Efficacy of wheat straw mulching in reducing soil and water losses from three typical soils of the Loess Plateau, China

Journal Pre-proof Efficacy of wheat straw mulching in reducing soil and water losses from three typical soils of the Loess Plateau, China Abbas E. Rahma, David N. Warrington, Tingwu Lei PII:

S2095-6339(19)30181-9

DOI:

https://doi.org/10.1016/j.iswcr.2019.08.003

Reference:

ISWCR 185

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International Soil and Water Conservation Research

Received Date: 5 May 2019 Accepted Date: 9 August 2019

Please cite this article as: Rahma A.E., Warrington D.N. & Lei T., Efficacy of wheat straw mulching in reducing soil and water losses from three typical soils of the Loess Plateau, China, International Soil and Water Conservation Research, https://doi.org/10.1016/j.iswcr.2019.08.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 International Research and Training Center on Erosion and Sedimentation and China Water and Power Press. Production and Hosting by Elsevier B.V.

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Efficacy of wheat straw mulching in reducing soil and water losses from three typical soils of the Loess Plateau, China 1,2

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2,3

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Abbas E. Rahma , David N. Warrington and Tingwu Lei

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1. College of Agricultural Studies, Department of Agricultural Engineering, Sudan University of Science and

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Technology, Khartoum, Sudan-shambat.Email:[email protected], [email protected]

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2. College of Water Conservancy and Civil Engineering, Shandong Agricultural University, Taian Shandong,

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P.R. China.

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3. State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water

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Conservation, Chinese Academy of Science and Ministry of Water Resources, Yangling, Shaanxi Province, 712100,

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P.R. China.

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Abstract

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Mulching the soil surface with a layer of plant residue is considered an effective method of

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conserving water and soil because it increases water infiltration into the soil, reduces surface

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runoff and the soil erosion, and reduces flow velocity and the sediment carrying capacity of

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overland flow. However, application of plant residues increases operational costs and so optimal

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levels of mulch in order to prevent soil and/or water losses should be used according to the soil

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type and rainfall and slope conditions. In this study, the effect of wheat straw mulch rate on the

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total runoff and total soil losses from 60-mm simulated rainstorms was assessed for two intense

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rainfalls (90 and 180 mm h-1) on three slope gradients typical conditions on the Loess Plateau of

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China and elsewhere.

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For short slopes (1 m), the optimal mulch rate to save water for a silt loam and a loam soil

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was 0.4 kg m-2. However, for a clay loam soil the mulch rate of 0.4 kg m-2 would be optimal only

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under the 90 mm h-1 rainfall; 0.8 kg m-2 was required for the 180 mm h-1.

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In order to save soil, a mulch rate of 0.2 kg m-2 on the silt loam slopes prevented 60% to 80%

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of the soil losses. For the loam soil, mulch at the rate of 0.4 kg m-2 was essential in most cases in

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order to reduce soil losses substantially. For the clay loam, 0.4 kg m-2 may be optimal under the

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90 mm h-1 rain, but 0.8 kg m-2 required for the 180 mm h-1 rainstorm. These optimal values

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would also need to be considered alongside other factors since the mulch may have value if used

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elsewhere. Hence doubling the optimal mulch rate for the silt loam soil from 0.2 kg m-2 or the

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clay loam soil under 90 mm h-1 rainfall from 0.4 kg m-2 in order to achieve a further 10%

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reduction in soil loss needs to be assessed in that context. Therefore,. Optimal mulch rate can be

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an effective approach to virtually reduce costs or to maximize the area that can be treated.

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Meantime, soil conservation should be aware that levels of mulch for short slopes might not be

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suitable for long slopes.

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Key words: straw mulch, soil losses, runoff, rain simulator

1. Introduction

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Soil erosion is a severe problem for most cultivated land in the world, and particularly on the

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Loess Plateau of China. The Loess Plateau is located in the upper and middle reaches of the

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Yellow River (from 100° 54' to 114° 33'E and 33° 43' to 41° 16' N). It covers a total area of

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624,000 km2 and the soils are derived from thick ancient loess deposits (Gao et al. 2016). Over 60%

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of the area of the Loess Plateau is subject to great soil and water losses, with a mean annual soil

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loss of 2000-2500 t km-2. Soil erosion by water has been the major cause for the losses of land

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nutrients and productivity. In recent years, off-site problems such as river/channel and reservoir

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sedimentation and waters pollution by sediment-borne chemicals have also become a major

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concern (Poesen et al., 2003; Wu et al., 2003; Udawatta et al., 2004). The severe soil erosion in

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the Loess Plateau is mainly caused by erosive rainfall events (Zhang and Zhu 2006; Wu et al.

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2016a)

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Soil erosion by water begins with the production of runoff water during a rainstorm when the

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infiltrability of the soil becomes lower than the rainfall intensity. One of the main factors

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affecting infiltrability is seal formation, which reduces the hydraulic conductivity of the soil

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surface layer. Seal formation on freshly cultivated field begins with the breakdown of surface

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clods and aggregates by both physical forces and physicochemical processes (Agassi et al., 1981;

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Lado et al 2004; Assoulineet al., 2006). The physical forces are primarily produced by raindrop

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impact, which also compact the soil surface, and slaking. Physicochemical dispersion is

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determined by the electrolyte concentration of the rainwater and by the concentration of elements

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in the soil, particularly sodium (Ma et al., 2014). Dispersion of clay results in free clay entering

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the surface soil pore system and partially blocking the pores.

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Slaking is caused by the explosive force of escaping air that was entrapped under pressure

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inside dry aggregates during wetting (Yoder, 1936; Panabokke and Quirk, 1957; Emerson, 1967;

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Zaher and Caron, 2008; Fajardo et al., 2016). It is most severe when dry aggregates are rapidly

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wetted. The force generated by slaking depends on the volume of air entraption inside the

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aggregates, the rate of clod wetting (Loch et al., 1994; Zaher et al. 2005; Chenu et al.; 2000; Fan

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et al., 2008; . Han et al., 2016; Lado et al., 2004), and the shear strength of wet aggregates

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(Nearing and Bradford, 1985; Fattet et al., 2011). Slaking depends on aggregate stability, which

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is directly related to organic matter, sesquioxides and clay contents (Kemper and Koch, 1966;

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Kay and Angers, 1999; Norton et al., 2006; Puget et al., 1995; Le Bissonnais and Arrouays,

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1997 ; Barthès et al., 2008; An et al., 2013). Mulching is referred to as the agronomic practice of

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covering the soil surface with straw for soil and water conservations and to favour plant growth

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(Jordán et al., 2011).

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The effectiveness of mulching in reducing runoff and soil loss can be attributed to three main

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aspects. Firstly, using mulch to protect the soil surface from the direct impact of raindrops,

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reduces splash erosion and soil detachment and, thereby, limiting the availability of detached soil

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readily being transported by runoff (Schwab et al., 1993; Lal, 1979; Gholami et al., 2013; Cook

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et al., 2006; García-Orenes et al., 2009 and 2012; Keesstra et al., 2016; Mwango et al., 2016;

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Prosdocimi et al., 2016a, 2016b) as well as reducing soil surface crusting, sealing and

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compaction (Cook et al., 2006; Jordán et al., 2010; Montenegro et al., 2013a, b; Zonta et al.,

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2012). It is therefore considered to be an effective way to control soil erosion by water (Gabet et

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al., 2003; Zhang et al., 2003). Secondly, mulch increases the hydraulic roughness of the soil

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surface, thereby reducing surface flow velocity which reduces soil detachment and the carrying

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capacity of the overland flow (Montenegro et al., 2013a, b; Shi et al., 2013; Foster and Meyer,

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1975; Cruse et al., 2011; Jordán et al., 2010; Miyata et al., 2009; Rahma et al., .2013). Thirdly,

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mulch entraps water and soil (Cerdà et al., 2016; Foltz and Wagenbrenner, 2010; Groen and

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Woods, 2008; Pannkuk and Robichaud, 2003; Prats et al., 2012, 2016b; Robichaud et al., 2013),

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especially in the beginning of a rainfall event when the mulch is dry and its capacity to retain

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water and soil particles is the highest.

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Many studies have evaluated the use of various plant residue mulches on soil erosion (Hou

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and Du, 1985; Luo et al., 1990; Jin et al., 1992; Achmad et al., 2003; Cook et al., 2006;

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Prosdocimi et al., 2016a; Sadeghi et al., 2015a; Shi et al., 2013).

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Factors affecting soil loss include mulch cover, rainfall intensity and rainfall duration, and

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slope gradient (Khan et al., 1988; Francis and Thornes, 1990; Jin et al., 2009; Lattanzi et al.,

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1974; Sadeghi et al., 2015b; Smets et al., 2008b; Auerswald et al., 2003).

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Reports have indicated that mulching is one of the most cost effective means of crop residue

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usage (Dickey et al., 1985; Shelton et al., 1995). Even so, mulch is of use for other purposes,

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such as feed for animal, fuel for cooking, or as a building material. Therefore, excess use of

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mulch to reduce water and soil losses is desirable.

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Applying straw mulch always reduced water and soil losses as compared to un-mulched soil.

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However, the mulch rates required to reduce soil and water losses to an optimal level depended

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on the soil type and the rainfall and slope conditions. The rates could also depend on whether the

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objective was primarily to reduce soil or water losses. However, although the advantageous

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effects of mulching with crop residues are known, further research is needed to quantify these

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effects, particularly in areas where soil erosion by water represents a severe threat. Arguably,

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there are still some uncertainties in the literature about how to maximize the effectiveness of

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straw mulch for reducing soil and water loss rates.

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First, the choice of vegetative residue cover type is essentially; this choice drives the

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application rate, cost and, consequently ,effectiveness of mulching (Robichaud et al., 2013a;

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Smets et al., 2008a, 2008b; Beyers, 2004; Erenstein, 2003; Lal, 1976; Prats et al., 2012;). Second,

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the appropriate application rate is another significant factor that substantially influences the

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effectiveness of mulching in reducing soil and water losses (Jordán et al., 2010; Lal, 1984;

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Lattanzi et al., 1974; Meyer et al., 1970; Mulumba and Lal, 2008; Prosdocimi et al., 2016a) as

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well as the percentage of area covered by mulch (Adekalu et al., 2007; Harold, 1942; Lal, 1977;

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Norton et al., 1985)

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Most of the studies research focusing on the efficiency of mulching to reduce runoff and

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erosion were carried out in the field. They involved natural rainfall conditions (Cook et al., 2006;

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Martinez-Raya et al., 2006; Mupangwa et al., 2007; Prats et al., 2012, 2014, 2016a, b; Robichaud

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et al., 2013; Are et al., 2011; Bhatt and Khera, 2006; Cawson et al., 2013) as well as simulated

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rainfall (Cerdà, 1997; Cerdà et al., 2016; Groen and Woods, 2008; Jordán et al., 2010; Mayor et

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al., 2009; Montenegro et al., 2013b; Robichaud et al., 2013) and applied concentrated flow from

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upslope (Robichaud et al., 2013; Harrison et al., 2016). Field studies research and, in particular,

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those under natural rainfall conditions, are typically high time-consuming and need demanding

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in resources and facilities, as they often require many years to obtain representative results of the

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aimed soil and rainfall conditions (Lal, 1994). However, experiments under laboratory conditions

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using soil flumes have been used to study runoff and soil erosion processes (Marzen et al., 2016;

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Prats et al., 2018; de Lima et al., 2003, 2013), to determine the impacts of mulching (Foltz and

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Wagenbrenner, 2010; Gholami et al., 2013; Montenegro et al., 2013a; Pannkuk and Robichaud,

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2003; Prats et al., 2015, 2017; Xu et al., 2017). The main usefulness of such laboratory

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experiments is that they allow systematic replication of a wide range of rainfall and terrain

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conditions (e.g., rainfall spatial and temporal characteristics, surface slope, and soil roughness)

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The objectives of this study were: (i) to determine the effect of wheat straw mulch at four

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different rates on seal formation, infiltration, runoff and soil loss under different rainfall intensity

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and slope conditions; (ii) to assess the efficacy of different rates of straw mulch cover in

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reducing soil and water losses during intense rainstorms typical of the Loess Plateau and other

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areas of the world; and (iii) to suggest optimal levels of mulch to reduce soil and water losses

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according to the soil type and rainfall intensity and slope conditions.

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2. Material and Methods

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Soil was collected from three locations in Shaanxi Province (a silt loam soil from Ansai, a

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clay loam soil from Yangling, and a loam from Chang Wu) on the Loess Plateau, which

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represented three common agricultural soils that differed in texture and are situated in the most

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productive part of the Plateau. Samples were collected from the upper 20 cm soil layer of

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cultivated land at each location. The soils were air-dried to a gravimetric moisture content of less

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than 8%. Large clods were manually broken apart and the soils were then passed through 2mm

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mesh. A representative subsample of the soils was used for chemical and mechanical analysis.

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Cation exchange capacity, exchangeable sodium percentage and organic matter content were

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determined according to standard methods. A Mastersizer 2000 (Malvern Instruments, Malvern,

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England) analyzer was used to determine particle size distributions. Basic soil properties are

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given in Table 1.

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Table.1 Basic properties of the soils used in the study

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Steel boxes measuring 1.0 m in length, 0.6 m in width, and 0.25 m in depth, which were

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supported on a mobile framework that could be set at various slope angles, was used to contain

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the soils during the experiments. Holes, 3 mm in diameter with a spacing of 10 mm, in the

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bottom of the box allowed air to escape; a layer of thin cloth covered the bottom of the box to

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prevent loss of soil through the holes. A funnel at the lower end of the box directed runoff into

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collection buckets.

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A rainfall simulator, similar to the one described by Meyer and McCune (1985), at the State

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Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, was used to perform

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the experiments which were conducted in Yangling, China (108°24′E and 34°20′N,521 m above

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sea level). The equipment was capable of generating simulated rainfall with deionized water over

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large areas. Simulated rainfall was projected sideways from eight nozzles situated 16 m above

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the ground, and then fall vertically towards the ground. Rainfall intensity was determined by

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valves linked to pressure gauges, controlled automatically by a computer monitoring an

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electronic rain gauge. The raindrop speed after calibration meets natural rainfall features

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Deionized water, used to simulate rainwater, was projected sideways from six nozzles, in two

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rows of three arrayed over the two long sides of the rectangular target area (4 m x 9 m) Fig. 1.

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The nozzles were 16 m above the ground so that the raindrops attained their terminal velocity

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before impact, about 98% of that of natural rain. Rainfall intensities were determined by a pump

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that was controlled by a computer connected to a rain gauge positioned in the center of the target

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area.

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Steel boxes measuring 1.0 m in length, 0.6 m in width, and 0.25 m in depth, which were

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supported on a mobile framework that could be set at various slope angles, were used to contain

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the soil materials during the experiments. The flumes were on a manual jack that could be

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positioned under the rainfall simulator and raised to attain a designed slope gradient.

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Drainage holes of 3 mm in diameter with a spacing of 10 mm were made at the bottom of the

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flume to facilitate the escape of soil-air; a layer of thin cloth covered the bottom of the box to

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prevent loss of soil through the holes. A funnel was attached to the end of each flume to direct

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sediment laden runoff water into collection buckets. With the flumes in a horizontal position, the

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air-dried soil material was uniformly packed into the boxes, by pouring a known mass of soil

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into a known volume of the box in layers of 2.5 cm thickness and tamping it down with a

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wooden paddle so that the bulk density was 1200 kg m-3. This bulk density was representative of

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freshly tilled soils on the Loess Plateau. The surface of each layer was scored rough before

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adding more soil to minimize discontinuity effects.

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Figure.1 Rain fall simulator.

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Following packing, mulch was spread uniformly over the soil surface at the rates of 0, 0.2,

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0.44, and 0.8 kg m-2. Wheat straw was collected from the field following the wheat harvest. The

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straw was air-dried and cut or broken to lengths of less than 30 cm, the purpose of this treatment

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because under the field condition the wheat crop was harvested using the harvester machine that

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produce wheat straw ranging from 25 to 30 cm length.

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The soil without mulch (0 kg m-2) served as the control. Prior to a simulated rainstorm, the

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boxes were moved into predetermined randomized positions, in two rows of six boxes each,

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under the rainfall simulator and the slopes were adjusted to the designated gradient. Three slope

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gradients were studied: gentle (5°, 8.7%), moderate (15°, 26.8%) and steep (25° 46.6%).

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Each rainfall experiment consisted of 60 mm of rainfall, which ensured that a dry soil layer

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would be maintained in the bottom of the box to avoid potential drainage problems and the pot

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effect. Two rainfall intensities were studied (90 and 180 mm h-1), which were typical of the more

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intense rainstorms on the Loess Plateau. Three rainstorms were used for each rainfall intensity,

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and each treatment was replicated three times. During each rainstorm, all the runoff was

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collected in buckets at intervals equivalent to 3 mm of rain depth, i.e., 20 samples were collected

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from each box Fig.2. Following the rainstorm, the buckets containing the runoff water and

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sediments were weighed. Sediments were allowed to settle overnight and clear water was

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removed. The wet sediments were dried at 105 ºC and weighed to give the soil loss per 3-mm

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rain depth interval. Runoff volume was calculated as the difference in mass of the bucket

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containing the sediment and water and the sum of the tare mass of the bucket and the mass of the

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sediment. Runoff volume was used to calculate the runoff depth (mm) and runoff rate (mm h-1).

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Infiltration amount (mm) was estimated from the difference between the rainfall depth, adjusted

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to the projected area of the box, and the runoff depth.

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Figure 2 Experimental equipment used to collect data under rainfall simulator.

3. Statistical analysis

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The experiment used a 3 Χ 3 Χ 3 Χ 2 Χ 4 factorial design (3 soils, 3 slopes, 3 replicates, 2

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rainfall intensities, and 4 independent variables, the mulch treatments). The runoff and soil loss

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data was tested for normality and then one-way analysis of variance was used to determine the

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mean effect of the treatments and their interactions. A Tukey HSD post-hoc test was used to

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separate between means at probably level of 5%

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4. Results and discussion

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4.1 Mulch rate and surface runoff

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The ANOVA (Table 2) indicated that mulch rate significantly affected the amount of runoff

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(P <0.01). Soil type, slope gradient and rainfall intensity, as well as some of their interactions,

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also significantly affected the runoff amount generated by a rainstorm of 60 mm depth (P < 0.01). 10

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Interactions between variables (Table 2) gave the effect of soil type, slope, rainfall

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intensity and mulch rate on soil loss, and runoff. There were two star significant differences (P ≤

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0.01) dependent variables interactions between two variables: there were two star significant

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differences (P ≤ 0.05) for soil type x mulch rate, soil type x slope, soil type x rainfall intensities,

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mulch rate x slope, mulch rate x rainfall intensity, slope x rainfall intensity and soil type x

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rainfall intensities in the soil loss and runoff. There were no significant difference (P ≥ 0.05) for

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interactions of soil type x slope, soil type x rainfall intensity, mulch rate x rainfall intensity, slope

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x rainfall intensity on runoff except mulch rate, soil type, mulch rate x slope on runoff. There

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were two star significant differences (P ≤ 0.01) dependent variables. Interactions involving three

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or more variables: there were no significant differences (P ≥ 0.05) for all the interactions. This

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means that as the number of interaction increases the significant differences decreases from (P ≤

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0.01) to (P ≤ 0.05). Where there was no significant difference (P ≥ 0.05) for soil loss, it was also

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not significant (P ≥ 0.05) for runoff and infiltration. Where variables interactions was significant

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(P ≤ 0.05) for soil loss, it was also significant for runoff that where there was no significant

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difference (P ≥ 0.05) for soil loss, it was also not significant (P ≥ 0.05) for runoff and where

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variables interactions was significant (P ≤ 0.05) for soil loss, it was also significant for runoff.

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This confirms findings by Adekalu, (2006) that where there was no significant difference (P ≥

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0.05) for soil loss, it was also not significant (P ≥ 0.05) for runoff and where variables

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interactions was significant (P ≤ 0.05) for soil loss, it was also significant for runoff.

238 239

Table.2 Analysis of variance for the runoff and total soil loss data generated in the rainfall simulator

240

a

DF, degrees of freedom; ns, ns—not significant; *, ** Significant at 5% and 1%, respectively.

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The total runoff amount generated by a 60-mm rainstorm decreased with increasing mulch

242

rate for all soil types under both rainfall intensities (Tables.3 and.4). In the case of the zero

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mulch rate (control), the order of the soil types producing decreasing amounts of runoff was the

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same for each slope and rainfall intensity: clay loam > loam > silt loam. For the three slopes, the

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ranges of the percentages of water lost as runoff during a 60-mm rainstorm at 90 mm h-1 were

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67%-81%, 59%-67%, and 22%-38% for the clay loam, loam and silt loam soils, respectively;

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when the rainfall intensity was 180 mm h-1 the corresponding ranges were 81%-94%, 68%-81%,

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and 42%-49%.

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The greater amounts of runoff generated from the soils where mulch cover was absent as

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compared to mulch covered soils indicate that a greater degree of surface sealing occurred. When

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a soil surface is exposed to raindrop impact the amount of runoff reflects the degree of surface

252

sealing. The hydraulic conductivity of the surface seal limits infiltration.

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In un-mulched soil, the runoff was significantly affected by rainfall intensity and slope

254

gradient, i.e., runoff increased with increasing rainfall intensity and slope gradient (Tables 3 and

255

4). The minimum and maximum runoff of 12.90 and 56.69 mm·m-2 for the clay loam, loam and

256

silt loam soils, respectively, were recorded during the lowest rainfall intensity under the lowest

257

slope and highest rainfall intensity with the steepest slope, respectively. Slope is an important

258

factor influencing the runoff generation process (Bracken et al., 2007; Mu et al., 2015) It

259

strongly affects the water storage on the soil surface (Onstad,1984; Kamphorst et al., 2000).

260

Therefore, changes in slope gradient could alter the runoff process. The results show that the

261

effect of slope on the runoff varied with the rainfall intensity (Tables 3 and 4 ). Runoff

262

significantly (α = 0.05) increased with increase in slope gradient during all rainfall intensities in

263

un-mulched soil. However, an increase in slope gradient from 5◦ to 15◦ had a non-significant 12

264

effect on runoff under the rainfall intensities of 180 mm·h−1, because during higher rainfall

265

intensities, the time required for runoff generation was too short to reflect the difference between

266

slopes (Cao et al., 20-15). Whereas, the same slope gradient (increase from 5◦ to 15◦) with a

267

rainfall intensity of 90 mm·h−1 had a significant effect on runoff. The effect could be due to the

268

permeable conditions of soil, which absorbed more water during lower rainfall intensity, and

269

delayed runoff on 5◦ slopes. The effect of increasing rainfall intensities at a specific slope level

270

was also compared and different patterns of runoff generation were recorded at different slope

271

levels (Tables 3 and 4). The runoff losses significantly (α = 0.05) increased with an increase in

272

the rainfall intensities from 90 to 180 mm·h−1 at all slope levels for the clay loam, loam and silt

273

loam soils, When rainfall begins, surface depressions progressively overflow and are connected

274

to nearby depressions resulting in overland flow (Onstad et al., 1984., Darboux et al., 2002;

275

Antoine et al., 2001; Yang et al., 2013). This process starts when the infiltration capacity during

276

a rainfall event is lower than the rainfall intensity (Yang et al., 2013).

277 278

Once formed, sealed soils generally have lower hydraulic conductivities and infiltration rates

279

and have higher shear strengths than unsealed soils although this very much depends on the type

280

of seal in place. These conditions combine to increase runoff and influence local erosion

281

processes (McIntyre, 1958; Assouline, 2004)..

282

In this study, the clay loam was the most susceptible to surface sealing while the silt loam

283

was the least susceptible resulting in more runoff from the former soil than from the latter. Our

284

results confirmed the previous findings by Le Bissonnaiset al. (2007) that the increase in clay

285

content could largely explain the increase in soil aggregate stability when organic C contents

13

286

were low. The susceptibility of the three soil types to surface sealing was controlled by a number

287

of factors. Surface sealing occurred because soil aggregates were first broken, the resulting loose

288

particles entered the soil pores and inhibited the flow of infiltrating water, while the porosity of

289

the surface layer was further reduced by compaction due to raindrop impact (McIntyre, 1958 ;).

290

Two physical processes would have occurred in these experiments that resulted in aggregate

291

breakdown, i.e., raindrop impact and slaking. Slaking occurred because of the explosion of air

292

entrapped under pressure within initially dry soil aggregates during rapid wetting, which

293

occurred in these experiment because of the high rainfall intensities used (Quirk and Panabokke,

294

1962; Shainberg et al., 2003; Han et al., 2016; Almajmaie et al., 2017). Aggregate breakdown

295

and surface sealing was enhanced by clay dispersion released by a physicochemical process due

296

to the low electrolyte content of rainwater (Agassi et al., 1981). The three studied soils all have

297

relatively high silt contents ranging from 43% to 66% (Table.1). Soils high in silt usually have

298

weak aggregates that are more susceptible to aggregates breakdown and consequently to surface

299

sealing. In addition, the soil aggregates were weak due to low organic matter contents that were

300

all less than 1%. Although clay content often increases aggregate stability, the presence of higher

301

amounts of clay also provide a source of free clay particles after dispersion that enter the soil

302

pores and inhibit infiltration. Therefore, sealing occurred to a greater degree in the soils with

303

more clay.

304

Table.3 Total surface runoff (mm) from three soils on different slopes under different mulch

305

rates during a 60-mm rainstorm with a rainfall intensity of 90 mm h-1

306

a

307

Table .4 Total surface runoff (mm) from three soils on different slopes under different mulch

308

rates during a 60-mm rainstorm with a rainfall intensity of 180 mm h-1

Values are means with (standard deviation).

14

309

a

Values are means with (standard deviation).

310 311

In mulched soil, the runoff losses increased with increasing rainfall intensity and the slope

312

gradient. The results, in Table (Tables 3 and 4), indicate that the maximum runoff of 45.13.

313

mm·m−2 was recorded for the rainfall intensity of 180 mm·h−1 at the 25◦ slope level under clay

314

loam soil. Straw mulch significantly (α = 0.05) reduced runoff volume as compared with the un-

315

mulched treatments (Figure 3.), indicating that a portion of rainfall was either infiltrated into the

316

soil or absorbed by the straw covered surface. The capacity of air-dried residues to absorb water

317

up to 4.8 times its original weight (Wu et al .,1995). Moreover, the conservation effect of straw

318

mulch reduced with increasing slope gradient during higher rainfall intensities (Figure 1a, b and

319

c). These trends indicate that a reduction in runoff losses decreased with an increase in rainfall

320

intensity in mulch treatments at a steep slope.

321 322

Mulch rate had a significant effect on runoff. Mean runoff losses were 34%, 25%, 10% and 6%

323

under mulch rates of 0, 0.2, 0.4 and 0.8 kg m-2, respectively, under the 90 mm h-1 rainfall; while

324

under 180 mm h-1, the corresponding runoff losses were 41%, 32%, 18% and 10%. When mulch

325

intercepts the raindrops, aggregate breakdown due to raindrop impact is reduced. With increased

326

mulch rates, the degree of cover protecting the soil surface from raindrop impact increased in this

327

study. The runoff reductions could be attributed to the protection provided by the mulch against

328

the direct impact of raindrops, promoting the dispersion of the kinetic energy of the raindrops,

329

preventing the destruction of soil aggregates and the compaction of the soil surface layer

330

(Gholami et al., 2013). Consequently, infiltration rates were maintained to a greater degree and,

331

for a given rainfall intensity and slope, the runoff was reduced (Bajracharya and Lal, 1998). 15

332

However, mulch can also reduce runoff by increasing surface roughness and enhancing

333

infiltration (Cook et al., 2006; Jordán et al., 2010; Montenegro et al., 2013a, b; Zonta et al., 2012;

334

Shi et al., 2013). Increasing the slope gradient and rainfall intensity tended to increase the

335

amount of runoff generated under each treatment. Similar findings have been reported by other

336

researchers, who also observed that increasing slope gradient was the important factor in runoff

337

losses even in the presence of mulch cover (Won et al., 2012; Adekalu et al., 2007. The degree of

338

surface sealing is a balance between seal forming processes and seal destruction processes

339

(Poesen et al., 1987)

340

Increasing rainfall intensity can both increase the frequency of raindrop compaction or the

341

degree of slaking (forming processes) and soil detachment (a destruction process). In this study,

342

seal forming processes may have been enhanced by the increase in rainfall intensity more than

343

the seal destruction processes.

344

Figure.3 Reductions in water loss due to mulch cover for three soil types from different slopes under two rainfall

345

intensities (90 and 180 mm h-1 represented by filled and unfilled symbols, respectively)

346

Reductions in water losses due to mulch cover may be represented by percentage reductions in

347

runoff using the control as a baseline (Fig.3). Figure 1 indicates clearly that although runoff was

348

substantially reduced by a mulch rate of 0.2 kg m-2 for all soil types, rainfall intensities and

349

slopes, it was further reduced substantially by a further increase in mulch rate. For the silt loam

350

(Fig.3a) and the loam (Fig.3c) soils, applying 0.4 kg m-2 effectively reduced runoff and a further

351

increase of mulch did not greatly enhance runoff reduction. Therefore, a mulch application of 0.4

352

kg m-2 would be optimal for these soils. However, for the clay loam soil, while the mulch rate of

353

0.4 kg m-2 would be optimal under the 90 mm h-1 rain, it was not for the higher intensity

16

354

rainstorm. Therefore, for clay loams likely to be subjected to rainfall intensities in the order of

355

180 mm h-1, a higher rate of mulch (0.8 kg m-2) would be optimal in order to reduce water losses.

356

However, some scholars (Won et al., 2012) reported that straw mulch (600 g · m− 2) had no

357

runoff during rainfall of 30 mm · h−1 on both 10◦ and 20◦ slopes, and negligible runoff in a

358

simulation of 60 mm · h−1 on the 10◦ slope. Our present study also shows that mulching

359

decreases runoff losses (Figure 3.). However, these data indicate that the application of straw

360

mulch to 0.4 kg m -2 during low rainfall intensity may not be economical. Indeed, runoff losses

361

from un-mulched soil during lower rainfall intensity were negligible compared to those under

362

higher rainfall events. During low rainfall intensities, more rainfall is intercepted by high mulch

363

rate, and also partly influences soil water conservation (Li et al., 2005).However, there was

364

threshold mulch thickness to avoid excessive water interception during low -intensity rainfalls

365

(Pérez, 2000). Although the magnitude of reduction in runoff losses decreased with an increase

366

in slope during high-intensity rains, the reductions were significant. These results conclusively

367

demonstrate the effectiveness of straw mulch in reducing runoff losses.

368

369

4.2 Mulch rate and soil loss

370 371

Mulch reduced soil losses from all soil types on the different slopes and under different

372

rainfall intensities (Tables.5 and.6). Soil losses increased with increasing slope and rainfall

373

intensity similar to the runoff increases, and were reduced by increasing mulch rates. In part,

374

when the runoff amount increases, greater soil losses are likely due to an increased capacity for

375

sediment transport. In addition, increasing the slope gradient or the rainfall intensity results in 17

376

higher runoff rates and faster flow velocities, which have a higher capacity for soil detachment

377

and a higher carrying capacity. Mulch reduces flow velocity of the runoff regardless of the runoff

378

rate (Rahma et al, 2013 ), which generally reduces the carrying capacity and soil losses, even

379

though mulch also increase roughness, which may increase detachment.

380

Mulch also reduces soil losses by reducing detachment due to raindrop splash. Furthermore, soil

381

detachment and transport are enhanced when the particles are smaller, while water stable

382

aggregates are larger and harder to erode. Under mulch, the reduction in raindrop impact results

383

in less aggregate breakdown, although slaking is still a factor, leading to fewer smaller or

384

erodible particles (Shi et al., 2013; Sirjani and Mahmoodabadi., 2014).

385

Mulch also reduced and delayed rill formation, in particular by decreasing runoff velocity

386

and its sediment transport capacity (Montenegro et al., 2013a, b; Shi et al., 2013). Also, by

387

protecting the soil surface from the direct impact of raindrops, mulching reduced soil detachment

388

by splash erosion and the amount of soil available for mobilization by runoff (Cerdà et al., 2016;

389

Foltz and Wagenbrenner, 2010; Gholami et al., 2013; Groen and Woods, 2008; Montenegro et

390

al., 2013a, b; Pannkuk and Robichaud, 2003; Prats et al., 2012, 2014, 2015, 2017; Robichaud et

391

al., 2013).

392

Moreover, the presence of a water layer on the soil surface controls the detachment rate

393

(Kinnell., 2005). As a result, the dissipation of raindrop kinetic energy greatly influences the

394

detachment and transport processes in rain impacted flows, and more of the raindrop energy is

395

dissipated in the water layer as flow depth increases, leading to a reduction in the soil erosion

396

rate (Kinnel.,2010) Thinner flow depth on the soil surface exposes the aggregates to raindrop

397

impact and exacerbates the soil loss losses on steep slopes.

18

398

The order of soil erodibility followed that of the soil clay content, i.e. soil losses were in the

399

order clay loam > loam > silt loam. Due to the high silt contents and low organic matter contents,

400

all the soils had weak aggregate stability, so the potential for clay dispersion and removal was

401

likely a factor in the soil erodibility.

402

Table.5 Total soil loss (kg m-2) from three soils on different slopes under different mulch rates

403

during a 60-mm rainstorm with a rainfall intensity of 90 mm h-1

404

a

Values are means with (standard deviation).

405

Similar to assessing the efficacy of mulch in reducing water losses, the efficacy in reducing

406

soil loss can be assessed from the percentage reductions in soil los using the control as a baseline

407

(Fig.4).

408

Figure.4 indicates clearly that the relation of mulch rate to soil loss reduction was different for

409

different soil types and was also different from the relation to water loss reduction shown in

410

Figure.3. In the case of the silt loam, a mulch rate of 0.2 kg m-2 would be sufficient to prevent 60%

411

to 80% of the soil losses, depending on the rainfall intensity and slope conditions (Fig.4a).

412

However, a further 10% reduction in soil losses could be achieved by increasing the mulch rate

413

to 0.4 kg m-2 but there would be no clear benefit from increasing the mulch rate to 0.8 kg m-2.

414

Furthermore, since the use of mulch for soil conservation prevents it use for other things,

415

doubling the mulch rate to achieve a soil loss reduction of 10% may not be considered to be

416

worthwhile.

417

Table. 6 Total soil loss (kg m-2) from three soils on different slopes under different mulch rates

418

during a 60-mm rainstorm with a rainfall intensity of 180 mm h -1

419

a

Values are means with (standard deviation).

19

420

However, that decision is best left to local land managers. For the loam soil (Fig.4c),

421

applying 0.2 kg m-2 would not be sufficient to substantially reduce soil losses for many of the

422

intense rainstorms on the steeper slopes, so that an application of 0.4 kg m-2 would be of greater

423

benefit than in the silt loam case. For the clay loam, 0.4 kg m-2 may be optimal under the 90 mm

424

h-1 rain, but not for the higher intensity rainstorm. Therefore, for clay loam soils likely to be

425

subjected to rainfall intensities in the order of 180 mm h-1, a higher rate of mulch (0.8 kg m-2)

426

would be optimal in order to reduce water losses. Furthermore, if the benefits of a further

427

increase of 10% soil reduction are worth increasing the mulch cover from 0.4 to 0.8 mm h-1, then

428

this would the optimal rate under all conditions for the clay loam soil would be 0.8 mm h-1.

429

Figure . 4 Reductions in soil loss due to mulch cover for three soil types from different slopes under two rainfall

430

intensities (90 and 180 mm h-1 represented by filled and unfilled symbols, respectively)

431

4.5. Conclusions

432

The efficacy of applying mulch in order to reduce soil and water losses from cultivated soils

433

with three different textures exposed to intense rainfall conditions was assessed. Applying mulch

434

always reduced water and soil losses as compared to a bare soil without mulch. However, the

435

levels of mulch required to reduce soil and water losses depended on the soil type and the rainfall

436

and slope conditions. The level could also depend on whether the objective was to primarily

437

reduce soil or water losses.

438

For short slopes (1 m), in order to save water, it was found that the silt loam and the loam

439

soils should have mulch applied at a rate of 0.4 kg m-2 and a further increase of mulch did not

440

greatly enhance runoff reduction. However, for the clay loam soil, while the mulch rate of 0.4 kg

441

m-2 would be optimal under the 90 mm h-1 rain, it was not for the higher intensity rainstorm.

20

442

Therefore, for clay loams likely to be subjected to rainfall intensities in the order of 180 mm h-1,

443

a higher rate of mulch (0.8 kg m-2) would be optimal in order to reduce water losses.

444

For short slopes, in order to save soil, it was found that the silt loam could be treated with as

445

low a mulch rate as 0.2 kg m-2 if preventing 60% to 80% of the soil losses, depending on the

446

rainfall intensity and slope conditions, was sufficient. However, a further 10% reduction in soil

447

losses could be achieved by increasing the mulch rate to 0.4 kg m-2 but there would be no benefit

448

from increasing the mulch rate to 0.8 kg m-2. For the loam soil, mulch at the rate of 0.4 kg m-2

449

was essential in most cases in order to reduce soil losses substantially. For the clay loam, 0.4 kg

450

m-2 may be optimal under the 90 mm h-1 rain, but not for the higher intensity rainstorm.

451

Therefore, for clay loam soils likely to be subjected to rainfall intensities in the order of 180 mm

452

h-1, a higher rate of mulch (0.8 kg m-2) would be optimal in order to reduce soil losses, as was the

453

case for the water losses from this soil. These optimal values would also need to be considered

454

alongside other factors since the mulch has value if used elsewhere. Hence doubling the optimal

455

mulch rate for the silt loam soil from 0.2 kg m-2 or the clay loam soil from 0.4 kg m-2 in order to

456

achieve a further 10% reduction in soil loss needs to be assessed in that context.

457

Acknowledgement

458

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Captions for Tables

Table.1 Basic properties of the soils used in the study Table. 2 Analysis of variance for the runoff and total soil loss data generated in the rainfall simulator Table.3 Total surface runoff (mm) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 90 mm h-1 Table .4 Total surface runoff (mm) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 180 mm h-1 Table.5 Total soil loss (kg m-2) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 90 mm h-1 Table . 6 Total soil loss (kg m-2) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 180 mm h -1

Table.1 Basic properties of the soils used in the study

Cation

Sodium

Organic

exchange

exchangeable

matter

capacity

percentage

content

Sand

Silt

Clay

(mmol kg-1)

(%)

(%)

(%)

(%)

(%)

62.2

3.9

0.5

17.6

66.3

16.2

Clay loam 110.6

4.2

0.8

24.9

43.4

31.8

Loam

5.4

0.9

28.3

45.4

26.3

Soil type

Silt loam

75.2

Particle size distribution

Table. 2 Analysis of variance for the runoff and total soil loss data generated in the rainfall simulator F-value Sources

DFa Soil loss

Runoff

Soil

2

22.9**

123.4**

Mulch

3

25.8**

193.4**

Slope

2

7.6**

21.4**

Rainfall intensity

1

11.6**

5.5**

Soil * Mulch

6

8.8**

11.2**

Soil * Slope

4

3.6**

1.0ns

Soil * Rainfall intensity

2

8.9**

0.5 ns

Mulch * Slope

6

1.6*

5.0**

Mulch * Rainfall intensity

3

4.7**

0.7 ns

Slope * Rainfall intensity

2

2.8*

0.2ns

Soil * Mulch * Slope

12

0.9ns

3.1 **

Soil * Mulch * Rainfall intensity

6

2.7**

0.1ns

Mulch * Slope * Rainfall intensity

4

0.5ns

1.6ns

Soil * Slope * Rainfall intensity

6

1.7*

0.9 ns

Soil * Mulch * Slope * Rainfall intensity

12

0.7ns

1.4 ns

Table.3 Total surface runoff (mm) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 90 mm h-1 Soil series

Silt loam

Clay loam

Loam

Mulch rate (kg m-2) Slope 0

0.2

0.4

0.8

Gentle

12.9 (0.44)a

10.61 (0.26)

1.01 (0.05)

1.00 (0.02)

Moderate

19.42 (0.56)

11.45 (0.36)

2.03 (0.08)

1.10 (0.05)

Steep

22.71 (0.63)

13.03 (0.52)

4.63 (0.31)

2.50 (0.05)

Gentle

40.51 (1.3)

30.57 (2.23)

8.58 (0.34)

6.93 (0.23)

Moderate

44.61 (1.12)

36.83 (2.30)

10.74 (0.76)

8.71 (0.67)

Steep

49.7 (1.39)

40.29 (2.24)

12.19 (0.95)

9.97 (0.67)

Gentle

35.09 (1.58)

25.24 (1.17)

5.82 (0.61)

3.83 (0.41)

Moderate

38.83 (1.86)

27.51 (1.28)

6.41 (0.20)

4.73 (0.82)

Steep

40.22 (1.17)

30.29 (1.36)

7.65 (0.15)

5.85 (0.92)

Table .4 Total surface runoff (mm) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 180 mm h-1 Mulch rate (kg m-2) Soil series

Slope 0

Silt loam

Clay loam

Loam

0.2

0.4

0.8

Gentle

25.15 (1.61)a

12.63 (1.10)

6.13 (0.86)

5.53 (0.38)

Moderate

27.09 (1.13)

14.93 (1.32)

8.29 (0.49)

7.80 (0.44)

Steep

29.45 (1.16 )

16.37 (1.12)

9.10 (0.58)

8.75 (0.49)

Gentle

48.74 (2.74)

40.47 (1.08)

20.24 (0.76)

9.45 (0.43)

Moderate

50.46 (2.95)

43.01 (1.41)

26.70 (0.80)

12.93 (0.59)

Steep

56.69 (3.12)

45.13(1.45)

29.64 (0.85)

14.43 (0.79 )

Gentle

40.44 (2.67)

33.53 (1.51)

8.47 (0.53)

7.91 (0.13)

Moderate

43.16 (2.74)

36.58 (1.22)

9.68 (0.59)

9.43 (0.22)

Steep

48.30 (2.86)

40.77 (1.14)

11.60 (0.71)

10.14 (0.35)

Table.5 Total soil loss (kg m-2) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 90 mm h-1 Mulch rate (kg m-2) Soil series

Slope 0

Silt loam

Clay loam

Loam

0.2

0.4

0.8

Gentle

0.15 (0.05)a

0.05 (0.00)

0.02 (0.00)

0.01 (0.00)

Moderate

0.26 (0.07)

0.08 (0.00)

0.03 (0.00)

0.03 (0.00)

Steep

0.71 (0.08)

0.12 (0.01)

0.04 (0.00)

0.04 (0.00)

Gentle

1.19 (0.13)

0.50 (0.05)

0.20 (0.00)

0.06 (0.00)

Moderate

1.64 (0.14)

0.70 (0.08)

0.30 (0.06)

0.09 (0.00)

Steep

2.43 (0.03)

0.99 (0.01)

0.44 (0.01)

0.16 (0.02)

Gentle

0.52 (0.01)

0.12 (0.00)

0.06 (0.00)

0.03 ( 0.00)

Moderate

0.64 (0.01)

0.23 (0.05)

0.07 (0.00)

0.04 (0.00)

Steep

0.88 (0.01)

0.41 (0.08)

0.09 (0.00)

0.05 (0.02)

Table . 6 Total soil loss (kg m-2) from three soils on different slopes under different mulch rates during a 60-mm rainstorm with a rainfall intensity of 180 mm h -1 Mulch rate (kg m-2) Soil series

Silt loam

Clay loam

Loam

Slope 0

0.2

0.4

0.8

Gentle

0.20 (0.50)b

0.08 (0.02)

0.04 (0.01)

0.02 (0.00)

Moderate

0.38 (0.61)

0.09 (0.001)

0.06 (0.01)

0.05 (0.00)

Steep

0.95 (0.93)

0.21 (0.010)

0.09 (0.00)

0.06 (0.00)

Gentle

2.07 (0.19)

1.50 (0.12)

1.30 (0.00)

0.08 (0.00)

Moderate

5.22 (0.20)

1.95 (0.14)

1.60 (0.05)

0.23 (0.01)

Steep

7.95 (0.40)

2.52 (0.17)

1.71 (0.09)

0.43 (0.04)

Gentle

9.70 (0.42)

0.53 (0.05)

0.08 (0.00)

0.09 (0.00)

Moderate

1.16 (0.15)

0.72 (0.06)

0.18 (0.00)

0.14 (0.01)

Steep

1.21 (0.17)

0.90 (0.10)

0.22 (0.04)

0.12 (0.02)

Captions for Figures Figure.1 Rain fall simulator Figure 2 Experimental equipment used to collect data under rainfall simulator.

Figure.3 Reductions in water loss due to mulch cover for three soil types from different slopes under two rainfall intensities (90 and 180 mm h-1 represented by filled and unfilled symbols, respectively) Figure . 4 Reductions in soil loss due to mulch cover for three soil types from different slopes under two rainfall intensities (90 and 180 mm h-1 represented by filled and unfilled symbols, respectively)

Figure.1 Rain fall simulator

Figure 2 Experimental equipment used to collect data under rainfall simulator.

Figure.3 Reductions in water loss due to mulch cover for three soil types from different slopes under two rainfall intensities (90 and 180 mm h-1 represented by filled and unfilled symbols, respectively)

Figure . 4 Reductions in soil loss due to mulch cover for three soil types from different slopes under two rainfall intensities (90 and 180 mm h-1 represented by filled and unfilled symbols, respectively)

Conflict of internist

This statement is to certify that all Authors have seen and approved the manuscript being submitted. We warrant that the article is the Authors' original work. We warrant that the article has not received prior publication and is not under consideration for publication elsewhere. On behalf of all Co-Authors, the corresponding Author shall bear full responsibility for the submission. This research has not been submitted for publication nor has it been published in whole or in part elsewhere. We attest to the fact that all Authors listed on the title page have contributed significantly to the work, have read the manuscript, attest to the validity and legitimacy of the data and its interpretation, and agree to its submission to the international soil and water conservation research Journal of . All authors agree that author list is correct in its content and order and that no modification to the author list can be made without the formal approval of the Editor-in-Chief, and all authors accept that the Editor-in-Chief's decisions over acceptance or rejection or in the event of any breach of the Principles of Ethical Publishing in the international soil and water conservation research Journal of being discovered of retraction are final. No additional authors will be added post submission, unless editors receive agreement from all authors and detailed information is supplied as to why the author list should be amended. Dr Abbas. E.Rahma