Tillage Practices and Their Potential to Impact the Soil Carbon Dynamics

Abstract
In dry regions, it is customary for farmers to use soil water conservation and/or water harvesting techniques. These practices have now become applicable to agriculturalists combating the adverse effects of drought on food production. In the semiarid areas of Zalingei in western Sudan, we quantified the soil erosion using traditional conservation measures and conducted experiments in two consecutive rainy seasons (2013 and 2014). A split-split plot design was used to quantify the respective influences of each variable on reducing soil erosion: A) three gentle gradients (Slope1 (0.98%), Slope2 (1.81%), and Slope3 (3.1%)); B) two cropping systems (mono-crop and mixed-crops); and C) five indigenous conservation tillage practices—chisel ploughing (CHP), cross slope tied bonding (CSTB), contour ridge with stone bonds (CRSB), cross slope bonding (CSB), and zero tillage (ZT). Our results showed that there were significant differences between the slopes in season 2 (2014); the soil eroded at Slope3 was more than that of Slope1 and Slope2 by 71% and 27%, respectively. Over two seasons, there were no significant differences between the cropping systems. Conversely, the erosion level observed with CHP was higher than with the other practices. However, the CSTB and CSB erosion levels were only higher in season 2 when compared with those of CRSB and ZT. The study concluded that under the above conditions, the rate of soil erosion was severe and exceeded the erosion tolerance. Based on these results, in western Sudan, CRSB and ZT may be the more effective indigenous conservation practices for the protection of agricultural soils and productivity.

INTRODUCTION
The level of soil degradation is increasing globally and is linked to an increased risk of drought . The growing human population faces increased pressure on its food supply, which is compounded by the limited availability of agricultural lands. To address these issues, the productivity of existing arable lands must be increased via better water use efficiency and soil conservation practices. . Moreover, climate change is also predicted to increase the incidence and severity of droughts in semiarid regions
The erosion of agricultural soils has long been a global environmental concern . Also, soil erosion is considered as one of the significant environmental problems worldwide due to its effects on water quality, soil productivity, and ecosystems. Rain flood erosion in agricultural lands strips the fertile topsoil on-site in arid and semiarid regions. Soil loss is often triggered and accelerated by poor or non-existing soil conservation practices promoting the transport of sediments and potentially hazardous pollutants via runoff to rivers and streams . In rainfed areas, the on-farm management of soil and water plays a critical role in achieving the full potential of soil conservation and water use . It is important to design conservation measures and strategies that are effective in controlling such soil losses on the Earth . Reasonable cultivation patterns combined with rainwater harvesting techniques can improve the management of degraded lands, which eventually leads to the sustainability of production, greater certainty, and lower marginal inputs. One type of soil conservation practices that are commonly used are conservation tillage practices (CTPs), which aim at no-tillage, little tillage, and cover crops . Other traditional or modern agricultural operations are used in conjunction with CTPs to reduce farmland degradation and to increase land productivity . In developing countries, the land is commonly fragile and degradable with poor vegetation as a result of the influence of frequent fires, tree logging, and free grazing. On top of these challenges, the area may suffer from climatic changes , rainfall fluctuations and the rapid advancement of the Sahara Desert. As a climatic adaptation strategy, rainfed, smallholder cultivation of grain crops is the predominant agricultural system in the hilly areas, where the terrace system is an interesting practice among indigenous water harvesting systems . However, the most effective method in conserving the soil surface is not well studied and reported so far in this area. Knowledge about the reduction in water and soil losses due to the practice of the terrace system is very limited in developing areas. Whether this traditional irrigation system helps in improving soil characteristics, crop production, and biodiversity richness in the area is also insufficiently quantified. Arid and semiarid areas account for about 60% of the national land in Sudan and account for approximately 8% of the world’s semiarid tropical regions . They are usually characterized by an unprotected soil surface, especially in clay areas, through which high-intensity rainfall cannot be infiltrated and thereby discharged dominantly by the surface runoff process . From these perspectives, this study aims to:
(A) evaluate the effect of the cultivation practices on the soil erosion;
(B) understand whether the cropping system affects water erosion, and
(C) recommend suitable field operations.
2. Method and Materials
2.1. Study Site
The field experiments were carried out during two consecutive seasons (2013 and 2014) at the experimental field of the University of Zalingei, western Sudan (between latitudes 12◦300–13◦300 N and 23◦300–23◦450 E) in the semiarid zone of North Africa. This is a mountainous area, where there are different types of waterways due to the land undulation, and large water streams (valley/wadi), streaming through from the Jebel Marra massif (approximately 100 km east of Zalingei) carrying fertile alluvial and volcanic soils [21]. The agriculture consists exclusively of small-scale family farms. Miehe [17] classified the soils of the area as Andosols with a bulk density of 1.32 g/cm3 . The climate is hot and rainy in the summer and harsh and dry during the winter. The average annual rainfall in Zalingei is approximately 600 mm/yr.; however, approximately two-thirds of it falls during July and August and about 90% from June to September, and there is very little or no rain from October to March [22,23]. The major crops for food are mostly grown during the rainy seasons (June to October) and include sorghum (Sorghum bicolor), millet (Pennisetum glaucum), and groundnuts (Arachis hypogaea), with vegetables mostly cultivated alongside the wadis’ lowland and flood plains during the winter. Water 2020, 12, 1554 3 of 11 Water 2020, 12, x FOR PEER REVIEW 3 of 12 Figure 1. The study area, located in the semiarid zone of North Africa. In the first rainy season (May–September 2013), the accumulation of rainfall was 542 mm, and it was 675 mm in the second season (June–October 2014); thus, the rainfall in the second season was 24.5% greater than in the first (Figure 2). The rain distributions were also varied, particularly during July and August of the second season, as a higher amount of rain precipitated, compared to that of the same period in the first season. To be exact, it was higher by 116.2% and 46% in July and August, respectively. Figure 2. The distribution of the rainwater per month (mm/m) and the accumulation of rainfall in each of the growing seasons, Season 1 (2013) and Season 2 (2014).
2.2. Experimental Design:
In three-factor field trials, split-split plots, each with 3 replicates, were used to quantify the influence of the following factors on soil erosion control: slope, cropping systems, and indigenous Water 2020, 12, 1554 4 of 11 conservation tillage practices. Thus, the field trial layout consisted of 90 plots, i.e., 3 whole plots × 2 subplots × 5 sub-subplots × 3 replicates. Three whole plots (each 35 m × 10 m), each with a different slope (S): S1 = 0.98%, S2 = 1.81%, and S3 = 3.1% (gradient measured by manual clinometer), were subdivided into 3 subplots (each 11 m × 10 m), each with a different cropping system (CS): mono-crop; sorghum and mixed crop; sorghum + groundnut. Each cropping type subplot was further subdivided into 5 sub-subplots (each 2 m × 5 m), each cultivated with a different conservation tillage practice (CTP): chisel ploughing (CHP), cross slope tied bonding (CSTB), contour ridge with stone bonds (CRSB), cross slope bonding (CSB), and zero tillage (ZT) (Figures 3A and 4). Water 2020, 12, x FOR PEER REVIEW 4 of 12 conservation tillage practices. Thus, the field trial layout consisted of 90 plots, i.e., 3 whole plots × 2 subplots × 5 sub-subplots × 3 replicates. Three whole plots (each 35 m × 10 m), each with a different slope (S): S1 = 0.98%, S2 = 1.81%, and S3 = 3.1% (gradient measured by manual clinometer), were subdivided into 3 subplots (each 11 m × 10 m), each with a different cropping system (CS): monocrop; sorghum and mixed crop; sorghum + groundnut. Each cropping type subplot was further subdivided into 5 sub-subplots (each 2 m × 5 m), each cultivated with a different conservation tillage practice (CTP): chisel ploughing (CHP), cross slope tied bonding (CSTB), contour ridge with stone bonds (CRSB), cross slope bonding (CSB), and zero tillage (ZT) (Figures 3A and 4). Figure 3. View of the prepared land of the experiment: (A) conservation tillage techniques; (B) collectors of soil particle deposits. Figure 4. The field layout of the simple methodology to determine the effect of conservation tillage on soil erosion. Chisel ploughing, CHP; cross slope tied bonding, CSTB; contour ridge with stone bonds, CRSB; cross slope bonding, CSB; and zero tillage, ZT. Arrows are in the flow direction.
2.3. Measurement of Soil Losses:
A small ditch (40 cm wide and 20 cm deep) was prepared, covered with a plastic sheet at the bottom end of each plot to receive the deposit of soil eroded, and transported by rain force out of the plot (Figures 3B and 4). After at least one day of rain events, the soil deposit was carefully taken from the field to the lab, where the samples were dried in an oven at 70 ◦C until a constant weight was obtained. At the rainy season’s end, the samples were weighed and the eroded soil mass per plot area was estimated.
2.4. Data Analysis
The procedure in Gomez and Gomez [24] was used for the analysis of variance (ANOVA) and all statistical analyses were performed using the SAS 9.4 software package (SAS Institute Inc., Cary, NC, USA). A least significant difference (LSD) of 5% probability was used in comparing the means of soil erosion for different treatment: (S), (CS), (CTP) (S × CS), (S × CTP), (CS × CTP) and (S × CS × CTP).
3. Results
3.1. Soil Erosion
There were significant differences in the mass of eroded soils among treatments with different land slopes in the second season (Figure 5A). However, the slope levels did not significantly affect the soil erosion in the first season. The cropping system factor also did not exhibit any significant variances among the treatment means over the two consecutive seasons (Figure 5B). The results showed differences as a result of the interaction between the land slope and the cropping system only in the first season (Figure 6A). The conservation tillage also significantly (p ≤ 5%) affected the soil erosion. There were no significant differences among treatments due to factor interactions over the two seasons: slope (S) × cropping system (CS) × conservation tillage practices (CTPs).
3.2. Effect of the Land Slopes and Cropping System
The slope factor clearly affected the magnitude of soil erosion, as we expected (Figure 5A). Only in the second season, S3 (3.1%) had significantly more eroded soil than S2 (1.81%); moreover, the masses of the eroded soil at the slopes were in the following order: S3 > S2 (by 27%), S2 > S1 (by 35%), and S3 > S1 (by 71%). However, there were no significant differences between S1 (0.98%) and S2 or S3 in the first season. The first season was unlike the second; the slope factor did not show any significant differences. The results did not show any significant differences between the mono- and mixed-crop systems (Figure 5B). Therefore, the two systems had no effects on soil erosion.
3.3. Effect of the Conservation Tillage Practices
The results in Figure 5C showed that the conservation tillage techniques in both seasons affected the soil water erosion significantly: in the first season, the CHP treatment differed significantly from CSTB and CRSB/CSB/ZT, whereas CRSB, CSB, and ZT were not markedly dissimilar from each other. In season 2, the soil loss measured in CRSB and ZT was significantly smaller than in the other three techniques. At the same time, the soil loss measured in CHP did not significantly differ from that in ZT; moreover, CSTB did not significantly differ from CSB. However, CHP and Z were not different from CRSB, but smaller than CSTB and CSB.

4. Discussion
Soil erosion occurs due to soil particle detachment, transportation, and sedimentation by erosion, such as water or wind. The problems caused by soil erosion can be both onsite and offsite, including nutrient lessening and limiting of crop production. As the topography affects these processes, the practices of CTP and CS also affect the soil erosion and crop production as a result. Not only is the quantity of annual rainfall essential to removing the soil particles from the original place, but the number of events, rain intensity in each event, and raindrop characteristics are also crucial factors. The mass of heavy rain with larger drops is more powerful for detaching and transporting the soil.


5. Conclusions
Our experiment provides a clear picture of how the conservation tillage practices (CTPs) affect the soil erosion of three slopes (S), and two cropping systems (CSs) in the hilly semiarid areas of the study area. According to the results of the experiment, we concluded that the conservation tillage practices (CTPs) strongly affected the soil erosion, such as the contour ridge with stone bonds and zero tillage. We found that the cropping systems (CSs) did not have significant effects; therefore, farmers need to apply more practices (i.e., CRSB) beside the CSs to conserve their land from soil erosion. We confirmed that the steeper the land, the more erodible the soil surface. The soil erosion due to cultivation practices in the study area is severe; therefore, the cultivation of cropland requires extensive 5. Conclusions Our experiment provides a clear picture of how the conservation tillage practices (CTPs) affect the soil erosion of three slopes (S), and two cropping systems (CSs) in the hilly semiarid areas of the study area. According to the results of the experiment, we concluded that the conservation tillage practices (CTPs) strongly affected the soil erosion, such as the contour ridge with stone bonds and zero tillage. We found that the cropping systems (CSs) did not have significant effects; therefore, farmers need to apply more practices (i.e., CRSB) beside the CSs to conserve their land from soil erosion. We confirmed that the steeper the land, the more erodible the soil surface. The soil erosion due to cultivation practices in the study area is severe; therefore, the cultivation of cropland requires extensive.