Browsing by Author "Paterson, Eric"
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Item Open Access A field system for measuring plant and soil carbon fluxes using stable isotope methods(Wiley, 2020-06-21) McCloskey, Christopher S.; Otten, Wilfred; Paterson, Eric; Ingram, Benjamin R.; Kirk, Guy J. D.There is a lack of field methods for measuring plant and soil processes controlling soil organic matter (SOM) turnover over diurnal, seasonal, and longer time-scales with which to develop datasets for modelling. We describe an automated field system for measuring plant and soil carbon fluxes over such time-scales using stable isotope methods, and we assess its performance. The system comprises 24 large (1-m deep, 0.8-m diameter) cylindrical lysimeters connected to gas-flux chambers and instruments. The lysimeters contain intact, naturally-structured C3 soil planted with a C4 grass. Fluxes of CO2 and their 13C isotope composition are measured 3-times daily in each lysimeter, and the isotope composition is used to partition the fluxes between plant and soil sources. We investigate the following potential sources of error in the measurement system and show they do not significantly affect the measured CO2 fluxes or isotope signatures: gas leaks; the rate of gas flow through sampling loops; instrument precision and drift; the concentration-dependence of isotope measurements; and the linearity of CO2 accumulation in the chambers and associated isotope fractionation resulting from different rates of 13CO2 and 12CO2 diffusion from the soil. For the loamy grassland soil and US prairie grass (Bouteloua dactyloides) tested, the precision of CO2 flux measurements was ± 0.04 % and that of the flux partitioning ± 0.40 %. We give examples of diurnal and seasonal patterns of plant and soil C fluxes and soil temperature and moisture. We discuss the limitations of the isotope methodology for partitioning fluxes as applied in our system. We conclude the system is suitable for measuring net ecosystem respiration fluxes and their plant and soil components with sufficient precision to resolve diurnal and seasonal patternsItem Open Access Measuring and modelling plant-driven soil carbon dynamics.(Cranfield University, 2021-01) McCloskey, Christopher S.; Otten, Wilfred; Paterson, EricPlant 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.Item Open Access On allowing for transient variation in end-member δ13C values in partitioning soil C fluxes from net ecosystem respiration(Wiley, 2021-09-24) McCloskey, Christopher S.; Otten, Wilfred; Paterson, Eric; Kirk, Guy J. D.The use of stable isotope analysis to resolve ecosystem respiration into its plant and soil components rests on how well the end-member isotope signatures (δ13C) are characterised. In general, it is assumed that end-member values are constant over time. However, there are necessarily diurnal and other transient variations in end-members with environmental conditions. We analyse diurnal and seasonal patterns of ecosystem respiration and its δ13C in a C4 grass growing in a C3 soil using fixed and diurnally varying plant and soil δ13C end-members. We measure the end-members independently, and we assess the effects of expected variation in values. We show that variation in end-members within realistic ranges, particularly diurnal changes in the plant end-member, can cause partitioning errors of 40% during periods of high plant growth. The effect depends on how close the end-member is to the measured net respiration δ13C, that is, the proportion of the respiration due to that end-member. We show light-driven variation in plant end-members can cause substantial distortion of partitioned soil organic matter (SOM) flux patterns on a diurnal scale and cause underestimation of daily to annual SOM turnover of approximately 25%. We conclude that, while it is not practicable to independently measure the full temporal variation in end-member values over a growing season, this error may be adjusted for by using a diurnally varying δ13Cplant.Item Open Access Tight coupling between photosynthesis and soil carbon turnover indicative of rhizosphere priming in the field(EGU: European Geophysical Union, 2022-05-27) McCloskey, Chris; Kirk, Guy J. D.; Otten, Wilfred; Paterson, EricWhile rhizosphere priming effects are well-documented under laboratory and controlled-environment conditions, their significance in undisturbed systems under field conditions is less clear. This is in part because it is impracticable to measure rates of rhizodeposition in the field with high resolution over a substantial period. We propose that photosynthesis, closely linked to rhizodeposition, can be used as a proxy for plant root activity.