Add below-ground organic matter before/after planting non-woody plants: brackish/saline wetlands
Overall effectiveness category Likely to be beneficial
Number of studies: 6
Background information and definitions
This section involves adding organic matter (i.e. remains or waste products of living organisms) below the ground surface (i.e. by mixing it into the sediment, or placing it into holes) to areas planted with marsh or swamp vegetation.
Organic matter could increase the initial survival and/or growth rate of introduced plants, helping them to establish. Organic matter directly supplies nutrients to growing plants, supplies carbon and energy to soil organisms (which can indirectly increase nutrient availability), helps bind the soil together, retains water during dry periods, and mediates soil temperature (Donahue et al. 1983; Weil & Brady 2016). However, 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). Substances than can be used to add organic matter to wetland soils include compost, sewage sludge, wood chips and seaweed extract.
Note that many studies testing this action do not separate the effects of adding organic matter and disturbing the soil/sediment. We have included such studies, as well as those that do separate the effects of these actions with appropriate controls.
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.
Weil R.R. & Brady N.C. (2016) The Nature and Properties of Soils, Fifteenth Edition. Pearson, USA.
Supporting evidence from individual studies
A replicated, randomized, paired, controlled study in 1990–1991 in a recently excavated estuarine marsh in California, USA (Gibson et al. 1994) found that tilling organic matter into plots before planting California cordgrass Spartina foliosa typically had no significant effect on cordgrass biomass, stem density or plant height. This was true after both one and two growing seasons. For example, after two growing seasons, plots amended with organic matter contained a similar cordgrass biomass to unamended plots in five of six comparisons (for which amended: 100–220 g/m2; unamended: 290–500 g/m2), a similar cordgrass density to unamended plots in five of six comparisons (for which amended: 70–100 stems/m2; unamended: 30–50 stems/m2), and cordgrass of a similar height to unamended plots in six of six comparisons (data not reported). 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 16 plots (four random plots/set), organic matter was tilled into the surface (3 kg/m2 straw or alfalfa). Eight plots (two random plots/set) were tilled but did not receive organic matter. The final four plots (one random plot/set) were not even tilled. Inorganic fertilizer was also added to some plots. 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
A replicated, paired, controlled study in 1999–2002 in an estuary in California, USA (O’Brien & Zedler 2006) found that mixing kelp compost into the sediment before planting California cordgrass Spartina foliosa increased its density and height. After three growing seasons, plots amended with kelp compost contained a higher density of California cordgrass (237 stems/m2) than unamended plots (126 stems/m2). The average height of California cordgrass was also greater in amended plots (48 cm) than unamended plots (37 cm). Methods: In winter 1999/2000, six pairs of 15 x 30 m plots were established during the excavation of a salt marsh. Kelp compost (an industrial waste product) was mixed into the top 30 cm of sediment in one plot/pair (2:1 sediment:compost ratio). No compost was added to the other six plots. In February 2000, plugs of California cordgrass (range 50–100 cm tall) were dug from a nearby marsh and planted (2 m apart) in the plots. In August 2002, cordgrass stems were counted and measured in four 0.25-m2 quadrats/plot (each with ≥15 stems). This study used the same marsh as (3), but a different experimental set-up.Study and other actions tested
A replicated, randomized, paired, controlled study in 2000–2002 in an estuary in California, USA (O’Brien & Zedler 2006) found that tilling kelp compost into plots before planting salt marsh plants increased their overall survival and size, but did not always have significant effects on the survival or size individual species. Over the first year after initial planting, dead plants were replaced by stock plants of a similar age. Fewer replacements were needed in composted plots (8 replacements/plot) than in uncomposted plots (tilled: 9.1; undisturbed: 9.7). Over the second year of the study, composted plots supported a higher number of surviving plants on average (3.5 survivors/plot) than uncomposted plots (tilled: 2.9; undisturbed: 2.8). However, the survival rate of individual species was similar under each treatment in 9 of 10 comparisons (for which composted: 42–92%; tilled: 31–72%; undisturbed: 19–86%). Across both years, surviving plants were typically larger in composted than uncomposted plots (data reported as an index combining height and lateral extent). This was true in four of four comparisons of the average size of plants per plot, and 16 of 20 comparisons of the average size of each species. Methods: In January 2000, one hundred and eight 2.24-m2 plots were established (in 6 sets of 18) on intertidal sediment excavated earlier that winter. Thirty-six plots (six random plots/set) received each soil treatment: tilling 40 L of kelp compost into the top 30 cm of soil, tilling only, or no disturbance (no compost or tilling). In December 2000, five greenhouse-reared plants (each a different species) were planted into each plot. Colonizing vegetation was removed until October 2001. Dead planted vegetation was replaced until December 2001 to maintain 36 plants/species/soil treatment. Survival, height and lateral spread of planted vegetation were recorded in August 2002. This study used the same marsh as (2), but a different experimental set-up.Study and other actions tested
A replicated, randomized, controlled study in 2005 in a greenhouse in California, USA (Cohen et al. 2009) found that adding alginate after planting California cordgrass Spartina foliosa had no significant effect on the number of shoots, plant height or plant biomass. After nine weeks, plants with added alginate had a statistically similar number of shoots (3.1 shoots/plant) to plots without added alginate (2.8 shoots /plant). Plants with and without added alginate were also of a statistically similar average height and above-ground biomass (data not reported). Methods: In spring 2005, twelve cordgrass plants were collected from salt marshes and planted in individual pots of natural wetland sediment. The pots were placed in a greenhouse with simulated tides. After 45 days acclimation, alginate (a carbon-rich seaweed extract) was added to a trench around six random plants (15 g/plant). A trench was dug around the other six plants but no alginate was added. The number of shoots (ramets) and the maximum height of each plant were measured for nine weeks after intervention. Plants were harvested, dried and weighed after nine weeks.Study and other actions tested
A replicated, controlled, before-and-after study in an estuarine wetland in eastern China (Guan et al. 2011) found that mixing reed debris into the sediment before sowing seeds of seablite Suaeda salsa increased seablite biomass, but had no significant effect on its density or height. Five months after sowing, plots amended with reed debris contained a greater above-ground biomass of seablite (771 g/m2) than unamended plots (396 g/m2). Meanwhile, there was no significant difference between treatments in seablite density (amended: 531 plants/m2; unamended: 365 plants/m2) or height (amended: 63 cm; unamended: 60 cm). Height was also statistically similar under both treatments for measurements taken 1–4 months after sowing (amended: 15–56 cm; unamended: 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 reed debris (2 kg/m2). 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
A replicated, randomized, controlled, site comparison study in 2010–2012 in a salt marsh in Georgia, USA (Cain & Cohen 2014) found that adding alginate to the sediment before planting smooth cordgrass Spartina alterniflora had no significant effect on cordgrass density or height. The total number of live cordgrass stems increased by a statistically similar amount in plots with added alginate (from 35 stems/m2 at planting to 345 stems/m2 after 1–2 growing seasons) and plots without alginate (from 30 to 369 stems/m2). The same was true for the height of the tallest cordgrass plants (with alginate: from 48 to 58 cm; without alginate: from 45 to 56 cm). After three growing seasons, planted plots had a statistically similar live stem density to mature natural marshes (427 stems/m2) but taller plants than mature natural marshes (29 cm), whether alginate was added or not. The study noted a high sediment organic matter content (15%) before alginate addition. Methods: In May 2010, twenty 1-m2 plots were established on an intertidal mudflat where cordgrass had died off. All plots were planted with swards of cordgrass from nearby natural marsh, in nine holes 45 cm apart. In 10 random plots, 10 g of alginate (a carbon-rich seaweed extract) was poured into each hole before planting. Cordgrass stems were counted, and the five tallest stems/plot measured, in each plot over three growing seasons. A nearby natural marsh was also surveyed.Study and other actions tested