**1. Introduction**

Nitrogen is an important element for plant growth in agro-ecosystems [1], but the effectiveness of applied fertilizer N in crops rarely exceeds 40% [2]. Chemical N fertilizer constitutes approximately 75% of the total EU input of reactive N [3], and between 40% and 70% of the fertilizer N applied is lost to the atmosphere or the hydrosphere [4]. The majority of applied N is lost from agriculture through ammonia (NH3) volatilization, gaseous emissions of nitrous oxide (N2O) and di-nitrogen (N2) and nitrate (NO3 −) leaching [5]. Soil NO3 −-N leaching and N2O emission are processes responsible for both N losses from agricultural soils as well as environmental pollution [6,7]. Beyond its powerful greenhouse effect, N2O is also a major ozone-depleting substance involved in the destruction of the protective ozone layer in the stratosphere [8].

The concentration of N2O, which is about 300 times more reactive a gas than CO2, has risen from a pre-industrial value of 270 ppb to 319 ppb in 2005 [9], primarily due to agricultural practices and increased use of industrial fertilizers [1,10]. It has been estimated that agricultural soils produce 2.8 (1.7–4.8) Tg N2O-N year−<sup>1</sup> and contribute approximately 65% of the atmospheric N2O loading [8,9]. In Europe, N2O emissions from agricultural soils contribute about 70% of the total annual N2O emissions (European Environment Agency, 2015). Application of N to soils as chemical or organic fertilizers stimulates nitrous oxide (N2O) emissions, mainly through the processes of denitrification

**Citation:** Guo, Y.; Naeem, A.; Mühling, K.H. Comparative Effectiveness of Four Nitrification Inhibitors for Mitigating Carbon Dioxide and Nitrous Oxide Emissions from Three Different Textured Soils. *Nitrogen* **2021**, *2*, 155–166. https:// doi.org/10.3390/nitrogen2020011

Academic Editor: Tida Ge

Received: 23 March 2021 Accepted: 9 April 2021 Published: 13 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and nitrification [11,12]. Nitrification is an aerobic process in which ammonium (NH4 +) is first oxidized to nitrite (NO2 −) and then nitrate (NO3 −) [1], and it plays a key role in the soil N cycle [13]. During the oxidation of NH4 <sup>+</sup> to NO2 −, N2O can be produced as an intermediate and liberated into the atmosphere [1]. Denitrification is an anaerobic microbial process in which organic carbon is used as an energy source and NO3 − is reduced to gaseous N compounds, including N2 and N2O [14].

Nitrification inhibitors (NIs) are used to improve the efficiency of N fertilizers through decreasing both NO3 − leaching and gaseous N emissions [14,15]. NIs can decelerate the rate of soil nitrification by deactivating the enzyme ammonia monooxygenase (AMO) responsible for catalyzing ammonia oxidation, the first and rate-limiting step of nitrification, which is produced by ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) [16]. As NO3 − is the initial required substrate for denitrification, the use of NIs decreases N2O emissions from both processes (nitrification and denitrification) [1].

The production of N2O, and in turn CO2 emission, in soil and its inhibition by NIs are complex processes, which can be influenced by different factors such as physicochemical characteristics of the soil. Among these, texture is a major soil characteristic that governs various soil properties, and hence the relative effectiveness of NIs may be different in different textured soils. Soil texture can influence the effectiveness of NIs by affecting their stability/persistence and absorption in soils. Some NIs such as 3, 4-dimethylpyrazole phosphate (DMPP) have been shown to affect N2O emission by decreasing soil pH [17], which can also vary with soil texture. Some studies have tested the effectiveness of NIs to reduce N2O emissions but results varied considerably because the studies were carried out under different soil conditions [1]. Thus, it is difficult to draw any specific conclusions about the N2O mitigation potential of NIs in different textured soils.

This study investigates the effectiveness of four NIs, namely, dicyandiamide (DCD), 3, 4-dimethylpyrazole phosphate (DMPP), nitrogenous mineral fertilizers containing the DMPP ammonium stabilizer (ENTEC) and active ingredients: 3.00–3.25% 1, 2, 4-triazole and 1.50–1.65% 3-methylpyrazole (PIADIN) under three vastly different textured soils (clayey, loamy and sandy). We hypothesized that (i) clay contents would have a positive effect on soil N2O emissions; and (ii) DMPP would have a better performance than DCD, PIADIN and ENTEC in reducing soil CO2 and N2O emissions and the conversion of NH4 +- N to NO3 −-N under a range of soils with different textures. The objective of this study was to evaluate the variation in soil CO2 and N2O emissions, and NH4 +-N and NO3 −-N concentrations following the application of the abovementioned NIs in clayey (Marsch), loamy (Östliches Hügelland) and sandy (Geest) soils.
