Add below-ground organic matter before/after planting trees/shrubs: freshwater wetlands
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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 2002–2005 in a created freshwater wetland in Virginia, USA (Bailey et al. 2007) found that the effects of adding organic matter to plots planted with tree saplings depended on the dose added. Four different doses of organic matter were used. After approximately three years, plots receiving the three highest doses contained a plant community characteristic of drier – but still wetland – conditions than unamended plots (data reported as a wetland indicator index). In plots receiving the two highest doses, birch Betula spp. saplings were significantly larger than in unamended plots (size calculated as an index combining height, stem diameter and crown diameter; data not reported). Plots receiving the highest dose had lower plant species richness (5.3 species/m2) than unamended plots (7.4 species/m2). The same was true for plant diversity (data reported as a diversity index). In all other comparisons, there were no significant differences between amended and unamended plots (see original paper for data). Further, at all four doses, above-ground vegetation biomass did not significantly differ between amended plots (580–790 g/m2) and unamended plots (604 g/m2). Methods: In June 2002, twenty 14-m2 plots were established, in four sets of five, on an 8-year-old created wetland. All plots were cleared and tilled. In July, organic matter (dry wood and garden compost) was mixed into the surface of 16 plots (four random plots/set, each with a different dose: 56, 112, 224 or 336 Mg/ha). The remaining four plots (one random plot/set) received no organic matter. In December 2002, ten saplings (five birch, five pin oak Quercus palustris) were planted in each plot and fertilized. Vegetation was surveyed in 2005. Plant species and cover were recorded monthly April–October. Surviving birch saplings were counted in June. Vegetation samples were cut in August, then dried and weighed.Study and other actions tested