The Effect of a Nitrogen Gradient on Above- and Below-Ground Plant Growth in Salt
Marshes
Peris N. Bentley
Mentor: Jennifer Martiny
The goal of this project was to study the effects of a nitrogen addition gradient on plant productivity
in Salicornia salt marshes along the California coast. Salt marsh environments are sensitive to
eutrophication, of which nitrogen (N) is a major component. Three California salt marshes that
differed in their sedimentary nitrogen content were selected for this research: Morro Bay Estuary
(MBE), Carpinteria Salt Marsh (CSM), and Tijuana River Estuary (TRE). Thirty-five plots were
established in July 2008 in each marsh (105 total plots). There were seven different treatments in the
nitrogen addition gradient and five replicate plots per treatment per marsh (0, 10, 20, 40, 80, 160, 320
g N/m2/yr). The plots were fertilized with slow release nitrogen in the form of urea every 10 weeks
for 14 months. To measure above-ground growth, a 12 cm2 subplot was marked by flags in each plot,
and all of the above-ground biomass in this subplot was collected. To measure below-ground growth,
a root ingrowth chamber was placed in each plot. Above-ground biomass increased significantly with
nitrogen addition and the nitrogen response curves were similar. In contrast, total (fine + heavy) root
growth did not change with increasing nitrogen; however, the heavy root response was dependent on
the marsh. This indicates that nitrogen is a limiting factor in plant growth and that plants prefer to
use the nutrient for above-ground expansion, possibly for increased photosynthesis.