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 ), afl uctuating rainfall regime, and a control. Water addition generated high temporal and spatial variation in soil moisture (0.3‐0.6 m 3 m -3 ), but had no significant effect on soil oxygen (O2) concentrations. Extractable nitrate (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 fluxes were 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 N2 Ow ere 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 in well-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.
Highlights
Humid tropical forests exert substantial influence on the atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4)
The importance of anaerobic biogeochemical cycling of C, nitrogen (N), manganese (Mn), iron (Fe), and sulfur (S) have long been documented in wetland ecosystems (Ponnamperuma 1972), the significance of anaerobic processes such as Fe reduction and methanogenesis has only recently been recognized in upland humid tropical forest soils (Peretyazhko and Sposito 2005; Teh and others 2005; Chacon and others 2006)
To account for spatial variation among plots, we report all redox species and gas flux data normalized to pre-treatment levels
Summary
Humid tropical forests exert substantial influence on the atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). These ecosystems generate the highest soil-atmosphere CO2 fluxes of any biome, provide the largest natural source of N2O, and are increasingly thought to provide a substantial net source of CH4 (Matson and Vitousek 1990; Raich and Schlesinger 1992; Carmo and others 2006; Bloom and others 2010). Global climate change is likely to affect soil C storage and greenhouse gas fluxes from humid tropical forest soils, yet the relative impact of different environmental drivers remains poorly understood. Understanding the environmental drivers of aerobic and anaerobic biogeochemical reactions in upland humid tropical forests could provide insight into dynamics of soil greenhouse gas fluxes from these ecosystems
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