Relative importance of macrophyte leaves for nitrogen uptake from flood water in tidal salt marshes

Nitrogen limits plant growth in most salt marshes. As foliar N-uptake makes a significant contribution to the overall N-requirements of submerged plant species such as (e.g.) seagrasses, we tested if foliar N-uptake was also significant in Spartina anglica Hubbard, a species that dominates the lowest, regularly flooded areas of salt marshes in the SW Netherlands. Foliar N-uptake was compared for plants from 2 estuaries with contrasting N-loads in their water column. N-uptake was quantified by (1) flooding detached leaves in test tubes, (2) spraying leaves still attached to the plants, and (3) flooding whole plants, with solutions containing either (NO3-)-N-15 or (NH4+)-N-15. We found that detaching the leaves from the plant underestimated NH4+ uptake by between 30 and 50%. Higher salinity also reduced foliar N-uptake. Uptake rates were higher for NH4+ than for NO3-, as has been found for many submerged and terrestrial angiosperms and marine algae. Methodology also had a major effect on the uptake rate, with flooding of intact plants yielding higher uptake rates than spraying attached leaves. However, in general, foliar N-uptake rates were low at the NO3- and NH4+ concentrations that are actually present in the tidal waters during the growth season, and may at most contribute to around 10% of the growth requirement. This percentage is much less than for seagrasses, but in line with data for some terrestrial systems. We conclude that in contrast to seagrasses, foliar N-uptake does not form a significant contribution to the overall N-requirements of S. anglica. This low N-uptake capacity of the S. anglica leaves appears to be a consequence of adaptations to survive tidal flooding.