Microbes, bacteria and fungi, are some of the primary decomposers of plant litter throughout the world and are thought to still play a pivotal role in decomposition in desert systems, even though it has extreme high temperatures and dry conditions.  One of the big questions around microbial decomposition in desert systems is when exactly are they active and what conditions do they need to be active?

Injecting gas samples into an infrared gas analyzer

To help determine when the microbes are active, I  measured microbial activity ( as respiration) in the lab at many different relative humidities and temperatures. To do this, I incubated different species of plant litter in serum bottles with different salt solutions. Different solutions of salts (Magnesium Nitrate, Sodium Bromide, Sodium Chloride, etc.) will produce different relative humidities in a sealed container (a serum bottle in this case). I then incubated these bottles in a growth chamber set at different temperatures and measured the respiration by injecting the gas from the bottle into an infrared gas analyzer.

A gas syringe used to inject CO2 into a gas analyzer

Once we know when the microbes are active in controlled lab conditions, we then need to see when these conditions are present in natural conditions. To do that, we closely track the microclimate conditions on the soil surface, where plant litter accumulates, such as temperature and relative humidity using small data loggers called ibuttons.

Litter samples in the field with ibuttons inside to measure temperature and relative humidity

Another variable we want to know in the field, are the plant litter moisture contents at different times of the year. Plant litter moisture contents are known to highly correlate with microbial activity. To measure litter moisture, we weigh litter as soon as it comes out of the oven to get the dry weight, then we weigh litter in the field to know how much water vapor it is taking up in different environmental conditions. We also do this in laboratory experiments when measuring microbial activity, so we can make estimates of microbial activity in the field at different relative humidities and temperatures.

Weighing samples in the field to get litter moisture contents

The coupling of the laboratory data with the field data will then provide us with a good estimate of when the microbes are active, if at all, throughout the year in natural conditions.

A scanning electron image of a fungal spore, Alternaria alternata, on a leaf litter hair from Lupinus sparsiflorus magnified 1200x