The effects of scale and context-dependency on the outcome of experiments investigating soil carbon dynamics
Crawford, J.A. 2017.
Abstract
Soil carbon dynamics and plant-soil interactions are an integral part of
ecosystem function and understanding the effects of perturbations on these
processes is vital if we are to predict the future of ecosystems under global
environmental change. However, it is often challenging to study complex
processes at the ecosystem-scale, due to high natural variability. Microcosm
experiments offer a way to study soil processes under controlled conditions but
common techniques to reduce environmental heterogeneity in laboratory
microcosms can also alter soil properties, which may affect the outcome of
experiments. This provides a challenge for research to develop a robust
understanding of soil processes and to establish how scale and context may alter
the outcome of experiments. The overarching aim of this thesis was to explore the
effect of context and experimental scale on the outcome of experiments
investigating soil carbon dynamics and plant-soil interactions. I conducted a series
of microcosm experiments exploring the effect of common soil processing
techniques on soil properties and function, as well as a comparative study across
three experimental scales. Soil pre-treatment by sieving and air-drying dramatically altered soil properties compared to fresh soil. None of the measured
soil properties recovered to fresh soil values during a 60-day microcosm
experiment. Despite consistent overall trends in soil properties, the recovery
trajectories varied among soils from different sites, which presents a challenge for
comparative studies using sieved and air-dried soils. Importantly, sieving and
drying also increased soil respiration, ion exchange rates and the magnitude of the
respiratory response to litter addition treatments. Finally, soil respiration and soil
properties differed substantially across experiments at different scales. Peak soil
carbon release by priming effects in response to litter addition was ten-fold higher
in microcosms compared to in situ mesocosms or field plots, and experimental
scale had a greater effect on soil respiration than litter addition treatments.
Microcosm studies remain a crucial part of ecological research into soil
carbon dynamics and plant-soil interactions. However, my results show that
experimental scale and context-dependency can alter the outcome of experiments.
Future research should aim to find a compromise between a reductionist approach
to test detailed mechanisms and representative experiments that better simulate
in situ conditions.
