Other biodiversity: Add sewage sludge to the soil

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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How is the evidence assessed?

Effectiveness

60%
Certainty

20%
Harms

0%

Calculating overall effectiveness category

Study locations

Key messages

Amphibians (0 studies)

Birds (0 studies)

Invertebrates (0 studies)

Mammals (0 studies)

Plants (2 studies): Two replicated, controlled studies from Spain found greater plant cover and faster tree growth in plots with added sewage sludge, compared to plots without it, in some or all comparisons. One of these studies found similar numbers of plant species in plots with or without added sewage sludge. The other one found more plant biomass in plots with added sewage sludge.

Reptiles (0 studies)

Implementation options (1 study): One study from Spain found faster tree growth in plots with composted or thermally dried sewage sludge, but not with digested sewage sludge, compared to plots without sewage sludge. Another one found no differences in pasture cover, tree growth, or numbers of species between plots with different types of sewage sludge.

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, controlled study in 2001–2003 in a degraded wood pasture in Catalonia, Spain (same study as (2)), found higher plant cover, more plant biomass, and faster tree growth in plots with added sewage sludge, compared to plots without it. Plants: Greater plant cover and more herbaceous biomass were found in plots with added sewage sludge, compared to plots without it (cover: 85–93% vs 60%; biomass: 2,700–2,800 vs 1,700 kg dry weight/ha). Faster oak tree Quercus humilis growth was found in plots with added sewage sludge (composted or thermally dried), compared to plots without it (41–42 vs 34 cm/year). Implementation options: No difference in tree growth was found in plots with added digested sewage sludge, compared to plots without it (39 vs 34 cm/year). Methods: There were five plots (20 x 5 m) for each of three sewage-sludge treatments (10 t dry matter/ha of composted, digested, or thermally dried sewage sludge) and there were five control plots (no sewage sludge). To restore the wood pasture, shrubs and small trees were crushed and scattered on the soil, and grass seeds were sown.
Study and other actions tested
Referenced paper
Tarrasón D., Ortiz O. & Alcañiz J.M. (2007) A multi-criteria evaluation of organic amendments used to transform an unproductive shrubland into a Mediterranean dehesa. Journal of Environmental Management, 82, 446-456.
A replicated, controlled study in 2001–2003 in a degraded wood pasture in Catalonia, Spain (same study as (1)), found greater pasture cover and faster tree growth in plots with added sewage sludge, compared to plots without. Plants: Greater pasture cover and faster tree growth were found in plots with added sewage sludge, compared to plots without it, in two of three comparisons (90–98% vs 80–90% cover; 0.25 vs 0.16–0.17 cm/year). Similar numbers of species were found in plots with or without added sewage sludge (data not reported). Implementation options: No differences in pasture cover, tree growth, or numbers of species were found in plots with dewatered, composted, or thermally dried sewage sludge (0.07–0.25 cm/year; other data not reported). Methods: Plots (100 m²) growing shrubs and trees such as Quercus ilex and Q. humilis through natural regeneration had added sewage sludge (dewatered, composted, or thermally dried) or no sewage sludge (five plots for each): dewatered (11 Mg/ha), composted (mixed with pinewood splinters and composted; 14 Mg/ha), or thermally dried (dried at 130 ^oC; 50 Mg/ha). All sewage sludge was anaerobically digested before being processed. All plots were seeded with grasses Lolium perenne, Festuca arundinacea and Dactylis glomerata, and were weeded to simulate grazing. Woody vegetation remnants were crushed and scattered over the soil surface. Cover and number of species was estimated using a line-intercept method (every 10 cm along 10 m transect) in June 2001 and 2002. Tree growth was measured in January 2001 and December 2001–2003.
Study and other actions tested
Referenced paper
Tarrasón D., Ojeda G., Ortiz O. & Alcañiz J.M. (2014) Can organic amendments be useful in transforming a mediterranean shrubland into a dehesa? Restoration Ecology, 22, 486-494.

Please cite as:

Shackelford, G. E., Kelsey, R., Robertson, R. J., Williams, D. R. & Dicks, L. V. (2017) Sustainable Agriculture in California and Mediterranean Climates: Evidence for the effects of selected interventions. Synopses of Conservation Evidence Series. 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:

Mediterranean Farmland

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