『Abstract
In this study we quantified the different forms of nitrogen,
organic carbon and phosphorus in two eutrophicated watercourses
flowing into a coastal salt marsh of the Mar Menor lagoon and
analysed the role of the water flow regime in the nutrient loads
flowing into the salt marsh. We discuss the degree to which the
soil-plant system in stands of Phragmites australis could
be affected by the discharges of nutrients and estimate the stocks
of nitrogen, phosphorus and organic carbon in different compartments
of the system. The base flow sustained an important discharge
of surplus water of agricultural origin enriched in dissolved
organic carbon (12.7 T y-1) and nitrogen (78.3 T y-1,
85％ N-NO3- and 15％ organic-N)
into the salt marsh, while inputs from wastewater-treatment plants
were of much lower magnitude (5.5 T y-1 of dissolved
organic carbon and 4.1 T y-1 of nitrogen, 57％ N-NH4+and 43％ organic-N). The annual loads
of phosphorus of agricultural origin and from urban wastewater
were 1.87 T y-1 and 0.97 T y-1, respectively.
The data show that the high amounts of inorganic nitrogen from
agricultural activity are absorbed by vegetation or denitrified,
while organic nitrogen probably helps to compensate for soil nitrogen
lost by mineralisation. The soils of the salt marsh may be considered
a sink for phosphorus flowing into it in wastewater. The tissues
of P. australis showed differing patterns of accumulation
and translocation of carbon, nitrogen and phosphorus; the concentrations
of these three elements changed with the season but apparently
were not affected by the eutrophicated water that the plants received.
Soil salinity, pH, Fe concentrations and phosphorus content had
little influence on litter quality. Dry stems were important reservoirs
of organic carbon since they persisted throughout the year, while
dry leaves were the main contributors to the litter, which was
mineralised partially during spring and summer. Calculations of
primary productivity showed a positive balance of carbon in the
below-ground biomass (595 g m-2 y-1), above-ground
(2610 g -2 y-1) and litter (260 g m-2
y-1). The average soil organic carbon concentration
decreased in one of the plots studied, probably because mineralisation
was favoured since the soil was dry most of the time. Hence, our
data suggest that although the high biomass production of Phragmites
favours carbon sequestration in plant biomass, soil organic carbon
losses in stands of this species may be very important throughout
the year.
Keywords: Nutrient stocks; Carbon sequestration; Eutrophication;
Net primary productivity; Mar Menor』
1. Introduction
2. Material and methods
2.1. Study area
2.2. Sampling in the watercourses
2.3. Sampling within the salt marsh
2.4. Analytical procedures
2.4.1. Management and analysis of the water samples
2.4.2. Management and analysis of soil samples
2.4.3. Management and analysis of plant samples
2.5. Statistical procedures
2.6. Calculation of loads
3. Results
3.1. Watercourses
3.1.1. Discharges and precipitation
3.1.2. pH, salinity, concentrations of nitrogen, phosphorus
and dissolved organic carbon and nutrient loads
3.2. Soils within the salt marsh
3.2.1. Particle size distribution, bulk density, total CaCO3 and total Fe
3.2.2. pH, salinity and redox potential (Eh)
3.2.3. Total organic carbon, nitrogen and phosphorus
3.3. Phragmites australis
3.3.1. Plant biomass and organic carbon concentrations
3.3.2. Nitrogen and phosphorus concentrations
4. Discussion
4.1. Characteristics of the watercourses and nutrient inputs
into the salt marsh
4.2. Litter decomposition and the soils of the salt marsh as
sinks or sources of nutrients
4.3. Nutrient absorption and translocation in P. australis
4.4. Nutrients, organic carbon storage and primary productivity
in the salt marsh
Acknowledgments
References