Controls on Soil Carbon Stocks in El Yunque National Forest, Puerto Rico

Johnson, Arthur H.; Xing, Hao Xing; Scatena, Frederick N.

doi:10.2136/sssaj2014.05.0199

Cited by 25 publications

(20 citation statements)

References 24 publications

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“…In contrast, light fraction C content likely had less impact on variation in bulk density. Another recent study in the Luquillo Experimental Forest found that soil C content strongly increased with soil C/N ratios (Johnson et al 2015). The sierra palm and elfin forests likely had a greater proportion of particulate C, as suggested by large increases in total soil C concentrations.…”

Section: Discussionmentioning

confidence: 89%

Reducing conditions, reactive metals, and their interactions can explain spatial patterns of surface soil carbon in a humid tropical forest

2015

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Humid tropical forests support large stocks of surface soil carbon (C) that exhibit high spatial variability over scales of meters to landscapes (km). Reactive minerals and organo-metal complexes are known to contribute to C accumulation in these ecosystems, although potential interactions with environmental factors such as oxygen (O 2 ) availability have received much less attention. Reducing conditions can potentially contribute to C accumulation, yet anaerobic metabolic processes such as iron (Fe) reduction can also drive substantial C losses. We tested whether these factors could explain variation in soil C (0-10 and 10-20 cm depths) over multiple spatial scales in the Luquillo Experimental Forest, Puerto Rico, using reduced iron (Fe(II)) concentrations as an index of reducing conditions across sites differing in vegetation, topographic position, and/or climate. Fine root biomass and Fe(II) were the best overall correlates of site (n = 6) mean C concentrations and stocks from 0 to 20 cm depth (r = 0.99 and 0.98, respectively). Litterfall decreased as reducing conditions, total and dead fine root biomass, and soil C increased among sites, suggesting that decomposition rates rather than C inputs regulated soil C content at the landscape scale. Strong relationships between Fe(II) and dead fine root biomass suggest that reducing conditions suppressed particulate organic matter decomposition. The optimal mixed-effects regression model for individual soil samples (n = 149) showed that aluminum (Al) and Fe in citrate/ascorbate and oxalate extractions, Fe(II), fine root biomass, and interactions between Fe(II) and Al explained most of the variation in C concentrations (pseudo R 2 = 0.82). The optimal model of C stocks was similar but did not include fine root biomass (pseudo R 2 = 0.62). In these models, soil C concentrations and stocks increased with citrate/ascorbate-extractable Al and oxalate-extractable Fe. However, soil C decreased with citrate/ ascorbate-extractable Fe, an index of Fe susceptible to anaerobic microbial reduction. At the site scale (n = 6), ratios of citrate/ascorbate to oxalate-extractable Fe consistently decreased across a landscape O 2 gradient as C increased. We suggest that the impact of reducing conditions on organic matter decomposition and the presence of organo-metal complexes and C sorption by short-range order Fe and Al contribute to Responsible Editor: Kate Lajtha.Electronic supplementary material The online version of this article (

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Section: Discussionmentioning

confidence: 89%

Reducing conditions, reactive metals, and their interactions can explain spatial patterns of surface soil carbon in a humid tropical forest

2015

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“…The juxtaposition of high Hg deposition and low trophic uptake is not necessarily at odds, as inhibition of methylmercury formation will limit uptake (Wiener et al 2006). However, landscape conditions at our site appeared to be conducive to methylation, including year-round warm temperatures (Murphy and Stallard 2012), above-average soil carbon (Johnson et al 2015), ample sulfur availability (Peters et al 2006), and occurrence of reducing microsites in surface soils (Hall et al 2016). The apparent paradox prompted directed field campaigns to (i) confirm the preliminary finding of low trophic uptake by assessing Hg in avian blood, and (ii) investigate Hg(II)-methylation dynamics within soil and sediment.…”

Section: Introductionmentioning

confidence: 81%

“…1). Soils are sandy Inceptisols derived from the quartz diorite bedrock (Johnson et al 2015). However, nearly all soils have aquic properties due to clayrich B horizons that impede infiltration (Johnson et al 2015).…”

Section: Site Descriptionmentioning

confidence: 99%

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Resolving a paradox—high mercury deposition, but low bioaccumulation in northeastern Puerto Rico

et al. 2019

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At a "clean air" trade winds site in northeastern Puerto Rico, we found an apparent paradox: atmospheric total mercury (THg) deposition was highest of any site in the USA Mercury Deposition Network, but assimilation into the local food web was quite low. Avian blood THg concentrations (n = 31, from eight species in five foraging guilds) ranged widely from 0.2 to 32 ng g−1 (median of 4.3 ng g−1). Within this population, THg was significantly greater at a low-elevation site near a wetland compared to an upland montane site, even when the comparison was limited to a single species. Overall, however, THg concentrations were approximately an order of magnitude lower than comparable populations in the continental U.S. In surface soil and sediment, potential rates of demethylation were 3 to 9-fold greater than those for Hg(II)-methylation (based on six radiotracer amendment incubations), but rates of change of ambient MeHg pools showed a slight net positive Hg(II)-methylation. Thus, the resolution of the paradox is that MeHg degradation approximately keeps pace with MeHg production in this landscape. Further, any net production of MeHg is subject to frequent flushing by high rainfall on chronically wet soils. The interplay of these microbial processes and hydrology appears to shield the local food web from adverse effects of high atmospheric mercury loading. This scenario may play out in other humid tropical ecosystems as well, but it is difficult to evaluate because coordinated studies of Hg deposition, methylation, and trophic uptake have not been conducted at other tropical sites.

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“…Palm forest occupies most life zones and it is typically located at relatively steep upper slopes. Cloud forest accounts for less than 10 % of the Luquillo CZO stands, and it may occupy high elevation interfluves and poorly drained soils (Johnson et al ., ; Waide et al ., ). The vegetation cover of Icacos and Mameyes watersheds is given in Figures (c) and (c), respectively (Helmer et al, ; PRGAP, ).…”

Section: Methodsmentioning

confidence: 97%

Hydro‐geomorphic behavior of contrasting tropical landscapes and critical zone response to changing climate

Dialynas

Bras

2018

Earth Surf Processes Landf

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The potentially important influence of climate change on landscape evolution and on critical zone processes is not sufficiently understood. The relative contribution of hydro‐climatic factors on hillslope erosion rates may significantly vary with topography at the watershed scale. The objective of this study is to quantify the hydro‐geomorphic behavior of two contrasting landscapes in response to different climate change scenarios in the Luquillo Critical Zone Observatory, a site of particular geomorphological interest, in terms of hillslope erosion and rainfall‐triggered landslides. We investigate the extent to which hillslope erosion and landslide occurrence remain relatively invariant with future hydro‐climatic perturbations. The adjacent Mameyes and Icacos watersheds are studied, which are underlain by contrasting lithologies. A high resolution coupled hydro‐geomorphic model based on tRIBS (Triangulated Irregular Network‐based Real‐time Integrated Basin Simulator) is used. Observations of landslide activity and hillslope erosion are used to evaluate the model performance. The process‐based model quantifies feedbacks among different hydrologic processes, landslide occurrence, and topsoil erosion and deposition. Simulations suggest that the propensity for landslide occurrence in the Luquillo Mountains is controlled by tropical storms, subsurface water flow, and by non‐climatic factors, and is expected to remain significant through 2099. The Icacos watershed, which is underlain by quartz diorite, is dominated by relatively large landslides. The relative frequency of smaller landslides is higher at the Mameyes watershed, which is underlain by volcaniclastic rock. While projections of precipitation decrease at the study site may lead to moderate decline in hillslope erosion rates, the simulated erosional potential of the two diverse landscapes likely remains significant. © 2018 John Wiley & Sons, Ltd.

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Controls on Soil Carbon Stocks in El Yunque National Forest, Puerto Rico

Cited by 25 publications

References 24 publications

Reducing conditions, reactive metals, and their interactions can explain spatial patterns of surface soil carbon in a humid tropical forest

Reducing conditions, reactive metals, and their interactions can explain spatial patterns of surface soil carbon in a humid tropical forest

Resolving a paradox—high mercury deposition, but low bioaccumulation in northeastern Puerto Rico

Hydro‐geomorphic behavior of contrasting tropical landscapes and critical zone response to changing climate

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