and to mineralization-linked cycling of sulphur and nitrogen.