When Wet Gets Wetter: Decoupling of Moisture, Redox Biogeochemistry, and Greenhouse Gas Fluxes in a Humid Tropical Forest Soil

Hall S. J., McDowell W.H., Silver W.L. When Wet Gets Wetter: Decoupling of Moisture, Redox Biogeochemistry, and Greenhouse Gas Fluxes in a Humid Tropical Forest Soil. Ecosystems. ISSN 1432-9840. DOI 10.1007/s10021-012-9631-2

Upland humid tropical forest soils are often characterized by fluctuating redox dynamics that vary temporally and spatially across the landscape. An increase in the frequency and intensity of rainfall events with climate change is likely to affect soil redox reactions that control the production and emissions of greenhouse gases. We used a 24-day rainfall manipulation experiment to evaluate temporal and spatial trends of surface soil (0–20 cm) redox-active chemical species and greenhouse gas fluxes in the Luquillo Experimental Forest, Puerto Rico. Treatments consisted of a high rainfall simulation (60 mm day-1), a fluctuating rainfall regime, and a control. Water addition generated high temporal and spatial variation in soil moisture (0.3–0.6 m3 m-3), but had no significant effect on soil oxygen(O2) concentrations. Extractablenitrate(NO3 -) concentrations decreased with daily water additions and reduced iron (Fe(II)) concentrations increased towards the end of the experiment. Overall, redox indicators displayed a weak, non-deterministic, nonlinear relationship with soil moisture. High concentrations of Fe(II) and manganese (Mn) were present even where moisture was relatively low, and net Mn reduction occurred in all plots including controls. Mean CO2 fluxeswere best explained by soil C concentrations and a composite redox indicator, and not water addition. Several plots were CH4 sources irrespective of water addition, whereas other plots oscillated between weak CH4 sources and sinks. Fluxes of N2O were highest in control plots and were consistently low in water-addition plots. Together, these data suggest (1) a relative decoupling between soil moisture and redox processes at our spatial and temporal scales of measurement, (2) the co-occurrence of aerobic and anaerobic biogeochemical processes inwell-drained surface soils, and (3) an absence of threshold effects from sustained precipitation on redox reactions over the scale of weeks. Our data suggest a need to re-evaluate representations of moisture in biogeochemical models.

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.

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils
Whendee L. Silver, Donald J. Herman and Mary K. Firestone
Vol. 82, No. 9 (Sep., 2001), pp. 2410-2416

The internal transformations of nitrogen in terrestrial ecosystems exert strong controls over nitrogen availability to net primary productivity, nitrate leaching into groundwater, and emissions of nitrogen-based greenhouse gas. Here we report a reductive pathway for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem. Using 15N tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in upland humid tropical forest soils averaging ;0.6 mg·g21·d21. Rates of DNRA were three times greater than the combined N2O and N2 fluxes from nitrification and denitrification and accounted for 75% of the turnover of the nitrate pool. To determine the relative importance of ambient C, O2, and NO3 concentrations on rates of DNRA, we estimated rates of DNRA in laboratory assays using soils from three tropical forests (cloud forest, palm forest, and wet tropical forest) that differed in ambient C and O2 concentrations. Rates of DNRA measured in laboratory assays ranged from 0.5 to 9 mg·g21·d21 in soils from the three different forests and appeared to be primarily limited by the availability of NO3, as opposed to C or O2. Tests of sterile soils indicated that the dominant reductive pathway for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the form of N generally favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem, the rapid and direct transformation of NO3 to NH4 via DNRA has the potential to play an important role in ecosystem N conservation.

Topographic control of soil microbial activity: a case study of denitrifiers

Florinsky, 1. V., S. McMahon, and D. L. Burton. 2004.
Topographic control of soil microbial activity: a case study of
denitrifiers. Geoderma 119:33-53.

Topography may affect soil microbial processes, however, the use of topographic data to model and predict the spatial distribution of soil microbial properties has not been widely reported. We studied the effect of topography on the activity of denitrifiers under different hydrologic conditions in a typical agroecosystem of the northern grasslands of North America using digital terrain modelling (DTM). Three data sets were used: (1) digital models of nine topographic attributes, such as elevation, slope gradient and aspect, horizontal, vertical, and mean land surface curvatures, specific catchment area, topographic, and stream power indices; (2) two soil environmental attributes (soil gravimetric moisture and soil bulk density); and (3) six attributes of soil microbial activity (most probable number of denitrifiers, microbial biomass carbon content, denitrifier enzyme activity, nitrous oxide flux, denitrification rate, and microbial respiration rate). Linear multiple correlation, rank correlation, circular–linear correlation, circular rank correlation, and multiple regression were used as statistical analyses. In wetter soil conditions, topographically controlled and gravity-driven supply of nutritive materials to microbiota increased the denitrification rate. Spatial differentiation of the denitrification rate and amount of denitrifying enzyme in the soil was mostly effected by redistribution and accumulation of soil moisture and soil organic matter down the slope according to the relative position of a point in the landscape. The N2O emission was effected by differentiation and gain of soil moisture and organic matter due to the local geometry of a slope. The microbial biomass, number of denitrifiers, and microbial respiration depended on both the local geometry of a slope and relative position of a point in the landscape. In drier soil conditions, although denitrification persisted, it was reduced and did not depend on the spatial distribution of soil moisture and thus land surface morphology. This may result from a reduction in soil moisture content below a critical level sufficient for transient induction of denitrification but not sufficient to preserve spatial patterns of the denitrification according to relief. Digital terrain models can be used to predict the spatial distribution of the microbial biomass and amount of denitrifying enzyme in the soil. The study demonstrated a feasibility of applying digital terrain modelling to investigate relations of other groups of soil microbiota with topography and the system ‘topography–soil microbiota’ as a whole.

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Nitrous Oxide Fluxes

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Nitrous Oxide Fluxes
William B. Bowden, William H. McDowell, Clyde E. Asbury and Amy M. Finley
Vol. 18, No. 2 (1992), pp. 77-99

Fluxes of N<sub>2</sub>O at the soil surface, dissolved N<sub>2</sub>O in near-surface groundwater, and potential N<sub>2</sub>O production rates were measured across riparian catenas in two rain forest watersheds in Puerto Rico. In the Icacos watershed, mean N<sub>2</sub>O fluxes were highest at topographic breaks in the landscape (∼40-300 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>). At other locations in the riparian zone and hillslope, fluxes were lower (≤ 2 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>). This pattern of surface N<sub>2</sub>O fluxes was persistent. In the Bisley watershed, mean suface N<sub>2</sub>O fluxes were lower (<40 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>) and no identifiable spatial or temporal pattern. Although the spatial patterns and intensities of N<sub>2</sub>O emissions differed between the two watersheds, surface soils from both sites had a high potential to reduce NO<sub>3</sub> to N<sub>2</sub>O (and perhaps N<sub>2</sub>). This potential declined sharply with depth as did soil %C, %N, and potential N-mineralization. Simple controls on denitrification (i.e. aeration, nitrate, and carbon) explained characteristics of potential N<sub>2</sub>O production in surface and deep soils from riparian and upslope locations. In the field, spatial patterns in these controlling variables were defined by geomorphological differences between the two watersheds, which then explained the spatial patterns of observed N<sub>2</sub>O flux.
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