The influence of subsurface hydrology on nutrient supply and smooth cordgrass (Spartina alterniflora) production in a developing barrier island marsh

The supply of nutrients from surface and subsurface water flow into the roo t zone was measured in a developing barrier island marsh in Virginia. We hy pothesize that high production of tall-form Spartina alterniflora in the lo wer intertidal zone is due to a greater nitrogen input supplied by a larger subsurface flux. Individual nitrogen inputs to the tall-form and short-for m S. alterniflora root zones were calculated from water flow rates into the root zone and the nutrient concentration corresponding to the source of th e flow. Total dissolved inorganic nitrogen (DIN) input las ammonium and nit rate) was then calculated using a summation of the hourly nutrient inputs t o the root zone over the entire tidal cycle based on hydrologic and nutrien t data collected throughout the growing season (April-August) of 1993 and 1 994. Additionally, horizontal water flow into the lower intertidal marsh wa s reduced experimentally to determine its effects on nutrient input and pla nt growth. Total ammonium (NTH,I) input to the tall-form S. alterniflora ro ot zone (168 mu moles 6 h(-1)) was significantly greater relative to the sh ort-form (45 mu moles 6 h(-1)) during flood tide. Total NH4+ input was not significantly different between growth forms during ebb tide, and total nit rate (NO3-) and total DIN input were not significantly different between gr owth forms during either tidal stage. During tidal flooding, vertical flow from below the root zone accounted for 71% and horizontal flow from the adj acent mudflat accounted for 19% of the total NH4+ input to the tall-form S. alterniflora root zone. Infiltration of flooding water accounted for 15% m ore of the total NO3- input relative to the total NH4+ input at both zones on flood tide. During ebb tide, vertical flow from below the root zone stil l accounted for the majority of NH4+ and NO3- input to both growth forms. A fter vertical flow, horizontal subsurface flow from upgradient accounted fo r the next largest percentages of NH4+ and NO3- input to both growth forms during ebb tide. After 2 yr of interrupted subsurface horizontal flow to th e tall-form S. alterniflora root zone, height and nitrogen content of leaf tissue of treatment plants were only slightly, but significantly, lower tha n control plants. The results suggest that a dynamic supply of DIN las infl uenced by subsurface water flows) is a more accurate depiction of nutrient supply to macrophytes in this developing marsh, relative to standing stock nutrient concentrations. The dynamic subsurface supply of DIN may play a ro le in spatial patterns of aboveground S. alterniflora production, but deter mination of additional nitrogen inputs and the role of belowground producti on on nitrogen demand need to also be considered.