Modify crop farming practices in watershed to reduce pollution: freshwater marshes
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
Background information and definitions
Crop farming in watersheds can lead to pollution of focal marshes. Soil and nutrients can run off arable land and accumulate in wetlands, affecting the plant community composition, plant diversity and distribution of plant species (Verhoeven et al. 2006; Bromberg Gedan et al. 2009). Extreme excesses can cause algal blooms or even fill in wetlands (Gleason & Euliss Jr. 1998). Crop farming can also contribute to salinization and acidification, by raising the local water table and bringing salts to the surface. This can occur through (a) excess irrigation or (b) replacing native, perennial deep-rooted vegetation with shallow-rooted, temporary crops that don’t take up as much water (NSW Government 2019). Alternatively, runoff of excess fresh irrigation water can pollute brackish/saline sites (Zedler 1983).
This action includes a range of specific management techniques, other than changes in fertilizer or herbicide use that are considered as separate actions, that might reduce pollution from arable farming in wetland catchments. For example, ploughing or harrowing parallel to slopes avoids creating channels that carry soil and nutrients. Terraces may help to retain soil and nutrients. Minimizing the number/duration of fallow periods, and planting cover crops during them, could minimize sediment/chemical runoff and infiltration of water. Water table rise could also be reduced by tailoring irrigation to the needs of crops, increasing irrigation efficiency (e.g. replacing leaking pipes) and installing systems to retain excess irrigation water on site (NSW Government 2019).
Bromberg Gedan K., Silliman B.R. & Bertness M.D. (2009) Centuries of human-driven change in salt marsh ecosystems. Annual Review of Marine Science, 1, 117–141.
Gleason R.A. & Euliss N.H. Jr. (1998) Sedimentation of prairie wetlands. Great Plains Research, 8, 97–112.
NSW Government (2019) Type of Salinity and their Prevention. Available at https://www.environment.nsw.gov.au/topics/land-and-soil/soil-degradation/salinity/type-of-salinity-and-their-prevention. Accessed 30 December 2020.
Verhoeven J.T.A., Arheimer B., Yin C. & Hefting M.M. (2006) Regional and global concerns over wetlands and water quality. Trends in Ecology and Evolution, 21, 96–103.
Zedler J.B. (1983) Freshwater inputs in normally hypersaline marshes. Estuaries, 6, 346–355.
Supporting evidence from individual studies
A replicated, site comparison study around 2010 of 48 ephemeral freshwater marshes in Nebraska, USA (O’Connell et al. 2013) reported that marshes undergoing restoration (surrounding cropland abandoned and agricultural topsoil removed) contained a different plant community to natural marshes (surrounded by permanent grassland) and degraded marshes (surrounded by cropland), with lower cover of wetland perennial plants and fewer wetland perennial species than the natural marshes. Results summarized for this study are not based on assessments of statistical significance. After 1–12 years, the overall plant community composition differed between restored, natural and degraded marshes (data reported as a graphical analysis). Perennial wetland species were underrepresented in restored marshes (43% cover; 10.1 species/marsh) compared to natural marshes (56% cover; 13.0 species/marsh). However, restored marshes had greater cover of these species than degraded marshes (35% cover; species richness not reported). Annual wetland species were “slightly” overrepresented in restored marshes compared to natural marshes in terms of abundance (data reported as a graphical analysis only). However, there was a similar number of these species in restored marshes (8.2 species/marsh) and natural marshes (8.0 species/marsh). Methods: Around 2010, vegetation was surveyed in 48 ephemeral playa marshes (along two transects crossing each marsh, in both the cool and warm seasons). Sixteen of the marshes were undergoing restoration under the Wetland Reserve Program. This involved abandoning the surrounding cropland and removing eroded agricultural topsoil from the marshes. The study does not distinguish between the effects of these interventions. Of the remaining marshes, 16 were in natural catchments and 16 were in degraded, farmed catchments.Study and other actions tested