Abstract

The aim of this study was to assess soil carbon (C), nitrogen (N), and phosphorus (P) stocks from fallow of two forage legumes: Centrosema pascuorum (Cp) and Macrotyloma uniflorum (Mu) on Alfisols of Guinea Savanna, Nigeria. The study was conducted at the Institute for Agricultural Research (IAR) experimental field Samaru Zaria, Nigeria (2008 to 2009). Treatment consisted of 3 plots (Cp planted, Mu planted, and control - natural vegetation regrowth). Plot size was 5 m * 3 m = 15 m² replicated three times for each treatment. Pre-experimental composite soil samples were taken with an auger at 0-15 depth. The soil samples collected were air-dried, grounded, sieved with a 2 mm sieve, and the less than 2 mm fraction was analysed for C, N, and P. The results of the analysis of soil organic C, total N, and available P were 6.1 gkg^-1, 0.53 gkg-¹, and 8.75 mgkg-¹ respectively. Soil pH was 5.9. The plots were left fallow for one year and again soil samples were collected at 0-15 cm depth and analysed. The results showed that Cp significantly improved and had higher soil available P (13.74 mgkg^-1) after one year followed by Mu (7.68 mgkg^-1) and control (4.6 mgkg^-1). On the other hand, the control plots significantly had highest soil organic C (5.9 gkg-¹) compared to 5.2 and 3.7 gkg-¹ from Cp and Mu. Similarly, higher total N (2.9 gkg-¹) was recorded from control plots compared to 1.4 and 0.5 gkg-¹ from Mu and Cp respectively. Results from this study indicate that one year fallow of cultivated Cp has potential to improve soil available P compared to Mu and natural vegetation regrowth. In terms of plant nutrient uptake, the N and P content of Mu was the highest (4.28 and 2.65 % respectively). The natural vegetation from the control plots had the lowest (1.95 %) N concentration. There was no significant difference in the P content of the natural vegetation from the control plots and Cp (1.92 and 1.84 % respectively). Fallow periods of more than a year of cultivated Centrosema pascuorum and Macrotyloma uniflorum or their incorporation into soils are suggested for further studies.

1. Introduction

The continuous use of inorganic fertilizers especially under intensive agriculture can intensify problem of soil degradation ^{1, 2}. Also, the availability and affordability of inorganic fertilizers to smallholder farmers constitutes major challenge to crop production. Fallow systems involving forage legumes grown or incorporated into soils is a sustainable method of crop production that enhances soil quality. This includes reduction in the reliance of external inputs such as the use of inorganic fertilizers. However, the increase in the demand for land for competing uses arising from increase in population might make fallow systems of land unrealistic. Therefore, the use of short fallow periods with legumes or grasses can be a sustainable soil quality management ³.

Adequate management of planted fallow legumes can significantly improve soil nutrient status of the soil mainly carbon, nitrogen, phosphorus concentration as well as organic matter ^{3, 4, 5}. Furthermore, it provides effective cover against erosion, suppress weeds and pests and improve soil physical conditions ^{6, 7}. Enhanced soil N provided by forage legumes have been reported to improve the growth and yield performance of cereals cultivated after them ^{8, 9, 10}. The aim of this study was to assess soil carbon, nitrogen, and phosphorus stocks from fallow of two forage legumes: Centrosema pascuorum and Macrotyloma uniflorum on Alfisols of Guinea Savanna, Nigeria.

2. Materials and Methods

The study was conducted at the Institute for Agricultural Research (IAR) experimental field Samaru Zaria. Samaru is located on longitude 7° 38’ E and latitude 11° 11’ N. It is characterised by a tropical continent type of climate with alternating wet and dry seasons, which vary markedly in intensity and duration with attendant effects on agricultural productivity. The soils are formed from loess materials overlying basement complex, covering an area of 43,000 km ¹¹. The clay is predominantly Kaolinitic ¹², and is typically high in iron (Fe) and aluminium (Al) oxides ¹³ and are of major agricultural importance ¹⁴.

The area has a uni-modal pattern of rainfall annually, beginning in April and ending in October. Rainfall records show that the month of August recorded the highest rainfall amounts. The area is also characterised by high average monthly maximum temperatures and low minimum temperatures within the year with temperatures peak normally attained in March/April. The maximum and minimum temperatures during the rainy season range from 14.12°C –23.17°C and 29.97°C – 38.43°C respectively ¹⁵. The hamattan periods of December to February are mostly associated with lower temperature. Samaru has a very wide range of mean monthly relative humidity (11-85 %) compared with locations at lower latitudes within Nigeria ¹⁵. The length of raining season however has been on the decline in recent years. The normal raining season had been from the month of February with steady rise through the months of August when it attains its highest peak and begins to decline gradually terminating in October.

The experimental field under natural fallow for a year was divided into three (3) main plots. Two (2) of these plots were cultivated with forage legumes for a year while the third plot continued as natural fallow (control plot) - treatment consisted of 3 plots (Cp planted, Mu planted, and control - natural vegetation regrowth). Plot size was 5 m * 3 m = 15 m² replicated three times for each treatment in Randomized Complete Block Design (RCBD).

Pre-experimental and post-cropping composite soil samples were taken with an auger at depths 0-5 and 5-10 cm. The samples collected were air-dried, grounded and sieved with a 2 mm sieve. The less than 2 mm fraction was analysed for some chemical (organic carbon, total nitrogen, available phosphorus, pH) and physical properties (bulky density, total porosity, and hydraulic conductivity).

Bulk densities for the soils were obtained by sampling with 5 cm by 5 cm cores as described by Anderson and Ingram ¹⁶. The core samples were oven dried at 105°C-110°C over 24 hours to constant weight, after which they were removed and their weights taken. The bulk densities were computed using the formula:

(1)

The saturated hydraulic conductivity was measured by the constant head method, using the 1CW laboratory permeameter (Eijikelkamp Agrisearch No. 09.02).

The pH of the soil samples were measured in water and in 0.01M CaCl₂ solution, using the potentiometric (glass electrode) method ¹⁷.

The organic carbon contents of soil samples were determined using the Walkley-Black wet oxidation method ¹⁸.

Total nitrogen was determined using the Kjeldahl method described by Bremner and Mulvaney ¹⁹.

Bray 1 Method was employed for extraction and Ascorbic acid in the presence of ammonium molybdate and antimony tartarate was used for colorimetric quantitative determination of the available P as described by Bray and Kurtz ²⁰ and Anderson & Ingram ¹⁶.

Biomass produced for the three fallow systems were estimated using the quadrant method of 1m by 1m. Forage legumes biomass were harvested, separated from the roots, weighed as wet weight then was oven dried for 48 hours and later reweighed for dry weight. Samples of these dried shoot were taken, ground, sieved and analysed for N and P.

Data collected were subjected to Analysis of Variance (ANOVA) using GENSTAT Discovery software. Means were separated using Least significant Difference (LSD) at 5% probability level (P<0.05).

3. Results and Discussions

The effect of fallow of forage legumes on selected physical properties (saturated hydraulic conductivity, bulk density, and total porosity) of Alfisols in Guinea Savanna of Nigeria (2008 – 2009 Field trials) is presented in Table 1. The results revealed no significant difference in the interaction between legumes cultivated and the depth at which soil samples were collected for the physical properties. After one year fallow, treatments did not show significant difference in bulk density. On the other hand, significant difference in the Ksat and total porosity was observed from the treatments applied. The control plots had lowest Ksat and total porosity (0.037 cmsec-1, and 46.48 % respectively) while the forage legumes plots were higher (Centrosema: Ksat = 0.145 cmsec-1, total porosity = 50.69 %; Macrotyloma: Ksat = 0.535 cmsec-1, total porosity = 51.20 %). This could be due to reduced bulk density, increased porosity and aeration caused by roots of the legumes, and increased organic matter from plant biomass as well as microbial activities ^{3, 21, 22, 23, 24}. There was no difference in Ksat and total porosity of Centrosema and Macrotyloma from one year fallow. Results showed higher (50.15 %) total porosity was observed at 5 – 10 cm depth compared to 48.76 % at 0-5 cm after one year fallow. This is in contrast to the results of Malgwi et al. ²⁵ that porosity decreases gradually with depth to a value of 30 % or less in the studied area. A factor that might have influenced the total porosity in this study is cropping and/or tillage system - less intensively used soils or mechanically cultivated. To an extent, the cropping and tillage system also have influence on macroporosity of soils ²⁶.

Soil carbon, nitrogen, and phosphorus concentrations from one year fallow of cultivated forage legumes on Alfisols of Guinea Savanna Nigeria is presented in Table 2. Highest pH (6.0), total nitrogen (2.9 g/kg), and organic carbon (5.9 g/kg) were recorded from the control plots. Highest phosphorus concentration (13.74 mg/kg) were recorded from centrosema plots followed by Macrotyloma (7.68 mg/kg), and control (4.64 mg/kg).

While the control plots had increase in pH from one year fallow period, plots planted to centrosema and macrotyloma had a decrease in pH. The pH of the soils is slightly acidic (5.8 – 6.0). The Nigerian guinea savanna soils are generally slightly acidic, less leached soils derived from precambrian crystalline basement complex rocks, the soils are generally coarse textured consisting of sandy loam or loam over gravelly clay loam ¹³.

There was an increase in the total nitrogen of the control and macrotyloma plots while centrosema plots had a decrease in total nitrogen from the one year fallow period. Furthermore, one year fallow period resulted in decrease in the carbon content of control and macrotyloma plots but increase in carbon was observed in the cetrosema plots. Also, it was evident that one year fallow period increased the phosphorus concentrations of plots planted to centrosma and Macrotyloma. Conversely, there was a decrease in the phosphorus concentration of the control plots from one year fallow period. The improved soil physical properties (Ksat and total porosity) observed in the centrosema and macrotyloma plots might have contributed to increased concentrations of phosphorus. Although it has been reported that the of fallow legumes can significantly improve soil carbon, nitrogen, phosphorus concentration as well as organic matter ^{3, 4, 5}, the results from this study only affirms for increase in phosphorus concentrations.

The nutrient uptake (nitrogen and phosphorus) from one year fallow (2008 – 2009) of forage legumes in Alfisols of Guinea Savanna Nigeria is presented in Table 3. The nitrogen and phosphorus content of Macrotyloma was the highest (4.28 and 2.65 % respectively). The natural vegetation from the control plots had the lowest (1.95 %) nitrogen concentration. The nitrogen content from one year fallow period observed in this study was higher compared to the range of 2.0 to 2.19 % reported from two years fallow period in the same region by Odunze et al. ²⁷. There was no significant difference in the phosphorus content of the natural vegetation from the control plots and centrosema (1.92 and 1.84 % respectively). The results indicated that Macrotyloma uniflorum had higher nutrient uptake (nutritional quality) compared to Centrosema pascuorum and natural vegetation in the studied location. In addition to the nutritional quality that Macrotyloma uniflorum can offer to livestock from their fallow period(s), their large groundcover can provide soil quality benefits such as soil aggregate development and soil moisture conservation ^{27, 28}.

4. Conclusions

This study assessed soil carbon, nitrogen, and phosphorus stocks from fallow of Centrosema pascuorum, Macrotyloma uniflorum, and natural vegetation regrowth on Alfisols of Guinea Savanna, Nigeria. The results revealed that one year fallow period resulted in decrease in the carbon content of natural vegetation regrowth and macrotyloma. Increase in carbon content was observed in the plots planted to cetrosema. Also, it was evident that one year fallow period increased the phosphorus concentrations of plots planted to centrosma and Macrotyloma. Conversely, there was a decrease in the phosphorus concentration of the control plots from one year fallow period. In terms of plant nutrient uptake, the results indicated that Macrotyloma uniflorum had higher nutrient uptake compared to Centrosema pascuorum and natural vegetation. Fallow periods of more than a year of cultivated Centrosema pascuorum and Macrotyloma uniflorum or their incorporation into soils are suggested for further studies.

References