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

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 m 3 m -3), but had no significant effect on soil oxygen (O 2 ) concentrations. Extractable nitrate (NO 3 -) 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 CO 2 fluxes were best explained by soil C concentrations and a composite redox indicator, and not water addition. Several plots were CH 4 sources irrespective of water addition, whereas other plots oscillated between weak CH 4 sources and sinks. Fluxes of N 2 O 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 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.

Landscape analysis of soil methane flux across complex terrain

“…As depth to water table increases, the volume in which oxidation can occur increases, thereby decreasing net CH4 efflux (Moore and Roulet, 1993), yet similar to Fiedler and Sommer (2000), we found that depth to groundwater table was not sufficient to predict the magnitude of riparian CH4 efflux. High 20 VWC is often associated with O2 depletion (Silver et al, 1999), yet we measured near atmospheric O2 even up to 60% VWC, similarly to Hall et al (2012) who suggest that high VWC does not necessarily lead to depleted O2, even when soil water content is above field capacity. Additionally, Teh et al (2005) show that laboratory experiments varying O2 does not result in significant changes in rates of methanogenesis.…”

Landscape analysis of soil methane flux across complex terrain

“…As depth to water table increases, the volume in which oxidation can occur increases, thereby decreasing net CH 4 efflux (Moore and Roulet, 1993), yet, similar to Fiedler and Sommer (2000), we found that depth to groundwater table was not sufficient to predict the magnitude of riparian CH 4 efflux in these well-drained soils. High VWC is often associated with O 2 depletion (Silver et al, 1999), yet we measured near-atmospheric O 2 even up to 60 % VWC, similarly to Hall et al (2012), who suggest that high VWC does not necessarily lead to depleted O 2 , even when soil water content is above field capacity. Additionally, Teh et al (2005) show that laboratory experiments varying O 2 do not result in significant changes in rates of methanogenesis.…”

Nitrification rates and associated nitrous oxide emissions from agricultural soils – a synopsis

“…Indeed, water content is widely used as a proxy for the aerobicity of soil even though water can contain significant amounts of dissolved oxygen, and despite decoupling of the actual drivers and water content having been demonstrated previously (e.g. Hall et al 2013). Water also affects the diffusion of solutes, not just gases, so the capture of these multiple effects is challenging.…”

“…* Water functions are not universal. Water functions are problematic because they attempt to account for multiple processes or mechanisms simultaneously (Hall et al 2013). As a solution, explicit modelling of oxygen transport and consumption (Cook et al 2013), for example, could be implemented in models.…”