The impact of soil preparation on the soil erosion rates under laboratory conditions

Introduction Conclusions References


Introduction
Soil, as one of the valuable natural resources, is nonrenewable at short time scale and should be studied with a multidisciplinar perspective (Brevik et al., 2015).Soil erosion is a result of the interaction of several factors which vary in space and time (Cerdà, 1998;Le Bissonnias et al., 2002;García-Orenes, 2010).Study of soil erosion and sediment yield in the watershed is one of the basic necessities to achieve integrated land management and soil and water conservation.The identification and quantification of the hydrological properties and processes that induce runoff and soil erosion in necessary to determine the amount of soil erosion (Cerdà et al., 1997;Cerdà, 1999;Ramos et al., 2000;Iserloh et al., 2012;Iserloh et al., 2013;León et al., 2013;Martínez-Murillo et al., 2013).Although, the measurement of runoff and sediment using rainfall simulators can be performed in the laboratory and field conditions, field measurements are usually costly and time consuming works.In addition, different methods of measuring runoff and erosion may lead to non-identical results that are not necessarily related to specific effects on studied variables (Bryan and Ploey, 1983;Boardman et al., 1990).Nowadays, the use of laboratory methods using rainfall simulators are considered more and more, because of ability to control the intensity and duration of rainfall which leads to increase the accuracy of data (Sadeghi, 2010).On the other hand, measuring runoff and soil loss at the plot scale have been of crucial importance from the beginning of the soil erosion research (Licznar and Nearing, 2003).The limitations of laboratorial studies of soil erosion leads to lack of confidence especially when the aim of research is to study some important factors affecting erosion (Toy et al., 2002) which may because of soil disturbance in laboratory.Although various methods for soil preparation have been proposed to perform laboratory soil erosion research (Ekwue, 1991;Romkens et al., 2001;Hawke et al., 2006;Ekwue and Harrilal, 2010;Kukal and Sarkar, 2010), all these methods have one major goal that the soil samples were placed in the experimental plots as homogeneous as possible (Hawke et al., 2006).Changes in the soil during sampling, transportation and various stages of preparation include air-drying, passing through a sieve, soil moisture content during the preparation process and finally compacting to increase the bulk density of the soil surface by roller may influence the results of runoff and erosion.For example, the significant effect of soil characteristics such as small relief and aggregate shape on the amount and spatial pattern of runoff (Kirkby, 2001) and of surface roughness on runoff and erosion (Gomez and Nearing, 2005) that have been approved before, can all be created or weakened and intensified by rolling the soil surface.Tillage, as one of the most important human factors that leads to soil disturbance, is also a way to disturb the soil and will create higher erosion rates (Novara et al., 2011;Gabarrón-Galeote et al., 2013;Haregeweyn et al., 2013, Sadeghi et al., 2015) and this also occurs when the soil is disturbed by changes in crops (Zhang et al., 2015).
Nevertheless, the textural and structural changes during soil preparation for experimental studies of erosion may not be the same with those in preparation for agriculture, forestry or gardening purposes, because of many differences in method of soil preparation.
The present research has been therefore conducted to evaluate the effects of soil preparation for experimental studies on runoff and soil erosion.The results of present research can hopefully be used to generalize the results of laboratory studies of soil erosion to natural conditions more accurately.

Study area
The field experiments were conducted in a south slope with sandy-clay-loam soil located in the longitude and latitude of 36˚ 27΄ 15˝ N and 51˚46΄ 27˝ E and the altitude of 1665 m in the vicinity of Kodir village in Educational and Research Forest Watershed of Tarbiat Modares University, in the north of Iran (Fig. 1).The degree of the slope at the experiments site was about 18%.The amount of organic matter, pH and EC of the studied soil were 2.167%, 7.9 and 157.6 dS mm -1 respectively.

Installation and preparation of plots
To achieve the study purposes, 18 field 1×1 m-plots were adopted in the study slope.The top 20 cm layer of the soil (Assouline and Ben-Hur, 200;Kukal and Sarkar, 2011;Khaledi Darvishan et al., 2012) was then collected for soil preparation using Kukal and Sarkar method (2011) with some modifications to maintain aggregate structure (Khaledi Darvishan et al., 2014).The collected soil was air dried to the optimum soil moisture sontent (Fox and Bryan, 1999).All plant residues and pebbles were removed from the soil (Agassi and Bradford, 1999) and finally, the soil was passed through 8.0 mm sieve (Ekwue and Harrilal, 2010;Defersha et al., 2011;Khaledi Darvishan et al., 2014).The prepared soil was then transferred into the 9 plots with the depth of about 15 cm.Because of the effects of soil bulk density on soil resistance against rain drops and runoff (Luk, 1985;Cerdà, 2002), a PVC pipe with diameter of 10 cm and filled with a mixture of sand and cement as a roller was used to compact the soil to achieve the natural bulk density of the soil.The other 9 plots were placed on the soil in natural condition and all plant tissues above the soil surface were removed using a small secateur.The initial soil moisture content is also among the factors affecting soil hydrological responses (Chow et al., 1988) that was about 29 volumetric% and relatively the same in all 18 plots.A view of the plots in both natural and disturbed soil conditions is shown in Fig. 2.

Rainfall simulation
According to Kojour synoptic rain gauge data and IDF curves, which is the nearest station to the study slope, three rainfall intensities of 40, 60 and 80 mm h -1 were selected with a constant duration of 15 min.A portable rainfall simulator was then used to simulate rainfall events using one or two nozzles of BEX: 3/8 S24W for various rainfall intensities with a constant height of 3 m above the soil surface.The median diameter of raindrops were 1.11, 1.05 and 1.03 mm, the mean velocity of raindrops were 4.38, 4.08 and 4.03 m s -1 and the kinetic energy of simulated rainfalls were 9.59, 8.32 and 8.12 J m -2 mm -1 for three studied rainfall intensities respectively.

Measuring Runoff, Sediment Concentration and Soil Loss
During each experiment, runoff was collected in the outlet of plots and sampled in five 3-min intervals after runoff commencement time.The time of fifth sample was exactly coincide with the time the rain had stopped and then, all the remained runoff was collected as the final sixth sample.The samples were transferred to the laboratory and sediment concentration was measured using decantation procedure, oven dried at 105°C for 24 h (Walling et al., 2001;Gholami et al., 2013;Ziadat and Taimeh, 2013).

Statistical analysis
The statistical tests were performed under experimental design of spilt plots and factorial experiments with two soil conditions and three rainfall intensities.The normality test was done for all variables of runoff, sediment concentration and soil loss.The runoff and soil loss datasets were transformed to logarithmic form to achieve normality distribution, because parametric tests on normal data seems to be more powerful to detect the differences than the nonparametric tests on non-normal data (Townend, 2002).
The ANOVA tests with considering the split plots design (Bihamta and Zare Chahouki, 2011) were used to evaluate the statistical differences between studied variables in undisturbed and disturbed soil condition.

Results
The results of average runoff variables, sediment concentration and soil loss for three replicates of both undisturbed and disturbed soil treatments in three studied rainfall intensities are shown in Tables 1 to 3 respectively.

Table 1
Table 2 Table 3 The statistical analysis of the effects of rainfall intensity and soil disturbance on sediment concentration and soil loss are shown in Table 4.

Table 4
Mean temporal variation of sediment concentrations in three replications of disturbed and undisturbed soil treatments are shown in Fig. 3 and increasing ratios (%) of runoff variables, sediment concentration and soil loss after preparing soil are shown in Fig. 4.

Fig. 3
Fig. 4 4 Discussion According to Table 1, weighted mean runoff coefficient of the average values of various time intervals were varied from 6.82 to 25.70 in undisturbed and from 25.08 to 57.17 in disturbed soil condition.The results revealed that soil preparation leads to significantly (p≤0.01)increase runoff coefficient (Table 4).
According to Table 2, weighted mean sediment concentrations of the average values of various time intervals were varied from 2.7 to 7.57 in undisturbed and from 10.38 to 12.41 in disturbed soil condition.According to Tables 2 and 4, the sediment concentration was significantly (p≤0.01)increased after soil preparation for laboratory erosion plots.One of the reasons of more sediment concentration in disturbed soil is the longer time-to-runoff which leads to more splash and particle separation before the flow of surface runoff.Consequently, in the first sampling after runoff commencement time, the available source of soil particles to be transport is more and leads to increse sediment concentration.But a few minutes after runoff commencement time, the available sediment source and consequently, the sediment concentration decreases.The effects of soil disturbance during preparation for laboratory erosion plots on runoff or soil loss was in agreement with previous studies which revealed the same effects of soil disturbance for agriculture and gardening purposes (Harold et al., 1945;Choudhary et al., 1997;Layon et al., 1999;Erkossa et al., 2005;Gomez and Nearing, 2005;Ziadat and Taimeh, 2013).The results was in agreement with Cao et al., (2013) who studied and modelled the interrill erosion on unpaved roads and Villarreal et al., (2014) who studied the effects of vehicle-based soil disturbance and compaction on soil erosion potential.Soil surface disturbance and compaction because of grazing can increase soil erosion (Palacio et al., 2014).In other words, soil disturbance -for any purposes especially for laboratory erosion plots-could decrease soil resistance against raindrops because of aggregates breakdown which respectively leads to more detachment, less infiltration, more runoff and more sediment concentration.Concentrations of runoff sediment after soil preparation confirmed that erosion depended directly on the sediment available on the soil surface that was in agreement with Ceballos et al., (2002).The presence of pebbles and gravels on soil surface as well as inside soil profile has been considered as an affective factor against the kinetic energy of raindrops (Jomaa et al., 2012).The roots and other plant residues can also play a significant role to physically decrease the kinetic energy of raindrops and improve aggregates stability (Monroe and Kladivko, 1987;Ghidey and Alberts, 1997;Martens, 2002).Removing all pebbles, gravels and plant residues could also been considered as another significant reason which leads to more sediment concentration in prepared soil for laboratory studies.All these results mean that more splash in prepared soil is one the main results of increasing sediment concentration.
Soil disturbance during all preparing steps vis.Sampling, transporting, spreading to be airdried, passing through 8 mm sieve, packing into the plots and compacting again are the main reasons to damage soil structure and aggregates breakdown even without removing any parts of the soil materials.
Using a sieve with larger mesh number (8 mm) may decrease the negative effects of soil preparing (Khaleidi Darvishan et al., 2014), but a significant part of effects which is connected with sampling, transporting and especially compacting the soil remains yet.
Longer Time to runoff in disturbed soil revealed that disturbing soil, even with compacting again, can cause a temporary increase in infiltration which itself leads to longer time-to-runoff (Table 1).But the main note is that the increasing infiltration is a temporary effect of disturbing soil and after a few minutes, more detachment can decrease the infiltration rate and leads to more runoff volume in the first 3-minute sampling interval after runoff commencement time (Fig. 3).The results showed that in all three rainfall intensities, sediment concentration in both disturbed and undisturbed soil treatments reached to the peak in the first sample of runoff and then gradually decreased.This result was in agreement with many other laboratory soil erosion researches (Assouline and Ben-Hur, 2006).
The significant effect of soil disturbance on soil loss may be due to eliminated surface gravel during sieving the soil.This may be because of the ability of gravel surface to reduce total amount of available sediment (Tailong et al., 2010) and also to decrease power erosivity of surface flow (Rieke-Zap et al., 2007;Tailong et al., 2010).Rock fragments, roots and plants debris on the soil surface and within the soil profile in undisturbed soil surface could protect the aggregate against raindrops or runoff flow.In this regard, Li et al., (1991), Ghidey and Alberts (1997) and Mamo andBubenzer (2001a and2001b) showed that root system helps the soil resistance and thus reduces the amount of soil loss.
According to Table 4, the increasing effects of rainfall intensity on runoff coefficient, sediment concentration and soil loss were significant.The significant effects of rainfall intensity on various runoff, sediment and soil loss variables have been emphasized by Romkens et al., (2001), Chaplot and Le Bissonnais (2003), Assouline and Ben-Hur (2006), Ahmed et al., (2012) and Defersha and Melesse (2012) too.
The results of statistical analysis (Table 4) showed that the interaction between rainfall intensity and soil disturbance treatment on sediment concentration was not significant that may be due to the limited studied levels of rainfall intensity (40, 60 and 80 mm h -1 ).

Conclusion
It can be generally concluded that the average and peak values and variation gradient of runoff and sediment concentration increased due to soil disturbance.The increasing rates of runoff coefficient, sediment concentration and soil loss due to the study soil preparation method for laboratory soil erosion plots, were 179, 183 and 1050% (2.79, 2.83 and 11.50 times), respectively.It's highly recommended to leave the prepared soil inside the plots at least for a few weeks before rainfall simulation instead of using roller, to increase the bulk density and improve structural condition of the soil.It may decrease the negative effects of soil preparing process caused by rolling the soil surface.The soil moisture content during the process especially after packing the prepared soil inside the plots is also very important and can leads to increase the bulk density in a shorter time.

Fig. 1
Fig. 1 Location of the study area in Kojour Watershed, Mazandaran Province, Iran

Table 1
The average time-to-runoff and runoff volume for three replicates of both undisturbed and disturbed soil treatments in three studied rainfall intensities

Table 3
The average soil loss for three replicates of both undisturbed and disturbed soil treatments in three studied rainfall intensities

Table 4
Statistical analysis of the effects of soil disturbance and rainfall intensity on sediment concentration and soil loss * and ** are the significant levels of 95 and 99%, respectively.