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Background: Manganese deficiency often becomes a yield limiting factor, particularly on calcareous soils, even though the total soil manganese content is usually sufficient. Although it is known that acidifying N fertilizers can improve Mn availability, the reason of this effect is still unknown. Aim: Our aim was to investigate the effect of stabilized ammonium fertilizers as a tool to distinguish between physiological-and nitrification-induced acidification. Method: Two pot experiments with Triticum aestivum L. and one soil incubation experiment using different nitrogen forms (CN = calcium nitrate, AN = ammonium nitrate, AS = ammonium sulfate, ATS = ammonium thiosulfate) with and without addition of nitrification inhibitors (DCD, Nitrapyrin, Piadin, DMPP) were conducted to examine the effect on Mn availability in the soil and Mn uptake by the plants at different development stages (EC 31 und 39). Results: With increasing fertilizer NH þ 4 content a higher Mn concentration was detected: CN: 32 μg Mn g -1 DM, AN: 39 μg Mn g -1 DW, AS: 55 μg Mn g -1 DM, ATS: 109 μg Mn g -1 DM. The addition of a nitrification inhibitor resulted in a significantly lower rhizosphere pH compared to the non-stabilized fertilizer. Surprisingly, the use of different nitrification inhibitors led to unchanged (CN, AN) or lower Mn concentrations of wheat. Especially in the NH þ 4 treatments (AS and ATS), this negative effect was very evident (AS+DCD: 42 μg Mn g -1 DM; ATS+DCD: 55 μg Mn g -1 DM). Conclusions: Mn availability was enhanced by ongoing nitrification process rather than physiological acidification. Compared to other N forms, ammonium thiosulfate led to the highest Mn availability in bulk soil.

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