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Measuring the temperature response of soil respiration from two distinct carbon pools in soil.

As soils contain a large store of terrestrial carbon, understanding the dynamics and stability of this important carbon reserve and how it may change with regards to future changes in temperature is of global interest. It is primarily understood that carbon exists in different pools within soil, but there is considerable debate around the number, size and contents of these pools. Despite this debate, when modelling carbon cycling, it can be erroneously assumed that the decomposition of carbon pools will behave the in the same way with regards to temperature and varying management practices. The first objective of this research was to develop and test a new protocol allowing the measurement of the temperature response of two distinct carbon pools in soil. A Horotiu silt loam was mixed with a ¹³C labelled rye-grass clover litter, and incubated for 5 and 20 hours at 30 discrete temperatures (~2 – 50 °C). Resulting CO₂ was separated into litter and soil organic carbon (SOC) sourced respiration rates using a mixing model, and then fit with macromolecular rate theory (MMRT). Litter-derived respiration had a lower temperature optimum (Tₒₚₜ) than SOC-derived respiration. It was suggested that decomposition of highly available labile litter is rate-limited by enzyme kinetics, which displays a clear temperature optimum. In contrast, decomposition of stable SOC is more limited by desorption processes and diffusion of carbon to microbes, prior to decomposition, and so exhibits Arrhenius behaviour as temperature increases. The second objective was to use the developed protocol to measure temperature response of respiration was measured for ¹³C-labelled new photosynthate carbon inputs and bulk SOC, partitioned from soils labelled under seasonal irrigated and dryland conditions. Additionally, mass litter inputs of both root and shoot material were incubated with unlabelled soil. Root and shoot litter inputs showed a similar response to temperature with a well-defined MMRT-like response (Tₒₚₜ of 45 °C and 38 °C respectively). In contrast to this, respiration from new photosynthate carbon and SOC had the same Arrhenius-like temperature response (Tₒₚₜ of 50 °C for dryland and 62 °C for irrigated soils). It was suggested that the new carbon inputs deposited through roots were rapidly incorporated into the soil and thus had a similar availability and temperature response as SOC. Consequently, carbon inputs through roots appear to be more stabilised than litter inputs (as either root or above ground fragments). Respiration from dryland soils had lower a Topt than respiration from irrigated soils, which diminished with the application of autumnal rainfall, most likely due to an increase in short-term turnover of carbon under irrigation. In a similar experiment, soils were labelled under constant conditions prior to the imposition of seasonal irrigation or dryland treatments. Again, there was no difference between the temperature response of new photosynthate carbon and SOC. Increases in short term carbon cycling caused greater respiration under irrigation compared to dryland soils. However, this increased respiration did not contribute to a noticeable change in temperature response. Overall this research demonstrated a reliable protocol for measuring the temperature response of two distinct carbon pools in soil. This approach can be used to examine the stability of new carbon inputs from different sources to soil. Future research using the developed methodology with different forms of 13C labelled carbon will expand knowledge on the temperature response of distinctive pools of carbon in soil, allowing continued improvement on carbon cycling modelling.
Type of thesis
Robinson, J. M. (2020). Measuring the temperature response of soil respiration from two distinct carbon pools in soil. (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14389
The University of Waikato
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