Reduce fertilizer or herbicide use: freshwater marshes

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

Effectiveness

50%
Certainty

30%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

An excess of fertilizers and herbicides can have negative effects on vegetation. Herbicides can kill plants directly. An excess of nutrients, such as nitrogen and phosphorous, can alter the competitive balance leading to domination by single species (Tilman et al. 1999) or algal blooms (Smith et al. 2006). For many marshes, problems are related to chronic spillover of chemicals from agricultural or domestic land. Nutrient pollution is especially severe where a large proportion of the land is cultivated, e.g. in Europe, eastern North America and southeast China (Verhoeven et al. 2006). In some cases, e.g. rice paddies, excess chemical application could affect vegetation during the growing season and/or fallow periods.

Simply applying less fertilizer or herbicide to land/water near focal sites, or to agricultural wetlands, could reduce these problems. Ultimately, reduced application could be driven by legislation, financial incentives and/or education. Studies of organic farming – an agricultural system that minimizes the use of synthetic fertilizers and pesticides and relies on techniques such as crop rotation, compost and biological pest control (European Commission 2019) – are included within this action.

For this action, “reduction” includes stopping fertilizer or herbicide treatments altogether. Note that studies comparing areas that remain untreated to areas that become treated are not summarized as evidence for this action.

Related actions: Manage fertilizer or herbicide application, without reducing the total amount applied. In practice, that action and the current one will often be used simultaneously.

^{European Commission (2019) What is Organic Farming? Available at http://ec.europa.eu/agriculture/organic/organic-farming/what-is-organic-farming. Accessed 29 October 2019.}

^{Smith V.H., Joye S.B. & Howarth R.W. (2006) Eutrophication of freshwater and marine ecosystems. Limnology and Oceanography, 51, 351–355.}

^{Tilman E.A., Tilman D., Crawley M.J. & Johnston A.E. (1999) Biological weed control via nutrient competition: potassium limitation of dandelions. Ecological Applications, 9, 103–111.}

^{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.}

Study locations

Key messages

One study evaluated the effects, on vegetation in freshwater marshes, of reducing the amount of fertilizer or herbicide used in the marshes or adjacent areas. The study was in Brazil.

VEGETATION COMMUNITY

Community composition (1 study): One replicated, site comparison study of rice fields in Brazil found that the overall plant community composition (excluding rice) was similar in organically farmed fields and conventionally farmed fields, but different from the community in nearby natural marshes.
Overall richness/diversity (1 study): The same study found that organically farmed rice fields contained a similar average richness and diversity of wetland plants (at any single point in time) to conventionally farmed rice fields, although more species were recorded in the organic fields over the year of the study. Organically farmed rice fields had lower wetland plant richness and diversity than nearby natural marshes.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, site comparison study of rice fields in Brazil found that organically farmed fields contained more wetland plant biomass than conventionally farmed fields over the year of the study, but less wetland plant biomass than nearby natural marshes.

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 in 2010–2011 involving eight rice fields in southern Brazil (Linke et al. 2014) found that organically farmed fields were similar to conventionally farmed fields in terms of wetland plant community composition, richness and diversity (but not biomass), but different from natural marshes. Across one year of cultivation, the overall plant community composition in organic fields was statistically similar to conventional fields (data reported as a graphical analysis). Fields under each farming treatment had statistically similar wetland plant species richness (organic: 3–6 species/0.9 m²/survey; conventional: 1–7 species/0.9 m²/survey) and diversity (data reported as a diversity index). However, a total of 27 wetland plant species were recorded in the organic fields, compared to 23 in conventional fields. The average biomass of wetland plants was higher in organic fields (1–18 g/m²) than conventional fields (<1–11 g/m²). Compared to nearby natural marshes, the organic rice fields supported a different plant community with fewer total species (natural: 55), lower species richness (natural: 7–11 species/0.9 m²), lower diversity, and less biomass (natural: 36–228 g/m²). Methods: Between August 2010 and August 2011, at six stages in the rice-growing calendar, vegetation was surveyed in four organically farmed rice fields (no artificial fertilizers or herbicides; weeds controlled by changing water level and tilling), four conventionally farmed rice fields, and four nearby natural ephemeral marshes. Wild wetland plant species (i.e. excluding rice and terrestrial species) were recorded in ten 30 x 30 cm quadrats/site/survey. Their above-ground parts were collected, then dried and weighed.
Study and other actions tested
Referenced paper
Linke M.G., Godoy R.S., Rolon A.S. & Maltchik L. (2014) Can organic rice crops help conserve aquatic plants in southern Brazil wetlands? Applied Vegetation Science, 17, 346-355.

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

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Clean waste water before it enters the environment Action Link	No evidence found (no assessment)	0	Synopsis Link
Add lime or similar chemicals: freshwater swamps Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
Retain/restore/create vegetation around freshwater marshes Action Link	Likely to be beneficial	4	Synopsis Link