Add below-ground organic matter: brackish/salt marshes

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

40%
Certainty

30%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

This action involves adding organic matter (i.e. remains or waste products of living organisms) below the ground surface, for example by mixing it into the sediment or placing it into holes. Specific substances than can be used include compost, sewage sludge, wood chips and seaweed extract.

The soil organic matter content of wetland soils may be reduced by disturbance. For example, drainage allows oxygen into the soil, whilst reprofiling removes surface layers rich in organic matter (Bruland et al. 2006). Organic matter can be an important component of wetland soils. It directly supplies nutrients to growing plants, supplies carbon and energy to soil organisms, helps bind the soil together, retains water during dry periods, and mediates soil temperature (Donahue et al. 1983; Weil & Brady 2016). Adding carbon-rich organic materials can indirectly modify nutrient availability by stimulating microbial activity, and so could help to manage invasive species where they benefit from an excess of certain nutrients (Reever Morghan & Seastedt 1999; Tilman et al. 1999; Perry et al. 2004).

To be summarized as evidence for this action, studies must evaluate the effects of adding organic matter without adding living vegetation. The organic matter should be used to help or manage existing vegetation, such as remnant patches of vegetation, or seedlings that germinate from seeds already present.

^{Bruland G.L., Richardson C.J. & Whalen S.C. (2006) Spatial variability of denitrification potential and related soil properties in created, restored, and paired natural wetlands. Wetlands, 26, 1042–1056.}

^{Donahue R.L., Shickluna J.C. & Robertson L.S. (1983) Soils: An Introduction to Soils and Plant Growth, Fifth Edition. Prentice-Hall, Englewood Cliffs, NJ, USA.}

^{Perry L.G., Galatowitsch S.M. & Rosen C.J. (2004) Competitive control of invasive vegetation: a native wetland sedge suppresses Phalaris arundinacea in carbon-enriched soil. Journal of Applied Ecology, 41, 151–162.}

^{Reever Morghan K.J. & Seastedt T.R. (1999) Effects of soil nitrogen reduction on nonnative plants in restored grasslands. Restoration Ecology, 7, 51–55.}

^{Tilman E.A., Tilman D., Crawley M.J. & Johnston A.E. (1999) Biological weed control via nutrient competition: potassium limitation of dandelions. Ecological Applications, 9, 103–111.}

^{Weil R.R. & Brady N.C. (2016) The Nature and Properties of Soils, Fifteenth Edition. Pearson, USA.}

Study locations

Key messages

One study evaluated the effects, on vegetation, of adding below-ground organic matter to restore or create brackish/salt marshes. The study was in the USA.

VEGETATION COMMUNITY

VEGETATION ABUNDANCE

Individual species abundance (1 study): One replicated, randomized, controlled, before-and-after study in a salt marsh in the USA found that plots amended with alginate contained a greater density of smooth cordgrass Spartina alterniflora than unamended plots after 6–52 weeks. However, amended and unamended plots contained similar smooth cordgrass biomass when it was sampled after 52 weeks.

VEGETATION STRUCTURE

Height (1 study): One replicated, randomized, controlled, before-and-after study in a salt marsh in the USA found that amending plots with alginate had no significant effect on smooth cordgrass height in the first 16 weeks after intervention, but that amended plots contained taller smooth cordgrass than unamended plots after 28–52 weeks.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A replicated, randomized, controlled, before-and-after study in 2007–2008 in a salt marsh in Georgia, USA (Cohen & Kern 2012) found that adding alginate generally increased the density and height, but not biomass, of smooth cordgrass Spartina alterniflora. Before intervention, plots contained 285–334 smooth cordgrass stems/0.5 m² and plants were 53–59 cm tall. After 6–16 weeks, smooth cordgrass density was greater in plots amended with alginate (282–367 stems/0.5 m²) than in unamended plots (224–313 stems/0.5 m²). However, smooth cordgrass height did not significantly differ between treatments (amended: 65–68 cm; unamended: 60–63 cm). After 28–52 weeks, smooth cordgrass density remained greater in amended plots (135–213 stems/0.5 m²) than in unamended plots (121–164 stems/0.5 m²). Cordgrass was also significantly taller in amended plots (23–49 cm) than in unamended plots (15–35 cm). Finally, after 52 weeks, above-ground cordgrass biomass did not significantly differ between treatments (amended: 3.4 g/0.25 m²; unamended: 6.3 g/0.25 m²). Methods: In July 2007, ten 0.5-m² plots were established in a cordgrass-dominated salt marsh. Alginate (a carbon-rich seaweed extract) was added to five plots (80 g/plot, across ten 2-cm diameter x 10-cm deep holes). In the other five plots, holes were dug but alginate was not added. Live stem density and the height of the five tallest plants were recorded immediately before intervention and biweekly afterwards. Smooth cordgrass was cut from plots after one year, then dried and weighed.
Study and other actions tested
Referenced paper
Cohen R.A. & Kern H. (2012) Alginate addition influences smooth cordgrass (Spartina alterniflora) growth and macroinvertebrate densities. Wetlands, 32, 51-58.

Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Reprofile/relandscape: brackish/saline swamps Action Link	Likely to be beneficial	5	Synopsis Link
Raise water level to restore/create brackish/salt marshes from other land uses Action Link	Unknown effectiveness (limited evidence)	2	Synopsis Link
Create mounds or hollows: freshwater marshes Action Link	No evidence found (no assessment)	0	Synopsis Link