**4.1 Response of microbial diversity to wet and dry periods at the Guánica Dry Forest**

Historical rainfall patterns contribute to the acclimatization and resilience of soil bacterial communities to low and high rainfall events. Bacterial Shannon index (3.9) was similar to values reported by Žifčáková et al. (2016) for a Norway spruce forest (S = 3.5) and lower than the one reported for a hardwood forest (S = 6–6.5) or dry heath in a tundra (S = 7.5) [25]. Our study demonstrates that soil bacterial richness, diversity, and equitability were impacted by rainfall patterns and not by tree species. Both bacterial richness and equitability were higher during the dry period, but bacterial diversity was not impacted by rainfall regime. Our trends imply that a total number of bacterial species do not change during low rainfall events in this forest, but changes occur in the quantity of each species and in their distribution, indicating that soil bacterial communities have adapted to low rainfall at the Guánica Dry Forest. There is indirect evidence that microbial communities do become resistant and function optimally under their historical rainfall regime [26–28]. Cruz-Martínez et al. (2009) [29] found that soil microbial communities were more resilient to long-term changes in rainfall after a 7-year rainfall amendment study. They stated that after 7 years soil microbial communities developed a degree of robustness or acclimatization to the rainfall amendments. Additionally, other studies have reported acclimatization of soil heterotrophic communities to experimental warming and seasonal variation [30]. Compositional changes exhibit historical legacy with regard to moisture regimes [27] suggesting that microbial communities will be shaped in part by the historical climate to which they are exposed [30].

In this study the bacterial communities under all tree species were dominated by *Proteobacteria* (57%), *Actinobacteria* (24–29%), and *Bacteroidetes* (4–7%). All of these bacterial phyla are ubiquitous and have been identified in desert soils [31], agroecosystems, and other types of forest environments [32–35]. *Proteobacteria* are one of the largest bacterial divisions within the prokaryotes and account for most of the known Gram-negative bacteria [36]. Although *Proteobacteria* are one of the top bacterial phyla that are found colonizing many soil types their high predominance at the GDF could be associated with high accumulation of litter and SOM that is reported in these soils. An interesting detail is that Proteobacteria dominated during both periods, with the exception that during the dry period they reduced their relative abundance, suggestion there association with soil moisture as found in other studies [28].

*Actinobacteria*, one of the largest bacterial phyla known, was the second most abundant phylum found in this study. This group is mainly composed of Grampositive bacteria [32–35] and are known to withstand in harsh environments, due to their metabolic, physiologic, and morphological diversity [36]. Soils of the area

*First Insights into the Resilience of the Soil Microbiome of a Tropical Dry Forest… DOI: http://dx.doi.org/10.5772/intechopen.90395*

under study have an accumulation of organic matter mainly composed of new and old leaf litter due to the harsh environmental conditions; this bacterial phylum is associated with the degradation of more complex substrates of SOM including lignin, which could explain their predominance in this forest soils [37–39]. Our data suggest that *Actinobacteria* were highly associated with phosphodiesterase at the Guánica forest. Other studies have also found that *Actinobacteria* populations (characterized by TRLFP) correlated with P content in semiarid environments [40]. Other studies have also demonstrated the production of phosphatases by cultivable soil *Actinobacteria* [41–43] serving as a further evidence of the trend observed in this study. I also found that Actinobacteria were the most abundant phyla during the dry period which is consistent with other reports. A similar trend was also found for a semiarid, high desert grassland north of Flagstaff, Arizona [44] and in another study, which evaluated African tropical forest soils and Chinese forest soils [45].

*Bacteroidetes*, the third most abundant phylum in this forest, are widely distributed in different habitats ranging from Antarctic ice, lakes, the gut of animals, and terrestrial environments [46]. Additionally, this phylum has the ability to withstand extreme desiccation conditions including droughts and UV light [47]. They have been found in fine dust traveling thousands of kilometers [48] and inside microaggregates; authors explain that in semiarid agroecosystems this could be a protection strategy employed by these microbes to endure extreme environmental conditions [48]. I found that *Bacteroidetes* were highly associated with the activities of two soil enzymes evaluated (acid phosphatase and alkaline phosphatase). These enzymes mineralize organic P forms into inorganic P forms. Another study provided evidence suggesting that environmental *Bacteroidetes* specialize in the mineralization of high molecular weight organic matter making them a key compartment for carbon fluxes and budgets in ecosystems [49]. *Bacteroidetes* are oligotrophic and are commonly associated with substrates rich in organic matter [50] as is the forest area where I based my study. The high abundance of *Bacteroidetes* could be associated with the rich organic matter present in the sampling area.

#### **4.2 Influence of tree species on the bacterial populations in this forest**

Even though the most abundant bacterial phyla identified under all tree species were the same (*Proteobacteria*, *Actinobacteria*, and *Bacteroidetes*), some were specific for each tree species. Indicator species analysis (ISA) revealed that specific bacterial species were present under *Ficus citrifolia* and *Pisonia albida* only during the wet period. This trend suggests that these species assemblages may play an important role in the soil ecosystem processes under these specific tree species. One species was *Devosia* spp. whose OUT frequency was 14 (ind val = 0.62; p = 0.009) in soils collected under *Ficus citrifolia. Devosia* spp. forms part of the α-*Proteobacteria*; this genus is a non-rhizobia nodulating, nitrogen fixer [51]. Three different species of *Brevundimonas* spp. also occurred in high frequency under *Ficus citrifolia* during the wet period as indicated by ISA. This genus is actually known to produce phosphodiesterase, a group of enzymes involved in the degradation of organophosphorus [52]. During the wet period, 32 different OTUs were identified exclusively for *Pisonia albida*, and at least 7 of them had a frequency of 18. This high frequency suggests that indicator species may be playing an important role in the soil dynamics of *Pisonia albida*. For instance, three of the most frequent species are nitrogen fixers (*Microlunatus* sp., *Rhodospirillacease* spp*.*, and *Paenibacillus*), and *Pisonia albida* was the tree species with the highest total available nitrogen in this study (data not shown here). Even though we did not cultivate the indicator species nor have information regarding the physiology of the indicator species identified for

to dry and wet periods (**Figure 3**). Samples that correspond to the wet period were associated with acid phosphatase, alkaline phosphatase, β-glucosaminidase, β-glucosidase, and arylsulfatase (**Figure 4**), whereas samples corresponding to the dry period are associated with phosphodiesterase (**Figure 4**). Microbial taxa

*Actinobacteria*, *Planctomycetes*, *Acidobacteria*, *Verrucomicrobia*, *Cyanobacteria*,

**4.1 Response of microbial diversity to wet and dry periods at the Guánica Dry**

warming and seasonal variation [30]. Compositional changes exhibit historical legacy with regard to moisture regimes [27] suggesting that microbial communities will be shaped in part by the historical climate to which they are exposed [30].

*Proteobacteria* (57%), *Actinobacteria* (24–29%), and *Bacteroidetes* (4–7%). All of these bacterial phyla are ubiquitous and have been identified in desert soils [31], agroecosystems, and other types of forest environments [32–35]. *Proteobacteria* are one of the largest bacterial divisions within the prokaryotes and account for most of the known Gram-negative bacteria [36]. Although *Proteobacteria* are one of the top bacterial phyla that are found colonizing many soil types their high predominance at the GDF could be associated with high accumulation of litter and SOM that is reported in these soils. An interesting detail is that Proteobacteria dominated during both periods, with the exception that during the dry period they reduced their relative abundance, suggestion there association with soil moisture as found in

In this study the bacterial communities under all tree species were dominated by

*Actinobacteria*, one of the largest bacterial phyla known, was the second most abundant phylum found in this study. This group is mainly composed of Grampositive bacteria [32–35] and are known to withstand in harsh environments, due to their metabolic, physiologic, and morphological diversity [36]. Soils of the area

Historical rainfall patterns contribute to the acclimatization and resilience of soil bacterial communities to low and high rainfall events. Bacterial Shannon index (3.9) was similar to values reported by Žifčáková et al. (2016) for a Norway spruce forest (S = 3.5) and lower than the one reported for a hardwood forest (S = 6–6.5) or dry heath in a tundra (S = 7.5) [25]. Our study demonstrates that soil bacterial richness, diversity, and equitability were impacted by rainfall patterns and not by tree species. Both bacterial richness and equitability were higher during the dry period, but bacterial diversity was not impacted by rainfall regime. Our trends imply that a total number of bacterial species do not change during low rainfall events in this forest, but changes occur in the quantity of each species and in their distribution, indicating that soil bacterial communities have adapted to low rainfall at the Guánica Dry Forest. There is indirect evidence that microbial communities do become resistant and function optimally under their historical rainfall regime [26–28]. Cruz-Martínez et al. (2009) [29] found that soil microbial communities were more resilient to long-term changes in rainfall after a 7-year rainfall amendment study. They stated that after 7 years soil microbial communities developed a degree of robustness or acclimatization to the rainfall amendments. Additionally, other studies have reported acclimatization of soil heterotrophic communities to experimental

associated with wet samples were Bacteroidetes, Proteobacteria, and Tenericutes. Microbial taxa associated with dry sampling points were

and *Chloroflexi*.

*Microorganisms*

**4. Discussion**

**Forest**

other studies [28].

**54**

*Ficus citrifolia* and *Pisonia albida*, we can infer that they contribute with relevant functions in this soil system. The dwarfed tree species selected for this study influenced the structure and diversity of specific bacterial populations.

of Puerto Rico, Río Piedras Campus, that participated in the field collection of soil samples. Special thanks to Dr. Verónica Acosta-Martínez for offering her comments

*First Insights into the Resilience of the Soil Microbiome of a Tropical Dry Forest…*

and suggestions in the preparation of this manuscript.

*DOI: http://dx.doi.org/10.5772/intechopen.90395*

**Conflict of interest**

**Author details**

**57**

Michelle Jovanne Rivera-Rivera\* and Elvira Cuevas

University of Puerto Rico, Rio Piedras, Puerto Rico

provided the original work is properly cited.

\*Address all correspondence to: yersiniamjr@hotmail.com

Department of Biology and Center for Applied Tropical Ecology and Conservation,

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

No conflict of interest.
