In simple terms: Post fire litters are richer in water soluble carbon and lead to increased microbial activity

Abstract:

Under conditions of increased fire season length and area affected by fire, stocks of carbon stored in forests are at increased risk of burning. While much research has investigated the immediate loss of above ground and below ground carbon stocks through combustion during a fire, there has been little research on subsequent organic matter cycling in post-fire environments. Fire can introduce new organic matter to the litter layer through the formation of a post-fire litter layer composed of debris from fire induced plant stress or death. This litter may have different chemistry and decomposition dynamics to the pre-fire litter due to the changed pathway from plant to ground and the narrow age range of the debris. In this study, litters collected from two vegetation types (Pinus and Eucalyptus dominated) and from adjacent areas either fire affected (FA) or not fire affected (NFA) were incubated as litter, or as water extracts of litter, in soils to determine fire induced changes in nutrient pools, microbial biomass and microbial activity. Post-fire litters contained more labile C (15 and 30 mg C g native/pine litter-1, respectively) than litters unaffected by fire (4 mg C g litter-1). Increased labile C concentration correlated (r2>0.95) with increased microbial activity without a concurrent change in nitrogen (microbial) or phosphorus (resin and microbial) pools. Our results suggest that labile C in post-fire litter can alter microbial carbon cycling and that effects may be more pronounced under pine compared to native forest.

This article is available by requesting it through ResearchGate, or by direct request (email).

Premise:

The experiments in this short communication came out of a relatively simple idea – is decomposition different for native and pine litters, do different soils behave differently, and does fire affect it anyway? In the grand scheme of hypothesis wrangling, it wasn’t the best frame to work in, but I’m sure that can be forgiven as it was the first successful piece of research that I personally directed (with supervisory input from my initial supervisory panel of course).

Note that this article was initially submitted to (and rejected from) Soil Biology and Biochemistry (SBB) – rejected on perfectly reasonable grounds, in my opinion. During the rejection process, it was clear that we’d really asked too many questions of the experiments as they stood; therefore the data was condensed down to more specifically address these questions:

  • Did the litters have different soluble carbon contents?
  • Did the litters have different soil respiration activities?
  • Is the soluble carbon content and the respiration significantly correlated?

So, why were we interested in these questions? Largely because no one else had asked them (i.e. blue-sky research) but also because decomposition of organic matter (e.g. leaf litter) releases carbon into the atmosphere and we (i.e. researchers in general) don’t know if those releases change in quantity or nature after a fire.

Experiments and results:

This paper follows two experiments, both of which use laboratory incubations of soil and litter products. In brief, for the first experiment, I took soil from burnt and not burnt native and pine forests (4 soils) and leaf litter from those same sites (4 litters), mixed them in a fully factorial fashion (4 soils x [4 litters + 1 control]), measured them into little PVC cores, and added water. These cores were stored in specially modified mason jars (yes – jam jars) and then destructively sampled at various times to analyse for soil mineral nutrients and microbial biomass. Note that most of the results from this experiment are basically meaningless and were not included in the final manuscript.

There were two interesting points however. Firstly, that undisturbed pine litter was the least decomposable while the fire affected pine litter was the most decomposable (over the first week of incubation), and secondly, that microbial biomass N was effectively non-existent.

In response to the first point, we conducted further characterisation of the litters and then the second experiment. The second point remained a mystery for a while (how can you have microbial biomass carbon, but not microbial biomass nitrogen?), but that mystery gets addressed later in a different paper.

The second experiment was effectively the same as the first, however I used water extracted from litters rather than the litters themselves. This experiment (largely) agreed with the first – that is that the fire affected pine litter to a greater degree and made it more decomposable over the first week. This relationship appears to be due to the greater amount of water soluble carbon in the fire affected pine needles than any other treatment.

What does this mean?

Fire affected litters are ‘suddenly’ dead instead of ‘normally’ dead – this means fire affected leaves might have a higher or richer proportion of simple organic resources when compared to their ‘normally’ dead counterparts. This in turn means that microbial decomposition can proceed faster: think pizza vs cardboard pizza box – you could eat both, but one is going to be faster and more delicious.

Overall, this means that post fire litters might pose a greater emissions risk than previously considered.