Repeated hurricane effects on pH, iron, and soil moisture in a tropical forest

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Christina Kiehl, Environmental Studies, Macalester College, Saint Paul, MN, Christine S. O'Connell, Environmental Studies, Macalester College, St. Paul, MN; Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA, Omar Gutiérrez del Arroyo Santiago, Departments of Biology and Environmental Sciences, University of Puerto Rico, Río Piedras, PR and Whendee Silver, Environmental Science, Policy, and Management, UC Berkeley, Berkeley, CA

Background/Question/Methods

Climate change is increasing the frequency and intensity of hurricanes in tropical forest regions. In the context of increasing hurricanes, it is vital to understand the possible compounded impacts of these disturbances. Here, we investigate iron (Fe) distribution within the soil profile of the Luquillo Long-Term Ecological Research (LTER) site’s Canopy Trimming Experiment (CTE). Samples were collected from two plots within the experiment; one that had experienced two artificial hurricanes in recent years, and one that had not. Samples were collected monthly from November 2017 to November 2018 at five distinct depths from 0-60cm. Fe is important for understanding tropical forest disturbance recovery. The availability of soil carbon (C) and phosphorus (P) are linked to the redox state of soil Fe, which fluctuate as soil redox cycles between oxic and anoxic conditions. Not only does the availability of C and P change, but their mobility shifts as well. Pulsed labile C inputs coupled with reducing conditions were found to stimulate Fe reduction as well as increase soil P mobilization. Therefore, understanding the distribution of Fe(III) and Fe(II) is vital to understanding the availability of P within an ecosystem.

Results/Conclusions
Total Fe showed a spike following Hurricane Maria in both the control and the treatment plots, with a more significant effect in the treatment plot. Both plots returned to pre-Hurricane Maria levels by November 2018. The control showed very little fluctuation in Fe (II) except for the 0-10 cm range. Although there was some variation throughout the year, there were no strong trends within the control. In the treatment, Fe (II) fluctuated significantly throughout the year. Between 0 and 30 cm, Fe (II) increased between January and March 2018. This increase became less pronounced and had later onset in deeper soil. Fe (III) showed less difference between the treatments than Fe(II). Fluctuations varied by depth, generally showing less variation deeper in the soil profile. Although Hurricane Maria passed over both treatment and control plots, the differences in observed Fe phase distribution suggest that there are some legacy effects from previous simulated hurricane disturbances that alter how soil Fe cycling responds to subsequent disturbance. Our results suggest that increased hurricane frequency results in an increase in total Fe to 30 cm depth and that it may lead to changing fluctuation patterns of Fe(II) and Fe(III).