Add inorganic fertilizer before/after planting non-woody plants: brackish/saline wetlands

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Source countries

Key messages

  • Seven studies evaluated the effects, on vegetation, of adding inorganic fertilizer to brackish/saline wetlands planted with emergent, non-woody plants. Four studies were in the USA. Two of these were based in the same marsh, but used different experimental set-ups. Two studies were in Canada. One study was in China.

VEGETATION COMMUNITY

 

VEGETATION ABUNDANCE

  • Overall abundance (2 studies): One replicated, randomized, paired, controlled study in intertidal brackish marshes in Canada found that adding fertilizer when planting wetland herbs typically had no significant effect on total live vegetation biomass, after two growing seasons. One replicated, paired, controlled, before-and-after study in salt-contaminated bogs in Canada found that overall vegetation biomass and cover were greater in fertilized than unfertilized plots, one year after introducing salt marsh vegetation.
  • Individual species abundance (6 studies): Six studies quantified the effect of this action on the abundance of individual plant species. For example, three replicated, randomized, paired, controlled studies in intertidal areas in the USA found that the abundance of cordgrasses Spartina spp. was typically similar in fertilized and unfertilized plots, 1–2 growing seasons after planting. This was true for density, biomass and/or cover. However, one controlled study on former borrow pits in the USA found that cordgrass Spartina spp. biomass was typically greater in fertilized than unfertilized plots, one growing season after planting. This study also found that fertilization typically reduced black rush Juncus roemarianus biomass, one growing season after planting.

VEGETATION STRUCTURE

  • Height (6 studies): Five replicated, controlled studies (four also paired, three also randomized) in brackish/saline wetlands in the USA, China and Canada found that adding fertilizer had no significant effect on the height of planted/sown wetland herbs after 1–2 growing seasons. One controlled study on former borrow pits in the USA found that fertilized smooth cordgrass Spartina alterniflora was taller than unfertilized smooth cordgrass, two growing seasons after planting.

OTHER

  • Survival (4 studies): Three replicated, randomized, paired, controlled studies in intertidal areas in the USA and Canada found that adding fertilizer had no significant effect on the survival of planted wetland herbs over 1–2 growing seasons. One controlled study on former borrow pits in the USA reported that adding standard fertilizer to planting holes reduced the survival of planted big cordgrass Spartina cynosuroides, after one growing season.

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

  1. A controlled study in 1979–1981 on reprofiled borrow pits in North Carolina, USA (Broome et al. 1982) found that adding fertilizer typically increased the biomass and height of planted cordgrasses Spartina spp., but typically reduced the biomass of planted black rush Juncus roemerianus and sometimes reduced survival of big cordgrass Spartina cynosuroides. After one growing season, fertilized cordgrasses produced more above-ground biomass than unfertilized cordgrasses in 40 of 53 comparisons (for which fertilized: 64–464 g/m2; unfertilized: 6–55 g/m2) with no significant difference in the other 13 comparisons (for which fertilized: 31–177 g/m2; unfertilized: 8–43 g/m2). In contrast, fertilized black rush produced less above-ground biomass than unfertilized black rush in four of six comparisons (for which fertilized: 4 g/m2; unfertilized: 18 g/m2) with no significant difference in the other two comparisons (for which fertilized: 11–12 g/m2; unfertilized: 18 g/m2). After two growing seasons, smooth cordgrass Spartina alterniflora was taller when fertilized (144–152 cm) than when not fertilized (113 cm). Black rush height was not measured. Finally, the study reported that adding standard fertilizer to the planting hole reduced survival of big cordgrass (standard fertilizer: 5–23%; slow-release fertilizer or unfertilized: 80% survival after one growing season). Methods: In June 1979 and 1980, greenhouse-grown or wild-harvested vegetation was planted into reprofiled borrow pits (salinity <20 ppt). Some plants were fertilized (one of 18 different type/dose combinations placed in planting holes, next to planting holes or mixed into soil surface). Other plants were left unfertilized. The study does not clearly report the experimental design (including numbers of plants and plots). In October 1980 and 1981, living vegetation was cut from 0.25-m2 quadrats then dried and weighed.

    Study and other actions tested
  2. A replicated, randomized, paired, controlled study in 1976–1977 on two intertidal mudflats in Texas, USA (Tanner & Dodd 1985) found that applying fertilizer after planting smooth cordgrass Spartina alterniflora typically had no significant effect on its survival, height, density or biomass. After one growing season, cordgrass survival was statistically similar in fertilized and unfertilized plots in 12 of 12 comparisons (fertilized: 18–89%; unfertilized: 9–85%). After two growing seasons, cordgrass height was statistically similar under both treatments in 11 of 12 comparisons (for which fertilized: 117–127 cm; unfertilized: 110–122 cm; other comparison shorter in fertilized than unfertilized plots). After 1–2 growing seasons, cordgrass density was statistically similar under both treatments in 20 of 24 comparisons (for which fertilized: 2–252 stems/m2; unfertilized: <1–252 stems/m2; other comparisons a mix of higher and lower density in fertilized than unfertilized plots). Above-ground cordgrass biomass was statistically similar under both treatments in 24 of 24 comparisons (fertilized: 23–1,840 g/m2; unfertilized: 20–1,700 g/m2). The same was true for live and dead biomass separately (12 of 12 comparisons; see original paper for data). Methods: In July 1976, fifty-four 12.5-m2 plots were established across two intertidal mudflats. Smooth cordgrass (20–100 cm tall) was transplanted into each plot (50 plants/plot, 50 cm apart. Thirty-six of the plots (18 random plots/mudflat) were fertilized after planting (NPK; 12 or 24 g/m2). The other plots were not fertilized. Cordgrass was monitored in October–November 1976 and 1977. Monitoring included counting stems, measuring representative flowering stems, and cutting, drying and weighing three cordgrass plants/plot. This study used the same marsh as (3), but a different experimental set-up.

    Study and other actions tested
  3. A replicated, randomized, paired, controlled study in 1977 on intertidal sediment in Texas, USA (Webb & Dodd 1989) found that applying fertilizer had no significant effect on the survival, cover or height of planted smooth cordgrass Spartina alterniflora, and typically had no significant effect on its density. After two months and/or one growing season, fertilized and unfertilized plots supported similar cordgrass survival in two of two comparisons, had similar cordgrass cover in two of two comparisons, and contained cordgrass of similar height in four of four comparisons (data not reported). Fertilized and unfertilized plots had similar cordgrass densities in three of four comparisons (data not reported). In the other comparison, fertilized plots had higher cordgrass densities on average (17 stems/m2) than unfertilized plots (15 stems/m2). Neither fertilizer dose nor timing of application affected any result. Methods: In 1977, three hundred 15-m2 plots were established (in 30 sets of 10) at varying elevations on created intertidal land (sediment deposited and graded, protected by a breakwater and fenced). All plots were planted with field-collected cordgrass in February or May (60 plants/plot). Two hundred and forty plots (eight plots/set) were fertilized in one of four ways: high dose (244 kg/ha of N, P2O5 and K2O) or low dose (122 kg/ha of N, P2O5 and K2O), all before planting or split before and after planting. The other 60 plots (two plots/set) were not fertilized. After two months (April and July) and one growing season (November), the central 30 cordgrass plants in each plot were surveyed. This study used the same marsh as (2), but a different experimental set-up.

    Study and other actions tested
  4. A replicated, randomized, paired, controlled study in 1990–1991 in a recently excavated estuarine marsh in California, USA (Gibson et al. 1994) found that adding inorganic fertilizer to plots planted with California cordgrass Spartina foliosa had no significant effect on cordgrass biomass, stem density or plant height after two growing seasons. In four of four comparisons per metric, there was no significant difference between treatments in above-ground cordgrass biomass (fertilized: 180–420 g/m2; unfertilized: 100–500 g/m2), cordgrass density (fertilized: 40–90 stems/m2; unfertilized: 30–100 stems/m2) or average cordgrass height (data not reported). Results were similar after one growing season, with no significant difference between fertilized and unfertilized plots in at least three of four comparisons per metric (see original paper for data). Methods: In February 1990, twenty-eight 5-m2 plots were established, in four sets of seven, alongside a tidal creek in a recently excavated salt marsh. In 12 plots (three random plots/set), ammonium nitrate fertilizer was tilled into the surface (105 g/m2). Twelve plots (three random plots/set) were tilled but not fertilized. The final four plots (one random plot/set) were not even tilled. Some plots were also amended with organic matter. In March 1990, each plot was planted with cordgrass plants from ten 4-L pots. California cordgrass stems were counted and measured until October 1991. Dry biomass was estimated from heights.

    Study and other actions tested
  5. A replicated, randomized, paired, controlled study in 1996–1997 in two degraded, intertidal, brackish marshes in Manitoba, Canada (Handa & Jeffries 2000) found that adding fertilizer increased the cover of one of two planted herb species, but did not significantly affect survival rates of either species or overall above-ground biomass. On all five survey dates across the second growing season after planting, creeping alkaligrass Puccinellia phryganodes cover was higher in fertilized plots (1,720–5,400 mm2/m2) than in unfertilized plots (1,020–4,870 mm2/m2). However, cover of estuary sedge Carex subspathacea never significantly differed between treatments (fertilized: 670–2,880 mm2/m2; unfertilized: 670–2,720 mm2/m2). On all five dates, survival rates were statistically similar under each treatment, for both alkaligrass (fertilized: 52–100%; unfertilized: 47–100%) and estuary sedge (fertilized: 24–58%; unfertilized: 23–50%). On at least two of three dates (results not clearly reported), live above-ground biomass was statistically similar under each treatment for both alkaligrass-dominated vegetation (fertilized: 47–178 g/m2; unfertilized: 29–99 g/m2) and sedge-dominated vegetation (fertilized: 1–4 g/m2; unfertilized: 1–4 g/m2). Methods: In June 1996, plugs of creeping alkaligrass and estuary sedge were transplanted from natural stands to 1-m2 plots within brackish marsh vegetation damaged by geese (one species/marsh; 12 plots/species; 42 plugs/plot). Two random quarters of each plot were fertilized with N and P at planting (10.5 g N/m2 and 4.5 g P/m2). Half of each plot was also mulched. All plots were fenced to exclude geese. Vegetation was surveyed in summer 1997. Survival and cover were monitored for planted plants in the centre of each plot. Vegetation samples were cut from the margins of each plot, then washed to remove dead biomass, dried and weighed.

    Study and other actions tested
  6. A replicated, controlled, before-and-after study in an estuarine wetland in eastern China (Guan et al. 2011) found that adding urea before sowing seeds of seablite Suaeda salsa increased seablite biomass, but had no significant effect on its density or height. Five months after sowing, fertilized plots contained a greater above-ground biomass of seablite (640 g/m2) than unfertilized plots (396 g/m2). Meanwhile, there was no significant difference between treatments in seablite density (fertilized: 292 plants/m2; unfertilized: 365 plants/m2) or height (fertilized: 59 cm; unfertilized: 59 cm). Height was also statistically similar under both treatments for measurements taken 1–4 months after sowing (fertilized: 12–47 cm; unfertilized: 12–51 cm). Methods: In May 2009, three pairs of 6-m2 plots were established in a degraded, unvegetated, hypersaline/alkaline wetland in the Yellow River estuary. Three plots were prepared by ploughing (to 20 cm depth) and mixing in urea (130 kg N/ha). The other three plots had been prepared by ploughing only. Approximately 5,000 seablite seeds were sown onto each plot, then watered. Vegetation was sampled in five 1-m2 quadrats/plot until October 2009. Biomass measurements involved samples of approximately 100 plants/plot.

    Study and other actions tested
  7. A replicated, paired, controlled, before-and-after study in 2011–2012 in two salt-contaminated bogs in New Brunswick, Canada (Emond et al. 2016) found that adding fertilizer before introducing vegetation increased overall vegetation cover and above-ground biomass, but had no significant effect on the height of transplanted herbs. After one year, fertilized plots contained more vegetation overall than unfertilized plots. This was true in terms of cover (fertilized: 25%; unfertilized: 13%) and above-ground biomass (fertilized: 161 g/m2; unfertilized: 86 g/m2). Meanwhile, the height of transplanted vegetation did not significantly differ between fertilized and unfertilized plots. This was true for chaffy sedge Carex paleacea (fertilized: 36 cm; unfertilized: 31 cm) and prairie cordgrass Spartina pectinata (fertilized: 25 cm; unfertilized: 19 cm). Methods: In summer 2011, eighty 9-m2 plots were established (in four blocks of 20) on bare, salt-contaminated peat (0.5–1.4 ppt). Sixty-four of the plots were planted with vegetation (chaffy sedge, prairie cordgrass, or mixed salt marsh plant fragments). Half of the plots were fertilized (9 g rock phosphate fertilizer in planting holes) and half were left unfertilized. Some fertilized and unfertilized plots were also limed. In July 2012, vegetation cover was recorded in one 4-m2 quadrat in each plot. Vegetation was cut from a 250-cm2 quadrat then dried and weighed.

    Study and other actions tested
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.

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Marsh and Swamp Conservation

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Marsh and Swamp Conservation
Marsh and Swamp Conservation

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Marsh and Swamp Synopsis

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