沙漠研究 24-1, 203-207 (2014)
Ἃ₍◊✲ 24-1, 203 - 207 (2014)
Journal of Arid Land Studies
Journal of Arid Land Studies－ DT11 Refereed Paper －㸫DT11 Refereed Paper㸫

Agro-ecosystem, Tillage, and Cropping Effects on Extractable Soil Nitrogen
and Organic Carbon in Ghana
B. DAVIES1), K. BOA2), J. AITKENHEAD-PETERSON*1), L. PITTS1) and W. PAYNE3)
Abstract: Extractable soil nitrogen species and organic carbon from four agro-ecosystems in Ghana (coastal savannah, forest,
transition and Guinea savannah) under three tillage (no-till, traditional till and zonal till) and four cropping treatments (maize,
maize-cowpea rotation, maize-mucuna rotation and maize-cowpea-mucuna relay) or (maize, maize-cowpea rotation, maize-cowpea
intercrop and cowpea-maize rotation) were examined. Soils were collected at 0-10 cm, 10-30 cm and 30-60 cm for analysis ofbaseline conditions prior to initiation of tillage and cropping treatments. Two years after treatments commenced, soils were collectedfrom a depth of 15 cm, air dried and sieved to 2 mm prior to acid extraction and analyses of nitrate-N, ammonium-N, dissolved
organic N (DON) and dissolved organic carbon (DOC). Within agro-ecosystems, cropping had a significant effect on DOC under
zonal tilling in the forest, and on NO3-N and NH4-N under traditional tillage and NO3-N under zonal tillage in the transition. In the
Guinea savannah, cropping had a significant effect on DON under DON under traditional and zonal tillage. Tillage had a significant
effect on DON and DOC in the forest, DOC in the transition and on NH4-N and DON in the Guinea savannah agro-ecosystems. The
data represent the second year after experimental treatments on the effects of tillage and cropping on soil nutrients in Ghana.
Key Words: Cropping, Extractable Soil Nutrients, Tillage, W. Africa

1. Introduction
In Sub-Saharan Africa agricultural production relies on the
productivity of smallholder farms (Xinshen et al., 2007).
Major constraints to farm productivity include soil degradation,
land competition, and climate change (Xinshen et al., 2007).
As in many rainfall scarce environments, soils in the semi-arid
Sahelian zone are inherently low in soil organic carbon (SOC),
nitrogen and phosphorus (Srinivasarao et al., 2012). In rural
appraisals, an increasing number of African farmers indeed
mention soil fertility decline as a major constraint to farming(Smaling et al., 2012). Therefore, a multi-pronged approachto improving Africa’s food security must include practices that
effectively improve local soil fertility.
Conservation agriculture (CA) in semi-arid West Africa iscurrently being promoted to mitigate the effect of droughts,
increase crop productivity and reduce production costs(Lahmar et al., 2012). The CA approach relies on thesimultaneous use of minimum (zonal) or no-tillage,
maintenance of a permanent soil cover and a diversified,profitable crop rotation (Lahmar et al., 2012) Research has
shown that soil organic matter (SOM), nutrients, pH, andtexture signify good soil quality (Reeves, 1997). Soil organiccarbon (SOC) is used as a measure of SOM because its
chemical structure and surface properties influence soil
* Corresponding Author: jpeterson@ag.tamu.edu
1) Department of Soil and Crop Science, Texas A&M University, USA
2) Farm Front Services, Nkawi-Toase, Ashanti, Ghana

structural stability and cation exchange capacity while also
serving as an energy source for soil biota (Logah et al., 2011).
This research aimed to evaluate the effects of tillage andcropping on soil extractable carbon and nitrogen in four
agro-ecological zones. Although the effects of tillage andcropping on soil fertility have been well researched, relatively
few studies have been reported on African based research.

2. Materials and Methods
2.1. Descriptions of agro-ecosystems
The coastal savannah agro-ecosystem is situated in the Ga
West district and Pokuase community. The soil is classified(World Reference Base for soil resources (WRB, 2006) as a
Haplic Lixisol formed on granite and has a loamy-sand texture
to 60 cm. The forest agro-ecosystem is in the Amansie West
district within the Ahwerewa community. The soil isclassified as a Leptic Lixisol formed on phyllite and has asilty-loam texture to 60 cm. The transition ecosystem is aforest-savannah transition situated in the Ejura-Sekodumase
district within the Ejura-Adiembra community. The soil isclassified as Leptic Lixisol formed on sandstone and has aloamy sand texture to 30 cm. The Guinea savannahagro-ecosystem is in the Tolon-Kumbungu district within the
Kumbungu-Kuko community. The soil is classified as a
Pisolithic Plinthosol formed on shalestone and has a silty loam
㸦Received, November 4th, 2013; Accepted, November 27th, 2013㸧

3) International Center for Agricultural Research in the Dry Areas, Addis Ababa, Ethiopia

204

texture to 60 cm.
Mean annual rainfall differs among the four
agro-ecosystems. Coastal savannah has a mean annual
rainfall of 800 mm, forest-1500 mm, forest-savannahtransition-1300 mm and the Guinea savannah 1100 mm.
2.2. Experimental design
In each agro-ecosystem, a split plot design was
implemented to test cropping × tillage and fertilizer
amendments. This paper discusses the preliminary results forthe cropping × tillage experiment. The main plots were 1) notill, 2) zonal or conservation tillage, and 3) traditional tillage.
The sub-plots were either 1) maize (Zea mays) only, 2) maize cowpea (Vigna unguiculata) rotation, 3) maize - mucuna(Mucuna pruriens) rotation, 4) maize - cowpea - mucuna relayor 5) cowpea - maize rotation and 6) maize - cowpea intercrop.
Soils were collected at 0-10 cm, 10-30 cm and 30-60 cm in2011 prior to planting and the tillage and cropping treatments.
Soils were analyzed for percent carbon and nitrogen and the
C:N ratio was estimated.
2.3. Soil collection and processing
2.3.1. Experimental soil collection (2012 - Year 2)
Two years after the tillage and cropping treatments were
initiated soils were collected for nutrient analyses. Soils werecollected using a 2 cm diameter soil probe to a depth of 15 cm.
Three soil cores were taken at each plot and bulked. Eachsample was collected across the central row of the plot.
Soils were air-dried and larger peds gently broken downusing a mortar and pestle prior to sieving to 2 mm. Soilsamples (3.5 g) were dissolved in 35 g of 0.1 M HCl (1:10
soil:HCl ratio) and shaken for two hours at 500 rpm on a rotary
shaker. Samples were then centrifuged for 15 minutes at
19,974 g-force and filtered using a Whatman GF/F filter(nominal pore size of 0.7 µm). Soil extracts were analyzed
immediately after extraction.
2.3.2. Chemical analyses
DOC and total dissolved nitrogen (TDN) were measured
using high temperature Pt-catalyzed combustion with a
Shimadzu TOC-VCSH and Shimadzu total measuring unit
TNM-1 (Shimadzu Corp. Houston, TX, USA).
Ammonium-N was analyzed using the phenate hypochlorite
method with sodium nitroprusside enhancement (USEPA
method 350.1; USEPA 1993) and nitrate-N was analyzed using
Cd-Cu reduction (USEPA method 353.2; USEPA 1997).
Colorimetric methods were performed with a Smartchem
Discrete Analyzer (Model 200 Westco Scientific Instruments
Inc., Brookfield, CT, USA). Dissolved organic nitrogen was
estimated by deducting NO3-N plus NH4-N from TDN. NIST

Table 1. Soil pH, carbon and nitrogen prior to tillage and cropping
experiment.
Agroecosystem
CoastalForestTransition

Guinea Savannah

Depth

C

N

(cm)

pH

(%)

(%)

C:N

0-10

6.2

0.40

0.04

10.00

10-30

6

0.30

0.04

7.50

30-60

5.9

0.10

0.02

5.00

0-10

6.3

2.50

0.2

12.50

10-30

6.3

0.70

0.07

10.00

30-60

5.6

0.70

0.07

10.00

0-10

4.8

0.35

0.03

11.70

10-30

4.9

0.35

0.03

11.70

30-60

4.7

0.29

0.02

14.50

0-10

5.3

1.00

0.06

16.70

10-30

5.3

0.60

0.04

15.00

30-60

5.6

0.80

0.04

20.00

traceable standards, check standards, and water blanks wereanalyzed every 12th sample for instrument precision andaccuracy.
2.3.3. Statistical analyses
Univariate analysis of variance was used to statisticallyanalyze the data with Į < 0.05 to examine effects ofagro-ecosystem, tillage and cropping across the whole dataset.
Univariate analysis of variance was used to statistically analyze
the effects of tillage, cropping and tillage × cropping
interactions within each agroecosystem zone. Two-sample,
2-tailed t-tests (Į < 0.05) were used to examine significant
differences resulting from tillage and cropping within eachagro-ecosystem.

3. Results
3.1. Baseline soil nutrientsResults for composite soil samples collected prior toplanting and initiation of tillage treatments indicated that soils
in the coastal savannah and forest agro-ecosystems
aremoderately to slightly acidic across the three depths, withpH ranges of 5.9 - 6.3 in the coastal savannah and 5.6 - 6.5 in
the forest. Soils in the transition site were very acidic (pH 4.3- 5.0), and those in the Guinea savanna site were acidic (pH 5.1- 5.6). Soil percent C and N tended to decline with depth(Table 1).
Soil C:N ratios are important indicators of whether
extractable nutrients might be transported to surface or groundwaters. Soil C:N ratios ranged from 10.0 to 5.0 in the coastal
savannah indicating a larger soil pool of N at 30-60 cm depth
compared to 0-10 cm depth. In the forest soil C:N ratio
ranged from 12.0 to 10.0 illustrating a steady C:N ratio withdepth. The transition zone had a soil C:N ratio of 11.7 to 14.0
and the Guinea savannah a C:N ratio of 16.7 to 20.0.

205

Table 2. Soil nitrogen species and DOC in the Coastal Savannahagro-ecosystem. ± = standard deviation.
Till
Till
Till
Till

Crop
Crop
Crop
Crop

NO
-N
NO
-N
NO
NO333-N
-N
3

NH
-N
DON
NH
-N
DON
NH
DON
NH444-N
-N
DON
4
-1
-1
-1
mg
kg
mg
kg
-1
mg
kg
mg kg

DOC
DOC
DOC
DOC

None
None
None
None

M
M
M
M

19.8±16.1
19.8±16.1
19.8±16.1
19.8±16.1

7.2±2.5
7.2±2.5
7.2±2.5
7.2±2.5

6.2±5.4
6.2±5.4
6.2±5.4
6.2±5.456.0±7.956.0±7.956.0±7.956.0±7.9

None
None
None
None

MC
MC
MC
MC

10.9±0.6
10.9±0.6
10.9±0.6
10.9±0.6

5.8±0.9
5.8±0.9
5.8±0.9
5.8±0.9

6.3±5.6
6.3±5.6
6.3±5.6
6.3±5.658.8±8.8
58.8±8.8
58.8±8.8
58.8±8.8

None
None
None
None

MM
MM
MM
MM

10.8±3.6
10.8±3.6
10.8±3.6
10.8±3.6

7.0±2.4
7.0±2.4
7.0±2.4
7.0±2.4

6.4±5.7
6.4±5.7
6.4±5.7
6.4±5.753.9±6.4
53.9±6.4
53.9±6.4

None
None
None
None

MCM
MCM
MCM
MCM

9.9±2.89.9±2.89.9±2.89.9±2.8

6.1±1.5
6.1±1.5
6.1±1.5
6.1±1.5

7.8±5.5
7.8±5.5
7.8±5.5
7.8±5.553.0±6.4
53.0±6.4
53.0±6.4
53.0±6.4

Trad
Trad
Trad
Trad

M
M
M
M

10.6±1.8
10.6±1.8
10.6±1.8
10.6±1.8

7.2±0.7
7.2±0.7
7.2±0.7
7.2±0.7

10.2±1.8
10.2±1.8
10.2±1.8
10.2±1.874.9±14.6
74.9±14.6
74.9±14.6

Trad
Trad
Trad
Trad

MC
MC
MC
MC

12.8±3.0
12.8±3.0
12.8±3.0
12.8±3.0

5.7±1.1
5.7±1.1
5.7±1.1
5.7±1.1

5.9±5.2
5.9±5.2
5.9±5.2
5.9±5.249.6±9.4
49.6±9.4
49.6±9.4
49.6±9.4

Trad
Trad
Trad
Trad

MM
MM
MM
MM

9.9±1.1
9.9±1.1
9.9±1.1
9.9±1.1

6.6±0.1
6.6±0.1
6.6±0.1
6.6±0.1

5.7±5.0
5.7±5.0
5.7±5.0
5.7±5.0

51.1±3.0
51.1±3.0
51.1±3.0
51.1±3.0

Trad
Trad
Trad

MCM
MCM
MCM

9.9±1.1
9.9±1.1
9.9±1.1

6.7±0.1
6.7±0.1
6.7±0.1

8.0±3.98.0±3.98.0±3.963.6±30.2
63.6±30.2
63.6±30.2

Zonal
Zonal
Zonal
Zonal

M
M
M
M

11.3±3.0
11.3±3.0
11.3±3.0
11.3±3.0

6.5±1.4
6.5±1.4
6.5±1.4

8.2±2.6
8.2±2.6
8.2±2.652.7±3.652.7±3.652.7±3.6

Zonal
Zonal
Zonal
Zonal

MC
MC
MC
MC

12.7±2.0
12.7±2.0
12.7±2.0
12.7±2.0

6.7±1.4
6.7±1.4
6.7±1.4
6.7±1.4

5.0±2.8
5.0±2.8
5.0±2.8
5.0±2.848.9±6.4
48.9±6.4
48.9±6.4
48.9±6.4

Zonal
Zonal
Zonal
Zonal

MM
MM
MM
MM

10.2±1.8
10.2±1.8
10.2±1.8
10.2±1.8

5.6±0.7
5.6±0.7
5.6±0.7
5.6±0.7

5.7±4.9
5.7±4.9
5.7±4.9
5.7±4.951.3±7.551.3±7.551.3±7.551.3±7.5

Zonal
Zonal
Zonal
Zonal

MCM
MCM
MCM
MCM

10.1±1.2
10.1±1.2
10.1±1.2
10.1±1.2

5.8±0.9
5.8±0.9
5.8±0.9
5.8±0.912.0±3.7
12.0±3.7
12.0±3.7
12.0±3.762.0±6.2
62.0±6.2
62.0±6.2

M = sole Maize; MC = Maize-Cowpea rotation; MM = Maize Mucuna
rotation and MCM = Maize-Cowpea-Mucuna relay.
Table 3. Soil nitrogen species in the Forest agro-ecosystem. ± =standard deviation. Differences in superscript lowercase
letters (ab) within each tillage groups shows significant effect of
cropping at Į<0.05. Differences in superscript letters (xz) within
cropping groups shows a significant effect of tillage at Į<0.05.

Table 4. Soil nitrogen species and DOC in the Transition
agro-ecosystem. ± = standard deviation. Differences insuperscript lowercase letters (ab) within each tillage groups
shows significant effect of cropping at Į<0.05. Differences insuperscript letters (xy) within cropping groups shows asignificant effect of tillage at Į<0.05.
Till
Till
Till

Crop
Crop
Crop

NO33-N
-N
NO
NO
33-N

NH44-N
-N
NH
NH
44-N

DON
DON
DON

DOC
DOC
DOC

-1
mg kg
kg-1-1
mg
kg
-1
mg
mg
kg

None
None
None
None

M
M
M
M

3.6±0.9
3.6±0.9
3.6±0.9
3.6±0.9

6.2±2.0
6.2±2.0
6.2±2.0
6.2±2.0

10.2±8.1
10.2±8.1
10.2±8.1
10.2±8.1

56.1±6.3
56.1±6.3
56.1±6.3
56.1±6.3

None
None
None
None

MC
MC
MC
MC

2.9±0.4
2.9±0.4
2.9±0.4
2.9±0.4

4.9±0.3
4.9±0.3
4.9±0.3
4.9±0.3

6.0±9.5
6.0±9.5
6.0±9.5
6.0±9.555.7±8.4
55.7±8.4
55.7±8.4
55.7±8.4

None
None
None
None

MM
MM
MM
MM

3.7±0.5
3.7±0.5
3.7±0.5
3.7±0.5

8.3±3.88.3±3.88.3±3.88.3±3.823.8±9.4
23.8±9.4
23.8±9.4
23.8±9.4

xxxx67.4±5.0
67.4±5.0
67.4±5.0
67.4±5.0

None
None
None
None

MCM
MCM
MCM
MCM

3.7±0.9
3.7±0.9
3.7±0.9
3.7±0.9

6.7±3.8
6.7±3.8
6.7±3.8
6.7±3.8

4.0±6.9
4.0±6.9
4.0±6.9
4.0±6.954.5±13.2
54.5±13.2
54.5±13.2
54.5±13.2

Trad
Trad
Trad
Trad

M
M
M
M

aaaa3.8±0.4
3.8±0.4
3.8±0.4
3.8±0.4

5.6±1.6
5.6±1.6
5.6±1.6
5.6±1.6

10.8±9.3
10.8±9.3
10.8±9.3
10.8±9.3

46.9±6.9
46.9±6.9
46.9±6.9
46.9±6.9

Trad
Trad
Trad
Trad

MC
MC
MC
MC

bbbb2.6±0.1
2.6±0.1
2.6±0.1
2.6±0.1

aaaa4.9±0.2
4.9±0.2
4.9±0.2
4.9±0.2

11.2±9.7
11.2±9.7
11.2±9.7
11.2±9.7

53.7±11.1
53.7±11.1
53.7±11.1
53.7±11.1

3.9±2.1
3.9±2.1
3.9±2.1
3.9±2.1

bbbb5.7±0.2
5.7±0.2
5.7±0.2
5.7±0.2

10.9±9.5
10.9±9.5
10.9±9.5
10.9±9.5

yyyy49.8±5.0
49.8±5.049.8±5.049.8±5.0

Trad
Trad
Trad
Trad

MM
MM
MM
MM

Trad
Trad
Trad
Trad

MCM
MCM
MCM
MCM

3.0±0.3
3.0±0.3
3.0±0.3
3.0±0.3

aaaa4.3±0.5
4.3±0.5
4.3±0.5
4.3±0.5

5.3±9.3
5.3±9.3
5.3±9.3
5.3±9.3

42.0±11.3
42.0±11.3
42.0±11.3
42.0±11.3

Zonal
Zonal
Zonal
Zonal

M
M
M
M

aaaa3.9±0.4
3.9±0.4
3.9±0.4
3.9±0.4

7.7±4.8
7.7±4.8
7.7±4.8
7.7±4.8

5.6±8.1
5.6±8.1
5.6±8.1
5.6±8.147.9±6.1
47.9±6.1
47.9±6.1
47.9±6.1

Zonal
Zonal
Zonal
Zonal

MC
MC
MC
MC

bbbb2.8±0.4
2.8±0.4
2.8±0.4
2.8±0.4

6.6±2.6
6.6±2.6
6.6±2.6
6.6±2.617.3±18.2
17.3±18.2
17.3±18.2
17.3±18.2

51.3±13.4
51.3±13.4
51.3±13.4
51.3±13.4

Zonal
Zonal
Zonal
Zonal

MM
MM
MM
MM

bbbb2.9±0.3
2.9±0.3
2.9±0.3
2.9±0.3

7.4±3.5
7.4±3.5
7.4±3.5
7.4±3.5

5.4±8.2
5.4±8.2
5.4±8.2
5.4±8.2

yyyy44.5±3.7
44.5±3.744.5±3.744.5±3.7

Zonal
Zonal
Zonal
Zonal

MCM
MCM
MCM
MCM

bbbb2.8±0.3
2.8±0.3
2.8±0.3
2.8±0.3

6.8±3.2
6.8±3.2
6.8±3.2
6.8±3.2

13.0±10.7
13.0±10.7
13.0±10.7
13.0±10.749.5±7.749.5±7.749.5±7.749.5±7.7

M = sole Maize; MC = Maize-Cowpea rotation; MM = Maize Mucuna
rotation and MCM = Maize-Cowpea-Mucuna relay.

None
None
None
None

M
M
M
M

8.6±2.8
8.6±2.8
8.6±2.8
8.6±2.810.4±3.910.4±3.910.4±3.910.4±3.922.2±2.3
22.2±2.3
22.2±2.3
22.2±2.3

xxxx113.9±9.6
113.9±9.6
113.9±9.6
113.9±9.6

None
None
None
None

MC
MC
MC
MC

9.2±2.2
9.2±2.2
9.2±2.2
9.2±2.2

8.9±2.78.9±2.78.9±2.78.9±2.7

17.0±2.917.0±2.917.0±2.917.0±2.990.0±20.2
90.0±20.2
90.0±20.2
90.0±20.2

Table 5. Soil nitrogen species in the Guinea Savannahagro-ecosystem. ± = standard deviation. Differences insuperscript lowercase letters (ab) within each tillage groups
shows significant effect of cropping at Į<0.05. Differences insuperscript letters (xz) within cropping groups shows asignificant effect of tillage at Į<0.05.

None
None
None
None

MM
MM
MM
MM

12.7±5.1
12.7±5.1
12.7±5.1
12.7±5.1

8.2±4.0
8.2±4.0
8.2±4.0
8.2±4.0

21.5±15.5
21.5±15.5
21.5±15.5
21.5±15.599.1±9.0
99.1±9.0
99.1±9.0

Till
Till
Till

None
None
None
None

MCM
MCM
MCM
MCM

9.3±3.6
9.3±3.6
9.3±3.6
9.3±3.6

9.8±5.7
9.8±5.7
9.8±5.7
9.8±5.7

18.0±4.2
18.0±4.2
18.0±4.2
18.0±4.2

126.9±72.4126.9±72.4126.9±72.4

Trad
Trad
Trad
Trad

M
M
M
M

14.7±4.5
14.7±4.5
14.7±4.5
14.7±4.5

8.7±4.98.7±4.98.7±4.98.7±4.914.9±12.7
14.9±12.7
14.9±12.7
14.9±12.7

yyyy89.1±8.6
89.1±8.6
89.1±8.6

Trad
Trad
Trad
Trad

MC
MC
MC
MC

13.0±2.4
13.0±2.4
13.0±2.4
13.0±2.4

12.5±7.0
12.5±7.0
12.5±7.0
12.5±7.0

17.9±1.717.9±1.717.9±1.717.9±1.787.2±28.3
87.2±28.3
87.2±28.3

Trad
Trad
Trad
Trad

MM
MM
MM
MM

9.8±2.69.8±2.69.8±2.69.8±2.6

8.4±3.88.4±3.88.4±3.88.4±3.8

x15.8±2.9xxx15.8±2.9
15.8±2.915.8±2.982.5±15.582.5±15.582.5±15.5

Trad
Trad
Trad
Trad

MCM
MCM
MCM
MCM

11.0±2.4
11.0±2.4
11.0±2.4
11.0±2.4

10.4±5.4
10.4±5.4
10.4±5.4
10.4±5.4

20.2±8.820.2±8.820.2±8.820.2±8.880.3±17.2
80.3±17.2
80.3±17.2

Zonal
Zonal
Zonal
Zonal

M
M
M
M

12.2±6.5
12.2±6.5
12.2±6.5
12.2±6.5

10.4±5.2
10.4±5.2
10.4±5.2
10.4±5.2

15.9±10.5
15.9±10.5
15.9±10.5
15.9±10.5

ayay
ay89.2±3.4
ay89.2±3.489.2±3.4

Zonal
Zonal
Zonal
Zonal

MC
MC
MC
MC

11.8±2.511.8±2.511.8±2.511.8±2.511.8±4.8
11.8±4.8
11.8±4.8
11.8±4.817.9±13.1
17.9±13.1
17.9±13.1
17.9±13.1

106.6±21.7
106.6±21.7
106.6±21.7

Zonal
Zonal
Zonal
Zonal

MM
MM
MM
MM

14.2±4.0
14.2±4.0
14.2±4.0
14.2±4.0

14.6±2.3
14.6±2.3
14.6±2.3
14.6±2.3

y26.6±3.1
yyy26.6±3.1
26.6±3.1
26.6±3.1

bbbb113.5±6.8
113.5±6.8113.5±6.8

Zonal
Zonal
Zonal
Zonal

MCM
MCM
MCM
MCM

8.6±2.6
8.6±2.6
8.6±2.6
8.6±2.6

9.0±4.0
9.0±4.0
9.0±4.0
9.0±4.0

20.2±2.5
20.2±2.5
20.2±2.5
20.2±2.5

101.9±17.3
101.9±17.3
101.9±17.3

Till
Till
Till
Till

Crop
Crop
Crop
Crop

NO
-N
NO
-N
NO
NO333-N
-N
3

NH
-N
NH
-N
NH
NH444-N
-N
4

DON
DON
DON
DON

DOC
DOC
DOC
DOC

-1
mg
kg
mg
kg
mg
mgkg
kg-1-1-1

M = sole Maize; MC = Maize-Cowpea rotation; MM = Maize Mucuna
rotation and MCM = Maize-Cowpea-Mucuna relay.

3.2. Effect of agro-ecosystem on soil N and C
Location, or agro-ecosystem, had a significant effect on soil
nitrogen species and DOC. The Guinea savannah hadsignificantly higher extractable nitrate-N (16.8 ± 3.0 mg kg-1)
and the forest-savannah transition significantly lower nitrate-N(3.3 ± 0.5 mg kg-1) compared to the coastal savannah (11.6 ±2.8 mg kg-1) and forest (11.3 ± 2.1 mg kg-1) agro-ecosystems.
Extractable ammonium-N was significantly higher (10.3 ±1.9 mg kg-1) in the forest compared to the other threeagro-ecosystems (range 6.3 ± 1.2 to 6.8 ± 0.9 mg kg-1). The
forest also had significantly higher DON (19 ± 3 mg kg-1) and
the Guinea savannah significantly lower DON (5.2 ± 3 mgkg-1) compared to the forest-savannah transition. Extractable

Crop
Crop
Crop

NO33-N
-N
NO
-N
NO
33

NH44-N
-N
NH
-N
NH
4

DON
DON
DON

DOC
DOC
DOC

mg kg
kg-1-1-1
mg
kg
mg
None
None
None

M
M
M

14.4±5.0
14.4±5.0
14.4±5.0

6.2±1.1
6.2±1.1
6.2±1.1

xxx5.5±2.7
5.5±2.75.5±2.743.5±9.0
43.5±9.0
43.5±9.0

None
None
None

MC
MC
MC

17.9±7.817.9±7.817.9±7.8

7.1±1.0
7.1±1.0
7.1±1.0

xxx5.5±4.9
5.5±4.9
5.5±4.951.7±8.2
51.7±8.2
51.7±8.2

None
None
None

MCI
MCI
MCI

22.0±6.0
22.0±6.0
22.0±6.0

6.6±1.4
6.6±1.4
6.6±1.4

2.3±4.0
2.3±4.0
2.3±4.046.2±9.546.2±9.546.2±9.5

None
None
None

CM
CM
CM

20.9±12.8
20.9±12.8
20.9±12.8

xxx6.7±0.5
6.7±0.5
6.7±0.5xxx2.3±4.0
2.3±4.0
2.3±4.052.3±16.2
52.3±16.2
52.3±16.2

Trad
Trad
Trad

M
M
M

20.5±7.920.5±7.920.5±7.9

6.4±1.4
6.4±1.4
6.4±1.4

ay0±0
ay
ay
0±0
0±0

44.2±8.544.2±8.544.2±8.5

Trad
Trad
Trad

MC
MC
MC

13.4±5.3
13.4±5.3
13.4±5.3

7.5±1.6
7.5±1.6
7.5±1.6

bbb6.1±5.0
6.1±5.0
6.1±5.044.3±15.2
44.3±15.2
44.3±15.2

Trad
Trad
Trad

MCI
MCI
MCI

14.4±7.6
14.4±7.6
14.4±7.6

5.8±1.65.8±1.65.8±1.6

bbb8.9±0.5
8.9±0.5
8.9±0.548.5±9.3
48.5±9.3
48.5±9.3

Trad
Trad
Trad

CM
CM
CM

14.3±4.914.3±4.914.3±4.9

7.0±1.2
7.0±1.2
7.0±1.2

bbb8.5±2.3
8.5±2.3
8.5±2.3

63.0±8.963.0±8.963.0±8.9

Zonal
Zonal
Zonal

M
M
M

16.9±11.4
16.9±11.4
16.9±11.4

5.8±0.6
5.8±0.6
5.8±0.6xxx4.3±4.1
4.3±4.1
4.3±4.1

46.0±10.8
46.0±10.8
46.0±10.8

Zonal
Zonal
Zonal

MC
MC
MC

13.2±1.6
13.2±1.6
13.2±1.6

7.5±2.4
7.5±2.4
7.5±2.4

ay2.6±2.3ayay
2.6±2.3
2.6±2.345.7±10.945.7±10.945.7±10.9

Zonal
Zonal
Zonal

MCI
MCI
MCI

17.5±3.1
17.5±3.1
17.5±3.1

8.7±3.2
8.7±3.2
8.7±3.2

6.7±2.8
6.7±2.8
6.7±2.865.7±40.2
65.7±40.2
65.7±40.2

Zonal
Zonal
Zonal

CM
CM
CM

16.7±6.4
16.7±6.4
16.7±6.4

yyy5.8±0.2
5.8±0.2
5.8±0.2

by9.3±1.2
byby
9.3±1.2
9.3±1.2

55.0±12.4
55.0±12.4
55.0±12.4

M = sole Maize; MC = Maize-Cowpea rotation; MCI = Maize-Cowpea
intercrop and CM = Cowpea-Maize rotation.

DOC in the forest soil was significantly higher (98 ± 14 mg
kg-1) than in the other three agro-ecosystem soils (range 50 ± 7
to 56 ± 7 mg kg-1).
3.3. Effect of tillage on soil N and C
There was no effect of tillage on extractable soil N and C in
the coastal savannah (Table 2). Tillage had a significant
effect on extractable soil nutrients in the forest, forest-savannahtransition and Guinea savannah agro-ecosystems two years

206

after tillage treatments were initiated but only for specificcropping treatments (Tables 3, 4 and 5). In the forest, zonal
tillage significantly increased DON in the maize-mucuna
rotation relative to the traditional tillage and no-till significantly
increased DOC in the maize only relative to the traditionaltillage (Table 3). In the forest-savannah transition, no-till
significantly increased extractable DOC compared to
traditional tillage in the maize-mucuna rotations (Table 4).
In the Guinea savannah agro-ecosystem, no-till
significantly increased extractable ammonium-N in the
cowpea-maize rotation compared to soil ammonium-N under
zonal tillage (Table 5).
Extractable DON was alsosignificantly affected by tillage in the Guinea savannah undermaize only cropping (Table 5). No-till significantly increased
the concentration of DON under maize-cowpea rotationcompared to zonal tillage. Zonal tillage had significantly higher
DON under cowpea-maize rotation compared to no-till undercowpea-maize rotation (Table 5). Tillage had no significant
effect on extractable soil nitrate-N or DOC in the Guinea
savannah (Table 5).
3.4. Effect of cropping on soil N and C
The type of crop grown appeared to have a greater effecton soil N and C dynamics than tillage, but that often depended
upon the type of tillage being practiced. There were no
significant effects of cropping in the coastal savannah (Table 2).
Under zonal tillage in the forest agro-ecosystem, extractable
DOC was significantly higher under the maize-mucuna
rotation (113.5 ± 6.8 mg kg-1) compared to maize only (89.2 ±3.4 mg kg-1; Table 3).
The forest-savannah transition also experienced
significanteffects of cropping. Under traditional tillage, thesoil beneath maize only exhibited significantly higher soil
nitrate-N concentrations (3.8 ± 0.4 mg kg-1) compared to themaize-cowpea rotation (2.6 ± 0.1 mg kg-1). Similarly under
zonal tillage, maize only had significantly higher nitrate-N
compared with the other cropping treatments. Ammonium-N
was significantly higher in the maize-mucuna rotationcompared to maize only and maize-cowpea-mucuna relay
under traditional tillage (Table 4).
Only DON was
significantly affected by cropping in the Guinea savannah.
Under traditional tillage DON was significantly under all
cropping treatments compared to maize only crops (Table 5).

4. Discussion
Soil C:N ratios from different biomes have a very large
range (Aitkenhead and McDowell, 2000). Across Ghana, theupper soil horizon C:N ratio was relatively low compared toother biomes found around the world. It can be expected thattillage and cropping will have an effect on soil nitrogen andcarbon. As an example, tillage aerates the soil, which isneeded for nitrification. The aeration induced by tillage will
also benefit soil heterotrophs to utilize and mineralize carbon
whereas no-till benefits carbon sequestration (Gonzalez-Chavez et al., 2010). Therefore, it is expected that under
traditional tillage nitrate should be high and DOC low. No-tillon the other hand has been shown to increase soil carbon
content (Gonzalez-Chavez et al., 2010). This expectation for
nitrate-N was not met in a 27 year study in Texas where higher
extractable nitrate-N was found under no-till and rotational
wheat compared to conventional tillage and rotational wheat(Carrillo-Gonzalez et al., 2013). This is further supported bya five-year study in Western Nigeria comparing no-till plots toones prepared under conventional tractor plowing. The study
found that the no-till plots had a higher organic matter content
in the surface soil horizons and higher concentrations of
nitrate-nitrogen, Bray-1 available phosphorus, and ammonium
acetate extractable cations such as Ca2+, Mg2+ and K+ than theplowed plots (Lal, 1976).
Soil extract concentrations can be different depending upon
the type of extracting agent used (Carrillo-Gonzalez et al.,2013). This can be a challenge when trying to compare one’s
results with those of other studies. For soils under along-term tillage and cropping experiments in Texas
examining DOC, DON, nitrate-N and ammonium-N, cold
water extraction of soil best simulated DOC observed in soilleachate (Carrillo-Gonzalez et al., 2013). Concentrations of
DOC under coastal savannah, transition and Guinea savannah
were much lower under no till (range: 44-127 mg kg-1) thanobserved in soil water extracts of a silty clay loam after 27
years of no-till and continuous wheat (116-272 mg kg-1) andno-till with a sorghum, wheat, soybean rotation wheat(133-290 mg kg-1) in Texas, USA (Carrillo-Gonzalez et al.,2013). The difference in extractable DOC may be due to adifference in %C in the two locations; in Texas %C ranged
from 0.9% (conventional tillage) to 1.6% (no-till) and in Ghanasavannah %C ranged from 0.4-1.0% prior to tillage andcropping treatments.
Dissolved organic nitrogen concentrations were similar in
our study (no-till: 2-24 mg kg-1; traditional tillage: 0-20 mgkg-1) compared to the long-term Texas study where DON
ranged from 4.0 to 32.6 mg kg-1 under no-till and 2.4 to 18.0
mg kg-1 under conventional tillage (Carrillo-Gonzalez et al.,2013). Nitrate-N concentrations in our study (8.6-20.9 mgkg-1) were similar to those reported in the Carrillo-Gonzalez(2013) study (10-30 mg kg-1) for all of our agro-ecosystems
except the transition zone where the nitrate-N concentrations
were lower. This suggests that after 27 years of no-till androtational cropping that nitrate-N concentrations and potential

207

for leaching will likely remain high in our plots.
Ammonium-N concentrations tended to be higher in our study(5-15 mg kg-1) compared to the concentrations in the Texas
study which had water extractable ammonium-N
concentrations ranging from 3.0-4.0 mg kg-1 (Carrillo-Gonzalez et al., 2013).
Glomalin related proteins are important for soil aggregate
formation and tend to be much higher under no-till relative toconventional tillage (Gonzalez-Chavez et al., 2010). Theseshould perhaps be an additional measure of soil quality infuture research to monitor potential for soil erosion.
Based on our results it was evident that the different
agro-ecosystems had significantly different nitrogen species
and organic carbon across all tilling and cropping treatments.
This suggests that a blanket directive of tillage and cropping
practices across all the agro-ecosystems in Ghana should not
be in place. Instead, tailored recommendations for tillage andcropping practices for each agro-ecosystem would likely be abetter option to maintain soil fertility and improve foodsecurity based on our results.
Acknowledgements
We thank the Howard Buffett Foundation for funding tosupport this research. We thank Nina Stanley for completingchemical analysis.

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