How do hot spots and hot moments in weathering, biogeochemical cycling, hydrologic processes, and atmospheric inputs drive landscape evolution and critical zone function in a humid tropical forest?

Our research is organized into four inter-related focal areas. Focal Area 1 explores the importance of knickpoints and different landscape positions as hot spots for weathering, soil development, and biogeochemical cycling. Focal Area 2 addresses the role of hot spots and hot moments in redox cycling that contributes to the dynamics of weathering, and to the retention and loss of C and nutrients in soils over a range of spatial and temporal scales. Focal Area 3 determines the role of hot moments in the transport of sediment, C, and nutrients in stream flow, and hot spots that determine the distribution of material across the landscape. Focal Area 4 scales up hot spots and hot moments in time and space using climate and hydrologic modeling, and identifies the role of key atmospheric inputs in clouds and rain. Taken together, the research proposed in LCZO2 will provide a well-integrated assessment of critical zone properties and processes that scale from microsites to catenas, watersheds, landscapes, and the region, and from minutes to hours, days, months, and years. The data collected and synthesized as part of LCZO2 will contribute to our understanding of the controls on weathering, soil development, C and nutrient storage and loss, soil and sediment transport, and ultimately landscape evolution and effects of climate change.

Focal Area 1: Hot spots and hot moments in the deep critical zone (Brantley Focal Area Lead)

Hypothesis 1.1: The higher chemical weathering flux and depletion of rock-derived elements from soils in Quartz Diorite(QD) above the knickpoint results from the penetration of high-O2 waters into fractures that promote rapid weathering. Below the knickpoint, relatively low-O2 waters effectively lower reaction rates. In contrast, in the Volcanoclastic(VC) rocks, O2 is consumed relatively high in the profile throughout the watersheds and deep dissolution of silicates outpaces deep Fe oxidation. As a result, VC-derived soils above and below the knickpoint show less variation than their QD-derived H1.1: The higher chemical weathering flux and depletion of rock-derived elements from soils in QD above the knickpoint results from the penetration of high-O2 waters into fractures that promote rapid weathering. Below the knickpoint, relatively low-O2 waters effectively lower reaction rates. In contrast, in the VC rocks, O2 is consumed relatively high in the profile throughout the watersheds and deep dissolution of silicates outpaces deep Fe oxidation. As a result, VC-derived soils above and below the knickpoint show less variation than their QD-derived counterparts. (Brantley, Comas, Buss)

Hypothesis 1.2: Hot spots of rock-derived nutrient availability are best predicted from denudation rates and lithology. The transition from reaction limitation (below the knickpoint) to supply limitation (above the knickpoint) will result in much higher phosphorus and cation availability lower in the landscape. (Porder)

Focal Area 2: Hot Spots and Hot Moments in Redox Dynamics and Associated Fe-C interactions (Silver Focal Area Lead)

Hypothesis 2.1: Patterns in rainfall, drainage, and biological activity drive the distribution of redox environments in the critical zone. (Silver)

Hypothesis 2.2a: Rapid, high magnitude redox fluctuations create hot spots and hot moments of decomposition by stimulating Fe reduction and associated C decomposition. (Silver, Thompson, Plante)

Hypothesis 2.2b: The storage and stabilization of soil organic matter in LCZO soils is controlled by hot spots of Fe-C interactions rather than the bulk mineral matrix. (Plante, Thompson, Silver)

Focal Area 3: Watershed scale hot spots and hot moments (Jerolmack Focal Area Lead)

Hypothesis 3.1: Particulate carbon, fine sediment and bed material each have different characteristic transit times within a watershed. Particles with short residence times are generated at hot spots in the landscape, and particles with long residence times are eroded and transported from relatively stable parts of the landscape during hot moments. Because of differences in landscape stability, these characteristic time scales will differ with position above or below knickpoints. (Willenbring, Jerolmack, Shanley, González)

Hypothesis 3.2: Floods are hot moments that may be treated as 'impulses' that drive sediment transport. The availability of sediment is strongly variable in space due to hot spots associated with physical landscape discontinuities, mainly knickpoints. Sediment transport hysteresis curves allow estimation of time- and space-varying sediment availability. Feedbacks between transport and topography maintain hot spots. (Jerolmack, Willenbring)

Hypothesis 3.3: Hot spots in stream chemistry are associated with recent landslides; hot moments are associated with high flow events that can dilute or enrich various solutes. Watershed lithology controls spatial and temporal variability of solute chemistry through its influence on landslides and subsurface flow paths. (McDowell, Shanley)

Focal Area 4: Hydrologic and Atmospheric Hot Spots and Hot Moments (McDowell Focal Area Lead) 

Hypothesis 4.1: The distribution of hydrologic hot spots like sediment sources and landslides will vary with watershed soils, vegetation, and channel knickpoints; the occurrence of hot spots will vary as a function of storm intensity and frequency (hot moments). (Bras, Wang, González)

Hypothesis 4.2: Orographic precipitation in the LM has decreased during historic times as a consequence of climatic warming. Orographic rains make a disproportionately large contribution to base flow (critical to municipal water supplies), and more so in VC than QD. Cloud level has likewise changed, resulting in smaller cloud inputs of moisture and nutrients to the Luquillo Mountains with important biotic consequences (Scholl, González, Gould, Shanley)

Hypothesis 4.3: Intercontinental transport of African dust alters incoming radiation and cloud formation, and provides nutrient inputs that are significant relative to those from rain events during periods without dust in the atmosphere (H4.2) (Mayol-Bracero, Scholl, González).