*2.2.6 Acid deposition*

*Advances in Forest Management under Global Change*

fertilization treatment (400 kg NH4NO3-N ha<sup>−</sup><sup>1</sup>

2012 in Sichuan Province, China [47].

hydrolyze to Fe(OH)3 or Al(OH)3 and release 3H+

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

hence reducing the pH buffering capacity of top soil.

<sup>−</sup> + H2O + 2H+

transfers to NO3

When plant roots absorb a NH4

*2.2.3 Microbial-mediated nitrification*

+

development of *C. oleifera*.

factors.

K+

*2.2.1 Precipitation*

*2.2.2 Plant physiology*

to plant growth [48, 54, 55].

For example, NH4

+ 2O2 → NO3

absorbing a NO3

, and Na+

92.14 ± 47.01 mg m<sup>−</sup><sup>2</sup>

**2.2 Soil acidification**

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

Our previous field study (1 year) found that soil N2O emissions were

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

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

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

Long-term precipitation increased the loss of base cations such as Ca2+, Mg2+,

cipitation promoted the accumulation of Al3+ and Fe3+ in soil, which could further

from soil, an H<sup>+</sup>

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

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+

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*).

+

Organic acid (R▬COOH) from root exudates can release an H<sup>+</sup>

<sup>−</sup> from soil will release an OH<sup>−</sup> into soil [53].

, reducing the soil pH buffering capacity. In addition, long-term pre-

.

, and Na+

) [53]. AOB, AOA, and fungi can participate in

<sup>−</sup> along with the 2H<sup>+</sup>

will release into soil; in turn,

release

after hydrolysis.

and accelerate leaching,

in control treatment and were 375.10 ± 60.30 mg m<sup>−</sup><sup>2</sup>

in

) from *C. oleifera* plantations [1].

**6**

(NH4 +

Acid deposition (water-soluble acid gases such as CO2 and sulfur dioxide) and N deposition (especially NH4 + -N) increased soil acidification [51]. Precipitation with H+ can replace the soil base cations such as Ca2+, Mg2+, K+ , and Na+ , which directly reduce the soil pH buffering capacity [51].

## *2.2.7 Other factors*

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+, K+ , and Na+ that generate from litter decomposition [61].

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

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 of application of 600 kg Urea-N ha<sup>−</sup><sup>1</sup> yr<sup>−</sup><sup>1</sup> , soil pH was significantly decreased (soil 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, stimulating soil N2O emissions [64].

The mechanism of soil acidification on the stimulation of soil N2O emissions is complex, which may include (but not limited to) the following points.

### *2.3.1 Chemical decomposition of nitrous acid*

Under acidic conditions, pH < 5.5, NO2 <sup>−</sup> (HNO2, pKa = 3.3) will naturally decompose into NO and/or NO2 (3HNO2 ⇌ 2NO + HNO3 + H2O or 2HNO2 ⇌ NO + NO2 + H2O) [65]. Soil NO can be further transformed to N2O with Fe2+ when it was not escaping soil [65].

#### *2.3.2 Shifts in microbial communities and abundance*

Generally, the abundance of AOB was lower in soil pH < 5.5 than that in neutral soil pH. Here, nitrification was weak and almost disappears at soil pH < 4 [66]. However, AOA could mediate the process of ammonia oxidation in extremely strong acidity soil (pH: 4.2–4.47) [43]. Another study reported that the abundance of AOB was positively correlated with pH (*R*<sup>2</sup> = 0.2807), while the abundance of AOA was negatively correlated with pH (*R*<sup>2</sup> = 0.2141) [67]. For example, AOA dominated in acid paddy soil (pH 5.6), while AOB dominated in alkaline soil (pH 8.2) [68]. Previous research indicated that fungi were the main microbial community that mediated N2O emissions in acid soil [69, 70]. Additionally, fungi-mediated denitrification accounted for 70% soil N2O emissions from a 100-year-old tea plantation (soil pH 3.8) [71].

In acid soils, the activity of N2O reductase was inhibited, leading to higher N2O emissions in lower soil pH [72]. Indeed, there was a positive correlation between the abundance of *nirS*, *nirK*, or *nosZ* and soil pH (4.0–8.0) and a negative correlation between N2O/(N2O + N2) and soil pH [73]. In agreement, N2O/(N2O + N2) was negatively correlated with soil pH (3.7–8.0) (*R*<sup>2</sup> = 0.759, *P* < 0.001), and lime addition decreased N2O/(N2O + N2) [74]. The ratio of N2O/(N2O + N2) increased with decreasing pH (5.57–7.06) (*R*<sup>2</sup> = 0.82) [75]. Consistently, soil pH was negatively correlated with N2O/N2 [76]. Intensive management consistently decreased soil pH and increased the ratio of N2O/(N2O + N2) [77]. Increasing dolomite dosage increased soil pH and hence increased the transcription of *nosZ* genes and reduced the potential of N2O production in acid soils [26].

#### *2.3.3 Microbes increased resistance to soil acidification*

Laboratory study showed that the potential of soil N2O emissions was increased with decreasing pH (soil pH ranging from 2.96 to 6.26) from tea plantations in Japanese [78]. In addition, higher soil N2O emissions and lower abundance of *nosZ* genes were observed in soil pH at 3.71 (control) than in pH at 5.11, 6.19, and 7.41 (lime amelioration) under NO3 <sup>−</sup>-N fertilization (50, 200, and 1000 mg kg<sup>−</sup><sup>1</sup> ) from a 100-year-old tea plantation [79]. Field study found a negative correlation between soil N2O emissions and pH (pH 3.6–5.9) (N2O-N = 636.6\* e<sup>−</sup>0.8028 \* pH, *R* = −0.93) from *Betula pendula* Roth forest [80]. Thus, denitrifying microorganisms may have been adapted extremely to acid soil environments, resulting in high N2O emissions when soil acidification happened.

#### **3. Sustainable forest management**

Soil amelioration (e.g*.*, application of lime, biochar nitrification inhibitors, and urease inhibitors) plays an important role in mitigation of soil acidification and N2O emissions.

#### **3.1 Lime**

Lime as an ameliorant was often used to amend acid soils in southern China due to increasing soil pH. It can relieve the toxic effect of soil Al3+ on plant growth by reducing soil exchangeable H+ [81]. Lime addition increased soil pH and salt saturation [82]. In addition, application of lime can reduce soil N2O emissions [40]. For example, under 60% WFPS or flooded conditions, dolomite addition at medium- or high-dose levels (1 or 2 g kg<sup>−</sup><sup>1</sup> soil) can reduce N2O emissions and increase the transcription of *nosZ* genes (N2O → N2) by increasing acid soil pH from a rice-rapeseed

**9**

H3SiO4

tion [107, 108].

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

nologies for biochar application in soil amelioration.

*3.2.1 Effects of biochar on soil nitrous oxide emissions*

*3.2.2 Effects of biochar on soil pH buffer capacity*

of weakly acidic functional groups can associate with H+

<sup>−</sup> (present at a high pH) can combine with H+

rotation system [26]. However, lime addition reduced the content of soluble organic carbon in the soil layer 10–30 cm [83]. Consistently, long-term lime addition increased the soil pH but stimulated the decomposition of soil organic carbon [84].

Biochar was stable in the soil from Amazon basin of Brazil, and biochar input improved soil fertility [85]. This discovery accelerated the development of tech-

properties of biochar were mainly determined by pyrolysis temperature [87]. Presently, biochar was widely used as a soil ameliorant in agriculture and forestry field. For example, our previous studies reported that *C. oleifera* fruit shells are ideal feedstock for producing biochar as they are rich in C and N [1, 88]. Biochar

1.Carbon recalcitrance of biochar can increase soil C pool. The potential of biochar in mitigation of greenhouse gas emissions was 1.0–1.8 Pg CO2-Ceq yr<sup>−</sup><sup>1</sup>

2.Biochar had excellent physicochemical characteristics in soil nutrient retention and utilization [90, 91] and water conservation [92]. Additionally, biochar can increase the plant resistance to Al3+ toxicity [81], the clone of arbuscular mycorrhizal fungi, and crop yield [93, 94]. It can decrease continuous cropping obstacles such as root-knot nematode [95] and *Ralstonia solanacearum* [96].

3.Biochar is rich in macro- and microelements [97], which can reduce the dosage

The physicochemical properties of biochar and soil can interactively influence soil N2O emissions [98]. However, the effects of biochar on soil N2O emissions varied, including positive effects [99], negative effects [100], and no effects [101]. Biochar addition increased soil N2O emissions with the release of N from biochar

[102]. By contrast, biochar reduced soil N2O emissions with (1) increased NO3

substances (pyrolysis by-products) on N-cycle microorganisms [106].

immobilization [103]; (2) increased copy numbers of *nos*Z gene [104, 105]; and (3) increased toxic effects of polycyclic aromatic hydrocarbons and other toxic

Biochar that increased soil pH buffer capacity may predominantly correlate with biochar riches in oxygen-containing functional groups in surface. The anions

pH. Meanwhile, exchangeable base cations can release into the solution, thus increasing soil pH buffer capacity [107, 108]. In addition, soluble silicon (Si) such as

[89].

<sup>−</sup>-N

, hence increasing soil

and generate H2SiO3 precipita-

Biochar is a carbon (C)-rich solid material by pyrolyzing of organic biomass such as crop straw, forestry by-products, urban waste, industrial by-products, animal manure, and urban sludge at low oxygen and high temperature (250–700°C) condition [86]. Biochar has been characterized by a high pH, specific surface area, degree of aromatization, and porosity. In addition, biochar is rich in C-containing functional groups (e.g., C–H, C–O, C=C and C=O) and relatively stable organic C. The physicochemical

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

includes the following advantages:

of fertilizer.

**3.2 Biochar**

rotation system [26]. However, lime addition reduced the content of soluble organic carbon in the soil layer 10–30 cm [83]. Consistently, long-term lime addition increased the soil pH but stimulated the decomposition of soil organic carbon [84].
