Add inorganic fertilizer: freshwater 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

15%

Calculating overall effectiveness category

Background information and definitions

Fertilizers can be used to manage nutrient availability and may speed up revegetation. Plant growth might be limited by a lack of nutrients overall, or of a specific nutrient, after drainage, mining, vegetation harvest or pollution. When one or two nutrients are overabundant, invasive plant species may benefit more than native species. Adding the less abundant nutrients may shift the competitive balance back towards native species (Tilman et al. 1999; Perry et al. 2004). Commonly added nutrients include nitrogen (N), phosphorous (P) and/or potassium (K). It may be sensible to add fertilizer when the focal site is not flooded, to reduce the risk of it dissolving or being washed away.

The effects of this action will be heavily dependent on the study context, especially initial site nutrient levels and the amount of fertilizer added. Adding fertilizer when nutrients are already abundant could cause more harm than good, encouraging the growth of undesirable plants or algae and even inhibiting plant growth (Weinbaum et al. 1992). Accordingly, studies testing the effects of nutrient enrichment as a threat (i.e. enrichment above normal or desirable levels) are not summarized as evidence.

Related actions: Add inorganic fertilizer to complement planting; Stimulate microbial breakdown of oil, including through fertilization.

^{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.}

^{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.}

^{Weinbaum S.A., Johnson R.S. & DeJong T.M. (1992) Causes and consequences of overfertilization in orchards. HortTechnology, 2, 112–121.}

Study locations

Key messages

One study evaluated the effects, on vegetation, of adding inorganic fertilizer to restore or create freshwater marshes. The study was in Germany.

VEGETATION COMMUNITY

Community composition (1 study): One replicated, paired, controlled, before-and-after study in wet grasslands in Germany reported that the effect of annual fertilization (for 20 years) on the average moisture preference of the vegetation varied between sites.
Overall richness/diversity (1 study): The same study reported that the effect of annual fertilization (for 20 years) on total plant species richness varied between sites.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, paired, controlled study in wet grasslands in Germany reported that plots fertilized every spring contained more vegetation biomass, after 4–18 years, than unfertilized plots.
Herb abundance (1 study): The same study reported that the effect of annual fertilization (for 20 years) on cover of herb groups (sedges, rushes, forbs, ferns, grasses and legumes) varied between sites.

VEGETATION STRUCTURE

Height (1 study): One replicated, paired, controlled, before-and-after study in wet grasslands in Germany reported that the effect of annual fertilization (for 20 years) on vegetation height varied between sites.

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, paired, controlled, before-and-after study in 1987–2007 in two wet grasslands in northwest Germany (Poptcheva et al. 2009) reported that fertilized plots contained more plant biomass than unfertilized plots after 4–18 years, but that fertilizer had no consistent effect on vegetation cover, height or species richness. In the first year of the study, above-ground vegetation biomass was statistically similar in fertilized plots (540–590 g/m²) and unfertilized plots (480–510 g/m²). However, after 4–18 years of intervention, above-ground vegetation biomass was significantly greater in fertilized plots (520–820 g/m²) than unfertilized plots (240–390 g/m²). Over 20 years, other vegetation metrics did not respond clearly or consistently to fertilization across the two wet grasslands (data reported as graphical analyses; statistical significance of differences not assessed). These metrics included cover of plant groups (e.g. sedges, rushes, forbs), vegetation height, species richness and community moisture preference. Methods: In 1987, two plots (each 200–250 m²) were established in each of two wet grassland sites (with non-peaty soils, and maintained as fertilized pasture prior to the study). From 1987, all four plots were mown twice each year (June/July and September). From 1989, one plot in each meadow was also fertilized each year in early spring (60 kg/ha P₂O₅ and 120 kg/ha K₂O). Vegetation was surveyed in mid-June. Cover/abundance of all plant species was recorded in four 4-m² quadrats/plot, every one or two years between 1987 and 2007. Vegetation was cut from eight 0.25-m² quadrats/plot in 1989, 1993, 1998 and 2007, then dried and weighed.
Study and other actions tested
Referenced paper
Poptcheva K., Schwartze P., Vogel A., Kleinebecker T. & Hölzel N. (2009) Changes in wet meadow vegetation after 20 years of different management in a field experiment (north-west Germany). Agriculture, Ecosystems & Environment, 134, 108-114.

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

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Lower water level to restore/create brackish/saline swamps from other land uses Action Link	No evidence found (no assessment)	Synopsis Link
Create mounds or hollows: freshwater swamps Action Link	No evidence found (no assessment)	Synopsis Link
Excavate brackish/saline pools Action Link	No evidence found (no assessment)	Synopsis Link