Context dependence in a tropical forest: Repeated disturbance reduces soil nitrate response but increases phosphate

Abstract

Hurricanes and other extreme events are increasing in many regions, yet their long‐term impacts on ecosystem function are uncertain. In forested ecosystems, soil solution chemistry provides an important tool to assess the impacts of disturbance on nutrient cycling and dissolved organic carbon dynamics. Here, we address the dependence of soil solution chemistry on disturbance regime using a novel combination of both experimental and observational results collected over a period of 16 years in montane tropical sites in the Luquillo Experimental Forest of Puerto Rico. Soil solution was sampled following various combinations of canopy trimming and detrital manipulation (2004), repeated manipulation (2014), drought (2015), and Hurricane Maria (2017). Soil solution was sensitive to disturbance but resilient, with return to baseline after 12–18 months. Any disturbance regime that involved loss of canopy and detrital inputs to the forest floor resulted in increased nitrate concentrations, but the response declined with repeated disturbance. Lysimeters in plots that had received no experimental manipulation had 1.5 times higher response to Hurricane Maria than those previously manipulated. The response to disturbance thus showed clear context dependence, with disturbance history affecting disturbance response. Among the nutrients and major ions, only nitrate showed a response to experimental manipulations, drought, and Hurricane Maria. In contrast to nitrate, soil solution potassium was unaltered by initial experimental manipulation but increased dramatically following drought and Hurricane Maria. Phosphorus only increased following Hurricane Maria and only in plots that had twice received experimental trimming and deposition of cut branches on the forest floor. Stoichiometry of dissolved organic matter also changed in these plots, with decreased carbon to nitrogen ratios. The potassium response suggests that damage to roots from tropical cyclones and drought is an important driver of the biogeochemical response to tropical storms. Dampening of soil nitrogen losses and increases in phosphorus losses following successive disturbance events indicates that increased frequency of tropical storms and droughts will result in fundamental alteration of soil biogeochemical cycles, with uncertain effects on forest structure.

Canopy opening increases leaf‐shredding arthropods and nutrient mineralization but not mass loss in wet tropical forest

“…We found N and P fluxes from green litter to soil were highest in bags with large mesh size in the trim + detritus treatment. Correspondingly, McDowell (in press) found in lysimeters located nearest our litterbags that nitrate accumulated more in groundwater in the trim + detritus than in the control treatment. The occurrence of a significant drought during this study could have uncoupled nutrient mineralization and root uptake, as observed previously following Hurricane Hugo (Lodge et al, 1994; McDowell et al, 1996).…”

Soil solution data from Bohemian headwater catchments record atmospheric metal deposition and legacy pollution

“…Lysimeters are used to collect soil solutions for analysis and can offer a path for investigating soil solution properties in their natural state. The study of soil solution hydrochemistry through lysimeters helps in understanding nutrient mobility and dynamics (e.g., Carey 2003 ; Johnson et al 2018 ; Petrash et al 2019 ; Makowski et al 2020 ; McDowell and Potter 2022 ), and can be used for evaluating the prevalence of toxicants in soils (e.g., Shaheen et al 2014 ; Worrall et al 1999 ). Many previous studies used lysimeters for developing the understanding on pollution processes, composition, and fluxes of vadose waters (Goss et al 2010 ).…”

Central European soil solution data record atmospheric metal deposition and legacy pollution

“…Complexity in integrating and interpreting the multiple factors that weigh on measurable chemical properties of soil solutions-at any temporal scale-means that studies based on lysimeters placed and "equilibrated" under natural conditions are rather unusual. Nonetheless, the hydrochemistry of soil solutions can be useful to assess nutrient mobility and dynamics (e.g., Carey, 2003;Johnson et al, 2018;Petrash et al, 2019;Makowski et al, 2020;McDowell and Potter, 2022), and for evaluating the prevalence of toxicants in soils (e.g., Shaheen et al, 2014;Worrall et al, 1999). Many other previous studies used lysimeters for the developing understanding on pollution processes, composition, and uxes of vadose waters (see Goss et al, 2010 for a review).…”