Biodiversity: Three replicated trials from Canada, Poland and Spain (including one also controlled, one also randomized and one also controlled and randomized) showed that adding mulch to crops (whether shredded paper, municipal compost or straw) increased soil animal and fungal numbers, diversity and activity. Of these, one trial also showed that mulch improved soil structure and increased soil organic matter.
Nutrient loss: One replicated study from Nigeria found higher nutrient levels in continually cropped soil.
Erosion: Five studies from India, France, Nigeria and the UK (including one controlled, randomized, replicated trial, one randomized, replicated trial, two replicated (one also controlled), and one controlled trial) found that mulches increased soil stability, and reduced soil erosion and runoff. One trial found that some mulches are more effective than others.
Drought: Two replicated trials from India found that adding mulch to crops increased soil moisture.
Yield: Two replicated trials from India found that yields increased when either a live mulch or vegetation barrier combined with mulch was used.
For this synopsis we consider higher numbers and a higher diversity of microorganisms as indicators of healthy soil. Protozoa are single-celled organisms which can move around, feed on organic matter (carbon and nitrogen compounds) and often also photosynthesise, which means they can fix carbon into organic compounds, providing food sources for other organisms. Nematodes are tiny, often microscopic worms. Arbuscular mycorrhizal fungi are a group of fungi that live around the roots of plants. By living together, the fungi and host plant benefit each other: the fungi can live in a habitat without having to compete for resources and have a supply of carbon from the plant, while they provide an enhanced supply of nutrients to the plant, improving plant growth, the ability to reproduce and tolerance to drought. Arbuscular mycorrhizal fungi colonise a wide variety of host plants including grasses, herbs, agricultural crops and legumes (Bardgett 2005).
Soil aggregates are groups of soil particles that bind to each other more strongly than to adjacent particles. Aggregate stability is the ability of soil aggregates to resist disintegration when disruptive forces associated with tillage and water or wind erosion are applied. Soil stability can is the ability of a soil to resist damage and chemical and physical change.
Bardgett R. (2005) The Biology of Soil: A community and ecosystem approach. Oxford University Press, Oxford.
A replicated experiment in 1970-1974 on sandy-clay to clay soil in Nigeria (Lal, 1976) found that mulched continuous maize Zea mays after ploughing had the lowest soil loss (0 t/ha) compared to continuous maize without mulch (41 t/ha) and bare fallow plots (230 t/ha). Runoff was also lower in the mulched treatment (2% of total annual rainfall) compared to bare fallow (36%). Slopes of 1, 5, 10 and 15% received the following treatments: bare fallow (conventionally ploughed); continuous maize (conventionally ploughed, mulched); continuous maize (conventionally ploughed, no mulch); maize-cowpea Vigna unguiculata rotation (zero-tillage); and cowpeas-maize rotation (conventionally ploughed). Plots were 25 x 4 m and were replicated five times on each slope. Soil and runoff water was collected from each plot after every rainstorm using a water collection system below ground level downslope of the plots.
A controlled, replicated experiment in 1970-1974 on sandy-clay to clay soil in Nigeria (Lal, 1976), found runoff was lowest under 6 t/ha straw mulch (2% of total annual rainfall), then 4 t/ha (4%) and 2 t/ha (10), compared to no mulch (50%). Soil loss from rainstorms where more than 25 mm fell was lowest under 6 t/ha mulch (0.4 kg/ha), then 4 t/ha (2 kg/ha) and 2 t/ha (16 kg/ha), compared to no mulch (143 kg/ha). Slopes of 1, 5, 10 and 15% received the following treatments: no mulch, 2, 4, and 6 t/ha straw mulch. Plots were 25 x 4 m and were replicated five times on each slope. Plots were ploughed, harrowed and mulched at the beginning of each growing season. Soil and runoff water was collected from each plot after every rainstorm using a water collection system below ground level downslope of the plots.
A replicated experiment in 1970-1974 on sandy-clay to clay soil in Nigeria (Lal, 1976), found lower nutrient loss in continuous maize Zea mays with mulch (2.3 kg/ha) continuous maize no mulch (17 kg/ha) and bare fallow (55 kg/ha). Slopes of 1, 5, 10 and 15% received the following treatments: bare fallow (conventionally ploughed); continuous maize (conventionally ploughed, mulched); continuous maize (conventionally ploughed, no mulch); maize-cowpea rotation (zero-tillage); and cowpea Vigna unguiculata-maize rotation (conventionally ploughed). Maize received 120, 26 and 60 kg/ha nitrogen, phosphorus and potassium respectively. Plots were 25 x 4 m and were replicated five times on each slope. Soil and runoff water was collected from each plot after every rainstorm using a water collection system below ground level downslope of the plots.
A randomized, replicated experiment in 1986 on sandy loam in East Malling, UK (Hipps et al. 1990) found that straw mulch reduced soil erosion by 85% in the alleys between trees, compared to the bare ground treatment (average of 0.45 t/ha lost). Erosion was highest between trees where soil had been compacted by traffic. There were two treatments in a 144 x 18 m mature apple Malus domestica orchard: bare soil mulched with 6 t/ha of straw and bare soil treated with 2.4 kg/ha simazine herbicide applied in spring. Each of the eight treatment plots was 36 x 9 m, replicated four times. Two troughs set into the soil downslope of all the plots measured soil loss.
A randomized, replicated experiment, in 1996-1998 on sandy, silty and clay soil in Ludhiana, India (Jalota et al. 2001) found higher soil moisture storage during dry conditions by applying straw mulch, (30.3 mm water/20 cm soil) compared to untreated coarse- and medium-textured soils (28.8 cm). Straw incorporation was better in rain-free conditions (26.7 cm) and rainy conditions (22.2 cm) in medium coarse-textured soils compared to untreated soil (24.2 and 21.1 cm). In the coarsest soil, tillage and straw mulching did not increase soil water storage any more than untreated soil. Below the tillage and straw incorporation treatments, soil water content was higher (0.1318 and 0.1314 m3 water/m-3 soil, respectively) relative to the untreated and mulched soils (0.1059 and 0.1180 m3/m3). There were four treatments on three soil types: untreated, tilled to 8 cm depth, straw mulch (rice Oryza sativa in September and wheat Triticum aestivum in April) at 6 t/ha, and straw incorporation. The treatments were replicated three times in 2.5 x 3.5 m, 5 x 3 m and 6 x 4 m plots, for fine, medium coarse, and coarse soil respectively. Mechanical weeding or herbicides (glyphosate) kept plots weed free. Soil water content was measured every 15-20 days.
A controlled, randomized, replicated experiment in 1994-2000 on gravelly-sandy loam in British Columbia, Canada (Forge et al. 2003) found that soil nematode and protozoan diversity was higher under mulches of shredded paper (56 Shannon diversity index) and shredded paper combined with municipal compost (54 Shannon diversity index), relative to the unmulched control (45 Shannon diversity index). Soil nematode diversity was reduced under mulches of municipal biosolids (solid processed sewage sludge) and alfalfa Medicago sativa hay compared to the control. Spartan apple Malus domestica trees were established in rows in 1994, and seven treatments were applied between rows: control (conventional management), municipal biosolids, shredded office paper, shredded office paper over municipal biosolids, shredded office paper over a composted mixture of biosolids and garden waste, alfalfa hay and black polypropylene mulch. Soil samples for nematode and protozoan community analyses were taken from each plot in October 1998, June 1999 and October 2000.
A controlled experiment in 1991-2000 on a sandy loam in vineyards in Champagne, France (Goulet et al. 2004) found higher soil particle stability in the topsoil under coniferous bark mulch (soil stability index of 15.2) and poplar bark mulch (soil stability index of 13.6) compared to an unmulched control (soil stability index of 10.5). The highest stability was found under a grass cover (soil stability index of 21.7). The conifer bark layer also increased stability in soils. Four treatments were tested, of which three were in 35 x 8 m plots: a bluegrass Poa pratensis cover between vine rows only, organic mixed mulch of coniferous Abies alba, Picea excelsa, Pinus sylvestri bark between and in vine rows (61 t/ha applied every three years), organic mulch of poplar Populus spp. bark (67 t/ha applied every three years), or bare soil between rows (15 x 8 m control plot). Soil under the grass cover was sampled in and between vine rows; the mulch and control treatments were sampled only between vine rows. All soils were sampled to 20 cm depth.
A replicated experiment in 2000-2004 on fine loamy soil in Dehradun, India (Sharma et al. 2010) found that live mulching with sunnhemp Crotalaria juncea or leucaena Leucaena leucocephala increased soil moisture content by 6.8-8.8% compared to no mulching. Combining both sunnhemp and leucaena increased soil moisture by a further 2.1-2.3% and increased overall grain yield by 15% of both maize Zea mays and wheat Triticum aestivum. A maize crop was grown followed by a wheat crop. In each main crop were four treatments in 130 m2 plots: no mulching (control), sunnhemp grown in situ (live mulching), leucaena prunings/twigs, and sunnhemp and leucaena combined. Each treatment had 29 m2 subplots with fertilizer applied at a rate of 0, 30, 60 or 90 kg N/ha for maize, and 0, 40 or 80 kg/ha for wheat. There were four replications.
A controlled, replicated experiment in 2005-2009 on silty loam soil in eastern Spain (García-Orenes et al. 2012) found that after five years, oat Avena sativa straw mulching had the highest levels of soil organic matter (4.5% of the soil), arbuscular mycorrhizal proteins (1,350 mg proteins/g soil) and aggregate stability (80%) compared to herbicide or ploughed plots (2% organic matter, 700 and 400 mg/g proteins, 41% and 57% aggregate stability, respectively). Plots with oat mulching also had lower soil erosion rates (0 Mg/ha/h soil loss) than herbicide or ploughed plots (0.97 Mg/ha/h and 0.01 Mg/ha/h, respectively). The other treatments had similar values to an abandoned land control. There were three replicates of five management treatments including: herbicide application; ploughing; ploughing then sowing oats; addition of oat straw mulch (0.25 kg/m2/year); and land abandonment (control). Plots were 6 x 10 m. Soil under native vegetation was used as a reference. Six soil samples from each plot were taken annually to 5 cm depth. Five rainfall simulations were also conducted during the summer drought period to test erosion on 1 m2 plots. Simulations lasted one hour at 55 mm/h.
A controlled, randomized, replicated experiment in 2007-2010 on sandy silt-loam in India (Ghosh et al, 2012) found lower soil loss and runoff from the palmarosa Cymbopogon martinii treatment with mulching, organic manures and minimal tillage (3.4 t/ha, 234 mm), than with no vegetation barrier (7.1 t/ha, 428 mm). The panicum without mulch treatment was less effective (5.2 t/ha, 356 mm) than mulched palmarosa. Maize Zea mays yield was 43% higher under minimal tillage with mulched palmarosa compared to no vegetation barrier with conventional tillage. The succeeding wheat Triticum aestivum yield was on average 73% higher in the palmarosa relative to panicum treatment, and 99% higher than with no vegetation barrier. It is not clear whether these results were due to organic amendments, mulching or reduced tillage. There were three replications of three treatments in a maize-wheat crop rotation: conventional tillage with no vegetation barrier but applying fertilizers and herbicides; conventional tillage with a panicum Panicum maximum vegetation barrier, fertilizers and herbicides; minimal tillage (30% crop cover retained) with a palmarosa vegetation barrier plus mulching and farmyard manure, vermicompost (produced by worms) and poultry manure applications. Plots were 100 x 20 m.
A randomized, replicated experiment in 2008 on silty soils in Lublin, Poland (Siczek & Frąc 2012) found that adding a straw mulch increased bacteria counts (3.5 billion colonies/kg) and activity compared to soil with no mulch (2.4 billion colonies/kg). There were three compaction treatments in a soybean Glycine max crop obtained using a wheel tractor: strongly compacted (5 passes); moderately compacted (3 passes); and uncompacted (0 passes) soil. There were six replicates. Within each treatment were 1.8 x 2.1 m plots with either no mulch or a straw mulch. Fertilizer was applied uniformly to all plots at 54-70-80 kg/ha NPK. Bacterial numbers and enzyme activities were measured in soil samples taken three times during crop development from the centre of the soybean rows.