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The absorptive nature of S by plants depends on desorption of S from soils. Understanding the kinetics desorption of sulphate as influenced by parent materials and biochar is critical for effective precision of S diagnosis and fertilization recommendations to ensure sustainable and profitable crop production and environmental protection. In this study, soils derived from three (3) different parent materials namely; Chad formation (CF), Basement complex rock (BCR) and Kerri-Kerri formation (KKF) in the Sudan savanna and biochar soils were investigated on the kinetics of sulphate desorption behaviour. In the desorption experiment, soils were allowed to adsorb sulphate as in the adsorption studies and the adsorbed sulphate was extracted by shaking for 30, 60, 150, 180 and 240 minutes with 15 ml Ca(H2PO4)2 solution containing 500 mg P L-1 respectively. Five kinetic models first-order, second-order, Elovich, fractional power and parabolic diffusion models were used to test best model for describing S desorption in the soils of varying parent materials. Results indicated that the mean values of both soil pH in water and CaCl2 were significantly (p>0.05) different between soil parent materials. The soils are low in organic carbon (range; 2.90 to 7.25 g/kg) in all the studied soils. The mean values of exchangeable bases and CEC were not significantly (p>0.05) different by soil parent materials. The mean values of the different forms of oxides were significantly different (p>0.05) by soil parent materials. The mean values of total sulphur and organic in all soils from three parent materials were significantly different (p > 0.05). The values of inorganic sulphur (Org.S) ranged from 18.33 to 123.78 mg/kg in all the soils across three parent materials, however the mean values were not significantly different (p>0.05). The results revealed that sulphate desorption decreased with increasing shaking time (range; 34.96 to 19.09 mg/kg) in all the studied soils. Rate of desorption was also characterized by an initial rapid desorption with > 65% being desorbed in the first 30 minutes which was considered as good index for describing S desorption maximum followed by a slow release that progressed gradually up to 240 minutes. The adsorption-desorption processes were irreversible and hysteretic in nature. The trend of magnitude of rate of desorption by the soil parent materials was in descending order: CF>BCR>KKF. The study showed that the parabolic diffusion and first-order models were found to describe S desorption data satisfactorily as characterized by relatively high R2 values and lowest S.E values by soil parent materials, respectively. Comparison of R2 values of the best isotherm equations indicated that both Langmuir and Freundlich isotherm models were suitable to describe S desorption data in all the studied soils, as can be seen from R2 greater than 0.98. However, in view of the low SE, Langmuir equation gave a better fit to experimental data in this study. Organic matter had a significant influence on modeling of S desorption in these soils. The results also revealed that the BC used in this study did not have significant effect on release of adsorb S (p>0.05). Based on the results, can be concluded that rate of sulphate desorption by these soils are time-dependent and mainly controlled by diffusion-controlled phenomena. It is recommended that sulphur management practice in the farm should be tied up with OM management. It is also recommended that affinity of BC to release plant nutrients should be tested              before        applying      to                        soils                  as                          amendment.



1.0       Introduction

The essentiality of sulphur (S) for plant growth has been known from the time of Liebig

(Tabatabai, 2005a), but compared with other major nutrients such as nitrogen (N), phosphorus

(P) and potassium (K) it has, until recently, received little attention. Plant adsorb sulphate (SO42-)

from the soil solution, therefore replenishment from organic and adsorbed sources is important in

maintaining sulphate supply to the plant (Sumner, 2000).

Importance of S as a plant nutrient was largely overlooked in soil science research, mainly

because the incidental supply of S via deposition and S- containing N and P fertilizer was usually

sufficient to meet crop demands. However, since concerns about intensive continuous cropping

were adopted among farmers in the decades, sulphur stock has decreased remarkably in some

soils in the world. In the 1980s, issues of acid rains were raised in Nigeria (Osu et al., 2013), and

sulphur emission from power plants and industries has significantly declined which has led to

major causes of S deficiency in some soils.

Over the last 2 decades, farmers deviate from using sulphur rich fertilizers to high analysis

fertilizers that contain less amount of sulphur which has masked many latent or incipient sulphur

deficiencies in cultivated land in Nigeria (Raji, 2008). The reduced input of S combined with

intensified farming and larger S removal in harvests depleted the S stocks in soils and in the

1990s, sulphur deficiency began to surface (Ceccotti, 1996).

Sulphur deficiency can cause marked yield loss and reduce the quality of crops and forage in

some soils of the world (Wang et al., 2002; 2008; Zhao et al., 2006a). In addition, S deficiency


decreases resistance to pathogens (Falk et al., 2007; Walters and Bingham, 2007) and reduces N

utilization efficiency (Ahmad and Abdin, 2000).

1.1       Problem statement

As sulphur is relatively cheap element, fertilization with S is now becoming standard practice

(Boye, 2011). However, knowledge of how to optimize the amount and timing of S fertilization

is still insufficient. Sulphur fertilization can decrease selenium (Se) content in crops, which is

problematic for animal and human nutrition in Se-poor soils (Stroud et al., 2010). Sulphur can

also induce eutrophication in freshwater wetlands, as the precipitation of iron-sulphide releases

iron-bonded phosphates (Lamers et al., 2002), the understanding of the kinetics of SO42-

desorption is critical in preventing such problems.

Sulphur is widely distributed in nature in both, organic and inorganic forms. Inorganic S is in the

readily available fraction for root uptake, or may be adsorbed on to soil colloids, but represents

an average of less than 5% of total S in the soil. The majority of S (> 95%) in soil is bound to

organic molecules and is only indirectly available to plants (Kertesz and Mirleau, 2004). In

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