Measuring and modelling plant-driven soil carbon dynamics.

Plant root activity and deposition of root carbon (C) into the rhizosphere are known to influence the turnover of existing soil organic matter (SOM) in so- called rhizosphere priming effects (RPE). Thereby soil microbes may access nutrients in SOM which are otherwise unavailable to them. However the magnitudes, drivers and mechanisms of these effects are poorly understood. In this thesis I develop a field system to measure such effects on diurnal, seasonal and longer timescales, and use it to explore RPEs and their drivers in contrasting soils under grass. The field system measures CO₂ fluxes and their ¹³ C isotope composition (δ¹³C) near continuously in large (0.8 m diameter, 1 m deep) lysimeters containing two naturally-structured C₃ soils planted with a C₄ grass. The difference in δ¹³C between C₃ SOM and C₄ plants is used to partition fluxes between plant and soil sources. The system’s accuracy and precision were sufficient to resolve diurnal and seasonal patterns in both plant and soil fluxes. Diurnal changes in plant δ¹³C can cause large partitioning errors. I show how, with long-term datasets with sufficient temporal resolution, part of the dataset can be used to allow for transient shifts in plant and soil δ¹³C. I explored the magnitude and mechanisms of RPEs in the two contrasting soils over two years, and the effect of differences in nitrogen supply. I used solar radiation as a proxy for photosynthesis, root activity and rhizodeposition. I found that seasonal and particularly diurnal patterns in SOM turnover were tightly coupled to solar radiation, and more so than in previously published studies. Model estimates of SOM turnover were improved by the inclusion of solar radiation as an explanatory variable alongside soil moisture and temperature, consistent with RPEs. There was no evidence for differences in RPEs with nitrogen supply in either soil.