Sole use of chemical fertilizers especially urea among farmers in Africa is a serious threat to achieving healthy soil conditions and sustainable food production. This exhaustive use of chemical fertilizers must be replaced with more sustainable approaches if soils in Africa would be able to produce enough healthy food for her ever-increasing human population. Inclusion of black carbon in soil fertility program among African farmers would be useful tool in achieving sustainable crop production in tropical soils dominantly composed of low activity clay minerals. The study compared the reactions of two black carbon types, biochar and locally sourced cooking charcoal, in a nutrient-degraded soil managed with urea fertilizer.
The results showed that sole urea treatment was the least effective in increasing soil organic carbon (SOC), macronutrient contents, maize yield, and nutrient uptake compared to soil pretreated with biochar and charcoal. Sole urea treatment depleted soil exchangeable calcium below the value obtained from the absolute control (with neither black carbon nor urea applied), leading to increased soil acidification. Similar Ca depletion and acidification potential of urea have been reported [24, 39–42].
Co-application of urea and other chemical fertilizers characterized by higher acidity equivalent with materials rich in basic materials has been recommended to buffer their acidity [43–47]. Conventional Liming materials such as limestone, slake and quick lime have been used extensively for this purpose. Their use, however, may not guarantee efficient carbon sequestration in tropical soils arising from their tendencies to encourage organic carbon decomposition and leaching from soils [45, 48, 49]. Alternative amendments such as plant residues, animal dung, compost and biochar are promising for achieving multiple soil ameliorative effects ranging from acidity neutralization, carbon sequestration, soil aggregate modification and improved microbial activities [26, 50, 51].
The biochar and charcoal studied are different black carbon types as indicated by their functional group distribution, nutrient composition, and reactions in soil. While the biochar has well defined spectra peaks at both the functional group and fingerprint regions when scanned on the FTIR, the charcoal was however, deficient in these. The intensity of peaks produced by biochar are reported to vary with biomass type, pyrolysis temperature and age in storage [52–54]. In this present work, especially for the biochar (from composted groundnut husk and wistar rat litter) tested, sharp peaks at wave numbers 1546, 1336 and 1070 representing C-C, C = C stretching for carbonyl functional group, C-H2 banding, and C-O stretching indicative of oxygenated functional groups [55] were observed at the functional group region. On the fingerprint region, peaks of 993, 846, 679, 497 and 406 representing mainly aromatic C-H stretching that indicate increasing condensation intensities in the biochar components [56] were observed. Similar dominance of sharp peaks at wave numbers below 1600 cm− 1 were reported by Sarafaraz et al. [57] for biochar produced from animal manure and crop residues. Biochar, produced from this composted agro-wastes, had higher carbon and macronutrient contents compared to the locally sourced cooking charcoal.
The differences in innate properties of the two black carbons influenced their individual reactions in the soil and contributed to the varying effects on carbon sequestration, nutrient availability, maize nutrient uptake, and biomass yield. The almost complete absence of peaks in the charcoal was not surprising due to the slow thermal destruction process involves in its production which spans into days [7]. This encourages higher heating value and poor active functional group which makes it more suited as fuel energy for cooking rather than as soil amendment.
Both biochar and charcoal outperformed the control (no black carbon nor urea) in improving soil organic carbon, macronutrient contents (except available P and exchangeable K with charcoal), and maize performance. However, biochar consistently outperformed charcoal in achieving these enhancements. The higher distribution of oxygenated functional groups, carbon, nitrogen, phosphorus, and basic cation contents in the biochar improved nutrient availability, holding, and release abilities in the soil. Evidence of biochar to improve nutrient retention in soils managed by chemical fertilizer have been reported [14, 26, 58, 81]. The carboxyl and oxygen containing functional groups in biochar were observed in several studies to be dominant mechanism in biochar to retain nutrients against leaching and volatilization from soil for improved plant uptake and yield [5, 59, 60]. This explains the improved nutrient uptake (N by 140.6% and P by 439.3%) and maize biomass yield by 144.7% in the biochar amended soil relative to absolute control in the urea treated soil. This probably translated to higher use efficiency of the nutrients supplied by the urea in the biochar amended soil over charcoal. On the other hand, charcoal, with virtually no functional group peaks, only supported N and P uptake and maize biomass yield increases by 48.1, 39.3 and 53.2% respectively compared to absolute control.
Sole urea application was not sufficient to achieve improved soil conditions for optimal maize growth and biomass yield in this present study. The soil studied was severely degraded having low concentrations of organic carbon and essential nutrients such as total nitrogen and phosphorus. This poor soil quality likely hindered the growth and biomass yield of the maize in sole urea treated soil.
One problem with using urea as a sole fertilizer in this context is possible occurrence of leachate losses of nutrients through the holes at the base of the poly pots. Similar nutrient leaching from field trials have been documented for urea and other commonly used chemical fertilizers [25, 61]. Furthermore, urea supplies mainly nitrogen, which is an essential nutrient for plant growth. However, in soils with a near-neutral pH, like the one studied, nitrogen from urea can undergo volatilization [27, 62–65]. This is a process where nitrogen is lost in the form of ammonia gas into the atmosphere making it unavailable to plants [66]. The loss of nitrogen through volatilization could have contributed to poor plant performance. Nitrogen losses from urea treated soils through volatilization and leaching have been reduced through compost and biochar pretreatments [41, 66, 82] and urea surface coating [32, 67; 83]. In severely degraded soils, where multiple nutrients are likely deficient, using urea alone (a single nutrient fertilizer) therefore may not have been sufficient to support optimal plant growth. Maize, like other crops, requires a range of nutrients, including phosphorus and potassium, in addition to nitrogen.
Pretreating the soil with biochar prior to urea application consistently outperformed the urea alone and charcoal based treatments. Biochar’s reputation to improve soil structure and nutrient retention have been well documented [15, 34, 68, 69]. These potentials facilitated reduced nutrient leaching, enhance nutrient availability to plants and efficient chemical fertilizer usage [66, 70, 71]. This combined approach provided by biochar pretreatment likely provided a more balanced and effective nutrient supply for maize plants in the degraded soil [26, 68]. The higher ash contents (49.23%) of the composted agro-waste biochar studied stood biochar out on its improved modification of the soil for improved crop performance. The higher ash contents known for biochar derived from animal manure feedstock serves as source of alkali and alkali metals needed for improved soil ability to retain macronutrients in its colloids and exchangeable sites [5, 34, 55, 72, 73]. This explains why the post-harvest soil and plant nutrients from biochar-based treatments were significantly higher than charcoal-based treatments. Cairns et al. [60] observed improved N and P retention in all the ash fortified biochar types they under studied. Furthermore, large surface area associated with biochar modifies the soil surface for enhanced capacity to hold and retain nutrients [74].
Biochar surface area are dominantly negatively charged thus conferring improved retention of cationic nutrients against losses from the percolating water [74]. Physical entrapment of negatively charged nutrient ions such as the nitrates unto the biochar surfaces also prevails in biochar with large surface area and many micropores. Gelardi et al. [5] reported significant proportion of nitrate entrapped unto biochar surface and pore which prevented their leaching and volatilization losses. The surface scanning of biochar using FTIR reveals the presence of various functional groups which are typically negatively charged [15, 16, 30]. These functional groups play a crucial role in controlling how biochar mitigates nutrient leaching from soils. Biochar’s functional groups are known to adsorb cationic nutrients, further enhancing nutrient retention in the soil [15, 75]. Earlier works on co-application of biochar and chemical fertilizers have highlighted the influence of biochar blending with chemical fertilizer to achieve slow nutrient release conditions [31, 35, 76, 77]. Significant reduction in N leachate losses were reported in biochar-chemical fertilizer co-application trials [16, 26, 28, 35, 78]. The functional group resulting from the pyrolysis of the biochar play significant role in this reaction. Biochar with fewer -OH functional group was reported to be more efficient in reducing rate of N release from nitrogen fertilizer [16, 30]. The high chemical affinity between the functional group of biochar with high O/C ratio and ammonium was reported by Gelardi et al. [5, 79] as dominant mechanism for ammonium ion retention in soils. All these unique characteristics of biochar consistently made it to outperformed cooking charcoal amendment in improving soil conditions for optimal nutrient retention in soils and uptake by plants.
This affirmation was supported by the consistent higher significant mean values biochar produced over charcoal across all the soil and plant parameters considered in the T-test conducted between the two black carbon types. The T-test results showed that the two black carbons are significantly different from each other in terms of their reactions in soils, potentials to sequester carbon, influence nutrient availability and maize performance. Charcoal was poorer in increasing soil carbon in the soil studied. This was similar to the submissions of Charvet et al. [7]; Leal et al. [13] where carbon in charcoal was implicated to be unstable with low fixed carbon values making it less recalcitrant compared to biochar. The charcoal studied also has the potential to increase sodium concentrations in the soil which could pose serious salt stress on the root cells of the plants. Reports of higher contents of polycyclic aromatic hydrocarbons (PAHs) and toxic particulate materials in charcoal arising from the lower temperature involved in its production have also been documented [8, 10–12, 80]. These innate toxicants in charcoal would of course result in soil contamination and impaired plant performance under prolong use as soil amendment.