Abstract
AbstractThere is a lack of field methods for measuring plant and soil processes controlling soil organic matter (SOM) turnover over diurnal, seasonal and longer timescales with which to develop datasets for modelling. We describe an automated field system for measuring plant and soil carbon fluxes over such timescales 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 three 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 that the system is suitable for measuring net ecosystem respiration fluxes and their plant and soil components with sufficient precision to resolve diurnal and seasonal patterns.Highlights We describe an automated system for measuring plant and soil carbon fluxes under field conditions. We exploit the large difference in isotope signatures between C3 and C4 soils and plants to partition the net flux. Possible sources of error are quantified and shown to be small. The system is capable of resolving diurnal and seasonal patterns.
Highlights
Measurements of soil-atmosphere carbon (C) fluxes necessarily conflate fluxes from plants and recent plant inputs with those from the decomposition of existing soil organic matter (SOM)
We describe an automated system for measuring plant and soil carbon fluxes under field conditions
In this paper we describe an automated field system for measuring plant and soil C fluxes separately using stable isotope methods, and we assess the limitations of the isotope methodology for partitioning fluxes as applied in our system
Summary
Measurements of soil-atmosphere carbon (C) fluxes necessarily conflate fluxes from plants and recent plant inputs with those from the decomposition of existing soil organic matter (SOM). To date only a few studies have exploited plant and soil δ13C differences to measure SOM turnover under field conditions (Millard et al, 2008; Snell et al, 2014; Moinet et al, 2018) These have relied on manual sample collection and processing, limiting the practicality of collecting long-term continuous datasets. The precision and instrumental drift of the CRDS measurements were measured by sampling a reference gas cylinder of medical-grade compressed air with 352 μmol CO2 mol-1 (BOC, UK) for 48 h and monitoring the absolute CO2 concentrations and δ13C values, and their drift over time This was used to inform the calibration regime detailed previously. It indicates the agreement with the expected relationship between the standard deviation of frequency fluctuations and the infinite-time average of the standard deviation
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