Phosphorus recovery from wastewater: Nitrogen K-edge micro-XANES spectroscopy unravels the effects of nitrification inhibitor on fertilizer phosphorus uptake of maize

Phosphorous is a main component in fertilizers, which are essential to feeding the growing population on earth.  Because phosphate rock and phosphorus (P) have become a scarce resource in the European Union, recovering P from wastewater and sewage sludge has become extremely important. However, the plant availability of these novel recycling P fertilizers is often lower than that of commercial P fertilizers. One promising way to increase the plant availability of P fertilizer is co-fertilization with nitrogen (N) in the form of ammonium and nitrification inhibitors, which can enhance the yield and P uptake of plants. To investigate the nitrification inhibitor effect, we performed a pot experiment with maize and several P fertilizers and an ammonium sulfate nitrate with or without a nitrification inhibitor. By applying the novel N K-edge micro-XRF and micro-XANES methods at the PHOENIX beamline on the soils from the pot experiment we verified that a nitrification inhibitor promotes ammonium fixation in fertilized soils. Thus, the delay in the nitrification process by the nitrification inhibitor and the possible slow-release of temporarily fixed ammonium in the soil resulted in a high amount of plant available ammonium in the soil solution. This development probably decreases the rhizosphere pH due to release of H⁺ by plants during ammonium uptake, which mobilizes phosphorus in the amended soil and increases the dry matter yield of maize. In particular, this is important for water insoluble apatite-based recycling P fertilizers, which are not directly plant available, and make these P fertilizers more competitive to commercial phosphate rock-based P fertilizers.

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