Add below-ground organic matter before/after planting non-woody plants: freshwater wetlands
Overall effectiveness category Trade-off between benefit and harms
Number of studies: 7
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, controlled study in 1992 in a greenhouse in Iowa, USA (van der Valk et al. 1999) found that mixing compost alone into mineral soil increased the number of shoots and above-ground biomass of planted tussock sedge Carex stricta seedlings, but typically had no significant effect on top of other soil amendments. After three months, sedge seedlings planted into a mixture of compost and mineral soil were larger (6.4 shoots/plant; 1.6 g/plant) than seedlings planted into mineral soil only (3.3 shoots/plant; 0.6 g/plant). Adding compost had no significant effect on sedge size in four of six other comparisons where it was an additional treatment (i.e. added to pots that were fertilized and/or amended with topsoil; see original paper for data). However, compost did increase sedge shoot density when added to fertilized pots (compost + fertilizer: 8.9 shoots/plant; fertilizer only: 5.4 shoots/plant) and sedge biomass when added to topsoil-amended pots (compost + topsoil: 2.1 g/plant; topsoil only: 1.6 g/plant). Methods: In March 1992, tussock sedge seedlings (6–8 weeks old) were planted into 144 pots (probably one seedling/pot). In half of the pots, compost was mixed in equal parts with whatever other soil was in the pots (mineral soil, sometimes mixed with topsoil). Some composted and uncomposted pots were also fertilized. All pots were watered to saturation. In June 1992, all sedge shoots were counted, harvested, dried and weighed.Study and other actions tested
A replicated, controlled study in 1992–1993 in a greenhouse in Iowa, USA (van der Valk et al. 1999) found that adding organic matter to pots increased the above-ground biomass of tussock sedge Carex stricta seedlings. After three months, sedge seedlings planted into a mixture of compost and sand had developed more above-ground biomass (0.12–0.46 g/plant) than seedlings planted into sand only (0.02 g/plant). Above-ground sedge biomass was greater in pots with higher proportions of compost (e.g. 90% compost: 0.46 g/plant; 50% compost: 0.33 g/plant; 10% compost: 0.12 g/plant). Methods: In October 1992, two- to four-week-old tussock sedge seedlings were planted into 144 pots (probably one seedling/pot). Of these pots, 128 contained some composted garden waste (16 pots for each of 8 proportions: 10%, 20%, 33%, 50%, 67%, 80%, 90% or 100% compost vs sterile sand). The final 16 pots contained sterile sand only. Plants were harvested, dried and weighed in January 1993.Study and other actions tested
A replicated, paired, controlled study in 1992 in a wet meadow restoration site in Iowa, USA (van der Valk et al. 1999) reported that adding compost to plots before planting tussock sedge Carex stricta seedlings increased the number of shoots they developed, whether compost was the only soil amendment or was additional to other amendments. Two weeks after planting, sedges assigned to each treatment had a statistically similar number of shoots (4.7–5.8 shoots/plant). After two months, sedge seedlings in plots amended with compost had more shoots (15.2 shoots/plant) than seedlings planted into unamended mineral soil (11.8 shoots/plant). The pattern was the same where compost was added to plots receiving other soil amendments, but statistical significance of these comparisons was not assessed (topsoil and/or fertilizer + compost: 14.3–15.5 shoots/plant; topsoil and/or fertilizer only: 11.5–12.2 shoots/plant). Methods: In June 1992, tussock sedge seedlings were planted into twelve sets of eight 1-m2 plots of mineral soil (topsoil had been removed). The number of seedlings/plot was not clearly reported. Composted garden waste was rototilled into the surface of half of the plots (four plots/set). Some plots were also amended with topsoil and/or fertilized.Study and other actions tested
A replicated, randomized, controlled study in 1991–1992 in an excavated freshwater wetland in Pennsylvania, USA (Stauffer & Brooks 1997) found that amending plots with leaf litter before planting lurid sedge Carex lurida tubers increased their survival. After one growing season, planted lurid sedge had a 79% survival rate in amended plots, on average, compared to only 38% than in unamended plots. Methods: In October 1991, lurid sedge tubers (number not reported) were planted into eight 6 x 6 m plots in a recently excavated wetland (formerly cropland). A 15-cm-thick layer of composted leaf litter was mixed into the top 15 cm of four plots before planting. The other four plots were not amended with leaf litter, and the soil was left undisturbed. All tubers were dug from nearby wetlands, planted 10 cm deep, then watered. Survival was last recorded in August 1992.Study and other actions tested
A replicated, randomized, paired, controlled, before-and-after study in 2004–2005 in two wet basins in Minnesota, USA (Iannone & Galatowitsch 2008) found that adding sawdust to plots before sowing a mixture of sedge meadow species typically had no significant effect on their germination or abundance after one growing season. Sixteen weeks after sowing, the germination rate was statistically similar in plots with sawdust (48%) and plots without sawdust (47%). The same was true in four of four comparisons at earlier dates. After 16 weeks, plots under each treatment contained a statistically similar total density of target species (sawdust: 460–1,300; no sawdust: 370–1,100 shoots/m2) and target grass-like plants (sawdust: 190–690; no sawdust: 150–780 shoots/m2). The effect of sawdust addition on the total density of target forbs depended on the presence/diversity of a cover crop (see original paper for details). The study also reported data on the abundance of individual target species. Sawdust addition had no significant effect on 5 of 10 species for any metric and in any conditions (see original paper for details). Methods: In October 2004, seventy two 1-m2 plots were established (in six sets of 12) across two experimental, vegetation-free wet basins. In half of the plots (6 random plots/set), the top 7 cm of soil was replaced with cedar Thuja sp. sawdust. The plots were then tilled. In May 2005, seeds of 10 target sedge meadow species were sown onto all 72 plots (total 2,250 seeds/m2). Some plots were also sown with other species, as cover crops and/or experimental invaders (reed canarygrass Phalaris arundinacea). Target vegetation was surveyed for 16 weeks after sowing. Seedlings were counted in five 100-cm2 subplots/plot. Shoot density and cover were monitored across the whole of each plot.Study and other actions tested
A replicated study in 2004–2006 of freshwater marshes alongside a recently reprofiled stream in North Carolina, USA (Sutton-Grier et al. 2009) found that adding compost to planted plots typically reduced plant species richness over three growing seasons, but had no significant effect on vegetation biomass. Total plant species richness was negatively related to the amount of soil organic matter in plots, both one and three growing seasons after amendment/planting. There was a similar but insignificant trend after two growing seasons. Above-ground vegetation biomass was not significantly related to the amount of soil organic matter three growing seasons after amendment/planting (data not reported after one and two growing seasons). However, there was a trend towards higher biomass in plots with more organic matter. For data and statistical models, see original paper. Methods: Around July 2004, twenty-one 20-m2 wetland plots alongside a recently re-meandered stream were planted with various herb species (one plant/m2). Fourteen plots had been amended with varying amounts of compost (a mix of topsoil, wood chips and sewage sludge) whilst seven plots had been amended with topsoil only. As a result, the organic matter content of the plots ranged from 6% to 25%. All plots were tilled after adding compost/topsoil. In September 2004–2006, all plant species were counted in 11 of the plots. In September 2006, vegetation was cut from all 21 plots (three 0.5-m2 quadrats/plot), then dried and weighed.Study and other actions tested
A replicated, randomized, paired, controlled study in 2012–2013 in a freshwater wetland in Wisconsin, USA (Doherty & Zedler 2015) reported that adding woodchips to soil before planting tussock sedge Carex stricta had mixed effects on sedge survival depending on soil moisture levels, but did not increase sedge growth, biomass or cover under either moisture level. Unless specified, statistical significance was not assessed. After two growing seasons and in a drier area, 67% of sedges survived when planted into mounds with woodchips vs only 27% in mounds without. However, in a wetter area, only 60% of sedges survived when planted into mounds with woodchips vs 93% in mounds without. In both areas, mounds with and without woodchips supported a statistically similar sedge growth rate (see original paper for data) and final above-ground biomass of surviving sedges (with: 2–15 g/plant; without: 2–8 g/ plant). Final sedge cover was lower in plots where sedges were planted into mounds with woodchips (11–18%) than mounds without (38%). Methods: In spring 2012, six pairs of 1-m2 plots were established in a wetland undergoing restoration. Five 16-cm-tall mounds were built in each plot. In half of the plots (one random plot/pair), the mounds were built with a mix of 50% woodchip and 50% soil. In the other plots, the mounds were built with soil only. Nursery-reared tussock sedge was planted into each mound (one plant/mound), then regularly watered and weeded. Survival and above-ground biomass of planted sedges, and total tussock sedge cover, were surveyed in June–August 2013. Biomass was dried before weighing. Growth rates were calculated from leaf lengths measured in 2012 and 2013.Study and other actions tested