Plant Assimilation

Most wetland plants absorb and assimilate significant amounts of P. The P concentration varies considerably between plant organs and species (Table 2) and ranges from 0.1% to 0.8% of the dry weight. The capacity of wetland plants to assimilate P depends on their growth rates, season of the year, total biomass per unit area, ionic composition of the water, water depth, sediment characteristics, and several biochemical and physico-chemical processes at the root-water-sediment interface. Some plants absorb and accumulate greater amounts of P when the availability increases (Tables 3a and 3b). If the plants are not harvested, P absorbed during the growth period is returned to the sediments and/or water after death and decomposition. Hence, some wetland plants have also been called as nutrient pumps. In case of perennial plants such as reeds and cattails, a part of the nutrients is translocated back to the roots and rhizomes before the death of aboveground shoots, and these nutrients are utilized for the early growth in the next growing season before fresh uptake occurs. These processes therefore limit the potential removal of P by the wetland plants.

Mechanisms for Nitrogen Removal

Total N in wastewaters occurs in both organic and inorganic forms. Whereas the organic N may be dissolved or particulate, the inorganic N is either in form of nitrates or ammonium ions. Same forms of nitrogen occur also in the wetland substrates. The nitrogen undergoes transformation along several pathways that govern its retention or cycling within and release from the wetland (Figure 3).

Unlike P, adsorption plays a minor role in case of N because only ammonium ions (NH^) are weakly bound to

Table 3a Ranges of phosphorus and nitrogen content (% dry weight) of some wetland plants under high nutrient loads

Free-floating species

Eichhornia crassipes 0.1-1.2 1.0-4.0

Salvinia molesta 0.2-0.9 2.0-4.8

Pistia stratiotes 0.2-1.2 1.2-4.0

Floating leaved species

Alternanthera philoxeroides 0.2-0.9 1.5-3.5

Ludwigia peploides 0.4-0.6 2.5-4.5

Marsilea mutica 0.5-0.7 2.3-3.6

Hydrocleys nymphoides 0.5-1.0 1.4-5.0

Hydrocotyle umbellate 0.2-1.3 1.5-4.5

Nymphoides indica 0.5-1.2 1.5-3.5

Submerged species

Ceratophyllum demersum 1.0-1.4 3.5-4.2

Elodea canadensis 0.7-1.1 4.0-4.1

Potamogeton crispus 0.6-1.0 3.5-4.0

Table 3b Ranges of phosphorus and nitrogen content (% dry weight) of some wetland plants under high nutrient loads

Phosphorus

Nitrogen

Trees

Leaf

Stem

Leaf

Stem

Acer rubrum

0.2-0.3

1.0-2.2

Magnolia virginiana

0.1-0.2

<0.1

1.9-2.5

0.4

Nyssa sylvatica

0.1

<0.1

1.9

0.1

Taxodium distichum

0.1-0.3

<0.1

<1

<0.1

T. ascendens

0.1-0.2

1.5-1.8

Emergent species

Leaf

Root

Rhizome

Leaf

Root

Rhizome

Cyperus involucratus

0.2-0.5

0.1-0.7

0.2-0.7

1.5-4.3

1.1-4.5

0.5-2.1

Phragmites australis

0.2-0.4

0.1-0.3

0.1-0.3

1.0-4.0

15-3.1

0.5-3.1

Typha spp.

0.1-0.5

0.2-0.7

0.1-0.7

0.5-3.2

0.4-5.2

0.2-4.0

Scirpus tabernaemontani

0.2-0.4

0.2-0.8

0.2-0.7

0.6-2.5

0.4-2.1

0.9-1.8

Bolboschoenus spp.

0.1-0.5

0.2-0.7

0.4-0.6

0.2-1.5

0.2-1.5

1.3-1.9

Baumea articulata

0.1-0.9

0.2-0.8

0.2-0.7

1.1-1.8

0.8-2.5

0.8-1.9

Figure 3 Major processes and pathways of nitrogen transformation in wetlands. Bu, biofilm uptake; DON, dissolved organic nitrogen; PON, particulate organic nitrogen; SMac, submerged macrophytes; Ni, nitrification; Phytopl, phytoplankton.

soil particles. Dissolved forms of N may however move across sediment-water interface through diffusion along the concentration gradient. Settlement of suspended organic particulates on the wetland sediments can remove large amounts of nitrogen. Further amounts of nitrogen can accumulate in the wetland in the form of partially decomposed plant detritus (peat) and gradually buried and incorporated into the soil profile.

Was this article helpful?

0 0
Project Earth Conservation

Project Earth Conservation

Get All The Support And Guidance You Need To Be A Success At Helping Save The Earth. This Book Is One Of The Most Valuable Resources In The World When It Comes To How To Recycle to Create a Better Future for Our Children.

Get My Free Ebook


Post a comment