Mechanisms and regulations of priming effects in soil

Författare

Jian Li

Summary, in English

Soil organic matter (SOM) plays a significant role in the global carbon (C) and nutrients cycles, not only since SOM is a major storage reservoir of C and plant nutrients, but also since microbial decomposition of SOM releases large quantities of CO2 to the atmosphere. The accelerated or reduced turnover rate of SOM after the input of labile C compounds is a phenomenon known as the priming effect (PE). When the labile C originates from root exudate, the effect is referred to as the rhizosphere PE. The rhizosphere PE has been reported to increase SOM decomposition by up to 380%, but also to reduce SOM decomposition by up to 50% compared to rootless soil. Although PE is an important regulator of SOM decomposition and nutrient cycling, little is known about the mechanisms and regulations of PEs.

In this thesis, I performed several greenhouse experiments with living tree seedlings to investigate how different tree species, light intensity, root exudation rate, elevated CO2 and nitrogen (N) fertilization influence the rhizosphere PE on SOM decomposition and gross N mineralization. I also performed an incubation experiment to compare the effect of increased C and N availability on the depolymerization of native SOM. I further measured the microbial growth rate, microbial community composition and potential enzyme activities to investigate the underlying mechanisms and role of different microbial groups in regulating the PE.

My results showed that the magnitude and direction of the rhizosphere PE differed between plant species. Light intensity also played an important role in regulating the rhizosphere PE, depending on the plant species present. My results further showed that the effect of N-fertilization and elevated CO2 on the partitioning of N between plants and microbes was of importance for SOM decomposition and PEs. Moreover, rhizosphere PE was not dependent on the root exudation rate. Instead, my studies suggest that variation in soil N availability is a key regulator of the PEs. My studies further revealed that fungi are responsible for the PEs, while potential enzyme activities are poor indicators of the PEs.

In conclusion, my studies suggest that soil C and N cycling are strongly affected by the PEs caused by living roots. Therefore, the regulation of PEs on SOM decomposition and N mineralization could play an important role in the terrestrial ecosystem's feedback to global change in response to e.g. elevated CO2 and N deposition. My results also demonstrated that the PE is regulated by multiple environmental factors, e.g. plant species, light intensity, atmospheric CO2 concentration, and soil N availability. In order to accurately predict how the PE on soil C and N cycling responds to environmental change, these factors need to be considered more in future studies.

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