Reaction of a ferralsol to the acidifying effect of nitrogen fertilization
Ngarukiyimana, Jean Bosco
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Soil fertility management is currently believed to be a major entry point for unlocking the productivity of crops in order to address food insecurity and poverty, particularly in Sub-Saharan Africa (SSA). Although fertilizers are renowned for improving the productivity of continuously cultivated soils in Africa, owing to the history of massive nutrient depletion, there are several feedbacks from farmers especially in Sub-Saharan Africa with claims that fertilizers actually spoil the soil by presumably degrading its productivity. One of the candidate factors behind negative perception is the unguided use of some otherwise acidity augmenting nitrogen (N) fertilizers, especially on hitherto acidic soils such as Ferralsols. Although acidification due to N fertilisers is generally associated with long term experiments, hardly any information exists on the possible ensuing reaction scenarios immediately following application of N fertilisers in an already acidic soil such as acidic ferralsol. The objectives of this study, therefore, were to (i) Determine the short term effect of urea fertilizer application on soil acidity (exchangeable H+ and Al3+ ions concentrations), mineral nitrogen ion species concentrations and plant response and (ii) characterize the sorption behavior of available P in the ferralsol applied with urea fertilizer. Two experimental setups were conducted, namely pot and laboratory experiments during 2017 at the Makerere University. The pot study was done in plastic pots of 10 liter capacity, at the Makerere University Agricultural Research Institute, Kabanyolo (MUARIK). Treatments included four N rates, namely 0, 200, 400 and 600 mg of urea per pot (equivalent to 0, 40, 80 and 120 kg N ha-1) applied in the form of urea and the study was replicated twice. A completely randomized design was adopted for this experiment, with three replicates using maize (variety Longe 10) as test crop. Data collected included concentration of [H+] and exchangeable Al3+; and nitrate and ammonium ion concentrations associated with urea hydrolysis and subsequent nitrification. Plant measurements included plant height and dry matter yield. The laboratory study treatments included equivalents of the four rates of urea in centrifuge tubes of 50 ml capacity, concurrently by going low and high basing on Bai, 2017 method seven levels of P, namely 2.5, 5, 10, 20, 30, 40 and 50 μg P ml-1 as KH2PO4 , equivalent to 5, 10, 20, 40, 60, 80 and 100 kg P ha-1, respectively were used.. Phosphorus adsorption measurements were made, and the Langmuir Model was used to obtain adsorption maximum (μg P g−1 soil) and bonding energy constant. From the pot experiment results, a hydrogen ion concentration curve was evident which depicted four phases of soil reactions, each one associated with different N species of urea mineralization. The phases were temporal in nature and were thus delineated into “short term” which lasted about 9 days from the time of urea application; “medium term which lasted 10 days after the end of the short term phase; “short term phase 2”, which lasted 10 days also after the expiry of the medium phase. After the expiry of the short term phase 2; and the long term phase, the latter of which was indefinite after short term phase 2. The first Short term phase was characterized by a drop in hydrogen ion concentration from 8.63 x 10-5 mol l-1 for urea applied at 0 mg per pot to 2.69 x 10-5 mol l-1 for urea applied at 400 mg per pot, which was attributed to hydrolysis of the urea. However, this was quickly surpassed by nitrification which was the product of oxidation of ammonium through nitrite, to nitrate. The resulting product of nitrification was acidification, leading to a surge in hydrogen ion concentration. The acidification phase marked the medium term phase, whose climax ended with the application of the second split dose of urea at day 19. Then the reactions that manifested in the first short term phase again became evident in this phase, though the drop in hydrogen ion concentration was not as low as that in the first short term phase, perhaps signifying the accumulation of soil acidity caused under the preceding phase. The phase after Day 19, which lasted for approximately 10 days, was codenamed short term phase 2. After this phase, the hydrogen ion concentration increased virtually indefinitely, and this indefinite phase was codenamed the long term phase. Generally, the profile of exchangeable Al3+ and nitrate concentrations was in synchrony with the pattern of the hydrogen ion curve, while that of ammonium acted in the inverse direction. It was then concluded that soil reactions in the short term phase of urea application are quite distinct from of the long term exposure of urea to the environmental conditions, and may be useful in the designing of more prudent and beneficial N fertilizer management strategies. With regard to the laboratory study, which was restricted to 10 days after application of urea, P sorption was enhanced by soil reactions within this short term phase. This was in direct contrast with the renowned behavior of soil P in an acid soil, when subjected to alkalinizing conditions. Other observations were that the Langmuir adsorption maximum capacity increased, as did the bonding energy constant (k) and phosphorus buffering capacity; which signifies increased P sorption. Further investigations are necessary to explore more the reasons for increased P sorption under conditions that would otherwise be favorable for P availability to plants in this Ferralsol.