*2.3.1 Acidification*

Degradation of soil chemical properties is the intensification of naturally processed weathering and substance leaching. Chemical soil degradation includes acidification, salinization and intoxication. Acidification is the most extensive process of forest soil degradation causing decline in site fertility [36]. Soil acidification is gradual decreased in neutralizing capacity. In nature, acidification is elicited mainly by water autoprotolysis, naturally acid atmospheric precipitation, organic acids activities, but also by formation of strong acids after reactions of water with atmospheric gases (CO2, SO2) or with some rock-forming minerals (chlorides, sulphates or carbonates). The resulting acids (formal HCl, H2CO3 and H2SO3) can cause very intensive decomposition of original minerals into salts [37].

The intensification of soil acidification was caused by fertilization, crop cultivation and industrial pollution. Industrially emitted CO2, SO2 and NOx create formal acids and soil bases are excessively depleted to neutralize them. The base loss slows down humus formation; on the other hand, raw humus is a significant source of organic acids. The slow-motion formation of humus is reflected in decrease of organo-mineral colloid genesis as a result of which the number of binding sites for exchange cations on the active surfaces of soil particles decreases. The decomposition of variable organo-mineral colloids limits base cations exchanges to stable mineral colloids. Nonetheless, mineral colloids can capture only 0.2–25% of exchangeable cations, unlike organic particles [38].

The soil resists acidification impacts by exchange reactions between inputs of acid-forming H3O+ and available sources of releasable cations. Soil cation sources are

#### **Figure 4.**

*Introskeletal erosion leaves rocky flows without surface humus instead soil, where plants can to root hardly, thus forest site gets features of disperse platforms with dwarf vegetation.*

active depending on pH (**Figure 5**). Intensified acidification of forest soils is naturally slowed down either by deciduous tree species or by weathering of the soil-forming substrates. The influence of tree species predominates in surface soil horizons while the influences of soil-forming substrate predominate in the subsurface horizons. Significant acidification in surface horizons of forest soils most often affects the transitional ecosystem types [39]. Even though the mitigating effect of tree species does not overcome impacts of weathering, the optimal tree species composition actively

#### **Figure 5.**

*Intervals of soil acidity (pH) and organic matter C/N ratio divide trophic (A – Oligotrophic; AB – Oligomesotrophic; B – Mesotrophic; BC – Mesotrophically nitrophilous; C – Nitrophilous; CD – Nitrophilousbase; BD – Mesotrophically base; D – Base) series among zones buffering acidification through specific neutralization. Data according to [39].*

reducing C/N prolongs weathering effects. On the other hand, soil cation release by weathering maintains intensified acidification as a reversible process. Weathering counteracts acidification by means of electrochemically controlled soil-forming substrate decomposition [12, 40, 41]:


The unnatural decomposition of soil minerals triggers irreversible acidification. Acidification may be mitigated merely after the removal of acidifying substances sources. The acidification of forest soils affected exchange zone the most, switching to active aluminum zone [42]. The damage to the forest ecosystem by release of active Al3+ followed due to occupation of exchange sites on soil particle surfaces instead of bases, the lack of which limited root growth. The roots were concentrated shallowly below the surface so that new focal points of biological activity and humus ceased to form deeper in the soil [43]. Introducing the other side of the fact, the marginally widespread transition from the aluminum zone to iron one was ensued by loss at the ability of forest ecosystems to restore from the damage (**Figure 6**).

Air pollution has significantly accelerated soil acidification, especially in the areas of forests transformed into homogeneous stands of coniferous tree species. While cultivation of homogeneous coniferous forests homogenized formation of acidic humus causing micropodzolization and increased base cation leaching, the pollution after acid deposition reduced not only the forest increment but also decomposition of organic matter [44]. Forest increment was reduced by direct damage to the assimilation apparatus, by stimulating sensitivity to seasonal drought or frost and by reduction in soil symbioses mediating nutrient deficiencies. The decline of mycorrhizal

*Soil Degradation Processes Linked to Long-Term Forest-Type Damage DOI: http://dx.doi.org/10.5772/intechopen.106390*

#### **Figure 6.**

*Irreversible damaged forests are characteristic by predominantingly dead tree storey and by absent young woods due to lost soil organic matter irreplaceably stabilizing moisture during seed germination.*

fungi was followed by increase in frequency of saprophytic to saproparasitic fungi, which diverted organic matter decomposition to complete leaching from the ecosystem [45]. The susceptibility of mycorrhizal symbioses to pollution resulted in limited accessibility to phosphorus necessary for nucleic acid synthesis [46]. The disturbed phosphorus cycle triggered decrease in increment as well as seed germination leading to forest self-organization loss [47].

#### *2.3.2 Salinization*

Soil salinization is the process of accumulating surpluses of mineral salts. Salinization of forest soils is a rare phenomenon, but it threatens 23% of agricultural land, mainly in arid areas [48]. Forest soils are salinized in areal or linear extent. Areal salinization is caused by high groundwater mineral levels, the use of saline water for irrigation, waste materials for fertilization or deposition of solids. Linear salinization occurs alongside roadsides maintained by chemical salting during winter or along river banks. The recent climate change is expanding areas of salinized soils with rising sea level along the coast or estuaries. On the other hand, the natural risk of soil acidification subdues consequences of salinization [49].

The impacts of salinization in forests are associated with extreme soil chemical properties. Salinization highlights malfunctions of water and nutrient uptake by plants. Above all, the disproportionate sodium input (sodification) disrupts ration among exchangeable bases in the soil environment, thereby disrupting effects of alkalization on soil structure. Significant Na<sup>+</sup> inputs displace other cations from soil sorption complex and disperse soil particles. Sodium displacement of cations results in deficient nutrition, but at the same time crushes soil structure, thus water availability fluctuates. Sodium surplus in plant tissues reduces osmotic pressure, whereby cells lose ability to absorb other substances from soil solution [27]. While conifers are susceptible to soil salinization, deciduous tree species are tolerant to it. The younger plants are more susceptible than the older ones.

Areal forest salinization is most at risk in floodplains due to variability of water flow. The regulation of water flow caused groundwater level fall in some river basins while it resulted to water level permanent increase in some other ones. The groundwater level decline was typically ensued by ecosystem desiccation due to the fact that riparian forests are mostly located in submontane locations with insufficient precipitation [50]. By contrast, rising groundwater levels after water regulation meant change in availability of mineral ions, with impacts on soil microbial activity and ability of the ecosystem to sequester carbon. The increase in level of saline water inflicts decrease in soil microbial activity and consequently decrease in vegetation growth [51]. On the one hand, decreases in growth processes are caused by loss of oxygen in soil environment, on the other hand, by increasing concentrations of Na+ , Cl<sup>−</sup> , SO4 2− ions, including Fe and Mn compounds. In particular, SO4 2− in the soil solution is converted to toxic sulphide when there is a lack of oxygen. Although Fe and Mn are biogenic elements that catalyze soil organic matter decomposition, bound in sulphides block microbial metabolism [52]. The main forest salinization danger with groundwater is inability to adapt on climate change due to spread of microaerobic conditions [53, 54].

#### *2.3.3 Intoxication*

Intoxication of soils with heavy metals, radioactive or petroleum substances is a rare but very hazardous process of cumulative pollution. Especially, heavy metals merely slowly participate in biogeochemical cycles and accumulate in the ecosystem because they are either microbiogenic (Cu and Zn), or xenobiotic (e.g. Cd, Co, Pb, Hg, Ni).

Soil intoxication occurs by deposition means. Sources of cumulative pollution are point or dispersed ones. The point sources of heavy metals are smelters, thermal power stations or municipalities by watercourses. The dispersed sources are represented by polluted water, inappropriate distribution of industrial or sewage sludge or operation of internal combustion engines. Fluvisols, which are among the most intoxicated soils, are usually located between watercourses and agricultural soils [48].

Xenobiotics are toxic to most organisms. They mainly affect energy balance of living cells and their division. Heavy metals mainly bring about halting respiration as a consequence of interactions with SH- groups at intracellular enzymes and their complexes, they disrupt semipermeability of cell membranes and their proton gradient. Soil environment pollution with heavy metals significantly reduces density of microbial occurrences and directly damages plants. The rate of soil contamination damages microbial activity significantly more than the differences in heavy metal contents among different sites [55]. However, the decline of susceptible species is being replaced by expansion of resistant species populations, including pests abundantly infesting damaged plants [56].

Resistant phytophagous arthropods adapt to the environment contaminated with heavy metals by searching for less contaminated nourishment, sufficient release of metals in excrements, by sloughing or by means of other tissues (in the adipose body, epithelium of the digestive tract) [57]. The importance of phytophagous insects for the movement of heavy metals in ecosystem lies in the fact that these invertebrates are an important link in food web that receive toxic substances directly from plants, especially Cd and Zn [58, 59], but no Ni and Fe [60].
