Add upland topsoil before/after planting non-woody plants: freshwater wetlands
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 3
Background information and definitions
Topsoil can be a source of soil organic matter and help to improve water retention (Bruland & Richardson 2004). This might benefit wetland vegetation, particularly when creating new marshes. Topsoil may often be added to complement planting (current intervention) rather than without planting: upland soil will probably not contain seeds or fragments of marsh plants. Caution: Topsoil may contain seeds or fragments of undesirable vegetation.
Bruland G.L. & Richardson C.J. (2004) Hydrologic gradients and topsoil additions affect soil properties of Virginia created wetlands. Soil Science Society of America Journal, 68, 2069–2077.
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 topsoil into mineral soil typically increased the number of shoots and above-ground biomass of planted tussock sedge Carex stricta seedlings, whether topsoil was the only soil amendment or was additional to other amendments. After three months, sedge seedlings planted into a mixture of topsoil and mineral soil were larger (8.8 shoots/plant; 1.6 g/plant) than seedlings planted into mineral soil only (3.3 shoots/plant; 0.6 g/plant). Adding topsoil also increased the size of sedge seedlings in four of six comparisons where it was an additional treatment (i.e. added to pots that were fertilized and/or amended with compost; see original paper for data). In the other two comparisons, topsoil did not have a significant additional effect on sedge size. 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, topsoil was mixed in equal parts with whatever other soil was in the pots (deeper mineral soil, sometimes mixed with compost). Some pots with and without topsoil 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, paired, controlled study in 1992 in a wet meadow restoration site in Iowa, USA (van der Valk et al. 1999) reported that adding topsoil to plots before planting tussock sedge Carex stricta seedlings had no clear effect on the number of shoots they developed. 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 topsoil had a similar number of shoots (12.2–15.5 shoots/plant) to seedlings in plots that had not been amended with topsoil (11.8–15.2 shoots/plant). This was true when topsoil was the only amendment to mineral soil plots (statistically tested), or when topsoil was an additional amendment to plots already amended with compost (not statistically tested). Methods: In June 1992, tussock sedge seedlings were planted into twelve sets of four 1-m2 plots of mineral soil (topsoil had been removed). The number of seedlings/plot was not clearly reported. Fresh topsoil was rototilled into the surface of half of the plots (two plots/set). Some plots were also amended with compost.Study and other actions tested
A replicated, randomized, paired, controlled study in 2010–2011 in six experimental wetland trenches in Alberta, Canada (Roy et al. 2014) found that adding peat/mineral soil to mine tailings did not reduce survival of planted water sedge Carex aquatilis over two growing seasons, and typically increased the biomass of surviving sedges. In two of four comparisons, pots of mine tailings mixed with peat/mineral soil supported higher sedge survival (50–67%) than pots of raw mine tailings (24–44%). There was no significant difference between treatments in the other two comparisons (peat/mineral soil: 74%; raw tailings: 54–69%). In three of four comparisons, the above-ground biomass of surviving sedges was higher in pots of mine tailings mixed with peat/mineral soil (2.1–2.8 g/trench) than in pots of raw mine tailings (1.1–1.5 g/trench). There was no significant difference between treatments in the other comparisons (peat/mineral soil: 2.2 g/trench; raw tailings: 2.2 g/trench). Methods: In June 2010, water sedges were collected from a natural marsh and randomly planted into 192 one-gallon pots (number of plants/pot not clearly reported). Half of the pots contained mine tailings amended with a mixture of peat and mineral soil (1 part tailings to 2 parts peat/mineral soil). Half of the pots contained pure mine tailings (dense sediments, low in organic matter, rich in salts and metals). The pots were placed into six experimental wetland trenches: 16 amended pots and 16 raw tailings pots/trench. Surviving plants were harvested at the end of the 2011 growing season. Biomass was dried before weighing.Study and other actions tested