*2.1.2.3 Soil O2 concentration*

Soil O2 concentration was closely related with soil moisture and soil mechanical composition. Generally, soil with higher water content and larger clay fraction had lower soil O2 concentrations. Lower soil O2 concentrations mainly promoted soil N2O emissions via denitrification [20, 35]. The production of N2O and NO was increased when O2 concentration decreased from 21% to 0.5% in a laboratory study [36]. Similarly, field study reported that soil N2O emissions increased with increasing soil O2 concentrations in wetland [37].

#### *2.1.2.4 Soil pH*

pH played an important role in the activity of microbes [38]. Indeed, soil acidification [39] and soil pH amelioration [40] significantly influenced soil N2O emissions. However, other researchers reported that there was no significant correlation between N2O emissions and pH [41].

pH influenced the activity of nitrification- and denitrification-related enzymes [42]. Generally, soil acidification increased N2O emissions [42]. Compared with ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) were higher in activity and resistance from acid soil [43]. However, the domination of AOB was increased by increasing soil pH [44]. Additionally, archaeal *amoA* genes had a wide pH range of about 3.7–8.65, which had high activity in extreme environments such as high temperature and extreme acid [45].

#### *2.1.3 Nitrous oxide emissions from* Camellia oleifera *plantation soils*

Our previous field study (1 year) found that soil N2O emissions were 92.14 ± 47.01 mg m<sup>−</sup><sup>2</sup> in control treatment and were 375.10 ± 60.30 mg m<sup>−</sup><sup>2</sup> in fertilization treatment (400 kg NH4NO3-N ha<sup>−</sup><sup>1</sup> ) from *C. oleifera* plantations [1].

#### **2.2 Soil acidification**

Acid soil (pH < 5.5) as a main soil type covers about 30% free ice land [46]. However, soil acidification has been becoming more and more serious [47]. Soil acidification should be taken into consideration due to its constraint in the sustainable development of agricultural sector [48]. In China, soil pH (except alkaline soils at pH 7.10–8.80) from crop fields reduced by 0.13–0.76 during the year 1980–2000 [49]. For example, soil pH (surface layer) decreased by 0.30 units from 1981 to 2012 in Sichuan Province, China [47].

With a long cultivation history, *C. oleifera* was widely cultivated in acid or strongly acid soil in subtropical China [7]. The optimum pH for the growth of *C. oleifera* is 5.5–6.5 [50]. However, acid deposition [51] and intensive N input [49] may stimulate soil acidification from *C. oleifera* plantations. Additionally, long-term N input may also increase the toxicity of aluminum (Al) [52], limiting the sustainable development of *C. oleifera*.

Soil acidification from *C. oleifera* plantations is mainly related to the following factors.

#### *2.2.1 Precipitation*

Long-term precipitation increased the loss of base cations such as Ca2+, Mg2+, K+ , and Na+ , reducing the soil pH buffering capacity. In addition, long-term precipitation promoted the accumulation of Al3+ and Fe3+ in soil, which could further hydrolyze to Fe(OH)3 or Al(OH)3 and release 3H+ .

#### *2.2.2 Plant physiology*

When plant roots absorb a NH4 + from soil, an H<sup>+</sup> will release into soil; in turn, absorbing a NO3 <sup>−</sup> from soil will release an OH<sup>−</sup> into soil [53].

Organic acid (R▬COOH) from root exudates can release an H<sup>+</sup> after hydrolysis. In addition, anions of organic acids (e.g*.*, citric acid and malic acid) can chelate with Al3+ in the soil and inhibit the root system that absorbs Al3+, alleviating Al3+ toxicity to plant growth [48, 54, 55].

Plants such as *C. oleifera* [56] can uptake Al3+ by roots, promoting the accumulation of Al3+ in surface soil via litter decomposition [57]. The accumulation of Al3+ can replace the base cations such as Ca2+, Mg2+, K+ , and Na+ and accelerate leaching, hence reducing the pH buffering capacity of top soil.

#### *2.2.3 Microbial-mediated nitrification*

For example, NH4 + transfers to NO3 <sup>−</sup> along with the 2H<sup>+</sup> release (NH4 + + 2O2 → NO3 <sup>−</sup> + H2O + 2H+ ) [53]. AOB, AOA, and fungi can participate in the process of nitrification [20]. Nitrification includes the pathway of ammonia oxidation to hydroxylamine, the pathway of hydroxylamine oxidation to nitrite, and the pathway of nitrite oxidation to nitrate (**Figure 1**) [58]. Ammonia can be oxidized by AOA or AOB to hydroxylamine via ammonia monooxygenase (*amo*).

**7**

*Nitrogen Cycling and Soil Amelioration in* Camellia oleifera *Plantations*

Oxidation of sulfur-containing organics will release 4H+

+

soil acidification (Ca(H2PO4)2 → CaHPO4 + H3PO4, H3PO4 → H+

Acidic fertilizers such as Ca(H2PO4)2 will gradually release H<sup>+</sup>

Hydroxylamine can be oxidized to nitrite by hydroxylamine oxidoreductase. Nitrite

Oxidation of sulfur mineral, for example, oxidation of FeS2, will produce 2H<sup>+</sup>

and accelerate leaching, reducing the pH buffering capacity of top

Acid deposition (water-soluble acid gases such as CO2 and sulfur dioxide) and N

For example, deforestation and other land uses can reduce litter accumulation in surface soil, hence declining the accumulation of base cations such as Ca2+, Mg2+,

Acid soils have been facing an increased risk of acidification due to human activities, especially intensive N fertilization [47, 49, 62]. For example, after 6 years

pH in control and fertilization treatment was 5.1 and 4.9, respectively) from a tea plantation in Yixing City, Jiangsu Province, China [63]. A meta-analysis of 1104 field data showed that a negative correlation between soil N2O emissions and pH (3.34–8.7) (N2O-N = −0.67x + 6.55, *R* = 0.22) is negatively related with N fertilization [9]. Moreover, deposition of sulfur dioxide increased soil acidification, stimu-

The mechanism of soil acidification on the stimulation of soil N2O emissions is

2HNO2 ⇌ NO + NO2 + H2O) [65]. Soil NO can be further transformed to N2O with

complex, which may include (but not limited to) the following points.

rally decompose into NO and/or NO2 (3HNO2 ⇌ 2NO + HNO3 + H2O or

yr<sup>−</sup><sup>1</sup>

).

input can replace the base cations such as Ca2+, Mg2+, K+

will generate NH3 (gas) and consume an OH<sup>−</sup>


, and Na+

, soil pH was significantly decreased (soil

<sup>−</sup> (HNO2, pKa = 3.3) will natu-

(2Organic-S + 3O2 +

,

, hence increasing

, which directly

+

+ H2PO4 → 2H+

<sup>2</sup>− + 4H+

*DOI: http://dx.doi.org/10.5772/intechopen.92415*

*2.2.4 Oxidation of sulfur-containing organics*

).

(2FeS2 + 7O2 + 2H2O → 2Fe2+ + 4SO4

<sup>2</sup>− + 4H+

*2.2.5 Intensive nitrogen fertilization*

+

+ PO4

+ OH<sup>−</sup> = NH3↑ + H2O) [60].

<sup>3</sup><sup>−</sup>).

reduce the soil pH buffering capacity [51].

of application of 600 kg Urea-N ha<sup>−</sup><sup>1</sup>

lating soil N2O emissions [64].

*2.3.1 Chemical decomposition of nitrous acid*

Fe2+ when it was not escaping soil [65].

Under acidic conditions, pH < 5.5, NO2

+

can replace the soil base cations such as Ca2+, Mg2+, K+

**2.3 Effects of soil acidification on nitrous oxide emissions**

that generate from litter decomposition [61].

soil [59]. Hydrolysis of soil NH4

2H2O → 2SO4

and Na<sup>+</sup>

(NH4 +

HPO4

H+

K+

Intensive NH4

<sup>2</sup><sup>−</sup> → 3H+

*2.2.6 Acid deposition*

*2.2.7 Other factors*

, and Na+

deposition (especially NH4

can be oxidized to nitrate by nitrite oxidoreductase.

Hydroxylamine can be oxidized to nitrite by hydroxylamine oxidoreductase. Nitrite can be oxidized to nitrate by nitrite oxidoreductase.
