**5. Conclusions**

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any correlation between available P and mycorrhizal marker 16:1 ω5c.

trends during the rainfall pulse.

**4.3 Soil enzymatic activity**

events. The fungal markers showed lower abundance during July and higher abundance during October and November. Saprophytic fungal marker (18:3 ω6c, 18:2 ω6c) and the mycorrhiza marker 16:1 ω5c were more susceptible to monthly rainfall variations (p < 0.0001) than any other microbial group. As a consequence of rainfall pulses, adventitious roots were observed in the soil substrate, which serves as a surface area for the establishment of arbuscular mycorrhizal fungi. Although arbuscular mycorrhizal fungi enhance P absorption in root systems, we did not find

Although fungal markers showed greater monthly fluctuations, the G+ bacterial marker *i*15:0 also responded significantly to monthly variations, specifically to water input. Dijkman [26] postulated that FAME marker (*i*15:0) is found in sulphate-reducing bacteria, which could explain our result. For example, the accumulation of water due to high rain pulses could make the soil habitat an anaerobic substrate, contributing to the proliferation of anaerobic and sulphate-reducing bacteria. Sulphate-reducing bacteria are widely spread in anaerobic habitats and play crucial roles in S and C mineralization [27]. For instance, during November, samples were collected after 2 days of continuous rain and the abundance of FAME marker *i*15:0 increased responding to rainfall pulses, which could support the possibility that certain sulphate-reducing bacteria were represented by this FAME marker and this would be in agreement with the C and S cycling enzyme activity

Our results show that enzyme activity is highly dependent on the soil microbial community structure of each tree species. Tree species was a strong modulator of soil enzyme activity when compared to monthly climatic variations. Although all enzymes tested were active under each tree species, activity of certain enzymes was higher under specific tree species. For instance, acid phosphatase, alkaline phosphatase and β-glucosidase activity was higher under *Tabebuia heterophylla,* and

Tree species idiosyncratic traits affect not only the structure of microbial communities but also their enzyme activities. Alkaline phosphatase under *Pisonia albida* correlated with many microbial groups (Gram-positive, Gram-negative, actinobacteria, protozoa and saprophytic and arbuscular fungal markers). Alkaline phosphatase under *Ficus citrifolia* only correlated with Gram-positive, saprophytic and arbuscular fungi and finally alkaline phosphatase under *T. heterophylla* correlated with Gram-negative, actinobacteria and saprophytic fungi. Each enzyme activity correlated with specific microbial FAMEs, which varied depending on the tree species. Our results agree with [28] where they found that phosphatase enzymes

Microbial phosphatase activity is crucial for the supply of inorganic phosphate in this system. Soils at the Guánica Dry Forest are P limited due to the high amount of carbonates from the underlying calcareous substrate. In this study, phosphatases were the most active enzymes, microbial communities are allocating resources to balance the P deficiency of the Guánica soils. When P deficiency is present in a system, increased phosphatase activity occurs as a response to P starvation [29]. A mechanism that compensates soil P deficiency is the inoculation of the arbuscular mycorrhizae, which are known to enhance plant P availability via the production of phosphatase enzymes [30]. We identified similar concentrations of the arbuscular mycorrhizae marker 16:1 ω5c for *Tabebuia heterophylla* and *Ficus citrifolia.* Idiosyncratic effect was observed for each tree species; for example, *Ficus citrifolia* enhanced the activity of phosphodiesterase and *Tabebuia heterophylla* enhanced the

β-glucosaminidase activity was the highest reported for *Pisonia albida.*

correlated with higher numbers of fatty acids [28].

**108**

Idiosyncratic effects of tree species coupled with extreme changes in water input contribute in shaping the soil microbial community structure and enzymatic activity at the Guánica Dry Forest. In this system, saprophytic fungi, arbuscular mycorrhizae and anaerobic Gram-positive sulphur-reducing bacteria seem to be more sensitive to rainfall pulses when compared to Gram-positive (including actinobacteria), Gram-negative and protozoan communities. Even though monthly variations play a significant role in microbial community structure, soil enzyme activities did not vary during the months sampled. Our findings demonstrate that, although mesoclimate is a determinant driver of ecosystems, tree species is a stronger modulator of the soil microbial dynamics at the Guánica Dry Forest. To our understanding, this is the first study that provides insight into the soil microbial community of the Guánica Dry Forest, a valuable contribution that will help elucidate strategies for better management and protection of the soil biota of the area.

## **Acknowledgements**

The project was funded by NSF Grant HRD-0734826 and is a contribution of the Centre of Applied Tropical Ecology and Conservation of the University of Puerto Rico in collaboration with the USDA-ARS in Lubbock, Texas. We appreciate the support of Mr. Larry Diaz, laboratory coordinator, and students from the Ecosystems Processes and Function laboratory of the University of Puerto Rico, Río Piedras Campus, that participated in the field collection of soil samples; Mr. Jon Cotton, laboratory technician, and students from Dr. Veronica Acosta-Martinez laboratory at the USDA-ARS Lubbock Texas, that participated in sample processing, and Mr. Dean Holder from the USDA-ARS Lubbock Texas, who offered training in the determination of soil texture. Special thanks to Dr. Francisco Calderon from the USDA-ARS Akron Colorado for kind participation in the revision of this manuscript. Thanks to Dr. Raymond Tremblay for commenting on the statistical part of the manuscript.

## **Conflict of interest**

No conflict of interest.

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