*3.4. Nitrous Oxide (N2O) Emissions*

Nitrous oxide emissions were significantly (*p* ≤ 0.01) affected by the application of liming materials. The N2O emission rate and the cumulative soil N2O emissions (329.52 μg kg−1) were highest in the control among all the treatments (Figures 7 and 8). The N2O emissions increased on day 1 following the addition of liming materials, and then started to decline, with variant magnitudes (Figure 7). The decrease in N2O emissions was sharper in Ca(OH)2 than that of the other liming treatments, and indeed in the control. The lowest emission rate and cumulative N2O emissions were observed in the Ca(OH)2 treatment. The cumulative N2O emissions in the Ca(OH)2, CaO, CaCO3, CaMg(CO3)2, and control treatments were 208.08, 237.60, 261.72, 287.64, and 329.52 μg kg<sup>−</sup>1, respectively (Figure 8).

**Figure 7.** Soil N2O emissions following the application of liming materials. Vertical bars denote the error bars of the means of three replicates. Values at the same time followed by different letters indicate significant differences between different treatments (*p* < 0.05).

**Figure 8.** Cumulative soil N2O emissions following the application of liming materials. Vertical bars denote the error bars of the means of three replicates. Different letters (from *a* to *e*) denote significant differences (*p* ≤ 0.05) among the means of treatments.

### **4. Discussion**

Acidic soils are generally considered as less efficient for high crop production. To ameliorate acidic soils, farmers usually amend them with liming materials. Application of liming materials has dual benefits of raising soil pH as well as supply essential elements, mainly Ca and Mg. In the present study, the liming materials used were Ca(OH)2, CaO, CaCO3, and CaMg(CO3)2. Application of all these liming materials obviously influenced N2O emissions, but the magnitudes of the N2O emissions dramatically altered with soil pH. In fact, high N2O emissions were observed at low pH levels (without lime application) in the acidic soil in the present study.

High magnitudes of N2O emissions from low pH soils can be explained by incomplete denitrification and less activity or complete inhibition of N2O–reductase. Nitrous oxide reductase (N2O–R) is the sole enzyme of the denitrification process, which reduces N2O to N2 at neutral, near neutral, or above 7 pH [18]. Therefore, higher magnitudes of N2O emissions are expected from soils at low pH relative to higher pH values because of the incomplete denitrification process [19,20]. In the present study, N2O emissions were perceptibly mitigated by the application of all selected liming materials. However, the highest decline in cumulative as well as emission rates of N2O occurred in the Ca(OH)2 treatment, and this was possible due to the highest pH value. Kunhikrishnan et al. [21] also indicated that the pH value could prominently control N2O production and emissions, and Bakken et al. [22] commented that the possible mechanism involved in low magnitudes of N2O emissions in limed soils at high pH values was pertinent to the activities of N2O–reductase. It has been shown that the application of liming materials improved the activities of N2O–reductase for N2O reduction [23], and magnitudes of soil N2O emissions are unswervingly controlled by pH [8,24]. These studies demonstrated that N2O–reductase was functional at higher pH

relative to low pH, which led to a complete denitrification process and low N2O emissions at high pH levels.

Results of raising soil pH regarding the effect on N2O emissions have been proposed by several researchers. Stevens and Laughlin [25] reported that raising the soil pH from 6.5 to 8 eminently reduced N2O emissions. Qu et al. [26] reported that acidic soils produced higher magnitudes of N2O emissions, whereas neutral pH soils showed less magnitudes of N2O emissions. Khan et al. [27] found that the application of calcium hydroxide to soil at the dose of 5.63 g kg−<sup>1</sup> soil significantly decreased N2O emissions by increasing soil pH from 5.2 to 7.6. Additionally, an 80-day laboratory study revealed that Ca(OH)2 amendment (1.1 to 5.6 g kg−<sup>1</sup> soil) substantially reduced N2O emission [28]. Moreover, some other experiments showed the following: application of Ca(OH)2 at the dose of 7.3 g kg−<sup>1</sup> soil mitigated cumulative emissions of N2O from 547 g ha−<sup>1</sup> to 46 g ha−<sup>1</sup> in a soil with a pH of 4.71 [29]. A 2-year research showed that increasing the pH from 4 to 5.5 by CaCO3 application dwindled N2O emissions from 0.96 mg m−<sup>1</sup> d−<sup>1</sup> to 0.88 mg m−<sup>1</sup> d−<sup>1</sup> [30]. The mitigation of N2O emissions from limed soils showed that pH plays an imperative role in regulating such N2O release to the atmosphere [31].

In the present study, the addition of liming materials greatly impacted mineral N concentrations displaying a quick decline of NH4 <sup>+</sup>–N with time, indicating that the nitrification process sped up, as linked to the concurrent rise of NO3 <sup>−</sup>–N concentrations. Higher NO3 −–N concentrations at relatively higher soil pH levels advocate that microbes consumed N2O as an electron acceptor instead of NO3 −–N. It can be observed from these results that complete denitrification occurred, rendering N2O to N2 conversion in all liming material treated soils, and thus correspondingly, low magnitudes of N2O emissions occurred. Moreover, it is interesting to report herein that the trend and behavior of N2O release from liming material amended soils corresponded with the changes in NH4 <sup>+</sup>–N and NO3 −–N concentrations, but the degree of the mitigation of N2O emissions did not follow the same pattern. The most rapid changes of mineral N dynamics were observed in the CaCO3 treatment, whereas the highest reduction of N2O emissions occurred in the Ca(OH)2 treatment. The discrepancies between the degrees of N2O emission magnitudes following liming material application is plausible because of the potential of soil pH manipulation.

In addition to mineral N dynamics, the application of all liming materials influenced dissolved organic C, which is believed to be a readily available C substrate for microbial growth prolongation and proliferation, leading to processing nitrification and denitrification producing N2O [32]. It is interesting to note that the changes in MBC comparing the end values with the starting values: ca. −10 mg/kg soil for the control versus are ca. +10 mg/kg for treatment Ca(OH)2. Dissolved organic C acted as a substrate for microbes, conjecturing that available C favored N2O reduction. Furthermore, high contents of MBC in the liming material added soils were detected when compared to the control, which indicated the likely high reduction of N2O emissions.

### **5. Conclusions**

The present research showed that the application of liming materials reduced magnitudes of N2O emissions. The pronounced and maximum reduction of soil N2O emissions occurred in the Ca(OH)2 treatment through increasing soil pH when compared to the other liming materials tested. The results suggest that N2O emission mitigation is more dependent on soil pH than on C and N dynamics when capering different liming materials. Moreover, ameliorating the soil acidity condition is a promising option to alleviate N2O emissions from acidic soils. The results can be considered of environmental relevance, and further research in this regard could be interesting, especially in the current context of global warming due to a variety of greenhouse gases released to the atmosphere from different compartments and due to various anthropogenic activities.

**Author Contributions:** Conceptualization, M.S. and R.H.; Formal analysis, M.S., Y.W., L.W., R.H. and S.L.; Investigation, M.S.; Methodology, M.S., Y.W., L.W., P.X., Z.S., X.X. and Y.J.; Software, M.S. and A.Y.; Writing—original draft, M.S.; Writing—review & editing, M.S., A.Y., A.N.-D. and S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to thank the funding bodies of the National Science Foundation of China (417501 10485), China Post-Doctoral Science Foundation (2017 M 622478), and National Key R&D Program (2017 YFD 0800102) for financially assisting the present research.

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