oxygen

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

Abstract: 
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.

Distribution of Nitrous Oxide and Regulators of Its Production across a Tropical Rainforest Catena in the Luquillo Experimental Forest, Puerto Rico

MCSWINEY, CLAIRE P.; MCDOWELL, WILLIAM H.; KELLER, MICHAEL 2001. Distribution of nitrous oxide and regulators of its production across a tropical rainforest catena in the Luquillo Experimental Forest, Puerto Rico. Biogeochemistry 56: 265-286.

Abstract: 
Understanding of N2O fluxes to the atmosphere is complicated by interactions between chemical and physical controls on both production and movement of the gas. To better understand how N2O production is controlled in the soil, we measured concentrations of N2O and of the proximal controllers on its production in soil water and soil air in a field study in the Rio Icacos basin of the Luquillo Experimental Forest, Puerto Rico. A toposequence (ridge, slope-ridge break, slope, slope-riparian break, riparian, and streambank) was used that has been previously characterized for groundwater chemistry and surface N2O fluxes. The proximal controls on N2O production include NO−3 , NH+4 , DOC, and O2. Nitrous oxide and O2 were measured in soil air and NO−3 , NH+4 , and DO were measured in soil water. Nitrate and DOC disappeared from soil solution at the slope-riparian interface, where soil N2O concentrations increased dramatically. Soil N2O concentrations continued to increase through the flood plain and the streambank. Nitrous oxide concentrations were highest in soil air probes that had intermediate O2 concentrations. Changes in N2O concentrations in groundwater and soil air in different environments along the catena appear to be controlled by O2 concentrations. In general, N processing in the unsaturated and saturated zones differs within each topographic position apparently due to differences in redox status.
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