Modify crop farming practices in watershed to reduce pollution: freshwater marshes

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

59%
Certainty

29%
Harms

0%

Calculating overall effectiveness category

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.}

Study locations

Key messages

One study evaluated the effects, on vegetation in freshwater marshes, of modifying crop farming practices in the watershed to reduce pollution. The study was in the USA.

VEGETATION COMMUNITY

Community composition (1 study): One replicated, site comparison study in the USA reported that freshwater marshes being restored by abandoning cropland in the watershed (along with removing topsoil from the marshes) contained a different overall plant community, after 1–12 years, to both natural and degraded marshes nearby.
Overall richness/diversity (1 study): The same study reported that freshwater marshes being restored by abandoning cropland in the watershed (along with removing topsoil from the marshes) contained fewer wetland plant species, after 1–12 years, than nearby natural marshes.

VEGETATION ABUNDANCE

VEGETATION STRUCTURE

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

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
Referenced paper
O’Connell J.L., Johnson L.A., Beas B.J., Smith L.M., McMurry S.T. & Haukos D.A. (2013) Predicting dispersal-limitation in plants: optimizing planting decisions for isolated wetland restoration in agricultural landscapes. Biological Conservation, 159, 343-354.

Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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