Soil Organic Carbon Concentrations and Stocks under Maize/Legume Cropping System in Alfisols of a Savanna Zone, Nigeria

Carbon sequestration in soil aggregates and Carbon stock (SOC) under Maize-Legume Cropping system in a Northern Guinea Savanna Alfisol, Nigeria trial was conducted in 2014 and 2015 cropping seasons. The experiment was a randomized complete block design (RCBD), replicated three times and treatments used were: Sole Maize (M), Desmodium (D) and Soybeans (S); Maizesoybeans intercrop (MS), Maize-Desmodium intercrop (MD), Maize Strip cropped with Soybean (MS 2:4) and Maize Strip cropped with Desmodium (MD 2:4). Data obtained were evaluated for Organic carbon, carbon stock, Bulk density and mean weight diameter of aggregates in the soil. Results obtained show that Mono-crop (Sole) Maize treatment gave significantly higher BD than other treatments at 8WAP and 16WAP, suggesting that soils under sole maize were degraded for sustainable crop production. Organic carbon sequestered over 2014 to 2015 was least under MD and highest under MD2:4 treatments and mean carbon stock sequestered in the macro and micro aggregates was highest under MD 2:4 (28.35 t Cha -1 ) and least under MD (8.82 t Cha -1 ). Soil Original Research Article Chidowe et al.; BJAST, 21(1): 1-12, 2017; Article no.BJAST.32538 2 organic carbon (SOC) sequestered in macro aggregates under MS (1.38 gkg -1 ) were significantly higher than the other treatments. Maize/Desmodium 2:4 treatment was inferred to have best improved soil conditions (quality/health) for sustainable crop production, mitigate climate change and global warming by sequestering carbon better than the other treatments.


INTRODUCTION
Soil is a vital resource for producing food and fiber needed to support an increasing world population [1]. However, degradation of soil as a consequence of improper land use management practices pose serious threat to sustainable agriculture, resulting in the need for appropriate soil management strategy. In grassland areas for example, implementing grassland management practices that increase carbon uptake by increasing productivity and/or reducing carbon losses (e.g. through high rates of off-take) can lead to net accumulation of carbon in grassland savanna soils by sequestering atmospheric carbon dioxide (CO 2 ). The potential to sequester carbon by improving grassland practices or rehabilitating degraded savanna grasslands is substantial because practices that sequester carbon in grasslands often enhance productivity [2]. Practices that sequester carbon in grasslands also tend to enhance resilience in the face of climate variability, and are thus likely to enhance longer-term adaptation to changing climates [2,3]. Therefore, management of soil organic carbon to maintain the soil in good health is a major concern and challenging task in the arid and semi-arid tropical regions, and Nigeria in particular. Strategy for increasing and sustaining crop yields at a high productivity level must include integrated approaches to management of soil carbon that recognizes soil as the foundation and storehouse of most plant nutrients essential for plant growth.
Concerns over global warming have led to several investigations on quality, kind, distribution and behaviour of soil organic carbon [4,5,6,7] that have led to various quantitative estimates of soil organic carbon [8,9,10]. Reporting of organic' carbon status of soils in terms of per cent distribution is one way [3]; yet, it does not show the stock and reservoir of organic carbon in a particular area. For this study soil organic carbon will be reported on unit area basis for a specified depth interval and described as soil carbon stock (SOC). Over exploitation of soil has resulted in exhaustion of intensive agricultural production systems, steady declining productivity [11] and impoverished soil quality. Therefore, the way in which soil carbon is managed will majorly impact on plant growth, soil fertility, agricultural sustainability and environmental conservation.
Soils of Nigerian Northern Guinea Savanna are intensively cultivated with maize, sorghum, cowpea, groundnut, cotton and soybeans, and have resulted in inherently poor fertility status [12,13,14], have poor moisture retention capacity, rich in low activity clays and sesquioxides [15] and have very low organic carbon content [16]. The soils are therefore in a degraded condition to support sustainable agricultural production and require appropriate integrated management practices that will enhance productivity of the soils. Due to the fragile nature of the soil, they degrade rapidly under continuous and intensive cultivation [17]. In the Nigerian Northern Guinea Savanna zone, soil is frequently tilled at land preparation, crop residues are harvested for fencing, fuel wood or livestock feed [18,19], are not returned to restore soil carbon stock and fertility. Continued intensive cultivation, coupled with annual nonreturn of crop residues to the soil has conferred impoverished soil productivity status and necessitated the study on 'soil sequestration and carbon stock under maize/legume cropping system in Alfisols of a Savanna zone, Nigeria. Commonly, cereal-based cropping systems in the Northern Guinea grassland Savanna of Nigeria practice legume relays into cereals, strip cropping of cereals with legumes, sole cropping of cereals and legumes. However, the focus for these management practices is largely on maximizing crop yield with little or no attention to resulting soil productivity status that would support subsequent cropping. The present study therefore intends to evaluate carbon sequestration and carbon stock of soils under varying cereal/legume practices with a view to determine most sustainable management practice(s) best enhanced soil productivity in the Northern Guinea Savanna zone Alfisols.

Description of the Study Area
This study was conducted during 2014 and 2015 rain-fed cropping seasons at the experimental farm of Institute for Agricultural Research (IAR), Samaru, Zaria (latitude 11°11'19.3"N and Longitude 7°37'02"E) in the Northern Guinea Savanna ecology of Nigeria (Fig. 1). Long-term mean annual rainfall of the study area is 986.5mm and is concentrated between May and October with a peak in August [20]. The mean daily air temperature (maximum and minimum) ranges between 15°C and 38°C [21]. Soil type of the study area was classified as Typic Haplustalf according to USDA Soil Taxonomy [22] as cited by [23] and Acrisol in the FAO-UNESCO legend as cited by [24] and [25]. The soils are low in inherent fertility, organic matter, cation exchange capacity (CEC) and dominated by low activity clays [12,26].  field was ploughed, harrowed and ridged at 75 cm between ridge distances and size of the field was 50m by 35m which is 1750m 2 (0.175 ha). Plot size was 6m by 11m (66m 2 ). One maize plant was allowed on crest of the ridge at 25cm intra row and 0.75cm inter row distances while soybean and Desmodium were both drilled along ridge at 5 cm intra row and 75 cm inter row spacing. Weeding was done manually at 3 and 6 weeks after planting (WAP), 60 kg Nha -1 , 60 kg P 2 O 5 ha -1 and 60 kg K 2 Oha -1 were basally applied at planting and top dressing was done with 60 kg Nha -1 at six weeks after planting with nitrogen sourced from Urea. Phosphorus was sourced from single super phosphate (SSP) and Muriate of potash was the source for potassium.

Soil Sampling Procedures
A total of 10 soil samples were taken from five points at depths 0-10 cm and 10-20 cm along two diagonals of the study field, homogenized, air-dried, ground and sieved through a 2mm sieve for laboratory analysis. The less than 2 mm fractions were analyzed for soil pH, particle size distribution, organic carbon, total nitrogen, available phosphorus, cation exchange capacity, exchangeable bases and exchangeable acidity to characterize initial properties of the soil. Also core soil samples were collected using 5 cm by 5 cm core samplers to determine bulk density of the soils. 50 g soil aggregates were obtained before trial establishment and in each treatment at harvest and assessed for aggregate stability and distribution using dry sieving methods. Carbon concentration in aggregate fractions was also determined from aggregate sizes in sieves and referred to as carbon sequestered in aggregates. Also, at 8 and 16 weeks after planting (WAP), core soil samples were collected using 5 cm by 5 cm core samplers for the analysis of bulk density to evaluate change in bulk density. Disturbed soil samples were obtained at 0-10 and 10-20 cm depths and core soil samples at 0-5 cm, 5-10 cm, 10-15 cm and 15-20 cm depths in plots/treatment and analyzed for organic carbon concentration and bulk density respectively at harvest to evaluate for soil organic carbon stock (SOC) in the area in each of 2014 and 2015.

Data Acquisition and Analytical Procedures
Particle size distribution was determined using the hydrometer method [27] and textural classes were obtained from textural triangle using the [28]) approach. Soil bulk density was determined by the [29] method. Aggregate stability was determined by dry sieving methods of [30], modified by [31]. Sieve sizes used were 1mm -2mm and aggregates in these sieve sizes were recorded and evaluated while the bulk soil samples (50 g) were sieved with 5mm sieve. Aggregates less than 0.25 mm were not evaluated in this study. Aggregate fractions distribution were determined and mean weight diameter (MWD) of the aggregates were calculated as shown in (1) by summing product of mean diameter of aggregates and proportion of soil in each aggregate-size class [31].
Where X i = proportional by weight of sand free aggregate W i = mean diameter of proceeding and preceding sieve Soil organic carbon (SOC) stock was determined as a product of soil carbon of each depth, multiplied by depth, bulk density and 10000m 2 and divided by 1000 i.e., Where SOC=Carbon Stock of soil (t C ha This consists of calculating carbon stock as a product of soil organic carbon (gkg -1 ), bulk density (Mgm -3 ) and depth (m) and multiplied by one hectare (1ha). Soil organic carbon sequestration was obtained from soil aggregates in each sieve size i.e., soil fraction contained in each of the sieve sizes. The soils used were obtained at depth 0-10 cm and 10-20cm depth. Soil pH was determined electrometrically at a ratio of 1:2.5 Soils to Water and CaCl 2 as described by [32]. Soil Organic Carbon was measured by wet oxidation method of Walkley and Black [33], and Available Phosphorus was measured by Bray No. 1 method described by [34] and [35]. Total Nitrogen was determined by the regular micro-Kjeldahl digestion method [36] and exchangeable acidity was determined by shaking soil in 0.01M KCl and filtrate was titrated with 0.1M NaOH [37]. Exchangeable bases (Ca, Mg, K and Na) were extracted with 1N NH 4 OA C [38].
Exchangeable Calcium (Ca) and Magnesium (Mg) were determined by EDTA titration methods [37]. Potassium (K) and Sodium (Na) was determined using flame photometry [39]. Cation Exchange Capacity (CEC) was determined by the1N Neutral Ammonium acetate (1N NH 4 OA C ) method described by [40] method. After harvest in each of 2014 and 2015, soil samples were obtained from each plot and analyzed for organic carbon, organic carbon in aggregates and carbon stock. Also, core samples were obtained from plots and analyzed for bulk density, using the 5 cm by 5 cm core rings. Data obtained was subjected to Analysis of variance (ANOVA) using General Linear Model (GLM) procedure of SAS 9.3 Software [41]. Differences between means were separated using Duncan's Multiple Range Test at 5% level of probability.

Initial Characteristics of Studied Soil
Bulk density (BD) of surface soils prior to experimentation range between 1.43 Mgm -3 to 1.57 Mgm -3 and is moderate in range to support sustainable agriculture (Table 1). Sand Fractions dominate the soil separates with values as high as 490 gkg -1 at the surface layers (0-10 cm) and 450 gkg -1 at the sub surface depths (10-20 cm). Silt value was 430 gkg -1 at the surface layers (0-10 cm), 460 gkg -1 in the sub surface layers (10-20 cm) and Clay value was 80 gkg -1 at the surface layers (0-10 cm) and 90 gkg -1 in the subsurface layers (10-20 cm). The Textural class according to USDA classification for surface and subsurface horizons was loam. Mean weight diameter (MWD) at the surface soil (0-10 cm) was 0.48 and was lower than that of subsurface (0.52) soil (10-20 cm) and suggests that surface soils could be more prone to erosion by wind and highly degraded for sustainable agricultural productivity. Soil pH in water was 5.80 at surface soils and 6.80 in the sub-surface depth, while pH in CaCl 2 at the soil surface was pH 4.89 and 5.20 in the sub-surface soils. The soils are therefore slightly acid and within the range for optimal nutrient uptake by plant roots [42]. Organic carbon values were higher at the surface (0-10 cm) layer (2.11 gkg -1 ) and lower at subsurface soil (10-20 cm) layer (1.99 gkg -1 ). Total Nitrogen of soils at surface layer was 0.50 gkg -1 and lower at the sub-surface with a mean value of 0.40 gkg -1 . Available phosphorus of the surface soils (0-10 cm) was 4.91 mg kg -1 and 4.99 mg kg -1 at the sub-surface soils (10-20 cm). Exchangeable calcium had a value of 2.20 cmol kg -1 at the surface layers (0-10 cm) and 2.30 cmol kg -1 at the sub-surface soils. Exchangeable Mg was higher at the sub-surface and lower at the surface soils with values of 0.62 cmol kg -1 and 0.59 cmol kg -1 respectively. Exchangeable K values were slightly low in both surface; 0.31 cmol kg -1 and 0.36 cmol kg -1 in the surface and sub-surface depths (Table 1). These values confirm [12,13,14,15]  ; suggesting that the soils had no acid problems. Cation Exchange Capacity (CEC) of the soils was 7.75 cmolkg -1 at the surface (0-10 cm) and 7.50 cmolkg -1 at sub-surface layer (10-20 cm). Low CEC values of the experimental area (< 10 cmolkg -1 ) suggests dominance of low activity clays and sesquioxides [43], as well as low soil organic carbon content (Table 1). Initial carbon stock shows low values of 3.02 and 3.12 t Cha -1 at the 0-10 and 10-20 com depths to conform poor soil quality status of soils of the Nigerian Savanna Alfisol [43], 16].  Table 2. Lowest bulk density value was obtained in plots cropped with sole Desmodium (D); 1.48 Mgm -3 at 8 WAP and 1.39 Mgm -3 at 16 WAP. Perhaps, sole maize treatment caused more compaction on the soils relative to the other treatments, while sole Desmodium uncinatum best improved soil bulk density for roots growth and ramification [44]). Soil properties and processes such as moisture retention, water flow, root development, nutrient cycling and the sustainability of micro and macro organisms are negatively influenced by high bulk density values [45,46]. Hence, soils under sole maize treatment (M) having high bulk density values, could impair moisture retention, water flow, root development, nutrient cycling [47,44] and sustainability of micro and macro organisms activity to bestow a degraded status to the soils. At 8 and 16 WAP, there was no significant difference among the treatment in 2015 on bulk density conditions, though values decreased (improved) below 2014 records; perhaps due to improved management practice adopted in 2015.

Effect of Cropping Systems on Mean Weight Diameter of Soil Aggregates and Percent Change Over 2014 and 2015
Maize  (Table 3). However, aggregate stability or the distribution of stable aggregates is important to maintain a balance of air and water in the soil system and the development of plant roots. Hence, wellaggregated soils in good physical condition maintain the balance of air and water required to promote many other soil properties [48]. Therefore, the greater mean weight diameter under Maize/Soybeans and Sole Soybean suggest balanced air and water and plant roots development soil conditions for sustainable crop production. There was no significant difference in MWD under the treatment in 2015, though MWD generally increased across all the treatments in 2015, with Maize/Desmodium giving higher MWD (1.282). Table 3 also reveals that change in mean weight diameter over 2014 and 2015 was highest under Maize/Desmodium intercrop, followed by Sole Desmodium. Therefore, Maize/Desmodium, followed by Sole Desmodium cropping systems could cause improved soil aggregation in Northern Guinea Savanna Alfisols for sustainable crop production.  Fig. 3 shows that in 2014, Sole Maize resulted in significantly (P<0.05) higher bulk density, soil organic carbon and carbon stock (SOC) than the other legume-based treatments; except MD 2:4, that contributed significantly higher (9.51gkg -1 ) soil organic carbon and (27.01 t Cha -1 ) carbon stock to the soil. In 2015 however, Sole maize treatment resulted in reduced organic carbon and SOC, while MD 2:4 treatment resulted in significantly higher organic carbon (10.10gkg -1 ) and SOC (29.69 t Cha -1 ) than the other treatments. Fig. 3 therefore reveals that treatment MD 2:4 recorded significantly higher mean soil organic carbon (9.81gkg -1 ) and SOC (28.35 t Cha . Assessing the soil for percent change over 2014 and 2015 cropping seasons reveal that soil under mono-cropped (Sole) maize depreciated in soil organic carbon (6.77 %), suggesting degradation of the soils for sustainable crop production by over 6.0% of the 2014 value; a condition that could exacerbate global warming, atmospheric carbon dioxide enrichment and climate change occurrence in the study area. Also, carbon stock under monocropped maize (M) for the period of study depicted negative value (-8.03%) to confirm that the keys to successful soil carbon sequestration are increased plant growth and productivity, increased net primary production and decreased decomposition [49], 50] because; for example, the legume-based treatments all resulted in positive carbon stock in soil (Fig. 3), as against sole maize (M) that gave negative carbon stock. Increasing soil organic matter (SOM) is widely recognized as a means to increase agricultural production [51]. It would therefore be inferred that mono-cropping (M) maize resulted in negative change in carbon sequestration (stock), to suggest that continuous maize mono cropping would cause adverse organic carbon depletion and impoverished carbon stock for enhanced global warming, climate change and degraded soil quality for sustainable crop production.  Table 4. Results on the aggregate sizes show that adoption of maize-soybean sequestered the highest OC concentration in macro aggregates in each of the two years (2014 and 2015) as well as the change of carbon sequestered in the macro and micro aggregate fractions. This was followed  by MD2:4 treatments that sequestered significantly higher organic carbon concentration in macro aggregates than the other treatments across the periods of observation. This suggests that the best strategies focus on the protection of soil organic carbon against further depletion and erosion, or the replenishment of depleted carbon stocks through certain management techniques [49] will involve legume/ Cereal cropping systems such as Maize/Soybean and Maize/Desmodium 2:4 systems. The least amount of organic carbon sequestered in macro aggregates was under MD intercrop. Also, M, D, MS 2:4 and MD 2:4 treatments sequestered more carbon in the macro aggregate fractions, suggesting that aggregates developed in these treatments would be more readily available for micro organisms to access. The Sole soybean and Maize/Soybean 2:4 treatments sequestered most carbon concentration in the micro aggregate fractions, while MD 2:4 treatments resulted in net negative carbon sequestration in micro aggregate fractions.

CONCLUSION
Findings from the study show that sole maize treatment caused more compaction of the soils relative to other treatments, while sole Desmodium uncinatum best improved soil bulk density. Maize/Soybean intercrop treatments resulted in significantly (P<0.05) highest organic carbon sequestered in macro aggregates in each of the two years (2014 and 2015) and was followed by Maize/Desmodium 2:4 treatments that were significantly higher than the rest other treatments with organic carbon sequestered in macro aggregates across the periods of observation. The least amount of organic carbon sequestered in macro aggregates resulted under Maize/Desmodium intercrop. Change in mean weight diameter of aggregates over the 2014 and 2015 was highest under Maize/Desmodium intercrop, followed by Sole Desmodium to suggest that these legume-based cropping systems could cause improved soil aggregates development in Savanna Alfisols for sustainable crop production. Sole (mono-crop) Maize resulted in significantly (P<0.05) higher bulk density and sequestered (P<0.05) higher organic carbon and carbon stock (SOC) than the other legume-based treatments in 2014; except MD 2:4, that contributed significantly higher (9.51 gkg -1 ) organic carbon concentration and (27.01 t C ha -1 ) carbon stock to the soil. However in 2015, MD 2:4 treatment resulted in significantly higher organic carbon concentration (10.10 gkg -1 ) and SOC (29.69 t C ha -1 ) than the other treatments. Over 2014 and 2015 cropping season, Sole (mono-crop) maize depreciated in soil organic carbon (6.77 %), suggesting degradation of the soils for sustainable crop production by over 6.0 % of the 2014 value to exacerbate global warming, atmospheric carbon dioxide enrichment and climate change in the study area. Also, carbon stock under sole maize for the period of study depicted negative value (-8.03 %) to confirm that the keys to successful soil carbon sequestration are increased plant growth and productivity, increased net primary production and decreased decomposition.
It is therefore inferred that Maize Strip Cropped with Desmodium (MD 2:4) had high Mean Weight Diameter (MWD 1.282), resulted in highest soil organic carbon concentration and carbon stock, sequestered high organic carbon in soil macro and micro aggregates and resulted in highest organic carbon concentration and soil carbon stock change over the study period more than the rest other treatments. These suggest therefore that MD2:4 treatments would best improve the soil conditions (quality/health) for sustainable crop production, mitigate climate change and global warming by sequestering soil organic carbon better than the other treatments.