In simple terms: The effect of fire affected Pinus radiata litter and char addition on soil nitrogen cycling

Abstract:

In pine forest litters, decomposition rate is directly affected by the pathway the needle followed to the ground, whether that was via programmed apoptosis and abscission or via stress induced loss through branch damage or tree death. Stress induced losses may occur due to fire damage, which leads to a post-fire litter layer composed of non-senescent debris that fell during or after the event. This litter may be physically or chemically different to pre-fire litter in ways that could affect litter decomposition and nutrient cycling. This study investigates decomposition and nitrogen cycling in soils amended with two litters, one from the floor of a pine forest not affected by fire (designated “non-fire affected” or “NFA”) and one from the floor of a pine forest recently affected by fire and where fire-affected trees had dropped their foliage post-fire (designated “fire affected” or “FA”). Litters were incubated in the presence or absence of field collected char for up to 94 days. These soil treatments were analysed for microbial activity (via soil respiration) and N pools (microbial, mineral, and potentially mineralisable). Soil and litter treatments were additionally incubated in the presence of ammonium nitrate solution to determine N absorption potential of the litters. Microbial activity was greatest in soils that received FA litter regardless of the presence or absence of char. Nitrogen pools were largely similar between the control (no litter) treatment and NFA litter treatments. Nitrogen was exceedingly low (92% of samples <2 μg-N g soil-1 where detected) or not detectable (37% of samples below detection limits) for most N pools in all FA litter treatments at most times. This study indicates that fire affected pine litter collected four months post fire has strong N absorption properties that are likely to affect the mineral N pools available for regeneration of forest growth.

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

Premise:

The experiments in this article were inspired by the short communication where we investigated the effects of soluble C on microbial activity. The main conclusions of that article showed that pine litter was strongly affected by fire. Therefore, we ran a similar experiment over a much longer period to explore just how different the two litters were. In the interest of creating a more ‘field-like’ environment, we also included char addition as a factor.

We had intended to measure the full suite of available nitrogen and phosphorus properties, but eventually settled on mostly nitrogen. This is because, as you will see, nitrogen is where the story was.

Experiments and results:

This paper follows two experiments, one main experiment and one additional experiment. Both experiments used incubations, however the second experiment was conducted under aerobic and anaerobic conditions. For the first experiment, I used a composite soil collected from fire affected and unaffected pine plantations (1 soil), litters collected from those same sites (2 litters), and a composite char collected from fire grounds across Old Kersbrook Forest, South Australia. Initially, there was also a water content factor, but we dropped that data as it was muddying the narrative. Soils, litters, and char were mixed in a fully factorial fashion (1 soil x [1 char + 1 control] x [2 litters + 1 control]), measured them into little PVC cores, and added water. These cores were stored in specially modified jars (the kind you get cheap jam in) and then destructively sampled at various times to analyse for soil mineral nutrients and microbial biomass.

Again, the fire affected pine litter caused more respiration (yay! Repeatability). However, the most interesting point in the results is that there was effectively no mineral N detected in fire affected litter treatments. This is the same observation we made in the microbial biomass N in the previous experiment. We also found that char didn’t really have an effect one way or the other.

In response to these results, we carried out the second experiment – incubating litters and soils in ammonium nitrate solution under oxic and anoxic conditions. In theory, under anoxic conditions, ammonium should remain in solution as there is not oxygen (or limited oxygen) available for denitrification. This worked for the soil and the not fire affected litter, but again, only very low amounts of nitrogen were detected from the fire affected litter.

Therefore, we argued that the fire affected litter is removing mineral nitrogen from the soil.

What does this mean?

That pine needles suddenly lost during or after a fire have a significantly different effect on soil nitrogen cycling. That is, that ‘suddenly’ dead needles have a capacity to absorb large amounts on mineral nitrogen while still providing enough carbon for microbes to be active. As nitrogen is an important macronutrient for plants, a lack of mineral N in the soil could then affect the community of plants that establish after a fire.

Which means that post fire ecological succession could be influenced by litterfall immediately after the fire event.

It also means that pine needles moved into streams during post fire erosion events could have interesting effects on water quality and water organisms’ responses. The addition of a large quantity of water-soluble C means fast growing algae have an advantage… but a lack of mineral N might remove that advantage.

An experiment for another day.