Effects of a nitrification inhibitor on nitrogen species in the soil and the yield and phosphorus uptake of maize

Highlights

• Yield and phosphorus uptake of maize increase with specific ammonium co-fertilization.

• Novel nitrogen K-edge micro-XANES spectroscopy indicated fixed ammonium in soil.

• Use of nitrification inhibitor led to higher amount of ammonium in the soil solution.

• Release of H⁺ by plants during ammonium uptake mobilized P from apatite-based P fertilizers in amended soils.

Abstract

Phosphorus (P) resource availability is declining and the efficiency of applied nutrients in agricultural soils is becoming increasingly important. This is especially true for P fertilizers from recycled materials, which often have low plant availability. Specific co-fertilization with ammonium can enhance P plant availability in soils amended with these P fertilizers, and thus the yield of plants. To investigate this effect, we performed a pot experiment with maize in slightly acidic soil (pH 6.9) with one water-soluble (triple superphosphate [TSP]) and two water-insoluble (sewage sludge-based and hyperphosphate [Hyp]) P fertilizers and an ammonium sulfate nitrate with or without a nitrification inhibitor (NI). The dry matter yield of maize was significantly increased by the NI with the Hyp (from 14.7 to 21.5 g/pot) and TSP (from 40.0 to 45.4 g/pot) treatments. Furthermore, P uptake was slightly increased in all three P treatments with the NI, but not significantly. Olsen-P extraction and P K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy showed that apatite-P of the water-insoluble P fertilizers mobilized during the plant growth period. In addition, novel nitrogen (N) K-edge micro-XANES spectroscopy and the Mogilevkina method showed that the application of an NI increased the fixation of ammonium in detectable hot spots in the soil. Thus, the delay in the nitrification process by the NI 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. This is especially important for water-insoluble apatite-based P fertilizers (conventional and recycled), which tend to have poor plant availability.

Introduction

Because phosphate rock and phosphorus (P) have become a scarce resource in the European Union (EU, 2017), recovering P from wastewater or sewage sludge has become extremely important (Schaum, 2018). However, the plant availability of these novel P fertilizers from recycled materials is often lower than that of commercial P fertilizers (Kratz et al., 2019). One promising way to increase the plant availability of P fertilizer is co-fertilization with nitrogen (N) in the form of ammonium, which can enhance the yield and P uptake of plants and make P fertilizers from recycled material more competitive with commercial phosphate rock-based P fertilizers (Rahmatullah et al., 2006; Vogel et al., 2018; Robles-Aguilar et al., 2019).

Previously it was shown that ammonium fertilizer instead of nitrates could increase the P uptake of plants (Rahmatullah et al., 2006; George et al., 2016; Pedersen et al., 2019). The increased mobilization of the P fertilizer in the soil occurs because ammonium decreases the rhizospheric pH due to release of H⁺ by plants during ammonium uptake (Thomson et al., 1993; Pedersen et al., 2019). The additional use of a nitrification inhibitor (NI) can delay the nitrification, making the ammonium available for a longer period in the soil solution after its application (Barth et al., 2019), which could further enhance crop yield (Rahmatullah et al., 2006; Pedersen et al., 2019).

Thus, the goal of this study was to investigate the effects of ammonium and an NI on the dry matter yield and P uptake of maize with different types of P fertilizers. In this study, we performed a pot experiment with maize in soils amended with one water-soluble and two water-insoluble P fertilizers and ammonium sulfate nitrate (ASN), with or without an NI. Afterwards, the plant availability of phosphorus, ammonium, and nitrate in the amended soils was analyzed by various chemical extraction tests and the Diffusive Gradient in Thin-films (DGT) method (Huang et al., 2016a, Huang et al., 2016b; Vogel et al., 2017). To examine possible soil reactions to P and N compounds, soil samples were collected from pots after a pre-incubation period of 7 days with the fertilizer but before sowing, and after growth and harvest of maize plants, for the investigation of the amount of P and N by K-edge X-ray absorption near-edge structure (XANES) spectroscopy. This technique was previously successfully used to identify P (Kizewski et al., 2011; Kruse et al., 2015; Vogel et al., 2018) and N (Jokic et al., 2004; Leinweber et al., 2007; Adams et al., 2016; Wang et al., 2019) compounds in soils. We extended this study to examine the soil microscale using N K-edge micro-X-ray fluorescence (micro-XRF) and micro-XANES methods.