# Fertilization and Soil Microbial Community: A Review
## 3 Results and Discussion
### Effects of Chemical Fertilizers on Biomass, Activity and Diversity of Soil Microorganisms
Moreover, SOM quality (e.g., organic acid, protein, humic acid and lignin content) and its biodegradability essentially influence microbial characteristics (e.g., specific population size, microbial activity and composition) [94; 96]. Consequently, any SOM depletion has negative consequences for microbial community richness together with lower plant health, growth and productivity. In general, organic fertilizers improve soil structure (in terms of particle-size fraction [97]), and they are responsible for a more balanced and stable nutrient supply, which can sustain a more diverse microbial community if compared to mineral fertilizers [98]. Moreover, organic fertilization is reported to increase microbial activity and SOM content and improve the chemical and physical properties of soil better than inorganic fertilization [99; 100; 101], preventing the decrease in soil pH due to mineral fertilizer application [90]. In any case, long-term inorganic or organic fertilization significantly decreased soil pH if compared to a non-fertilized control [86], although soil pH changes due to organic amendments depend on the amendment used [102]. For example, some biochar was found to increase soil pH [102], whereas manure could in general decrease soil pH [103]. Dehydrogenases are respiratory enzymes that oxidase organic compounds allocating two hydrogen atoms from these compounds to electron acceptors, producing energy [58]. These enzymes are present in all soil microorganisms and are not present as a free form, representing only the activity of live microbial cells. Consequently, dehydrogenase activity has been considered as an indicator of soil microbiological activity [58; 67; 68; 104]. Dehydrogenase activity (DHA) was found to be lower in soils that had received high (160 N, 120 P\({}_{2}\)O\({}_{5}\), 160 K\({}_{2}\)O) amounts of NPK fertilization [98], suggesting that these enzymes are highly sensitive to the inhibitory effects associated with high mineral fertilization. In addition, long-term P-deficiency fertilization can significantly decrease DHA together with soil microbial biomass and bacterial diversity [90]. Although an NPK balanced fertilization can increase DHA, the higher increase is always found with organic fertilization [71]. Applying phosphorus-based fertilizers has been shown to lead to seasonal variations in microbial activity, as well as in the abundance of specific bacterial and fungal phospholipid fatty acid (PLGA) indicators of soil microbial biomass [105; 106; 107]. In their experimental study, [PERSON] and [PERSON] [108] examined the response of maize bacterial, fungi and archaeal communities to compost and inorganic fertilizers. The results showed that both fertilizers influenced the maize rhizosphere microbial community but the organic amendments provided the most stable microbial community; these results were also found by [PERSON] et al. [109], in which higher levels of NPK treatments (60 kg of NPK fertilizer as N/ha) negatively affected the microbial community structure and abundance in an agricultural soil. On the other hand, long-term fertilization with organic amendments can both mitigate the negative effects and exploit the positive effects of climate change on crop production, enhancing soil quality and improving crop productivity, as was observed by several authors (e.g., [PERSON] et al. [110] in northeast China). In an overall view, although it is not possible to summarize all the beneficial effects of organic fertilizers because they are very different in type of production, content in essential nutrients (NPK), pH, structure, etc., they always improve soil structure and organic matter content of soil, together with an increase in microbial communities. Moreover, thanks to the soil quality improvement, they also favor microbial community abundance and activity. On the other hand, although mineral fertilizers, apart from their environmental side effects (GHG emission, soil and water pollution), provide essential nutrients, long-term application contributes to soil depletion.