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.

Citation

Hall, S.J., McDowell W.H., Silver W.L. (2013): When Wet Gets Wetter: Decoupling of Moisture, Redox Biogeochemistry, and Greenhouse Gas Fluxes in a Humid Tropical Forest Soil. Ecosystems. DOI: 10.1007/s10021-012-9631-2

This Paper/Book acknowledges NSF CZO grant support.