RJOAS March 2025
by Nigussie Ashenafi (Department of Soil and Water Management, Wondo Genet Agriculture Research Center, Shashemene, Ethiopia)
The use of integrated soil management practices is vital to improve the soil’s fertility and function. Despite all these advantages of using integrated management practices on soil properties, their short-term impacts were not properly investigated in the soils of the study areas. Thus, the present study was carried out on two soils to evaluate the short-term effects of tillage methods, cropping systems, and nitrogen fertilizations and their interaction on organic carbon (OC) and total nitrogen (TN) stocks. Three-factors such as tillage methods, cropping system and nitrogen fertilization were laid out as a split-split plot arrangement in a randomized complete block design with three replications. Soil management practices have varying effects on soil OC and TN stocks. In the surface soil layers, MT gave higher OC and TN stocks compared to CT in both soils/locations. Similarly, N treatments had significant effects on OC and TN stocks at both locations in 0-20 cm but not in the 20-40 cm soil depth. The application of sole compost (20 t ha-1) provided the maximum (5.17 kg m-2) and (8.77 kg m-2) OC stocks for Cambisols and Phaeozems, respectively. Maximum TN stocks of 0.51 kg m-2 on Cambisols and 0.83 kg m-2 on Phaeozems were achieved from the integrated and inorganic sole N treatments, respectively. The study highlights that MT along with a legume-based rotation system and the use of organic input or integration with mineral N fertilizer are possible alternatives to improve soil OC and TN stocks and thus ensure sustainable production.
The degradation of soil fertility and concomitant food insecurity are significant challenges in sub-Saharan Africa (SSA), with 23% of the population being undernourished and more than 35 million humans expected to be food insecure by 2050 (Lal et al., 2015; FAO and ECA, 2018). In sub-Saharan Africa (SSA), low agricultural productivity is largely attributed to low soil fertility (Nduwumuremyi et al., 2013). The depletion of soil fertility and extensive soil degradation are the main constraints limiting crop productivity and natural resource conservation in Ethiopia (Zeleke et al., 2010). Wakene et al. (2007), who indicated that low soil fertility is the major bottleneck in resource-poor farmers for sustainable agricultural production and productivity, reported a similar observation. Therefore, adopting farming systems, which might be robust in improving soil fertility and health, is a crucial strategy for sustainable crop production.
Soil management actions can shake the sustainable use of soil resources through their influence on soil quality, stability, and resilience. Successful soil management to maintain soil quality depends on the understanding of how the soil responds to agricultural practices over time (Wakene, 2001). In the past several decades, agricultural management practices consisting of intensive tillage, continuous cropping, and low-level input application have resulted in the degradation of soil and environmental qualities by increasing erosion and nutrient leaching (Gebrekidan, 2003; Kimetu et al., 2008; Vanlauwe et al., 2010). Furthermore, in the face of climate change (CC), where rainfall patterns change and temperatures are rising, climate-smart agriculture technologies could help smallholder farmers (FAO, 2010).
Either tillage has a major effect on the carbon pool, negative with conventional plowing or positive when conservation tillage is applied (IPCC, 2000). The primary objective of tillage in crop production is to produce suitable soil physical conditions for seed germination and plant growth. However, tilling soil frequently can be a prime cause of soil structure degradation due to the gradual loss of soil organic matter, leading to soil erosion and compaction and low moisture availability for plants (Gupta et al., 2002).
Conventional tillage has been reported to deteriorate soil properties (Mangalassery et al., 2015; Nivelle et al., 2016). Davidson and Ackerman (1993) reported that generally up to 40% of the total carbon in the top 30 cm of the soil is lost due to tillage compared to uncultivated lands. Moreover, plowing the field in a conventional method leads to hardpan formations (Wasaya et al., 2011). Conventional tillage can accelerate the rate of organic matter decomposition and thus reduce the soil quality (Parras-Alcántara et al., 2015). For example, a lower bulk density was observed in conventionally tilled soils, induced a greater total porosity (Lipiec et al., 2006). Similarly, Tiritan et al. (2016) observed a negative effect of conventional tillage on soil properties. On the contrary, conservation tillage is known to improve soil physicochemical properties (Divito, 2012; Busari et al., 2015) and soil quality (Plaza et al., 2013). Soil organic carbon concentrations significantly improved under reduced tillage along with green manure application (Garcia-franco et al., 2015).
Cropping systems have been reported to enhance and/or maintain soil quality by influencing the soil environment primarily through differences in the quantity and quality of crop residue, as well as root exudates returned to the soil (McDaniel et al., 2014; Tiemann et al., 2015). Previous findings revealed that diversifying crops into rotations could result in relatively higher organic carbon content (Gal et al., 2007; Abdollahi et al., 2015). In contrast, continuous monocropping along with conventional tillage can negatively affect soil organic carbon contents and thereby poor physical, chemical, and biological soil properties (Mariangela and Francesco, 2010; Wyngaard et al., 2012).
Nitrogen dynamics in soils was markedly influenced by tillage practice as compared to N fertilization (Salinas Garcia et al., 1997). Previous studies showed that soils under conventional tillage have higher mineral N but lower TN than those under conservation tillage (Verachtert et al., 2009; Wu et al., 2009). However, the total N content of the soils in conservation tillage mostly depends on the amount of crop residues retained on the surface of the soil and the sampling depth (Govaerts et al., 2007). Therefore, careful use of soil resources along with proper tillage methods are essential for physical and chemical components of soil fertility to ensure sustainable agricultural productivity.
Soil organic matter (SOM) storage in agricultural systems is determined by the balance of carbon addition from crop residues versus carbon losses through SOM decomposition (Awale et al., 2013). Thus, the degree to which a tillage technique influences SOM turnover is generally determined by the frequency and timing of soil disturbance, depth of soil disturbance, and degree of soil-residue mixing (Cookson et al., 2008; Machado, 2011). The soil organic matter loss contributes to a variety of soil degradation processes, including erosion, compaction, salinization, nutrient insufficiency, loss of biodiversity and desertification, all of which are accompanied by a decrease in soil fertility (Lal, 2015). The effects of soil and crop management practices on SOC and TN dynamics, in part, depend on soil properties and environmental factors, such as soil texture, clay mineralogy, topography, and climate (Campbell et al., 1996).
Although individual effects of tillage methods, cropping systems and nitrogen fertilization on soil properties, particularly organic carbon and total nitrogen stocks, have been studied so far and documented elsewhere, their interaction has not properly investigated in the study areas. Furthermore, conventional tillage coupled with continuous monocropping, and inadequate external input application is a significant constraint on crop production in the central rift valley of Ethiopia. Therefore, the present study was initiated to examine the short-term effects of tillage methods, cropping systems, and nitrogen fertilization and their interaction on selected soil chemical properties, including organic carbon and total nitrogen stocks. We hypothesized that reduced tillage along with crop rotation and integrated application of organic and inorganic nitrogen would improve the soil carbon and nitrogen stocks.
Field experiments were carried out during two growing seasons (2019 and 2020) in Hawassa Zuria and Meskan districts of the Central rift valley of Ethiopia. The Hawassa Zuria site is geographically located at 07° 1 ’0.83 "N Latitude and 38°22’ 26" E Longitude with an altitude of 1713 m above sea level (asl) is found 287 km south of Addis Ababa. Mainly characterized by a semi-arid climate with long-term an average annual rainfall of 957.5 mm, of which 81% falls during the growing season (April to October) and an annual mean temperature of 21 ◦C. The trial site at Meskan is found at 08°05 '33 "N Latitude and 38°26’ 75" E Longitude with an altitude of 1841 m asl. (Table 1), 135 km south of Addis Ababa. The experimental site is categorized under a semi-arid climate with a long-term average annual rainfall of 987 mm, of which 84% falls during the growing season (April to October) and an annual mean temperature of 20.4 0C.
The soil types for the field trial were Cambisols and Phaeozems, according to the WRB soil classification system (IUSS Working Group, 2015). The soil in Hawassa Zuria has loam textural class, medium organic carbon and total nitrogen as rated by Landon (1991), very low available P (Havlin, 1999), and medium CEC as rated by Hazelton and Murphy (2007) (Table 1). The soil at Meskan has a clay textural class with high OC, medium TN, high available P, and CEC as presented in Table 1.
Two tillage practices were evaluated: conventional tillage (CT) and minimum tillage (MT). The two tillage practices were combined with two cropping systems: haricot bean-maize rotation (RC) and maize monocropping (MM). In addition, four nitrogen levels (0, 20 t ha-1 compost, 46 kg N ha-1 + 10 t compost ha-1, and 92 kg N ha-1) were combined with tillage practices and cropping systems. Treatments were arranged in a split–split plot with tillage practices as main plot, cropping systems as a sub plot and nitrogen levels as sub-sub plot factors, RCBD, with three replications, making 48 sub-sub plots for each experimental location. Each sub-sub plot was 4.8 m by 3 m (14.4 m2) and accommodated six maize rows with inter-and intra-row spacing of 80 and 25 cm, respectively.
From field experiments, soil sampling and analysis were performed twice. First, before the imposition of the treatments, three representative soil samples were prepared from 0-20 and 20-40 cm soil depths per experimental site during March 2019 to determine the initial fertility status of the experimental sites (Ashenafi Nigussie et al., 2021). At the end of experimentation, which is after the second crop harvest (November 2020), three random soil samples per sub-subplot were collected from 0-20 and 20-40 cm soil depths at each experimental location, following the standard soil sampling procedure. After manual homogenization, representative composite subsamples per experimental unit and site were prepared for soil organic carbon and total nitrogen analysis.
The bulk density of the soil was estimated by dividing the oven-dried soil sample taken with a core sampler by the volume of the core sampler (Blake and Hartge, 1986). Soil organic carbon was determined using the wet oxidation method (Walkley and Black, 1934), while total nitrogen content was determined following the Kjeldahl procedure described by Van Reeuwijk (1992). The soil OC stock (kg C m- 2) was computed using the method described by Batjes (1996).
Similarly, the total nitrogen stock (kg N m-2) was calculated by multiplying the percentage of total nitrogen (g / kg), the bulk density of the soil (kg m-3) and the defined depth classes (0–20 and 20-40 cm) for each treatment.
The effect of the treatments on soil OC and TN stocks was statistically analyzed using SAS computer software version 9.3 (2014).
The surface soils had a bulk density of 0.94 and 0.99 g cm-3 for the Cambisols at Hawassa Zuria and Phaeozems at Meskan sites, respectively. In both soils/sites, bulk density showed increasing tendency with soil depth (Table 1). The organic carbon stocks in the surface soil (0-20 cm) were 4.8 and 8.43 kg m-2 at Hawassa Zuria and Meskan on Cambisols and Phaeozems soil types, respectively (Table 1). The highest organic carbon stock was observed at the Meskan site compared to Hawassa Zuria soil; this may be attributed to the presence of higher clay separates in the former than in the latter. Batjes (1998) showed that the clay content was the main environmental factor that controlled the behavior of OM in the soil. The total nitrogen stocks of the surface soil depth (0-20 cm) were 0.49 and 0.75 kg m-2 for Cambisols and Phaeozems in Hawassa Zuria and Meskan, respectively (Table 1). In contrast to BD, the soil OC and TN stocks decreased in general with increasing soil depth (Table 1).
The effects of tillage and cropping systems on soil bulk density were insignificant (Table 2). However, maximum values of 0.99 and 1.1 g cm-3 for the Hawassa Zuria and Meskan sites were recorded from minimum tillage. In the present study, a relatively lower bulk density in surface soil (0-20 cm) was recorded in conventional tillage, which might be due to the formation of a loose soil structure by breaking soil aggregates. On the contrary, the application of N fertilizers brought a significant variation to the bulk density in the surface layer in Hawassa Zuria but not at Meskan. The lowest bulk density was recorded in the compost-only treated plot in Hawassa Zuria (Table 2). This was due to the improvement of soil structure by the applied organic input (compost).
The tillage methods had a significant effect on the OC stock in the surface soil layer (0-20 cm) at Hawassa Zuria (Cambisols) but not at Meskan (Phaeozems). Despite a non-significant difference of tillage methods in OC at Meskan, MT resulted in numerically higher OC stocks in both soils compared to CT (Table 2). The beneficial effect of MT on the OC stock was most likely due to the incorporation of residues from previous crops, which were less susceptible to breakdown or microbial attack. On the contrary, the lower OC stock resulting in CT might be due to accelerated oxidation of OM by stirring the soil and decomposition, which is favored by strong microbial activities (Six et al., 2002; Murage et al., 2007; Zheng et al., 2018). Tillage methods had a significant effect on the OC stock at the depth of the subsurface soil (20-40 cm) in Meskan but not at Hawassa Zuria. Although a non-significant variation was observed between tillage methods on OC in subsurface soil depth (20-40 cm) in Hawassa Zuria (Cambisols), CT resulted in a slightly higher OC stock than MT, suggesting that both the above and belowground crop residues were incorporated into the depth of the soil depth by tillage operation. Our result is in line with the previous studies (Six et al., 2002; Conceição et al., 2013).
In terms of cropping systems, there was no substantial variation in the OC stock between soil depths and soils/sites (Table 2). Despite the non-significant effect of cropping systems on OC stock, maize monocropping maintained a slightly superior OC stock at both sites than the haricot bean-maize rotation system, due to improved root biomass retention in the soil. However, different N treatments had a significant effect on the OC stock at both locations (soils) in the surface soil depth (0-20 cm) but not in the subsurface soil layer (20-40 cm) (Table 2). At Hawassa Zuria, the application of sole compost (20 tons ha-1) resulted in the maximum (5.17 kg m-2) OC stock, which was statistically similar to the integrated and sole inorganic N treatments (Table 2). The soil OC stock increased by 3.8 and 2.8% due to sole application of compost and integrated N treatments, respectively, compared to the unfertilized plot. Similarly, at Meskan, the maximum OC stock was recorded in the sole inorganic N treatment, which was statistically similar to the sole compost treatment. Compared to the unfertilized plot, the only compost and inorganic N treatments improved soil OC stock by 6.3% and 6.6%, respectively.
Tillage methods significantly affected the soil TN stock at the surface soil depth (0-20 cm). There was a tendency for better total N stock caused by MT at the 0-20 cm soil depth in both locations. This was due to the protection of organic N within the micro- and macro aggregates and thereby reduced N losses through leaching and mineralization of organic matter. Similarly, cropping systems had a significant effect on TN stock in surface soil depth in Meskan, but not at Hawassa Zuria.
The effect of N fertilization was highly significant (p < 0.001) on the depth of the TN stock at the surface soil (Table 2). The maximum TN stocks of 0.51 kg m2 on Cambisols at Hawassa Zuria and 0.83 kg m2 Phaeozems at Meskan soils were attained from the integrated and sole inorganic N treatments, respectively. At subsurface soil depth, at Hawassa Zuria, the TN stock was significantly affected by N fertilization, and the maximum (0.41 kg m-2) was obtained in the plot amended with integrated N treatment. However, in Meskan, no significant variation was observed among the N treatments at 20-40 depth. At the 20-40 cm soil depth, the effect of N fertilization on the TN stock was not analogous in the two soils. In this study, the influence of N fertilization was more visible in loamy-textured Cambisols than in clay-textured Phaeozems, suggesting that the nitrate form of N leached from the above soil layer, which was aggravated by CT and soil texture.
At Hawassa-Zuria, the three-way interaction of TM×CS×NF significantly (p ≤ 0.05) affected BD and OC stocks at the 0-20 cm soil depth, whereas at Meskan, the three-way interaction brought significant (p ≤ 0.05) variations on BD and OC and TN stocks (Table 3). The highest OC stock value was reported at Hawassa Zuria when MT interacted with a legume-based rotation system and sole compost, while at Meskan the highest OC and TN stocks were observed when MT interacted with a rotation system and sole inorganic N treatment (Table 3).
When compared to the initial OC stock, the combined application of MT + RCS + sole compost and MT + RCS + integrated N treatment improved soil OC stock in the surface soil depth by 9.8% (0.47 kg m-2) and 8.1% (0.39 kg m-2) at Hawassa Zuria, respectively. The combined application of MT + RCS + sole inorganic N improved OC and TN stocks by 9.25% (0.78 kg m-2) and 20% (0.15 kg m-2) at Meskan, respectively.
In the current investigation, different soil management practices have varying effects on soil OC and TN stocks. In the surface soil layer (0-20 cm), MT gave higher OC and TN stocks compared to CT in both locations. Similarly, the application of N fertilizers brought significant impacts on the OC and TN stocks at both locations in the 0-20 cm soil depth. At the 0-20 cm soil depth, the three-way interaction brought significant variation on OC stock at Hawassa-Zuria (Cambisols) and soil OC and TN stocks at Meskan (Phaeozems). In conclusion, MT along with a legume-based rotation system and use of organic input or integration with mineral N fertilizer could be recommended for both soils to improve soil OC and TN stocks and thus ensure sustainable production.
The author expresses his gratitude to the Ethiopian Institute of Agricultural Research and the Agricultural Growth Program II for funding the experiment. He also appreciates the technical and field support provided by the staff members of the Soil and Water Management Research Process at the Wondo Genet Agricultural Research Center.
Original paper, i.e. Figures, Tables, References, and Authors' Contacts available at http://rjoas.com/issue-2025-03/article_06.pdf