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.

ozone peaks associated with a subtropical fold and with a subtropical tropopause fold and with the trade wind inversion: a case study from the airborne campaign TROPOZ II over the caribbean in winter

Gouget, H., J.-P. Cammas, A. Marenco, R. Rosset, and I. Jonquieres,
Ozone peaks associated with a subtropical tropopause
fold and with the trade wind inversion: a case study from the
airbone campaign TROPOZ II over the Caribbean in winter,
J. Geophys. Res., 101 , 25,979{25,993, 1996.

Aircraft measurements of ozone, methane, carbon monoxide, relative humidity, and equivalent potential temperature were performed during the TROPOZII campaign. During the aircraft descent down to Pointe-á-Pitre (16.3°N, 61.5°W), at 2100 UTC on January 12, 1991, two ozone peaks (75 ppb) are observed, one at an altitude of 7.5 km and the other at 3.0 km. A physicochemical interpretation for each ozone peak is proposed in connection with the meteorological context, using radiosounding data, total ozone content from TOMS/NIMBUS 7 and diagnoses issued from analyses by the European Centre for Medium-Range Weather Forecasts, Reading, England. The stratospheric origin of the 7.5-km ozone peak is inferred from negative correlations between ozone and its precursors and from diagnoses based on potential vorticity and ageostrophic circulations depicting the structure of the tropopause fold embedded in the subtropical jet front system. Using an appropriate method to isolate cross- and along-front ageostrophic circulations, we show that much of the observed structure of the tropopause fold can be ascribed to transverse and vertical circulations associated with the irrotational part of the flow. Though the downward extent of the subtropical tropopause fold (400 hPa) is restricted in comparison with typical extratropical tropopause ones (700 hPa), the present results suggest that subtropical tropopause folds may significantly contribute to the global stratosphere-troposphere ozone exchange. The origin of the 3.0-km ozone peak trapped just below the trade wind inversion cannot be ascribed precisely. Analogies with other measurements of dust and aerosols transported over the Atlantic or Pacific in the summer season are discussed. Various possibilities are examined: (1) an earlier stratospheric intrusion event, (2) long-range transport by the trade winds of biomass burning species emitted over West Africa, and (3) fast photochemical ozone formation occurring just below the trade wind inversion within already polluted air parcels originating from remote regions (United States and Gulf of Mexico) after eastward and southward transport around the western Atlantic anticyclone.

Impact of experimental drought on greenhouse gas emissions and nutrient availability in a humid tropical forest

We excluded throughfall from humid tropical forests in Puerto Rico for a period of three months to determine how drought affects greenhouse gas emissions from tropical forest soils. We established five 1.24 m2 throughfall exclusion and five control plots of equal size in three sites located on ridges, slopes, and an upland valley dominated by palms (total of 30 plots). We measured weekly changes in carbon dioxide (CO2) and bi-weekly changes in nitrous oxide (N2O) and methane (CH4) in response to manipulation. We additionally measured the effects of throughfall exclusion on soil temperature and moisture, nutrient availability, and pH. Rainout shelters significantly reduced throughfall by 22 to 32 % and decreased soil moisture by 16 to 36% (top 10 cm). Rates of CO2 emissions decreased significantly in the ridge and slope sites (30%, 28%, respectively), but not the palm during the experimental drought. In contrast, the palm site became a significantly stronger sink for CH4 in response to drying (480% decline relative to controls), while CH4 fluxes in the ridge and slope sites did not respond to drought. Both the palm and ridge site became a sink for N2O in response to drought and the slope site followed a similar trend. Soil pH and available P decreased significantly in response to soil drying; however, available N was not affected. Variability in the response of greenhouse gas emissions to drought among the three sites highlights the complexity of biogeochemical cycling in tropical forested ecosystems, as well as the need for research that incorporates the high degree of spatial heterogeneity in experimental designs. Our results show that humid tropical forests are sensitive to climate change and that short-term declines in rainfall could result in a negative feedback to climate change via lowered greenhouse gas emissions and increased greenhouse gas consumption by soils.

Soil Oxygen Availability and Biogeochemistry along Rainfall and Topographic Gradients in Upland Wet Tropical Forest Soils

Silver W, Lugo AE, Keller M (1999) Soil oxygen availability and
biogeochemistry along rainfall and topographical gradients in
upland wet tropical forest soils. Biogeochemistry 44:301–

We measured soil oxygen concentrations at 10 and 35 cm depths and indices of biogeochemical cycling in upland forest soils along a rainfall and elevation gradient (3500– 5000 mm y−1; 350–1050 masl) and along topographic gradients (ridge to valley, 150 m) in the Luquillo Experimental Forest, Puerto Rico. Along the rainfall gradient, soil O2 availability decreased significantly with increasing annual rainfall, and reached very low levels (<3%) in individual chambers for up to 25 consecutive weeks over 82 weeks of study. Along localized topographic gradients, soil O2 concentrations were variable and decreased significantly from ridges to valleys. In the valleys, up to 35% of the observations at 10–35 cm depth were <3% soil O2. Cross correlation analyses showed that soil O2 concentrations were significantly positively correlated along the topographic gradient, and were sensitive to rainfall and hydrologic output. Soil O2 concentrations in valley soils were correlated with rainfall from the previous day, while ridge sites were correlated with cumulative rainfall inputs over 4 weeks. Soils at the wettest point along the rainfall gradient had very high soil methane concentrations (3–24%) indicating a strong influence of anaerobic processes.We measured net methane emission to the atmosphere at the wettest sites of the rainfall gradient, and in the valleys along topographic gradients. Other measures of biogeochemical function such as soil organic matter content and P availability were sensitive to chronic O2 depletion along the rainfall gradient, but less sensitive to the variable soil O2 environment exhibited at lower elevations along topographic gradients.
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