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}