Geochemical Model of Redox Reactions in a Tropical Rain Forest Stream Riparian Zone: DOC Oxidation, Respiration and Denitrification

Jiménez R.A., Geochemical Model of Redox Reactions in a Tropical Rain Forest Stream Riparian Zone: DOC Oxidation, Respiration and Denitrification. Master's Capstone and Thesis. University of Pennsylvania, 2011.

A geochemical equilibrium model was used to quantify Dissolved Organic Carbon (DOC) electron donors during aerobic respiration and denitrification in a tropical stream riparian zone of the Luquillo Experimental Forest, Puerto Rico. DOC electron donors were measured across three general redox zones (Oxic: slope, Transitional: slope-riparian interface and Anoxic: riparian-floodplain) of the Icacos watershed. Model results suggest that nitrate and oxygen are completely reduced after approximately 10.1 mg/L of DOC have reacted with an initial ground water solution. In order to reach the observed mean oxygen concentration of 3.79 mg/L in the Oxic zone from the modeled equilibrium oxygen concentration of 9.46 mg/L, approximately 5.33 mg/L of DOC need to be oxidized. Additionally, 2.06 mg/L of DOC are oxidized in order to reach the observed mean oxygen concentration of 1.6 mg/L in the Transitional zone. In order to reach the observed mean Anoxic zone oxygen concentration of 1.27 mg/L from the observed mean Transitional zone oxygen concentration, an additional 0.309 mg/L of DOC are oxidized. From modeled equilibrium concentrations of oxygen (9.46 mg/L), approximately 8.8 mg/L of DOC are oxidized by oxygen before nitrate becomes more thermodynamically favorable as the electron acceptor and begins decreasing in concentration. Model simulations suggest that 1.19 mg/L of DOC reduce the observed mean nitrate concentration of 0.47 mg/L found in the Oxic zone to the lowest observed mean nitrate concentration of 0.01mg/L found in the Transitional zone. Differences between the observed DOC concentrations in the field and the modeled DOC concentrations needed to reach zone levels of oxygen and nitrate suggest that field reported values for DOC electron donors could represent residual or unused electron donors. Results also indicate that between 8.68 mg/L and 10.7 mg/L of DOC oxidation, 0.42 mg/L of dissolved N2 are produced, HCO3 increases from 0.33 mg/L to 2.64 mg/L and CO2 concentrations decrease from 13.8 mg/L to 13.7 mg/L before continuing to increase. This pronounced interval of DOC oxidation at which denitrification occurs and beyond which CO2 continues increasing suggests a specific range at which denitrifiers metabolize versus a larger range at which a general heterotrophic population metabolizes.

Urban influences on the nitrogen cycle in Puerto Rico

Ortiz-Zayas, J. R., E. Cuevas, O. L. Mayol-Bracero, L.
Donoso, I. Trebs, D. Figueroa-Nieves, and W. H. Mcdowell.
2006. Urban influences on the nitrogen cycle in Puerto Rico.
Biogeochemistry 79:109–133.

Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where experimental data are available. In tropical developing countries, however, experimental studies are lacking. This paper summarizes available data and assesses the impacts of human activities on N fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape transformations over the last century associated with rapid socioeconomic changes. N yield calculations conducted in several watersheds of different anthropogenic influences revealed that disturbed watersheds export more N per unit area than undisturbed forested watersheds. Export of N from urban watersheds ranged from 4.8 kg ha)1 year)1 in the Rı´o Bayamo´ n watershed to 32.9 kg ha)1 year)1 in the highly urbanized Rı´o Piedras watershed and 33.3 kg ha)1 year)1 in the rural-agricultural Rı´o Grande de An˜ asco watershed. Along with land use, mean annual runoff explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan Area receives primary treatment before it is discharged into the Atlantic Ocean. These discharges are N-rich and export large amounts of N to the ocean at a rate of about 140 kg ha)1 year)1. Data on wet deposition of inorganic N (NHþ4 þ NO 3 ) suggest that rates of atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and mobile sources of NOx (NO+NO2) and N2O generated in the large urban centers may be responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively characterize the local N cycle. More research is required to assess rates of atmospheric N deposition, N fixation in natural and human-dominated landscapes, N-balance associated with food and feed trade, and denitrification.

Linking Species and Ecosystems: Different Biotic Assemblages Cause Interstream Differences in Organic Matter

Linking Species and Ecosystems: Different Biotic Assemblages Cause Interstream Differences in Organic Matter
C. M. Pringle, Nina Hemphill, W. H. McDowell, Angela Bednarek and James G. March
Vol. 80, No. 6 (Sep., 1999), pp. 1860-1872

Here we test the hypothesis that differences in macrobiotic assemblages can lead to differences in the quantity and quality of organic matter in benthic depositional environments among streams in montane Puerto Rico. We experimentally manipulated biota over a 30–40 d period in two streams with distinctly different macrobiotic assemblages: one characterized by high densities of omnivorous shrimps (Decapoda: Atyidae and Xiphocarididae) and no predaceous fishes, and one characterized by low densities of shrimps and the presence of predaceous fishes. To incorporate the natural hydrologic regime and to avoid confounding artifacts associated with cage enclosures/exclosures (e.g., high sedimentation), we used electricity as a mechanism for experimental exclusion, in situ. In each stream, shrimps and/or fishes were excluded from specific areas of rock substrata in four pools using electric “fences” attached to solar-powered fence chargers. In the stream lacking predaceous fishes (Sonadora), the unelectrified control treatment was almost exclusively dominated by high densities of omnivorous shrimps that constantly ingested fine particulate material from rock surfaces. Consequently, the control had significantly lower levels of inorganic sediments, organic material, carbon, and nitrogen than the exclusion treatment, as well as less variability in these parameters. Tenfold more organic material (as ash-free dry mass, AFDM) and fivefold more nitrogen accrued in shrimp exclosures (10.6 g AFDM/m2, 0.2 g N/m2) than in controls (1.1 g AFDM/m2, 0.04 g N/m2). By reducing the quantity of fine particulate organic material and associated nitrogen in benthic environments, omnivorous shrimps potentially affect the supply of this important resource to other trophic levels. The small amount of fine particulate organic matter (FPOM) that remained in control treatments (composed of sparse algal cells) was of higher quality than that in shrimp exclosures. This is evidenced by the significantly lower carbon-to-nitrogen (C/N) ratio (an indicator of food quality, with relatively low C/N indicating higher food quality) in the control relative to the shrimp exclosure treatment. In contrast, the stream with predaceous fishes (Bisley) was characterized by very low numbers of shrimps, and macrobiota had no significant effect on benthic sediments, organic matter, C, N, and C/N. All parameters were highly variable through time, with levels and ranges in variability similar to the shrimp exclusion treatment in the Sonadora. Our experimental results are consistent with findings of an independent survey of six streams in four different drainages. Four streams that had an abundance of omnivorous shrimps, but lacked predaceous fishes, had extremely low levels of fine benthic organic and inorganic material. In contrast, two streams that had low densities of shrimps and contained predaceous fishes had significantly higher levels. Results show a strong linkage between species and ecosystem characteristics: interstream differences in the quantity and quality of fine benthic organic matter resources were determined by the nature of the macrobiotic assemblage. Furthermore, patterns in the distribution of shrimp assemblages reflected landscape patterns in the benthic depositional environment among streams.

Short-Term Disappearance of Foliar Litter in Three Species Before and After a Hurricane'

Short-Term Disappearance of Foliar Litter in Three Species before and after a Hurricane
Neal H. Sullivan, William B. Bowden and William H. McDowell
Vol. 31, No. 3 (Sep., 1999), pp. 382-393

Litter disappearance was examined before (1989) and after (1990) Hurricane Hugo in the Luquillo Experimental Forest, Puerto Rico using mesh litterbags containing abscised Cyrilla racemiflora or Dacryodes excelsa leaves or fresh Prestoea montana leaves. Biomass and nitrogen dynamics were compared among: (i) species; (ii) mid- and high elevation forest types; (iii) riparian and upland sites; and (iv) pre- and post-hurricane disturbed environments. Biomass disappearance was compared using multiple regression and negative exponential models in which the slopes were estimates of the decomposition rates subsequent to apparent leaching losses and the y-intercepts were indices of initial mass losses (leaching). Cyrilla racemiflora leaves with low nitrogen (0.39%) and high lignin (22.1%) content decayed at a low rate and immobilized available nitrogen. Dacryodes excelsa leaves had moderate nitrogen (0.67%) and lignin (16.6%) content, decayed at moderate rates, and maintained the initial nitrogen mass. Prestoea montana foliage had high nitrogen (1.76%) and moderate lignin (16.7%) content and rapidly lost both mass and nitrogen. There were no significant differences in litter disappearance and nitrogen dynamics among forest types and slope positions. Initial mass loss of C. racemiflora leaves was lower in 1990 but the subsequent decomposition rate did not change. Initial mass losses and the overall decomposition rates were lower in 1990 than in 1989 for Dacryodes excelsa. Dacryodes excelsa and C. racemiflora litter immobilized nitrogen in 1990 but released 10-15 percent of their initial nitrogen in 1989, whereas P. montana released nitrogen in both years (25-40%). Observed differences in litter disappearance rates between years may have been due to differences in the timing of precipitation. Foliar litter inputs during post-hurricane recovery of vegetation in Puerto Rico may serve to immobilize and conserve site nitrogen.

The Effects of Natural and Human Disturbances on Soil Nitrogen Dynamics and Trace Gas Fluxes in a Puerto Rican Wet Forest

The Effects of Natural and Human Disturbances on Soil Nitrogen Dynamics and Trace Gas Fluxes in a Puerto Rican Wet Forest
P. A. Steudler, J. M. Melillo, R. D. Bowden, M. S. Castro and A. E. Lugo
Vol. 23, No. 4, Part A. Special Issue: Ecosystem, Plant, and Animal Responses to Hurricanes in the Caribbean (Dec., 1991), pp. 356-363

We examined the effects of two disturbances (Hurricane Hugo and forest clearcutting) on soil nitrogen dynamics and on the exchanges of N20, CO,, and CH, between soils and the atmosphere of a subtropical wet forest in Puerto Rico. The disturbances resulted in prolonged increases in ammonium pools and short-term increases in rates of net N-mineralization and net nitrification. Nitrous oxide emissions increased following both disturbances. The most dramatic increase was observed 4 mo after clearcutting; N 2 0 emissions (109.49 pg N/m2-hr) from the cut plot were about two orders of magnitude higher than emissions from the reference plot (1.71 pg N/m2-hr). Carbon dioxide emissions from both disturbed plots (mean 102.47 mg C/m2-hr) were about 30 percent lower than the reference (mean 15 1.28 mg C/m2-hr). Soils at all sites were generally sinks for CH,. Methane uptake, however, was suppressed by both disturbances. This suppression may be related to disturbance-induced changes in the nitrogen cycle, as we have previously observed in temperate zone forests.

Control of Nitrogen Export from Watersheds by Headwater Streams

Peterson, B.J. et al. 2001. Control of Nitrogen Export from Watersheds by Headwater Streams.
Science 6 April 2001:
Vol. 292 no. 5514 pp. 86-90
DOI: 10.1126/science.1056874

A comparative 15N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.

Effects of Hurricane Disturbance on Groundwater Chemistry and Riparian Function in a Tropical Rain Forest

Effects of Hurricane Disturbance on Groundwater Chemistry and Riparian Function in a Tropical Rain Forest
William H. McDowell, Claire P. McSwiney and William B. Bowden
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 577-584

The long-term response of shallow groundwater chemistry to the canopy disturbance and defoliation associated with Hurricane Hugo was studied at two sites in the Luquillo Experimental Forest, Puerto Rico. The sites differed in bedrock type, dominant vegetation, and availability of pre-hurricane data. At the primary study site, the Bisley catchment, hurricane disturbance resulted in increased concentrations of NO3 -, NH4 +, dissolved organic N, base cations, Cl-, and SiO2 in groundwater within 5 mo of the hurricane. The largest relative change in concentration occurred for K+, which increased from 0.7 to as high as 13 mg/L, concentrations were still 1.3 mg/L 5.5 yr after the hurricane. Most other solutes had returned to background levels within 1-2 yr of the hurricane. At the secondary study site, the Icacos catchment, NO3 - concentrations peaked at 1.1 mg/L one yr after the hurricane and decreased to nearly zero 5.5 yr after the hurricane. At both sites, NO3 - concentrations were higher in upslope wells than in those closer to the stream. Overall, riparian processes appear to reduce but not eliminate hydrologic losses of N following catastrophic disturbance. The nature of the long-term biogeochemical response to disturbance in this tropical rain forest ecosystem is similar to that observed in some montane temperate forests, and the time course of recovery appears to be associated with the speed with which vegetation regrows following disturbance.

A nitrogen budget for late-successional hillslope tabonuco forest, Puerto Rico

A Nitrogen Budget for Late-Successional Hillslope Tabonuco Forest, Puerto Rico
Tamara J. Chestnut, Daniel J. Zarin, William H. McDowell and Michael Keller
Vol. 46, No. 1/3, New Perspectives on Nitrogen Recycling in the Temperate and Tropical Americas (Jul., 1999), pp. 85-108

Nitrogen budgets of late successional forested stands and watersheds provide baseline data against which the effects of small- and large-scale disturbances may be measured. Using previously published data and supplemental new data on gaseous N loss, we construct a N budget for hillslope tabonuco forest (HTF) stands in Puerto Rico. HTF stands are subject to frequent hurricanes and landslides; here, we focus on N fluxes in the late phase of inter-disturbance forest development. N inputs from atmospheric deposition (4-6 kg N/ha/yr) are exceeded by N outputs from groundwater, gaseous N loss, and particulate N loss (6.3-15.7 kg N/ha/yr). Late-successional HTF stands also sequester N in their aggrading biomass (8 kg N/ha/yr), creating a total budget imbalance of 8.3-19.7 kg N/ha/yr. We surmise that this imbalance may be accounted for by unmeasured inputs from above- and belowground N-fixation and/or slow depletion of the large N pool in soil organic matter. Spatial and temporal variability, especially that associated with gaseous exchange and soil organic matter N-mineralization, constrain the reliability of this N budget.

C and N dynamics in the riparian and hyporheic zones of a tropical stream, Luquillo Mountains, Puerto Rico

C and N Dynamics in the Riparian and Hyporheic Zones of a Tropical Stream, Luquillo Mountains, Puerto Rico
Tamara J. Chestnut and William H. McDowell
Journal of the North American Benthological Society
Vol. 19, No. 2 (Jun., 2000), pp. 199-214

Hydrologic and chemical characteristics were determined for both riparian and hyporheic subsurface flow along a 100-m reach of a sandy-bottom tributary of the Rio Icacos in the Luquillo Experimental Forest, Puerto Rico. Hydrologic data (vertical hydraulic gradient and hydraulic conductivity of streambed sediments) and the topographic and morphological features of the watershed indicated diffuse inputs of groundwater from the near-stream riparian zone along this site. Cumulative groundwater discharge, determined by tracer dilution techniques, was ∼1.5 L/s or 10% of the total stream discharge. Spatial heterogeneity in hydrologic and chemical properties of riparian and hyporheic sediments was large. Hydraulic conductivity explained much of the variation in NH<sub>4</sub>-N and dissolved organic carbon (DOC) concentrations, with highest concentrations in sites having low conductivity. A mass-balance approach was used to examine the influence of the near-stream zone on nutrient transport and retention. Outwelling riparian groundwater had the potential to increase stream N concentrations by up to 84% and DOC concentrations by up to 38% along our 100-m reach. Because stream concentrations were constant downstream despite this input, we conclude that significant N and C retention or loss were occurring in the near-stream zone. Lotic ecosystems and their associated riparian groundwater can have a quantitatively significant impact on the nutrient budgets of tropical headwater catchments.

Decay rate and substrate quality of fine roots and foliage of two tropical tree species in the Luquillo Experimental Forest, Puerto Rico

Bloomfield, J., K. A. Vogt, and D. J. Vogt. 1993. Decay rate and substrate quality of fine roots and foliage of two tropical tree species in the Luquillo Experimental Forest, Puerto Rico. Plant and Soil 150:233-245.

Decomposition rates, initial chemical composition, and the relationship between initial chemistry and mass loss of fine roots and foliage were determined for two woody tropical species, Prestoea montana and Dacryodes excelsa, over a gradient of sites in two watersheds in the Luquillo Experimental Forest, Puerto Rico. At all locations, fine roots decayed significantly more slowly than foliage during the initial 6 months. Substrate quality of the initial tissue showed marked differences between roots and foliage when using cell wall chemistry, secondary chemistry and total elemental analysis as indices. Quantity of acid detergent fiber (ADF) (non-digestible cell wall fiber) and lignin content were higher for roots than leaves: D. excelsa roots had 55.3% ADF and 28.7% lignin while leaves had 36.2% ADF and 11.8% lignin; P. montana roots had 68.0% ADF and 26.8% lignin while leaves had 48.5% ADF and 16.1% lignin. Aluminum concentrations were higher in fine roots (843 mg kg -x in D. excelsa, 1500 mg kg -x in P. montana) than leaves (244mg kg -x in D. excelsa, 422mgkg -1 in P. montana), while calcium concentrations were higher in foliage (5.5 mg g-1 in D. excelsa, 7.8 mg g-X in P. montana) than roots (3.4mgg -1 in D. excelsa, 3.1 mgg -x in P. montana). Nitrogen did not show any trend with tissue or species type. A linear model between mass remaining after 6 months and initial tissue chemistry could be developed only for calcium (r 2= 0.64).
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