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Providing evidence to improve practice

Action: Soil: Add slurry to the soil Mediterranean Farmland

Key messages

Organic matter (4 studies): Three studies (two replicated, randomized, controlled; one meta-analysis) from Spain and multiple Mediterranean countries found similar amounts of organic matter in soils with or without added slurry. One replicated, randomized, controlled study from Spain found more organic matter in soils with added slurry, compared to soils without it, in some comparisons.

Nutrients (4 studies): Two replicated, randomized, controlled studies from Spain found more nitrate in soils with added slurry, compared to soils without it. Two replicated, randomized, controlled studies from Portugal and Spain found similar amounts of nitrate in soils with or without added slurry. One of these studies also found more ammonium, but another one did not.

Soil organisms (2 studies): One replicated, randomized, controlled study from Spain found more microbial biomass in soils with added slurry, compared to soils without it, but another one did not.

Soil erosion and aggregation (1 study): One replicated, randomized, controlled study from Spain found more stable soils in plots with added slurry, compared to plots without it, in some comparisons.

Greenhouse gases (8 studies)

  • Carbon dioxide (3 studies): Of three replicated, randomized, controlled studies from Spain, two studies found higher carbon dioxide emissions in soils with added slurry, compared to soils without it, but one study did not.
  • Methane (4 studies): One replicated, randomized, controlled study from Spain found that less methane was absorbed by soils with added slurry, compared to soils without it. Three replicated, randomized, controlled studies from Spain found similar methane fluxes in soils with or without added slurry.
  • Nitrous oxide (6 studies): Five replicated, randomized, controlled studies from Spain found higher nitrous oxide emissions in soils with added slurry, compared to soils without it, in some or all comparisons. One replicated, randomized, controlled study from Spain found similar nitrous oxide emissions in soils with or without added slurry.

Implementation options (3 studies): One replicated, randomized, controlled study from Spain found no differences in organic matter or greenhouse-gas emissions between plots with different amounts of slurry. One replicated, randomized, controlled study from Spain found similar amounts of nitrogen in soils with or without added slurry. One replicated, randomized, controlled study from Spain found similar nitrous oxide emissions in soils with digested or untreated pig slurry. One replicated, randomized, controlled study from Spain found similar carbon dioxide and methane emissions in soils with digested or untreated slurry.

Supporting evidence from individual studies

1 

A replicated, controlled study in 1998–1999 in irrigated arable farmland in Spain found more nitrate in soils with added slurry, compared to soils without it. Nutrients: More nitrate was found in soils with added slurry, compared to soils without it (40–90 vs 10–22 mg/kg). Methods: Plots (10 × 11 m) had added pig slurry (165 kg/ha) or no added fertilizer (three replicates each). Slurry was incorporated into the soil, five days after application, using a rotocultivator (0–5 cm depth). Soil samples were taken during the first 15 days after application and every 2 weeks thereafter.

 

2 

A replicated, randomized, controlled study in 2002 in irrigated arable farmland in Spain found higher nitrous oxide emissions in plots with added slurry, compared to plots without it. Greenhouse gases: Higher nitrous oxide emissions were found in plots with added slurry, compared to plots without it, in one of two comparisons (1.1 vs 0.5 g N/m2). Implementation options: Similar nitrous oxide emissions were found in plots with surface application, compared to injection, of slurry (0.8 vs 1 g N/m2). Methods: Plots (3 x 3 m) growing tall fescue Festuca arundinacea had pig slurry (surface application or injection, 200 kg N/ha) or no fertilizer (three plots each). Each plot had a lysimeter (1 x 1 m, 0.75 m depth) to measure leaching. Slurry was injected (5 L/m) or applied with a watering can. Water (5 L/plot) was added to the control plots. All plots were sprinkler-irrigated (June–August: daily; September: twice/week). Soil cores were taken from the centre of the plots (0–10 cm depth). Gas samples were taken (chambers, 30 cm diameter, 30 cm height) twice/day for 1–4 days after slurry application, every 2–3 days from 7 to 40 days after application, once/week in July and August, and every fortnight in September–December.

 

3 

A replicated, randomized, controlled study in 2004 in a maize field in the Jarama river basin, Spain, found higher nitrous oxide emissions from soils fertilized with slurry, compared to unfertilized soils. Greenhouse gases: Higher nitrous oxide emissions were found in soils with slurry, compared to unfertilised soils (untreated pig slurry: 8.3 vs 6.0; digested pig slurry: 7.7 vs 6.0 kg N/ha). Implementation options: No difference in nitrous oxide emissions was found between soils fertilized with digested pig slurry or untreated pig slurry (7.7 vs 8.3 kg N/ha). Methods: There were three plots (40 m2) for each of two treatments (untreated pig slurry or anaerobically digested thin fraction of pig slurry) and one control (no slurry). Both slurries were applied at a rate of 175 kg available N/ha. Nitrous oxide was measured in closed chambers (two chambers/plot, one within a maize row, one between rows; 35 cm diameter, 23 cm height; one sample/week, April-September).

 

4 

A replicated, randomized, controlled study in 2006 in a barley field in the Henares river basin, Spain, found no difference in greenhouse-gas emissions between soils with or without added slurry. Greenhouse gases: No differences in greenhouse-gas emissions were found between soils with or without added slurry (digested slurry, carbon dioxide: 465 vs 411 kg C/ha; methane: –287 vs –294 mg C/m2; untreated slurry, carbon dioxide: 447 vs 411; methane: –229 vs –294). Implementation options: No differences in greenhouse-gas emissions were found between soils fertilized with digested slurry, compared to untreated slurry (carbon dioxide: 465 vs 447 kg C/ha; methane: –287 vs –229 mg C/m2). Methods: There were three plots (30 m2) for each of two treatments (anaerobically digested thin fraction of pig slurry or untreated pig slurry) and three control plots (no slurry). Slurry was applied in January (125 kg available N/ha). Plots were cultivated (5 cm depth) to incorporate the slurry. Barley was planted in January and harvested in June. Greenhouse-gas emissions were measured with closed chambers (35 cm diameter, 25 cm height, 1–4 measurements/plot/week, 23 January–28 November).

 

5 

A replicated, randomized, controlled study in 2007–2009 in an irrigated onion field near Madrid, Spain, found that more nitrous oxide was emitted from, and less methane was absorbed by, plots with added slurry, compared to plots without it. Greenhouse gases: More nitrous oxide was emitted from plots with slurry, compared to plots without it (1 vs 0.4 kg/ha), and less methane was absorbed by plots with slurry (–0.5 vs –1 kg/ha). Methods: Plots (20 m2) had anaerobically digested pig slurry (110 kg N/ha) or no fertilizer in 2007 and 2008 (three plots for each). Slurry was immediately incorporated into the soil (10 cm depth), using a rotocultivator. Plots were irrigated 1–2 times/week (608–618 mm/year). Greenhouse-gas samples (closed chambers, 19 litre volume, 10 mL samples, 0, 30, and 60 minutes after closing) and soil samples (0–10 cm depth) were collected four times/week in the first two weeks after fertilizer was applied, twice/week during the first month, and once/week until the end of cropping season.

 

6 

A replicated, randomized, controlled study in 2007 in an irrigated melon field in Spain found lower nitrous oxide emissions from plots with added slurry, compared to plots without it. Greenhouse gases: Higher nitrous oxide emissions were found in plots with added slurry, compared to plots without it (1–3 vs 2–3 kg/ha). Methods: Plots (4 x 5 m) growing melon Cucumis melo (6,950 plants/ha) had digested pig slurry or no slurry, and were either drip or furrow irrigated (three plots for each). Slurry was applied using a hose pipe (175 kg N/ha). Additional fertilizers were added immediately after (phosphorous: 50 kg/ha; potassium: 150 kg/ha). Slurry and fertilizer were incorporated into the soil (15 cm) using a rotocultivator. For furrow irrigation (2 L/min), there were five furrows/plot (80 cm width, 15 cm depth, 100 cm apart). For drip irrigation (3 L/h), there were two lines/subplot (1.8 m apart). Irrigation was applied 20 times, on a weekly basis. Gas samples were taken weekly until irrigation, daily for the first week after fertilizer application, 2–3 days/week for the first month, and then weekly.

 

7 

A replicated, randomized, controlled study in 2006–2008 in a cereal field in the Castelo Branco region, Portugal, found similar amounts of nitrate in soils with or without added slurry. Nutrients: Similar amounts of nitrate were found in plots with and without added slurry (1–22 vs 1–10 mg NO3–N/L water). Methods: Water in the soil was collected in porous ceramic suction cup samplers (four/plot; 0.6–0.7 m depth; 50 kPa for 24 hours), whenever drainage occurred (October–November and April–May; 16 samples in total). Cattle slurry was added to three treatment plots (5.6 x 8 m), but not three control plots, in spring. Maize was grown in spring–summer, and oats were grown in autumn–winter.

 

8 

A meta-analysis from 2013 of studies in Mediterranean climates found similar percentages of organic carbon in soils with or without added slurry. Organic matter: There was no difference in organic carbon between soils with or without added slurry (2% higher in soils with slurry). Methods: Slurry included liquid pig and cattle manure, both raw and digested. The Web of Knowledge database was searched, using the keywords, “Mediterranean”, “soil”, and “conventional”, and 3 data sets from 3 studies of slurry amendment were found and meta-analysed. The most recent studies included in this meta-analysis were published in 2011.

 

9 

A replicated, randomized, controlled study in 2008–2011 in a wheat-barley field in Catalonia, northeast Spain, found similar amounts of organic matter and soil organisms, but greater soil stability, in soils with added manure, compared to soils without it. Organic matter: Similar amounts of organic carbon were found in soils with or without added slurry (amounts not reported). Soil organisms: Similar amounts of microbial biomass (measured as carbon) were found in soils with or without added slurry (655–1,372 vs 591–900 mg C/kg soil). Soil erosion and aggregation: More water-stable macroaggregates, and larger macroaggregates, were found in soils with added slurry in (200 kg N/ha) compared to soils without it, in one of two comparisons (0.43–0.44 vs 0.39 kg water-stable macroaggregates/kg soil; 3.30 vs 3.02 mean weight diameter). Methods: There were three plots (5 x 12 m) for each of two treatments (pig slurry, added at 100 or 200 kg N/ha) and there were three control plots (no slurry). Crops were planted in October (with a seed drill) and harvested by the end of June. Soil samples were collected seven times, from March 2010 to July 2011, with a flat spade (0–5 cm depth, two samples/plot).

 

10 

A replicated, randomized, controlled study in 2002–2012 in a rainfed cereal field in Spain found more organic matter and nutrients in soils with added slurry, compared to soils without it. Organic matter: More organic matter was found in plots with slurry, in one of five comparisons (1.9% vs 1.6%). Nutrients: More nitrogen, phosphorus, and potassium was found in plots with added slurry (nitrogen, in one of five comparisons: 0.14% vs 0.12%; phosphorus, in three of five comparisons: 52–78 vs 31 mg/kg; potassium, in two of five comparisons: 408–528 vs 279 mg/kg). Similar pH levels were found in plots with or without added slurry (pH 8.3–8.4). Soil organisms: Similar numbers of oribatid mites were found in plots with or without added slurry (2,404–5,448 vs 4,304 individuals/m2). Methods: Plots (11 x 12.5 m or 7 x 12.5 m) had slurry (pig: 30 or 55 t/ha/year; sow: 25, 55, or 80 t/ha/year) or no fertilizer (12 replicates of each, but three replicates with sow slurry at 25 t/ha/year). Plots had reduced tillage (disc-harrowing, 15 cm depth) or no tillage (with herbicide). Straw was removed from all plots. Soil samples were collected in October 2011, February 2012, and May 2012 from plots without fertilizer and plots with 25 t/ha/year (three cores/plot, 0–5 cm depth). The other plots were sampled in May 2012.

 

11 

A replicated, randomized, controlled study in 2010–2013 in rainfed barley fields in Spain (same study as (13)), found higher carbon dioxide emissions in plots with added slurry, compared to plots without it. Organic matter: Similar amounts of carbon were found in plots with or without added slurry (98–110 vs 83–96 Mg/ha). Greenhouse gases: Similar amounts of methane were absorbed by plots with or without added slurry (–1 vs –2 kg C/ha). Higher carbon dioxide emissions were found in plots with added slurry (4,294–4,586 vs 3,227 kg C/ha). Implementation options: Similar amounts of carbon were found in plots with less or more slurry (98–110 vs 100–107 Mg/ha). Similar amounts of methane were absorbed by plots with less or more slurry (–1 vs –2 kg C/ha). Similar carbon dioxide emissions were found in plots with less or more slurry (4,294 vs 4,586 kg C/ha). Methods: Plots (40 x 12 m) had pig slurry (75 or 150 kg N/ha) or no fertilizer (three plots for each). Plots had conventional tillage (mouldboard plough: 25 cm depth; cultivator: 15 cm depth) or no tillage. Soil samples were collected at the end of the experiment (two samples/plot; 0–75 cm depth).

 

12 

A replicated, randomized, controlled study in 2010–2012 in a rainfed barley field in Spain found more nitrate and higher carbon dioxide emissions in plots with added slurry, compared to plots without it. Nutrients: Similar amounts of ammonium were found in plots with or without slurry (2.6 vs 1.9 mg/kg). More nitrate was found in plots with slurry (89 vs 20 mg/kg). Soil organisms: Similar amounts of microbial biomass (measured as carbon) were found in plots with or without slurry (859 vs 893 mg/kg), but more microbial biomass (measured as nitrogen) was found in plots with slurry (338 vs 177 mg/kg). Soil erosion and aggregation: Similar amounts of water-stable aggregates were found in plots with or without slurry (0.2 vs 0.1–0.2 g). Greenhouse gases: Higher carbon dioxide emissions were found in plots with slurry (1,669 vs 1,218 µg/kg macroaggregates/hour). Similar amounts of methane were absorbed by plots with or without slurry (–0.1 vs –0.2 µg/kg macroaggregates/hour). Similar nitrous oxide emissions were found in plots with or without slurry (1 vs 0.6 µg/kg macroaggregates/hour). Methods: Plots had pig slurry (150 kg N/ha) or no fertilizer (three plots each; plot size not clearly reported). Plots had conventional tillage (20 cm depth) or no tillage. Soil samples (0–5 cm depth) and gas samples (15 mL) were collected in March 2012.

 

13 

A replicated, randomized, controlled study in 2010–2013 in rainfed barley fields in Spain (same study as (11)) found more ammonium and higher nitrous oxide emissions in plots with added slurry, compared to plots without it. Nutrients: Similar amounts of nitrate were found in plots with or without added slurry (59–107 vs 65 kg/ha). More ammonium was found in plots with added slurry (12–16 vs 3 kg/ha). Greenhouse gases: Higher nitrous oxide emissions were found in plots with added slurry, compared to plots without it, in one of two comparisons (0.2 vs 0.1 mg/m/d) Methods: Plots (40 x 12 m) had pig slurry (75 or 150 kg N/ha) or no fertilizer (three plots for each). Plots had conventional tillage (mouldboard plough: 25 cm depth; cultivator: 15 cm depth) or no tillage. Soil samples were collected at the end of the experiment (two samples/plot; 0–75 cm depth).

 

14 

A replicated, randomized, controlled study in 2003–2004 in an irrigated maize field in Spain found similar amounts of nitrogen in plots with different amounts of added slurry. Implementation options: Similar amounts of nitrogen were found in plots with different amounts of added slurry (21–80 kg N/ha). Methods: Plots (30 x 40 m) had pig slurry (30, 60, 90, or 120 Mg/ha) or no fertilizer (three plots for each). Slurry was immediately covered after application. Lysimeters (2.6 x 2 m; 1.5 m depth) were installed in each plot, five years before the study. Each lysimeter was drip-irrigated, simulating flood irrigation (May to mid-September, with 7–12 intervals). Soil samples were collected after harvest (0–120 cm depth).

 

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.