Content and Distribution of Nitrogen Forms in Some Black Cotton Soils in Akko LGA, Gombe State, Nigeria
A study was conducted to determine the content and distribution of nitrogen forms in some soils of Akko Local Government Area (LGA) of Gombe State, Nigeria. A total of 60 composite soil samples were collected from 0-15 and 15-30 cm depths from purposively selected representative locations in the study area and analyzed using standard procedures. Results obtained showed that the soils were generally low in nitrogen, with organic nitrogen accounting for about 72.8% of the total nitrogen in the soils. The nitrogen forms followed the trend: total nitrogen>total organic nitrogen >NH4-N>NO3¯-N>NO2¯-N in content, while each of the nitrogen forms was more or less uniformly distributed in the study area. Owing to the low available nitrogen content in the soil (<12% of the total N), it is recommended that supplementary nitrogen application and improvement of the drainage conditions of all the soils be employed to improve the nitrogen status of the soils.
Received: April 20, 2011;
Accepted: May 24, 2011;
Published: July 12, 2011
Nitrogen is one of the most widely distributed essential elements in nature.
Of all essential nutrients, nitrogen appears to have the most pronounced effect
on plant growth and development (Ayeni, 2011). It is
an important component of many important structural, genetic and metabolic compounds
in plant cells and is a major component of energy transfer compounds such as
ATP (which allows cells to conserve and use energy released in metabolism) and
nucleic acids such as DNA, the material that allows cells to grow and reproduce
(Brady and Weil, 2005a; Wiseman and
Halliwell, 1996). Nitrogen mainly enters the soils through the organic matter.
Plants and animal wastes decompose, adding nitrogen to the soil through the
action of bacteria that convert organic nitrogen into plant-usable nitrogen.
Nitrogen can also be lost when bacteria change the nitrate into atmospheric
nitrogen and fertilizers are turned into gasses which consequently return to
the atmosphere (Lang, 2010). Nitrogen in soils exists
in three general forms: organic nitrogen, ammonium ions and nitrate ions. About
95 to 99% of potential available nitrogen in the soil is in organic forms; either
as plants and animals residue in the relatively stable soil organic matter,
or in living soil organisms, mainly microbes. It was reported (Brady,
1996) that available nitrogen in soils is often supplemented by nitrogen
released from soil organic matter or organic materials added to the soil.
In Nigeria today, there is a gradual drift from traditional to a more scientific
agriculture. Consequent upon this, the increasing unit cost of nitrogen and
the fragility of soils in Nigeria (Mustapha, 2007b),
amongst others, the evaluation of the nitrogen forms in the soils becomes imperative.
Such evaluations will ensure prudent and sustainable nitrogen application/utilization.
This approach is useful now as efficient fertilizer use, based on recommendations
from soil testing that recognizes inherent variability in soil properties is
the tool for achieving Nigerias scientific agricultural rejuvenation (Mustapha,
2007a). The recommendations will differ from the blanket rates made over
large geographical area which farmers are being encouraged to adopt (Ayodele
and Omotoso, 2008). Recent studies have shown the inappropriateness of these
blanket recommendations; being wasteful thus causing environmental pollution
by, among others, irrational overuse of chemicals such as chemical fertilizers
(Sokouti and Mahdian, 2011).
With the foregoing in view, this study was undertaken to determine the contents and distribution of nitrogen forms in the Black Cotton soils of Akko LGA, Gombe State, Nigeria.
MATERIALS AND METHODS
The study area: The study was conducted between May and August, 2010
in Akko LGA; located along Gombe-Adamawa road, about 30 km away from Gombe town.
It is located at 12°30N and 11°45E, within the northern
Guinea Savanna Zone of Nigeria. The geology of the area is said to be tertiary
continental sandstone to the west of the Keri-Keri escarpment, clays and siltstones.
The climate is characterized by two distinct wet and dry seasons. The wet season
starts in May and ends in October while the dry season starts in November and
ends in April. The annual rainfall is about 800 to 900 mm per annum with mean
annual temperature ranging from 30 to 32°C (BSADP, 1982).
Soil sampling and handling: A total of 60 composite soil samples were collected at 0-15 and 15-30 cm depths from 15 different, purposively selected representative locations in Akko LGA of Gombe State, Nigeria. Each composite soil sample was made of 5 sub samples.
The collected soil samples were properly labeled and stored in polythene bags and taken to the laboratory. In the laboratory, each sample was separately dried in air and then ground using porcelain pestle and mortar. The ground soil samples were sieved with 2 mm sieve and the fine earth fractions, collected in separate bags, were used for all the laboratory analyses.
Laboratory analyses: The processed soil samples were subjected to standard
laboratory analyses as described by Page et al. (1982).
Particle size distribution was determined using the hydrometer method (Bouyoucos,
1951) while soil pH was determined in water at a 1:1 soil to water ratio
using glass electrode pH meter. Organic carbon was determined by the wet oxidation
method (Walkley and Black, 1934). Total Nitrogen was
determined by the micro-Kjeldhal method as described by Juo
(1979), while the organic and inorganic forms of nitrogen were determined
using the method described by Black (1965).
For the purpose of interpretation, critical limits provided by Esu
(1991) were used for organic carbon and total nitrogen.
Data analysis: Data generated were subjected to simple descriptive statistics,
including range and means (Harry and Steven, 1995). Analysis
of variance was employed using the Minitab computer software to determine significant
differences between means. Means that were statistically different were separated
using the Least Significant Difference (LSD).
RESULTS AND DISCUSSION
Physico-chemical properties: The particle size distribution of the soils
as shown in Table 1 indicates that the soils have relatively
high sand (mean = 46.9%) and clay (mean = 30.1%) contents; giving the soils
a generally sandy clay to clay texture.
|| Particle size distribution in the Black Cotton soils in Akko
LGA, Gombe state, Nigeria
|NS = Not significant
Clay content significantly (p<0.05) increased with soil depth. This is
expected as some of the increase with depth in clay may be as a result of removal
of the fraction by surface run-off and also by illuviation. This is a common
phenomenon in soil in this agro-ecology as was also reported by Voncir
et al. (2008). The results also indicate that all the fractions varied
significantly (p<0.05) between the locations indicating wide variability
in the fractions between soils from different parts of the LGA. Except for clay,
however, all the other fractions did not vary significantly (p>0.05) with
The soil reaction ranged from pH 5.7-6.5 (mean = 6.1) indicating slightly acidic
reaction. Though generally acidic, the pH values varied significantly (p<0.05)
between the locations and depths considered. The upper 0-15 cm was more acidic
(pH = 5.8) than the lower 15-30 cm (pH = 6.4). This could be attributed to the
removal of basic cations from the surface of the soils to the lower depths as
was also observed by Voncir et al. (2008) and
Kolo et al. (2009) and probably the use of acid-forming
fertilizers such as urea for agricultural purposes.
Organic carbon: Results in Table 1 show that organic
carbon content was generally low (Esu, 1991) through the
locations; with values ranging from 0.39 to 1.56 (mean = 0.83) g kg-1.
Even though the surface 0-15 cm soils contained more organic carbon (0.99 g
kg-1) than the lower 15- 30 cm (0.75 kg-1), the differences
were not statistically significant (p>0.05). Similar low organic carbon values
have been reported by Lombin (1983) for the Nigeria
Savannah soils and Mustapha and Nnalee (2007) for soils
in the northern guinea Savanna zone of Nigeria.
|| Distribution of nitrogen forms in the Black Cotton soils
of Akko LGA, Gombe state, Nigeria
It is noteworthy, however, that locations M. Bakari, W. Yola, Kembu and Y. Shehu contained medium levels of organic carbon ranging from 1.00 to 1.56 g kg-1. It is probable that farmers in these locations apply quite appreciable quantities of organic residues to their farmlands as a means of improving the fertility owing in part to the high cost of inorganic fertilizers.
Total nitrogen: The total nitrogen content of the soils (Table
2) followed a similar trend as the organic carbon content. The values were
generally low ranging from 560.0 to 1915 mg kg-1 and were not statistically
different between the locations (Table 2). The distribution
of the total nitrogen, however, varied significantly (p<0.05) with soil depth.
This variability could be attributed to the organic matter content on surface
soil resulting from liter fall and other means of organic matter deposition
on the soil surface. The values obtained are typical of the Nigeria savanna
soils as reported by Lombin and Esu (1987).
Organic nitrogen: Table 2 shows that the organic nitrogen
content ranged from 451.5-969.5 (mean = 754.8) mg kg-1. Even though
there appeared to be a wide variability in total organic nitrogen content in
the soils studied between locations, the differences were not statistically
significant (p>0.05). Importantly, however, is that the organic-N fraction
forms the bulk of the total nitrogen in the soils; accounting for about 72.8%
of the total. Brady and Weil (2005b) had earlier reported
that about 95-99% of the soil nitrogen is in organic compounds that protect
it from loss; thus leaving it largely unavailable to plants. The results obtained
from this study tend, to a large extent, agree with this assertion.
The top 0.15 cm is in the soils, contained significantly (p>0.05) higher
(888.9 mg kg-1) amount of organic nitrogen than the soils in the
15-30 cm stratum (630.7 mg kg-1). This may not be unconnected to
the higher organic matter content in the top 0-15 cm soil as was observed in
the soils studied (Table 1).
Total inorganic nitrogen: Total inorganic nitrogen in the soils studied (Table 2) varied widely ranging from 56.5-215.0 (mean = 110.3) mg kg-1 and contributed about 11.6% to the total nitrogen present in the soils. The wide variability may not be unconnected to the variability in organic carbon contents earlier observed in the soils studied and probably the variable use of agricultural inputs, especially nitrogen fertilizers, in the area studied.
Depth wise, inorganic nitrogen content appeared to decrease from 126.2 to 94.4 mg kg-1 with increasing depth. This decrease was, however, not statistically significant (p>0.05).
Ammonium-bound nitrogen: Of the three main forms of inorganic nitrogen
forms (NH4 -N, NO3-N and NO-2-N), ammonium
nitrogen formed the bulk (range = 35-119.8; mean = 83.9 mg kg-1),
accounting for about 76.1% of the total inorganic nitrogen and 8.7% of the total
nitrogen contents of the soils (Table 2). The results corroborate
the reports of OLeary et al. (2002) that
NH4-N, the most common form of N taken up by plants through the roots,
is the most abundant form of inorganic nitrogen soils. USEPA
(1994) attributed the high contents of NH4-N in soils to the
amount of organic matter, microbial activities and the soil pH. Even though
NH4-N was fairly similar in distribution between the locations considered
in the LGA, it significantly (p<0.05) varied with depth; following similar
trend as organic and inorganic nitrogen.
Nitrate-nitrogen: Nitrate-bound nitrogen (range = 12.5-21.0; mean =
18.5 mg kg-1) did not vary significantly (p<0.05) between the
locations in the study area, though it did with depth (Table 2).
It was next to NH4-N; accounting for about 16.8% of the total inorganic
nitrogen and 1.8% of the total nitrogen content of the soil. The preponderance
of NH4-N over NO3¯ N in the present study may not
be unconnected to the pH of the soils in the study area. Chude
et al. (2004) reported that while NO- 3-N predominates
in higher pH levels, NH4+-N are more common in low (acid)
pH levels. It is noteworthy that plants take up nitrogen from the soil solution
mainly as NH4 and NO3- ions. However, owing to its ease
of mobility in water, nitrate (NO3-) could be of concern in underground
water contamination which affects human and ruminant health (Bahmani
et al., 2009); hence the need to constantly monitor its accumulation
(Scott and Daryl, 1993).
Nitrite-nitrogen: The results of the present study (Table 2) indicate that NO-2-N is the lowest in content of all the inorganic nitrogen forms studied. It did not significantly (p<0.05) vary both with location and with depth in the soils. Ranging from 2.8 to 6.3 (mean = 4.13) mg kg-1, NO-2-N accounted for only 3.7% of the total inorganic nitrogen and about 0.5% of the total nitrogen of the soil.
Results of the present study indicate that the soils studied were low in nitrogen
available for plant uptake, with organic nitrogen form accounting for the bulk
(about 72.8%) of the total nitrogen in the soil. The content of the nitrogen
found in the soils followed the trend: total nitrogen >total organic nitrogen>total
inorganic nitrogen N>NH4+- N>NO3-N>N02¯
N. Owing to the very low nitrogen levels in the soil and low percentage (<12%)
of this as available for plant uptake, supplementary application of nitrogen
is recommended for successful crop production. The need for proper monitoring
of the accumulation of NO3- is further emphasized as it can contaminate
underground waters and be toxic to, especially newborns, causing anoxia or internal
suffocation. Results of the study further indicate that the pH and drainage
conditions of the soils be improved to stimulate microbial degradation of organic
matter and consequently release the organic nitrogen for plant use.
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