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Providing evidence to improve practice

Action: Soil: Exclude grazers Mediterranean Farmland

Key messages

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Organic matter (1 study): One replicated site comparison in shrublands in Spain found less carbon in soils at ungrazed sites, compared to cow-and-sheep-grazed sites.

Nutrients (3 studies): Three replicated studies (one controlled, two site comparisons) from the USA and Spain found less nitrogen in soils in ungrazed areas, compared to sheep- or cattle-grazed areas, in some or all comparisons. One of these studies found more phosphorus in soils at ungrazed sites, compared to grazed sites.

Soil organisms (1 study): One controlled study on a streambank in the USA found more nematodes and more diverse nematode communities in an area with goats and sheep excluded.

Soil erosion and aggregation (0 studies)

Greenhouse gases (3 studies): One replicated site comparison in shrublands in Spain found more carbon dioxide in soils (soil respiration) in ungrazed plots, compared to sheep- or cattle-grazed plots. One replicated, randomized, controlled study in grassland in the USA found similar amounts of carbon dioxide in soils (soil respiration) in ungrazed and cattle-grazed sites. One replicated, randomized, controlled study in wet grasslands in the USA found less methane in soils in ungrazed plots, compared to cattle-grazed plots.

Implementation options (1 study): One replicated site comparison in shrubland in Spain found less carbon and nitrogen in untilled soils that were grazed, compared to ungrazed, but found no differences in tilled soils that were grazed or ungrazed.

Supporting evidence from individual studies

1 

A site comparison in 1991 in annual grasslands on the Central Coast, California, USA, found less nitrogen but more phosphorus in ungrazed sites, compared to grazed sites. Nutrients: Less nitrogen was found in ungrazed sites, compared to grazed sites (0.09% vs 0.11% total Kjeldahl N), but more phosphorus was found in ungrazed sites (18 vs 11 mg P/kg soil). Methods: European domestic cattle were introduced to Monterey County in 1770. In 1937, grazers were excluded from one landscape (the Hastings Natural History Reservation), but not from an adjacent landscape. In 1991, 43 sites in the ungrazed grassland and 37 sites in the grazed grassland were sampled (methods not clearly reported, but soil samples were collected at 5–10 cm depth in a different part of this study).

 

2 

A replicated, randomized, controlled study in 1999–2001 in grazed wetlands in northern California, USA, found lower methane production in plots from which grazers were excluded, compared to cattle-grazed plots. Greenhouse gases: Methane production was lower in ungrazed plots, compared to grazed plots (2.6 vs 8.5 mg CH4–C/m2/hr). Methods: Experimental plots were established in four grazed wetlands in 1999, with cattle excluded from one plot, but not from another plot, in each wetland. Methane emissions were measured monthly in March–September 2002.

 

3 

A controlled study in 2005–2008 in restored riparian habitat on a farm in the Central Valley, California, USA, found more nematodes overall, more bacteria-feeding nematodes, and more diverse communities of nematodes, in plots without grazers, compared to plots grazed by goats and sheep. Soil organisms: More nematode biomass and higher nematode diversity were found in plots without grazers, compared to plots with grazers (831 vs 557 µg/100 g soil; diversity reported as Shannon diversity index). More bacteria-feeding nematodes were found in plots without grazers, compared to plots with grazers (178 vs 86 nematodes/100 g soil), but similar numbers of fungus-feeding (168 vs 194), omnivorous and predatory (29 vs 21), and plant-feeding (180 vs 176) nematodes were found in plots with or without grazers. Methods: Grazers were introduced to half of a streambank in 2005 (14 animals/ha), but they were excluded by a fence from the other half. Soil samples were collected from the grazed area and the ungrazed area in December 2007 and March–April 2008 (0–30 cm depth).

 

4 

A replicated, randomized, controlled study in 2008–2010 in grasslands in central California, USA, found no differences in soil respiration between plots with or without cattle excluded. Greenhouse gases: Potential microbial respiration rates did not differ between plots with or without cattle excluded (9–12 µg CO2/g/day). Methods: Ten sets of plots were established in grassland that had been grazed for decades: five plots in 2008 and five plots in 2009. Half of the plots were fenced to exclude cattle and half were left open and typically grazed in winter (approximately 0.25 cow-calf pairs/ha).

 

5 

A replicated, controlled study in 2012–2013 in grasslands in central California, USA, found less soil nitrogen in plots with cattle excluded, compared to grazed plots. Nutrients: Less soil nitrogen was found in ungrazed plots, compared to grazed plots (14–16 vs 17–24 mg NH4 and NO3 combined). Methods: Sixty 1 x 1 m plots were established in summer 2012: half in an area grazed at 0.25 cow-calf pairs/ha, and half in an area fenced in 2012 to exclude cattle. Soil cores (5 cm diameter, 0–10 cm depth) were collected in April 2013.

 

6 

A replicated site comparison in 2008–2010 in shrubland in central Spain found less carbon and nitrogen, and higher carbon dioxide emissions, in soils in ungrazed sites, compared to sheep-and-cattle-grazed sites. Organic matter: There was less carbon in soils in ungrazed sites, compared to grazed sites, in one of two comparisons (in untilled soils: 3.8–9.6 vs 6.5–15.8 Mg C/ha). Nutrients: There was less nitrogen in soils in ungrazed sites, compared to grazed sites, in one of two comparisons (in untilled soils: 0.3–0.7 vs 0.8–1.3 Mg N/ha). Greenhouse gases: More carbon dioxide was lost through soil respiration in ungrazed plots, compared to grazed plots (720–740 vs 640–655 g C/m2/year). Implementation options: Differences in carbon and nitrogen, due to grazing, were found in untilled soils, but not in tilled soils (see above for data on tilled soils). Methods: Eight holm oak Quercus ilex trees were selected in each of two areas grazed by sheep and cattle and in two ungrazed areas. Soils surrounding four trees in each area were tilled in April 2008. Soil respiration was measured nine times in July 2008–February 2010. Soil samples were collected in February 2010 (to measure carbon and nitrogen).

 

Referenced papers

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.