*4.2. Effect of NIs*

Except for CO2-C emission from Geest soil and N2O-N emission from ENTEC-treated soil, NIs significantly decreased CO2-C and N2O-N emissions in all other cases, but the magnitude of the inhibitory effect did not show any specific consistent trend with the soil texture (Tables 3 and 4). Different results have been reported by previous researchers regarding the relative efficacy of NIs in varying textured soils. Barth et al. [18] found that NIs are more efficient in light soils than heavy soils, whereas Akiyama et al. [34] found that the effectiveness of NIs was relatively consistent across the various soil types. According to Fisk et al. [35], the efficacy of DMPP and DCD diminished with the addition of soil organic matter, and Marsden et al. [36] reported decreased efficacy with higher clay content. Volpi et al. [14] found that NIs were effective only in the soil with the highest nitrification activity and the lowest clay content.

In our study, we found that the comparative effectiveness of the NIs in inhibiting CO2-C and N2O-N emissions depended on soil type (Tables 3 and 4). DCD had the highest inhibitory effect on soil nitrification in clay soil of Marsch, followed by loam soil of Östliches Hügelland and lowest in sandy soil of Geest (Table 4). Ernfors et al. [37] reported that the efficacy of DCD was soil-specific, whereas Wakelin et al. [38] found that the efficacy of DCD was not related to the soil type. The lowest efficacy of DCD in sandy soil as found in our study may be explained by the lowest decomposition of this NI in sandy soil as compared to heavy textured soils [1]. Moreover, McGeough et al. [7] found that DCD may not be an effective NI under heterotrophic nitrification, which is proportionally the dominant form of nitrification in sandy soil as compared to a heavy textured soil.

DMPP was reported to be highly specific and more effective in inhibiting nitrification as compared to DCD [39]. We found that the inhibitory effect of DMPP on N2O emission was more pronounced in the sandy soil of Geest than in the loamy and clay soils (Table 4). In short-term incubation experiments, Barth et al. [40] found that decreasing sand content reduced the efficacy of DMPP in retarding NH4 <sup>+</sup> oxidation. They found that the adsorption

capacity of DMPP is positively correlated with clay content, and that a lower effectiveness of DMPP may be due to the adsorption of the NI on silt and clay particles. In their later study, the same group of researchers reported that DMPP had more pronounced inhibition of N2O emission in sandy than in loam soil [41]. The same explanation as given above for the effect of DMPP stands true for PIADIN, which showed the same trend of effectiveness in different textured soils as that of DMPP. DMPP has relatively low mobility [39–43], mineralizes slowly and thus has a longer-lasting inhibitory effect on nitrification than DCD [44,45].

Soil texture may affect organic carbon turnover by adsorption of organic carbon onto surfaces of clay or organic complexes [46]. Thus, the effect of different NIs on organic carbon decomposition could also be affected by soil texture. Our results showed that DCD did not influence the soil organic carbon decomposition in Marsch soil, and thus CO2-C emission from DCD-treated soil was the same as that from control soil (Table 3). DMPP had the best inhibitory effect on the organic carbon decomposition in Marsch soil. The NIs DCD, DMPP and PIADIN effectively inhibited the decomposition of organic carbon in Östliches Hügelland soil and resulted in the lowest CO2-C emissions (Table 3). ENTEC had the smallest inhibitory effect on the soil organic carbon decomposition in Östliches Hügelland soil.

#### **5. Conclusions**

CO2-C and N2O-N emissions were higher from N-fertilized clayey soils than from lighter textured loamy and sandy soils. DCD was proven the most effective NI in decreasing CO2-C and N2O-N emissions and inhibiting nitrification in the clayey and loamy soils. On the other hand, DMPP and PIADIN could decrease CO2-C and N2O-N emissions more effectively in the sandy soil. ENTEC remained the least effective in inhibiting CO2-C emission, whereas it did not inhibit N2O-N emission from the studied soils.

It is concluded that clayey soil has more gaseous emissions and NIs perform differently depending on the soil texture to inhibit soil nitrification and gaseous emissions. DCD is a better NI for clay and loamy soils while DMPP and PIADIN are better for sandy soil to inhibit soil nitrification and gaseous emissions. ENTEC is ineffective in all soil textures.

**Author Contributions:** Conceptualization, K.H.M.; methodology, software, writing—original draft preparation, Y.G.; writing—review and editing, A.N. and K.H.M.; supervision, K.H.M.; funding acquisition, K.H.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ph.D. project funding from the Society of Energy and Climate (EKSH) of Schleswig-Holstein, grant number 14/12-24, and by a George Forster Post-Doctorate Fellowship grant from the Alexander von Humboldt Foundation.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to restrictions.

**Acknowledgments:** K.H.M. and Y.G. acknowledge Ph.D. project funding from the Society of Energy and Climate (EKSH) of Schleswig-Holstein (14/12-24). A.N. is thankful to the Alexander von Humboldt Foundation for a George Forster Post-Doctorate Fellowship grant.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

