Dynamic zones of higher soil oxygen caused by roots

Dissolved Oxygen Profiles

Zostera marina species of seagrass. Image source: http://www.uicnmed.org/medras/en/12.htm

Anyone who has attempted an experiment in soils knows how challenging it can be. Why, and how, then would a scientist measure soil oxygen in profiles less than 1mm from the root surface?

The challenge was accepted by Jensen and colleagues from the University of Copenhagen, Denmark. They decided to measure sediment oxygen profiles perpendicular and parallel to the roots of the seagrass, Zostera marina, to determine the extent and rate of oxygen loss from the roots.

Oxygen loss from roots of aquatic plants has important ecological and biogeochemical consequences. The underwater substrate is typically anoxic or anaerobic so any input of oxygen into the sediment will alter microbial species composition. Increase oxygen can also affect sulphate reduction and release of hydrogen sulfide (H2S), among other biogeochemical processes.

Jensen et al collected Z. marina specimens from the field, along with sediment and marine water. They proceeded to set up a lab experiment imposing various levels of light (photosynthetically active radiation) on the seagrass and to measure sediment oxygen profiles perpendicular and parallel to the root tip.

Oxygen profiles were measured with Unisense oxygen microelectrodes connected to a motorised profiling system. A fluorescence-lifetime imaging system was also used to map the extent of oxygen loss from the seagrass roots.

 

Anaerobic Oxygen Concentration

An example of a Unisense laboratory based microprofiling system measuring oxygen gradients of samples at a microscale.

 

The oxygen microsensors had a measuring tip ranging in diameter from 10 to 130μm. The motorised microprofiling system moved the sensors in 1mm incremental steps starting at the root tip. The microprofiling system can also move in smaller increments, such as 100μm steps, if required. These micro sized sensors enabled the scientists to measure oxygen profiles at a very fine scale in the rhizosphere.

 

soil sediment photosynthesis

An example of a microsensor tip at the surface of photosynthetic biofilm. These microsensors can measure at the micron (μm) scale for very fine spatial resolution of environmental parameters.

 

The research found that, measuring backwards and parallel to the root tip (that is, along the root surface going from the young, growing portion to the mature portion), oxygen concentration was maximum at 2mm from the root tip and was virtually zero around 6mm from the root tip. Measurements perpendicular to the root surface, at 2mm from the root tip, found that oxygen concentration was zero at approximately 1mm distance into the sediment (see Figure 4B in Jensen et al 2005).

The microsensor results were supported by the fluorescence-lifetime imaging system. This system clearly showed that there was a specific zone of higher oxygen concentration around the root tip, as seen in Figure 5 in Jensen et al 2005.

The fluorescent image gives an idea of how oxygen can have an impact on sediment ecology and biogeochemistry. Within the “halo” of higher oxygen concentration around the root tip, aerobic microbes can potentially exist whereas anaerobically adapted microbes will be found in the black-coloured sediment. The biogeochemistry between these two areas will also differ.

The “halo” of oxygen can also act as a protective shield for the root tip against H2S from entering and damaging the root.

Most interestingly, the root tip was found to grow at 5mm length per day. Therefore, this “halo” is effectively moving through the anoxic sediment at 5mm per day bringing a highly dynamic effect to a system that would otherwise have been viewed as largely inert.

 

conclusion

Oxygen microsensors can be used to measure fine scale patterns of oxygen concentration in an otherwise challenging experimental environment. Although this case study showed this is possible in seagrass sediments, it would be interesting to apply such an experimental approach to other plants and ecosystems.

 

related case studies and articles

 

relevant instrumentation

 

reference

Jensen, SI et al. 2005. Oxic microzones and radial oxygen loss from roots of Zostera marina. Marine Ecology Progress Series, 293: 49-58.