1. Introduction
The farming practices in Ethiopia have to cope with minimizing risks of soil degradation, such as intensive and repeated tillage, complete crop residue removal, often intensive stubble grazing, and biomass burning that affect soil health. Repeated tillage using the Mahresha ard drawn by two oxen has been reported to be the main cause of land degradation in Ethiopia (Bezuayehu et al., 2002). This physical soil disturbance by tillage destroys soil structure, aggravates soil erosion, depletes the soil organic carbon (SOC), reduces soil fertility and thus, reduces crop productivity (Bezuayehu et al., 2002; Araya et al., 2016). Tillage practices can affect SOC through changing the different soil characteristics such as soil oxygen levels and thus, microbial activity (Lal, 2004a). For example, extensive hyphal network of Arbuscular mycorrhizal fungi (AMF) plays an important role in soil conservation by improving soil aggregation (Helgason et al., 2010), increased uptake of nutrients, especially phosphorus and improving water uptake (Miller, 1992). Repeated tillage, on the other hand, disturbs the hyphal network of AMF (He et al. 2003; Mozafar et al., 2000; Helgason et al. 1998). The farming system in the highlands of northern Ethiopia is a subsistence mixed crop-livestock system. Livestock production is an essential part of the farming system mainly for provision of draught power. However, the presence of high livestock population in the region has affected the ecosystem processes through overgrazing including the cropland resulting in biological disturbance, reduced root mass, increased runoff, and increased soil temperature that can aggravate rainwater loss by evaporation.
Maintaining and building soil health is an essential component of long-term sustainable agricultural management practices. For this reason, research on long-term management practices has been directed to devising measures of the health of soil, which could be used to monitor its condition and inform its management so that degradation is avoided. Soil health is defined as the continued capacity of the soil to function as a vital living ecosystem that sustains plants, animals and humans (Doran and Zeiss, 2000). Soil health and soil quality are often used interchangeably. According to Kibblewhite et al. (2007), use of the term soil quality will generally be associated with a soils’ fitness for a specific use. Soil health is used in a broader sense to indicate the capacity of soil to function as a vital living system to sustain biological productivity, promote environmental quality, and maintain plant and animal health (Kibblewhite et al., 2007). On the other hand, Bünemann et al. (2018) conclude that the distinction between soil quality and soil health developed from a matter of principle to a matter of preference and they, therefore, consider the terms equivalent. Assessment of the soil health system may be achieved using diagnostic tests that include abiotic and biotic soil properties and soil functions. Abiotic soil properties are indicative of the state of the habitat (i.e. physical and chemical conditions such as soil bulk density, aggregate stability, pH, cation exchange capacity, etc.) and the levels of key energy and nutrient reservoirs (e.g. ratios of organic matter fractions and nutrient balances), while biotic soil properties describe the community composition, populations and biotic activities such as soil respiration and enzymatic activities of key functional groups of soil organisms. Soil ecosystem services depend on soil properties and their interaction, and are mostly influenced by its use and management. Only a few studies have linked soil properties to ecosystem services. The majority of these studies were relating soils to the defined soil functions that ultimately determined the delivery of ecosystem services. Adhikari and Hartemink (2016) reported that soil functions include availability of nutrients and water, carbon sequestration, food production, physical stability and support of plant systems and human structures, and promotion of biodiversity and habitat. They concluded that future evaluations of ecosystem services should focus on soil functions. However, there is no single definition of soil function (Hatfield et al., 2017).
Conservation agriculture (CA) involves three practical principles: reduction in tillage with a final goal to achieve zero tillage; retention of adequate levels of crop residue to cover at least 30% of the soil surface and use of crop rotations (FAO, 2018). CA has been widely promoted to overcome continued soil degradation and restore the soil physical, chemical and biological properties, and functioning of degraded soils (Powlson et al., 2016; Friedrich et al., 2012). CA practices can affect SOC either directly by increasing plant root biomass and the amount of C potentially returning to the soil as crop residues, or indirectly by changing the mineralization rate of SOC (Lal, 2004a). Farmers in the US, Canada, Latin America, Europe and certain parts of South Asia, for soil and water management and improved crop yield (Friedrich et al., 2012), have thus successfully adopted CA systems. Yet, acceptance of the practice is persistently low in Africa south of Saharan particularly in Ethiopia (Giller et al., 2009).An in situ soil and water conservation (SWC) tillage practice that integrates the principles of CA can reduce cropland degradation, improve soil health, thereby increasing crop productivity, and facilitate uptake of CA in Ethiopia. In this study, CA-based systems consider permanent raised beds (PRB) and contour furrowing (CF) as integral elements of CA. The shift from conventional tillage to CA-based system can alter the aforementioned soil functions. In this case, for developing sustainable agricultural practices, it is crucial to understand the multiple interactions between the soil biota and their abiotic environment as affected by soil management practices.
Our understanding of the linkage between soil properties and soil functions and the resultant ecosystem services is incomplete (Sarukhán et al., 2005). The ability of the soil to provide these functions will depend upon the state of the soil properties. Generally, Vertisols, the soil of interest in this study, have a high water holding capacity but are also characterized by high runoff rates and periodic waterlogging problems (Araya et al., 2015; Sayre, 1998). Therefore, introducingin situ SWC tillage practices as a fourth principal components of CA systems (PRB and CF) can be useful to improve soil health. Previous studies from the same experimental plots focused on evaluating impacts of CA-based systems on soil water erosion, and soil water balance in the crop root zone (Opolot et al., 2016; Araya et al., 2015). The soil losses and runoff were significantly higher in CT systems while soil water storage was higher in CA-based systems (Araya et al., 2016). They also reported that permanently raised beds in CA systems protects the crops from water-logging, while excess water is drained from the raised bed to the furrows that enhance soil water storage. The assessment using selected indicators of soil properties and function becomes a key issue for assessing the impact of sustainable agricultural practices on soil health. In this study, the effects of long-term CA-based systems on soil health using abiotic and biotic soil properties and soil function indicators was evaluated.
Therefore, the objectives of this study were to determine the impact of long-term in situ SWC tillage practices in CA systems on soil health using selected abiotic and biotic soil properties, and soil function indicators in northern Ethiopia. We hypothesize that CA and in situ SWC improve soil health compared to conventional tillage systems.
2. Materials and Methods
2.1. Description of the study area
A field study was conducted in permanently kept plots (2005-2013) at farmers’ rain fed fields in semi-arid area at Gum Selasa (13°14’N, 39°32’E) and an altitude of 2100 m a.s.l. in northern Ethiopia (Fig. 1).
The soil type under the experimentation in the study site is a Vertisol (Araya et al., 2016). The mean annual rainfall over 33 years (1971–2013) at Gum Selasa in Adi Gudom town (3 km away from the experimental plot) was 498 mm with more than 89% falling from June to September (Araya et al., 2016). The mean average temperature is 19.4 ºC.
Mixed farming system is dominant which includes livestock and subsistence crop production. Oxen are the only source of draft power used for plowing. The traditional ox drawn mahresha ard plow is made up of a metal and wood (Fig. 2). Three to four tillage operations are conventionally done with an oxen-drawn ard to control weeds, improve infiltration and prepare a fine seedbed, particularly for tef (Eragrostis tef ). The temporal pattern of plowing depends on the availability of oxen, type of crop and rainfall. The most cultivated crops include tef, barley (Hordeum vulgare ), wheat, and grass pea (Lathyrus sativus ).
2.2. Experimental Treatments and Layout
In brief, the experimental treatments consisted of conventional tillage, contour furrowing and permanent raised bed (Fig. 3). The experiment was arranged in a randomized complete block design (RBCD) with three replications. The crops grown in rotation throughout the experimental period from 2005-2013 were wheat, teff, wheat, barley, wheat, teff, grass pea, teff and wheat, respectively. The three treatments that include tillage and crop residue management practices are described below.
(1) Conventional tillage (CT): soil was plowed at least three times in a similar pattern with the local tillage practices and the crop straw was completely harvested without leaving residue on the soil surface. Although aftermath overgrazing is a common conventional practice in the study area, mimicking of this practice was not adopted in the experimentation because the plots were too small for animal movement. The depth of first and second plowing was about 15 cm, while 10 cm at planting. (2) Contour furrowing (CF): it involved plowing only once at sowing, 30% of the total crop residue was left as standing stubble to cover at least 30% of the soil surface and furrows were made at 1.5 m interval along the contour. The depth of plowing was about 10 cm. (3) Permanent raised beds (PRB): contour furrows were made at 35 cm interval with 30% of the total crop residue retained as standing stubble to cover at least 30% of the soil surface. Furrows were refreshed at planting while there was no tillage on the raised beds that were maintained undisturbed for 9 years (2005-2013).
The experiment was conducted under rain fed conditions with plot size of 5 m × 19 m. The slope gradient of the experimental plot was 3%. In 2013, wheat was sown in the first week of July at the start of the long Kiremt rainy season using manual broadcasting at the rate of 120 kg ha-1. Plowing and reshaping of furrows was made using oxen-drawnmahresha ard plow at planting after broadcasting the seeds. This led to move the soil and seeds from the furrow to an upper position on the beds. N and P fertilizers were applied uniformly to all plots at the rate of 46 P2O5 kg ha-1 and 64 N kg ha-1. Glyphosate at 2 l ha-1 (360 g a.i. l-1) was sprayed each year starting from the third year (2007) to control weeds emerged before planting in CA-based (CF and PRB) systems a week before sowing; it effectively kills annual and perennial weeds. Hand weeding was used to control weeds that emerged after planting in all treatments.
2.3. Soil chemical analysis
Composite soil samples from three sampling locations per plot were taken from the experimental plots at 0–10 cm soil depth at the end of August 2013. The soil samples were used to analyze soil pH (1:2.5 H2O), total soil N (Kjeldahl N, Bremner and Mulvaney, 1982), Olsen phosphate (Olsen, 1954), CEC (ammonium acetate extraction, Scholenberger and Simon, 1945) and CaCO3 (acid neutralization, De Leenheer, 1959). Soil organic C (SOC, Walkley and Black, 1934) was determined at three soil depths (0–10, 10-20 and 20–30 cm) from a composite sample of three sampling per plot.
Although the bulk density (BD) of the Vertisol in our study, which is subjected to swell and shrink, changed from 1 to 1.71 Mg m-3 with soil moisture content (Araya et al., 2016), value of 1.35 Mg m-3 was taken to calculate soil organic C stocks.
\(SOCS=\text{OC}_{\text{conc}}*BD*D*A*0.001\)
where SOCS is the soil carbon stock (Mg ha-1),\(\text{OC}_{\text{conc}}\) is the soil carbon concentration, D is the depth of the soil sample (m), A is an area of 10000 m2, BD is soil bulk density (Mg m-3) for the soil sampling depth and 0.001 is the conversion factor from kg to Mg (Mg Kg-1).
SOC sequestration was calculated using the change in soil carbon stock (SOCS) over years (between 2006 and 2013) divided by the number of years for the same period (7 years):
\begin{equation} \text{SOC\ sequestration}=\frac{\left(SOCs\ in\ 2013-SOCs\ in\ 2006\right)}{2013-2006}\nonumber \\ \end{equation}
where SOCS is the soil carbon stock (Mg ha-1).
The data for 2006 was taken from Araya et al. (2016).
2.4. Soil physical analysis
Bulk density was derived from the soil shrinkage characteristics curve (SSCC) measured by the balloon method as described in Cornelis et al. (2006) for shrinking and swelling soils. Soil water retention curve (SWRC) was established using two soil samples taken per plot by measuring soil-water contents at matric potentials of 0,-1.0, -2.9,-5.9, -9.8, -33, -100, and -1500 kPa on 100 cm3 undisturbed soil cores using tension tables (Eijkelkamp Agrisearch Equipment, Giesbeek, The Netherlands) for high soil matric potentials (0 to -9.8 kPa) and pressure chambers (Soilmoisture Equipment, Santa Barbara, CA, USA) for low soil matric potentials (-33 to -1500 kPa), following the procedure outlined in Cornelis et al. (2005). A single ring with constant head permeameter using the Mariotte system of a Guelph permeameter (Soilmoisture Equipment) was used to measure field saturated hydraulic conductivity (KFS) with two replications per plot as described in Cornelis et al. (2005). Soil aggregate stability index (SI) was determined at 0-15 cm and 15-30 cm soil depth using the wet sieving method (Kemper and Rosenau, 1986).
Water retention data were used to calculate plant-available water capacity, air capacity and macroporosity. The plant-available water capacity, PAWC (m3 m-3), indicates the amount of water the soil can store and provide for plant roots (White, 2006). This was calculated as:
\begin{equation} \text{PAWC}=\theta_{\text{FC}}-\theta_{\text{pwp}}\nonumber \\ \end{equation}