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Article

Soil Aggregation and Associated Organic Carbon and Total Nitrogen in a Sandy Loam Soil under Long-Term Tillage Effects

by
Mouna Mechri
1,2,*,
Khedija Bouajila
2,3,
Noura Ziadi
4,*,
Taqi Raza
5,
Hayet Beltayef
6,
Neal S. Eash
5,
Naceur Jedidi
7,
Azaiez Gharbi
6,
Abeer Hashem
8 and
Elsayed Fathi Abd_Allah
9
1
National Institute of Field Crops, Boussalem 8170, Tunisia
2
National Agronomic Institute of Tunisia, Tunis 1082, Tunisia
3
Soil Management, Regional Commissary for Agricultural Development of Beja-Northern Tunisia, Ministry of Agriculture, Hydraulic Resources and Fisheries, Beja 9000, Tunisia
4
Agriculture and Agri-Food Canada, Quebec Research and Development Centre, 2560 Hochelaga Blvd., Quebec, QC G1V 2J3, Canada
5
The Department of Biosystems Engineering & Soil Science, University of Tennessee, Knoxville, TN 37996, USA
6
Research Laboratory: Support for Sustainable Agricultural Productivity in the North West Region, El Kef Higher School of Agriculture, El Kef 7119, Tunisia
7
Water Research and Technology Center, University of Carthage, Soliman 8020, Tunisia
8
Botany and Microbiology Department, College of Science, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia
9
Plant Production Department, College of Food and Agricultural Sciences, King Saud University, P.O. Box 2460, Riyadh 11451, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Agronomy 2023, 13(10), 2520; https://doi.org/10.3390/agronomy13102520
Submission received: 2 August 2023 / Revised: 31 August 2023 / Accepted: 4 September 2023 / Published: 29 September 2023
(This article belongs to the Section Soil and Plant Nutrition)
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Abstract

:
In Tunisia, climate change impacts that lead to the degradation of soil resources are considered to be a major limiting factor on socio-economic development. These impacts are exacerbated by the intensive plowing and cultivation practices used by Tunisian farmers, which expedite the depletion of soil organic matter (SOM), leading to changes in the physio-chemical properties of soil and consequently promoting soil erosion. In fact, the decrease in soil organic carbon (SOC) stocks affects soil’s fertility and the ability to regulate climate change. The objective of this study, which was conducted in Le Krib in the Siliana region of northwestern Tunisia, was to evaluate the effects of two cropping systems, consisting of durum wheat (Triticum aestivum) and oats (Avena sativa), and two types of tillage, no-till (NT) and mouldboard plowing (MP), on different soil aggregate classes (>2000 µm, 2000–250 µm, 250–180 µm, 180–53 µm and <53 µm) and soil physio-chemical properties, as well as the resulting effects on the carbon and nitrogen concentrations in these aggregates. The results showed that the carbon content of all soil aggregate classes was influenced by interactions between the previous crop and tillage regime. The clay-silt fraction had higher carbon concentrations under no-till and mouldboard plowing management. Furthermore, the previous crop and tillage type and their interactions had significant effects on nitrogen concentrations in micro-aggregates. The highest nitrogen concentrations (2846.6 ppm) were found in micro-aggregates in soils where the previous crop was durum wheat and mouldboard plowing was used, while the lowest concentrations (1297 ppm) were obtained in soils where the previous crop was oats and mouldboard plowing was used.

1. Introduction

In the context of sustainable agriculture, farmers aim to modify their practices in order to reduce chemical input and tillage costs, preserve soil biodiversity, promote soil carbon sequestration, and lower greenhouse gas emissions [1]. One of the major challenges faced by the current agricultural system is reconciling the aims of sustainable agriculture with the need to maintain economic viability by ensuring competitive yields and crop quality [2]. As a result, organic matter has been assigned a more important role in maintaining soil quality. However, the intensification of cropping systems has led to a decrease in the organic matter content of cultivated soils, thus reducing soil fertility and increasing soils’ vulnerability to degradation [3]. In such situations, adverse climatic conditions and heavy tillage such as frequent plowing often lead to a decrease in soil organic matter (SOM), soil permeability, water retention, and soil structural stability by transforming macro-aggregates into micro-aggregates [4,5]. This process results in reduced microporosity and bulk density and increased compaction and overall nutrient imbalance [5,6]. Given that soil organic carbon (SOC) is a key indicator of soil health, changes in SOC content lead to modified soil aggregation processes [7,8]. SOC has also been reported to influence soil aggregation because of its binding function [9], with soil aggregation in turn affecting SOC sequestration. Therefore, SOC enhancement is a strategy that can be used to mitigate the adverse effects of climate change on soil function [10,11]. For these reasons, soil aggregation and SOC sequestration have gained attention among soil and climate scientists [12]. Stevenson [13] noted that SOM is an important indicator of soil quality due to its essential role in the physical protection of the soil surface, as well as in water retention and structural stabilization [14]. Belmekki1 [15] and Mechri et al. [16] reported that, globally, this inherent property of soil is influenced by agricultural production practices.
In Tunisia, the use of intense cropping and monoculture systems has led to a reduction in the organic matter content of cultivated soils, and thus in soil fertility and productivity [17,18]. No-till direct seeding and crop residue management practices [19,20,21,22] have been recently introduced in the country to maintain soil health. After a few years of implementation, these practices have been reported to result in increased SOC reserves [20,21,22,23]. Several research studies have shown that water-stable aggregate fractionation and fractionation density can be used to understand the impact of soil management practices on carbon dynamics [24,25], since aggregates are more sensitive to changes in soil management [26,27,28]. For example, Puget et al. [26] reported that macro-aggregates form due to enrichment with fresh organic matter and are recycled faster than micro-aggregates.
A number of researchers have been studying the impact of different cropping practices on carbon cycling and carbon sequestration under different climate regions. Studies have shown that the rate of transformation of organic matter from a free light fraction to an occluded light fraction and a heavy fraction is linked to soil aggregate turnover [29]. However, fewer studies have been conducted in arable climates to evaluate the interaction between different tillage practices and the resulting aggregate classes under different cropping systems. This aspect needs particular attention in arable climate regions like Tunisia.
The objective of this study is to evaluate the effects of previous cropping systems (durum wheat and oat) and two types of tillage regimes (no-till direct seeding [NT] and mouldboard plowing [MP]) on carbon and nitrogen concentrations in various soil aggregate classes.

2. Materials and Methods

2.1. Description of Experimental Site

The study was conducted at an 8-ha experimental site in a farm field (36.3862979 N, 9.1856103 E) in Le Krib in the Siliana region of northwestern Tunisia. The site was established in 1999 on sandy-loam soil; the slope is <2%, with little elevation or alluvial contribution. Initially, the soil had a SOC content of 0.7% and a pH of 7.8 in the subsurface layer (0–40 cm). During the cropping season 2003–2004 at the end of 4 years of no-tillage and mouldboard plowing (MP) systems the average of organic carbon content was equal to 0.9% and 1% in the subsurface layer (0–20 cm), for mouldboard plowing (MP) and no-tillage (NT) systems respectively [30]. The site was divided into eight 1-ha plots, where two different tillage systems (NT and MP) and four different rotation systems (wheat-fava bean, fava bean-wheat, oats-fava bean, and fava bean-oats) were used, respectively. For the objectives of this study, we will focus on the fava bean-oats and fava bean-wheat rotation systems under NT and MP (Figure 1).
The region is characterized by a warm and dry summer temperate Mediterranean climate according to the Köppen–Geiger classification [31]. The annual average temperature at Le Krib is 18.4 °C and the average annual precipitation is 454.8 mm. During the experimental years (2000–2009), the recorded annual average precipitation is 281.9–610.0 mm. While the highest average precipitation on the research site was recorded 929 mm between September 2008 and May 2009 given in Table 1.
Soil Sampling:
Soil samples from plots with different combinations of rotation systems (fava bean-oat and fava bean-wheat) and tillage regimes (no-till and mouldboard plowing) were collected at harvest in May 2013 at a depth of 0–20 cm using the methodology developed at the Soil Science Lab of the Kef Higher School of Agriculture (ESAK) in Tunisia. Each treatment was divided into three parts (1/3 ha), and 10 samples were taken from each part according to the “a” or “z” design, as indicated in Figure 2. Composite samples were obtained to characterize the whole soil; the remaining part was kept undisturbed in the laboratory in order to perform the physical soil analyses.

2.2. Chemical and Physical Analysis

(a)
Chemical analyses
The pH was measured using a pH meter [32] in a suspension of soil in water (20 g of soil in 50 mL of distilled water, after stirring for 2 h, followed by a rest state of 30 min); total carbon, by using the Walkly and Black method [33]; and total nitrogen in the whole-soil samples, by dry combustion using an Elementar CN analyzer (Elementar Analysensysteme GmbH, Hanua, Germany). The total nitrogen in the aggregates was determined by using the Kjeldahl method [34,35,36]. The C:N ratio was calculated according to the method used by Ma et al. [37].
The pH, total carbon, and total nitrogen analyses of the whole-soil samples were performed at Agriculture and Agri-Food Canada’s Quebec Research and Development Centre, Canada. The samples were air-dried, ground, and passed through 2 mm mesh. The analysis of the carbon and nitrogen content in the aggregates was performed at the soil laboratory of the Le Kef Regional Commissary for Agricultural Development.
(b)
Physical analysis
The hydrometer method was used for particle size analysis [38]; the distribution of aggregate classes was determined using the method described in Messiga et al. [39]. Wet-sieving was employed to obtain three water-stable aggregate-size classes and clay-silt sized particles [40]. Briefly, sieves (2000 μm, 250 μm, 180 μm, and 53 μm) were stacked together and immersed in a 5-L beaker containing deionized water. Then, 50 g of air-dried soil was capillary-wetted to field capacity to limit slaking following immersion. The wetted soil was placed on top of the first sieve and the whole apparatus was shaken up and down at 100 rpm for 10 min. The material retained on the sieves—<2000 μm, <250 μm (macro-aggregates), <180 μm (meso-aggregates), and <53 μm (micro-aggregates) was collected in iron cups. The water used for wet-sieving was left to rest without a flocculating agent, and 48 h after decantation, the supernatant was poured off and the clay-silt-sized particles were collected. The method developed by Meijboom et al. [40] was preferred because one sample can be processed at the same time using a small amount of water in a beaker to achieve aggregate separation. In contrast, the method developed by Cambardella and Elliott [41] is implemented with multiple samples simultaneously and requires large buckets containing more water. The macro-, meso-, and micro-aggregates and the clay-silt-sized particles were oven-dried at 60 °C for 48 h, weighed, and stored. Each fraction was analyzed for the organic carbon and total nitrogen content. The material collected on the >2000-μm sieve consisted mainly of coarse particles and non-decomposed fragments of plant residue and was discarded.

2.3. Statistical Data Analysis

Data collected for aggregate, carbon and nitrogen associated with aggregates were statistically analyzed using SAS Institute, 2001 (Analytical Software, SAS Institute, Cary, NC, USA, 2001; [42]). Two way analysis of variance (ANOVA) using multi factorial design: tillage (NT and MP), previous crop (durum wheat and oats); tillage, previous crop, and tillage + previous crop as fixed effects. Each treatment was replicated three time three replicates and Student-Newman-Keuls test (p < 0.05) was applied to compare the treatment means.

3. Results

The physiochemical properties of the experimental soil after 13 years of long-term tillage effects are shown in Table 2. The detail of sample preparation to analysis of physiochemical properties oof experimental sites is discussed in Section 2, Materials and Methods.
The effects of two cropping systems, consisting of durum wheat (Triticum aestivum) and oats (Avena sativa), and two types of tillage, no-till (NT) and mouldboard plowing (MP), were evaluated on different soil aggregate classes, on soil physio-chemical properties, and on the carbon and nitrogen concentrations in these aggregates. After 13 years, our results show that the soil in plots with oats as the previous crop was slightly more alkaline (pH > 7.0) than those with durum wheat. The total organic carbon concentration (TOC%) was higher in soils in NT plots than those in MP plots: 1.22% and 1.16% in soils in plots with previous crops of durum wheat and oats, respectively. Moreover, this value was 0.96% and 0.91%, respectively, in soils managed under an MP regime with previous crops of durum wheat and oats. The total soil nitrogen (TN %) concentration was higher in soils in NT plots than those in MP plots: 0.095% and 0.088% in soils in plots with previous crops of durum wheat and oats, respectively. On the other hand, the total soil nitrogen (TN%) concentration was only 0.070% in soils managed under a MP regime with durum wheat as the previous crop, slightly higher than the value in soils managed under the same regime with oats as the previous crop (0.0593%). C/N ratios of 12.89 and 13.18, respectively, were obtained in soils managed under a NT regime with durum wheat and oats as the previous crops. However, C/N ratios of 13.18 and 15.34, respectively, were measured in soils managed under a MP regime with previous crops of durum wheat and oats.

3.1. Distribution of the Different Classes of Aggregates

Five different classes of aggregates were obtained. An ANOVA was performed to determine the different aggregate classes. The results are shown in Table 3 and Table 4.
In the same column, the values with the same letter do not differ significantly according to the SNK test (p = 0.05). PC: previous crop; durum wheat (DW), oats (O); tillage method (TM); no-tillage (NT), moldboard plowing (MP).
According to the statistical analyses, the distribution of particles larger than 2000 µm is influenced by the tillage method (p < 0.0001) and the previous crop (p = 0.0003) (Table 3). Particles of this size made up 7.70% of the soil in plots with durum wheat as the previous crop but only 6.13% of the soil in plots with oats as the previous crop. In terms of tillage, these percentages were 8.52% and 5.32%, respectively, for the NT and MP plots (Table 4). The >2000-µm fraction was composed mainly of coarse mineral particles, fragments, and undecomposed crop residues.
The percentage of macro-aggregates (2000–250 µm) was not influenced by the previous crop but instead by the tillage regime (p < 0.0001) and its interactions (p = 0.0012) with the crop (Table 3); indeed, the highest percentage of macro-aggregates was recorded in soils from NT plots with wheat as the previous crop (54.6%), while soils managed under a MP regime with the same crop had the lowest values (35.6%).
The highest percentage of meso-aggregates (250–180 µm) was obtained in plots with a previous crop of oats, with no difference by tillage regime (17.5%) (Table 4), while the lowest percentage was recorded in soils managed under a MP regime with wheat as the previous crop (13.8%) (Figure 3).
Micro-aggregate (180–53 µm) content was influenced by the tillage regime and previous crop grown (p = 0.0280) (Table 3), and represented 35.9% of the contents of soils from MP plots previously planted in wheat, but only 17.9% of soils from NT plots, regardless of the crop (Figure 3).
Finally, the percentage of clay and silt particles (<53 µm) was influenced solely by the tillage regime (p < 0.0001) (Table 3), with soils from NT plots containing 4.38% of this aggregate class, compared to 7.18% for soils from MP plots (Figure 3). NT plots were associated with the highest values of aggregates larger than 250 µm and the lowest values of aggregates smaller than 180 µm.
According to Pearson’s correlation coefficient, we observed a significant positive correlation between SOC and the percentage of macro-aggregates (0.84); however, the correlations between SOC and the percentage of micro-aggregates and the fraction (clay + silt) were negative and significant (−0.88 and −0.90, respectively).

3.2. Carbon Associated with Aggregates

The carbon content of all classes of aggregates is influenced by the interaction between the previous crop and the tillage method (Table 5). For example, for macro-aggregates (2000–250 µm), the statistical analysis showed that the carbon content was influenced by the interaction between the tillage regime and the previous crop (p < 0.0001), with macro-aggregates from NT plots previously planted in wheat containing 1.38% carbon, while those from MP plots previously planted in oats contained 0.57% (Figure 4).
Concerning meso-aggregates (250–180 µm), the statistical analyses showed that those from NT plots with a previous crop of wheat had twice the carbon content of those from MP plots (Figure 4). In micro-aggregates (180–53 µm), the previous crop had no effect on the soil carbon percentage, while the tillage method and its interaction with the crop had a significant effect. Micro-aggregates in soils from NT plots with oats as the previous crop had the highest carbon content (1.23%), while those from MP plots had the lowest carbon content (0.64%). Regardless of the tillage method and the previous crop, macro-aggregates had a higher carbon content (0.93%) than meso-aggregates (0.87%) (Figure 4).
The carbon content of particles smaller than 53 µm (clay + silt) is determined by the previous crop and tillage regime: Particles smaller than 53 µm (clay + silt) in soils from NT plots with oats as the previous crop had the highest carbon content (1.81%), while those from MP plots with oats as the previous crop had the lowest (1.07%) (Figure 4).

3.3. Total Nitrogen Associated with Aggregates

Statistical analyses of the aggregate classes showed that, in macro-aggregates (2000–250 µm), the nitrogen content is influenced solely by the tillage regime (p = 0.0039) (Table 5). Macro-aggregates in soils from NT plots contained 64.92 ppm of nitrogen, while those from MP plots contained 74.75 ppm of nitrogen (Table 6).
Regarding meso-aggregates, the statistical analyses showed that their nitrogen content was higher (4490 ppm) in MP plots with wheat as the previous crop than in MP plots with oats (1378 ppm) (Figure 5). The highest levels of total nitrogen were found mainly in meso- and micro-aggregates from MP plots.
In the case of micro-aggregates, those in soils from MP plots with a previous crop of wheat had the highest nitrogen content (2846.6 ppm), while those from NT plots with an oats crop had the lowest (1297 ppm) (Figure 5). In the case of particles smaller than 53 µm (clay + silt), the nitrogen content was highest in soils from NT plots previously planted with oats (304.6 ppm) and was lowest in soils from MP plots planted with the same crop (118.6 ppm) (Figure 5). This indicates that the nitrogen content of soils is significantly influenced by previous crops and tillage methods and their interactions, as well as the size of soil particles (p = 0.0044).

4. Discussion

The results presented in Figure 3 show that increasing the tillage intensity decreases the mass of macro-aggregates. In general, soils in plots under a no-till direct seeding regime have a higher macro-aggregate mass (and lower micro-aggregate mass) than soils in plots with a conventional regime (mouldboard plowing), indicating that no-till direct seeding practices are the most effective in improving soil aggregation. The study results are in line with a number of other studies conducted in other regions [39,43,44]. Six et al. [45] reported that, in zero tillage systems (direct seeding), large macro-aggregates contributed from 54% to 63% of total macro-aggregates and that aggregate turnover was reduced, possibly due to less disturbance of the top layers. Belmekki et al. [15] found that soil quality (aggregate stability and organic matter) improved under no-tillage conditions. Dou and Hons [46] reported that, in other cropping systems, the percentage of water-stable macro-aggregates (>250 µm) was higher with direct seeding than conventional sowing methods. Jiao et al. [47] noted that direct seeding combined with manure spreading increased soil aggregation and carbon concentrations. According to Loss et al. [48], soil aggregation is a good indicator of soil physical quality, particularly macro-aggregate mass (>2 mm).
The higher percentage of macro-aggregates found under no-till regimes has been attributed to a reduction in macro-aggregate turnover under this type of regime [45]. Macro-aggregate mass in the soil’s surface layer is probably increased by the layer’s high biomass content and root activity [49]. Zotarelli et al. [50] found that cereal root systems positively influenced macro-aggregate formation, and a similar effect was observed in our experiment. Conversely, soil under MP tillage had the highest effective micro-aggregate content. These results are explained by the fact that MP tillage methods affect the largest aggregates first, which results in a greater mass of smaller aggregates, results that are similar to those of Kumari et al. [51].
Messiga et al. [39] also observed that, although the carbon content of macro-aggregates under a NT regime was higher than that in soil where conventional seeding methods were used, the same effect was not found for meso-aggregates. Our results show that, in general, macro-aggregates have a greater carbon content than meso- and micro-aggregates. The carbon content in the particle fraction (clay + silt) was higher in soil under the NT regime than in soil under the MP regime. Dou and Hons [46], who studied different cropping systems, including a wheat monoculture and a wheat-soybean-sorghum rotation, also observed higher carbon concentrations in the particle fraction (clay + silt) of topsoil under NT than MP. These results could be explained by the greater physical protection of organic carbon in this fraction under NT than under MP [46,52]. Contrary to our results, Cambardella and Elliott [41] did not observe any differences in carbon content in this fraction (clay + silt) under the two tillage methods. However, these results could be explained by the different cultivation systems and intensity in the two studies. The sequence order for our study was wheat-bean and oat-bean instead of a fallow-wheat system; however, the sampling depth was the same: 0–20 cm. Zotarelli et al. [53] reported no differences in the carbon content of the different aggregate classes; this could be attributed to the clay mineralogy of their study soil, which was dominated by 1:1 clays and is a type of soil that does not follow the hierarchy of aggregates [9]. The same authors also found that, 14 years after a direct seeding technique was adopted, the carbon concentrations in the different classes of aggregates had improved to the point that they were comparable to the concentrations in the macro-aggregates in the native soils.
The distribution of carbon among water-stable aggregate classes confirms the hierarchy concept formulated by Tisdall and Oades [9]. Furthermore, the higher carbon concentrations found in the clay-silt fraction under NT compared to MP are in line with the results obtained by Angers [54], who suggests that the particles in the clay-silt fraction of silty-clay soils are saturated with carbon and make up the components of macro-aggregates, uniting to form micro-, meso-, and macro-aggregates.
Zotarelli et al. [53] found that the total nitrogen content in the macro-aggregates in the upper layer (0–5 cm) of soil was higher in NT plots, but the reverse was true at a depth of 5–20 cm, where these values were higher in MP plots. These results are in agreement with our results for the 0–20 cm layer. The same trend was observed in our study for all the other classes of aggregates, with aggregates from MP plots containing the highest total nitrogen content, with the exception of the clay-silt fraction. As tillage is involved in manipulating the soil, it changes the dynamics of soil nutrients and soil structure.
Our study results showed that tillage methods have significantly influenced the aggregate stability, which is the most important property of the soil in achieving sustainable crop production and maintaining the soil health. It was recorded that no tillage practice with legume has increased the concentration of the organic matter, which is very important for soil health and fertility. Good soil health and fertility level is an indicator of good soil microbiology like fungi community, which improve the soil aggregation by improving the soil physiochemical properties. Our study results provide insights into sustainable crop production practices that improve the soil nutrients status, maintain the soil structure, and conserve the soil moisture level which are necessary to maintain the soil healthy and productive for long term. On other hand, tillage practice increases the production cost and poses negative impacts on the environment. The implementation of no tillage is a best strategy to protect the soil organic matter. By reducing the soil disturbing activities, microbial community [55] organic matter and other nutrients can be preserved which contribute to soil resilience enhancement. Such practices (e.g., no tillage, cover crops) not only protect soil structure but also support to achieve long term agricultural system sustainability.

5. Conclusions

After 13 years of the study, the no-till (direct seeding) regime had significantly altered the soil properties. It improved soil aggregation and structure, leading to an increase in carbon concentrations as a result of carbon sequestration. A greater percentage of macro-aggregates was found in plots with a no-till regime rather than mouldboard plowing. The highest organic carbon concentration was associated with the clay-silt fraction. The results show that the minimal disturbance of the soil under a no-till regime, in combination with a cereal-legume rotation, allows the soil to function as a CO2 sink (sequestration) instead of a CO2 source (emission). These tillage practices influence the environment and can significantly influence the climate if such tillage practices continue by farmers. Therefore, using no-till practices to protect SOM in soil aggregates is a key component in soil resilience.

Author Contributions

Data curation, M.M.; Funding acquisition, M.M. and E.F.A.; Investigation, M.M.; Methodology, M.M., N.Z. and A.G.; Supervision, N.Z., N.J. and A.G.; Validation, K.B. and N.J.; Visualization, T.R. and H.B.; Writing—original draft, M.M., K.B., T.R. and H.B.; Writing—review and editing, K.B., N.Z., N.S.E., N.J., A.H. and E.F.A. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to extend their sincere appreciation to The Tunisian Ministry of Higher Education and Scientific Research for the grant awarded to carry out the internship at the Agriculture and Agri-Food Canada, Quebec Research and Development Centre. The authors would like to extend their sincere appreciation to the Researchers Supporting Project Number (RSP2023R134), King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to extend their sincere appreciation to Adnen Abdrabba, the owner of the experimental site on a farm field. The authors would like to extend their sincere appreciation to the Researchers Supporting Project Number (RSP2023R134), King Saud University, Riyadh, Saudi Arabia.

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 13 02520 g001
Figure 1. The experimental design of the site Krib Siliana region; durum wheat (DW), oats (O); no-tillage (NT), moldboard plowing (MP). (Photo obtained from google earth on the date of 29 May 2023).
Figure 1. The experimental design of the site Krib Siliana region; durum wheat (DW), oats (O); no-tillage (NT), moldboard plowing (MP). (Photo obtained from google earth on the date of 29 May 2023).
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Figure 2. Soil sampling area and design.
Figure 2. Soil sampling area and design.
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Figure 3. Distribution of water-stable aggregates and (silt + clay)-sized particles in the 0–20 cm layer of tillage (no-tillage (NT), moldboard plowing (MP)) and the previous crop (durum wheat (DW), oats (O) after wet-sieving. [Error bars represent ± one standard deviation of the mean].
Figure 3. Distribution of water-stable aggregates and (silt + clay)-sized particles in the 0–20 cm layer of tillage (no-tillage (NT), moldboard plowing (MP)) and the previous crop (durum wheat (DW), oats (O) after wet-sieving. [Error bars represent ± one standard deviation of the mean].
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Figure 4. Effect of tillage methods (no-tillage (NT), moldboard plowing (MP)) and the previous crop (durum wheat (DW), oats (O)) on the carbon content associated with aggregates.
Figure 4. Effect of tillage methods (no-tillage (NT), moldboard plowing (MP)) and the previous crop (durum wheat (DW), oats (O)) on the carbon content associated with aggregates.
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Figure 5. Effect of tillage methods (no-tillage (NT), moldboard plowing (MP)) and the previous crop (durum wheat (DW), oats (O)) on the content of total nitrogen associated with aggregates.
Figure 5. Effect of tillage methods (no-tillage (NT), moldboard plowing (MP)) and the previous crop (durum wheat (DW), oats (O)) on the content of total nitrogen associated with aggregates.
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Table 1. Annual rainfall distribution (mm) in the Krib-Siliana area.
Table 1. Annual rainfall distribution (mm) in the Krib-Siliana area.
Cropping Season2000/20012001/20022002/20032003/20042004/20052005/20062006/20072007/20082008/2009
Precipitation (mm)388.7281.9585.0610.0468.6435.6461.6490.0929
Table 2. Physiochemical properties of soil, collected from Krib, Tunisia.
Table 2. Physiochemical properties of soil, collected from Krib, Tunisia.
ParametersDurum Wheat/NTDurum Wheat/MPOat/NTOat/MP
Sand (%) 78.45
Silt (%) 11.1
Clay (%) 10.95
Textural class Sandy-Loam
soil pH7.04 ± 0.047.23 ± 0.097.51 ± 0.067.68 ± 0.16
Organic carbon (%)1.22 ± 0.070.96 ± 0.071.16 ± 0.070.91 ± 0.05
C/N ratio12.89 ± 1.5113.71 ± 1.7213.18 ± 0.8815.34 ± 0.15
Total nitrogen (%)0.0946 ± 0.0040.070 ± 0.0020.088 ± 0.002 0.0593 ± 0.002
N-NO3 (mg kg−1)3.66 ± 0.231.61 ± 0.203.86 ±0.091.63 ± 0.13
N-NH4 (mg kg−1)7.30 ± 0.105.81 ± 0.5611.43 ± 0.254.47 ± 0.49
Phosphorus (mg kg−1)74.12 ± 3.8867.38 ± 3.3564.32 ± 3.2145.74 ± 2.12
Potassium (mg kg−1)195.24 ± 4.55153.0 ± 11.69137.57 ± 9.5199.08 ± 20.85
(±): standard deviation from the average value presented (n = 3). NT: No-till; MP: mouldboard plowing.
Table 3. Analysis of variance (ANOVA) of the different classes of aggregates according to the method of tillage, the previous crop, and their interactions.
Table 3. Analysis of variance (ANOVA) of the different classes of aggregates according to the method of tillage, the previous crop, and their interactions.
Factors>2000 µm2000–250 µm250–180 µm180–53 µm<53 µm
PC0.0003ns0.0096nsns
TM<0.0001<0.0001ns<0.0001<0.0001
PC × TMns0.00120.00540.0280ns
ns = not significant at the 5% level, PC: previous crop; tillage method (TM).
Table 4. The different aggregate classes in percentage according to the previous crop and the method of tillage.
Table 4. The different aggregate classes in percentage according to the previous crop and the method of tillage.
Factors>2000 µm2000–250 µm250–180 µm 180–53 µm <53 µm
PCDW7.70 ± 2.02 a45.13 ± 10.51 a14.68 ± 1.39 a26.95 ± 9.96 a5.53 ± 1.64 a
O6.13 ± 1.57 b44.00 ± 5.72 a16.45 ± 1.31 b27.39 ± 7.06 a6.03 ± 1.53 a
TMNT8.52 ± 1.13 a51.87 ± 3.39 a15.70 ± 2.14 a19.53 ± 2.47 b4.38 ± 0.53 b
MP5.32 ± 0.77 b37.27 ± 2.16 b15.43 ± 0.94 a34.80 ± 1.80 a7.18 ± 0.47 a
(±): standard deviation from the average value presented (n = 6). In the same column, the values with the same letter do not differ significantly according to the SNK test.
Table 5. Analysis of variance (ANOVA) of carbon (C) and total nitrogen (N) concentration associated with aggregates according to tillage, previous crop, and their interactions.
Table 5. Analysis of variance (ANOVA) of carbon (C) and total nitrogen (N) concentration associated with aggregates according to tillage, previous crop, and their interactions.
Factors2000–250 µm250–180 µm180–53 µm<53 µm2000–250 µm250–180 µm180–53 µm<53 µm
CN
PC<0.0001<0.0001ns<0.0001ns<0.0001<0.00010.0075
TM<0.0001<0.0001<0.0001<0.00010.0039<0.0001<0.0001<0.0001
PC × TM<0.0001<0.0001<0.0001<0.0001ns<0.0001<0.00010.0044
ns = not significant at the 5% level, PC: previous crop; tillage method (TM).
Table 6. Carbon(C) and total nitrogen (N) concentrations associated with aggregates according to previous crop and tillage method.
Table 6. Carbon(C) and total nitrogen (N) concentrations associated with aggregates according to previous crop and tillage method.
Factors2000–250 µm250–180 µm180–53 µm<53 µm2000–250 µm250–180 µm180–53 µm<53 µm
C (%)N (ppm)
PCDW1.05 ±
0.18 a		0.79 ±
0.2 b		0.90 ±
0.37 b		1.64 ±
0.03 a		67.17 ±
6.47 a		3580.83 ±
996.6 a		2391.67 ±
449.7 a		177.77 ±
62.5 b
O0.82 ±
0.32 b		0.96 ±
0.36 a		0.94 ±
0.27 a		1.44 ±
0.4 b		72.50 ±
6.8 a		1388.50 ±
37.2 b		1307.67 ±
27.5 b		211.67 ±
102.9 a
TM NT1.23 ±
0.09 a		1.13 ±
0.17 a		1.15 ±
0.17 a		1.74 ±
0.08 a		64.92 ±
4.7 b		2035.17 ±
697.3 b		1627.50 ±
338.9 b		269.00 ±
41.1 a
MP0.64 ±
0.05 b		0.61 ±
0.02 b		0.69 ±
0.07 b		1.340.2 ±
0.29 b		74.75 ±
4.9 a		2934.17 ±
1705 a		2071.83 ±
849.8 a		120.43 ±
16.5 b
(±): standard deviation from the average value presented (n = 6). In the same column, the values with the same letter do not differ significantly according to the SNK test (p = 0.05), PC: previous crop, durum wheat (DW), oats (O), tillage method (TM), no-tillage (NT), moldboard plowing (MP).
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Mechri, M.; Bouajila, K.; Ziadi, N.; Raza, T.; Beltayef, H.; Eash, N.S.; Jedidi, N.; Gharbi, A.; Hashem, A.; Abd_Allah, E.F. Soil Aggregation and Associated Organic Carbon and Total Nitrogen in a Sandy Loam Soil under Long-Term Tillage Effects. Agronomy 2023, 13, 2520. https://doi.org/10.3390/agronomy13102520

AMA Style

Mechri M, Bouajila K, Ziadi N, Raza T, Beltayef H, Eash NS, Jedidi N, Gharbi A, Hashem A, Abd_Allah EF. Soil Aggregation and Associated Organic Carbon and Total Nitrogen in a Sandy Loam Soil under Long-Term Tillage Effects. Agronomy. 2023; 13(10):2520. https://doi.org/10.3390/agronomy13102520

Chicago/Turabian Style

Mechri, Mouna, Khedija Bouajila, Noura Ziadi, Taqi Raza, Hayet Beltayef, Neal S. Eash, Naceur Jedidi, Azaiez Gharbi, Abeer Hashem, and Elsayed Fathi Abd_Allah. 2023. "Soil Aggregation and Associated Organic Carbon and Total Nitrogen in a Sandy Loam Soil under Long-Term Tillage Effects" Agronomy 13, no. 10: 2520. https://doi.org/10.3390/agronomy13102520

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