HAPPY BIRTHDAY HOT DIRT! One year later! What did we find with soil warming?
Its been one year since we started the heating at the Lyon Arboretum deep soil warming experimental field site! Data was collected every week on soil respiration, soil moisture and soil temperature at depths of 20, 40, 60, 80, and 100cm. Our inital hypothesis was:
"If soil respiration follows the Arrhenius equation, then as temperature increases, soil respiration rates will initially rapidly increase and then level off, indicating a threshold of warming which is controlled by Al/Fe organo-mineral complexes especially at depth."
Meaning that as the temperature of the soil increases we will see an increase in soil respiration like all other deep soil warming experiments. However, instead of the threshold of warming being controlled by nutrient availability (once microbes exhaust the carbon in the soil they will not respire more, even with augmented soil temperatures) the threshold will be controlled by these minerals from the volcanic ash origins of the soil which bond to soil carbon making it inaccessible to the microbes.
These minerals are called "poorly and non-crystalline minerals" meaning they have amorphous or poorly order structure when compared to crystalline minerals which have an ordered structure. The irregular structure of the poorly and non-crystalline minerals means more surface area for the soil carbon to bond to and be protected from microbial degradation, hence the control by the "organo-mineral complexes." These organo-mineral complexes increase with depth and therefore we can expect more C to be protected with higher poorly and non-crystalline mineral concentrations.
This is exactly what we found! Warmed soils only had soil respiration response at the most shallow sampling depth of 20cm of the soil profile, all other depths had no response. This is unlike any other soil warming experiment , which all depths responded to warming. This result suggests Hawaiian Andisols are a unique end member soil to soil warming experiments given their unique mineralogy. Our modeling apporach suggests that when the non crystalline minerals (Al + 0.5Fe) increase the there is little to no response of soil respiration, suggesting possible protection from organo-mineral associations. Data supporting the hypothesis, NEATO!
Figure 1: The mean soil respiration plotted against the non-crystalline mineral concentration for all depths (shapes) at all levels of heating (colors). The black line is the model fit showing how as non-crystalline minerals increase soil respiration decreases.
Whats next? More studies are needed to identify specific controls of mineralogy at depth. There are plans for a controlled soil incubation study as well as a long term microbial characterization study in the works, so stay tuned! If we see this hypothesis consistently proven through other studies, Andisols (the name for this specific study soil) could be managed as a climate change mitigation tool due to their lack of response from the expected rise in temperatures from climate change. Andisol conservation could offer a potential low cost and easy to implement option to carbon sequestrations under a changing climate, while longer term solutions to destructive human behavior are enacted.
