K. Billmark

δ18O of water vapour, evapotranspiration and the sites of leaf water evaporation in a soybean canopy

Stable isotopes in water have the potential to diagnose changes in the earths hydrological budget in response to climate change and land use change. However, there have been few measurements in the vapour phase. Here, we present high-frequency measurements of oxygen isotopic compositions of water vapour (delta(v)) and evapotranspiration (delta(ET)) above a soybean canopy using the tunable diode laser (TDL) technique for the entire 2006 growing season in Minnesota, USA. We observed a large variability in surface delta(v) from the daily to the seasonal timescales, largely explained by Rayleigh processes, but also influenced by vertical atmospheric mixing, local evapotranspiration (ET) and dew formation. We used delta(ET) measurements to calculate the isotopic composition at the sites of evaporative enrichment in leaves (delta(L,e)) and compared that with the commonly used steady-state prediction (delta(L,s)). There was generally a good agreement averaged over the season, but larger differences on individual days. We also found that vertical variability in relative humidity and temperature associated with canopy structure must be addressed in canopy-scale leaf water models. Finally, we explored this data set for direct evidence of the Péclet effect.

Canopy-scale kinetic fractionation of atmospheric carbon dioxide and water vapor isotopes

Received 18 August 2008; revised 16 October 2008; accepted 21 October 2008; published 4 February 2009. [1] The carbon and oxygen isotopes of CO2 and the oxygen isotopes of H2 Oa re powerful tracers for constraining the dynamics of carbon uptake and water flux on land. The role of land biota in the atmospheric budgets of these isotopes has been extensively explored through the lens of leaf-scale observations. At the ecosystem scale, kinetic fractionation is associated with molecular and turbulent diffusion. Intuitively, air turbulence, being nondiscriminative in diffusing materials, should act to erase the kinetic effect. Using the first canopy-scale isotopic flux measurements, we show just the opposite: that in the terrestrial environment, air turbulence enhances the effect, rather than suppressing it. The sensitivity of kinetic fractionation to turbulence is striking in situations where the canopy resistance is comparable to or lower than the aerodynamic resistance. Accounting for turbulent diffusion greatly improves land surface model predictions of the isoforcing of 18 O-CO2 and transpiration enrichment of leaf water in 18 O-H2O in field conditions. Our results suggest that variations in surface roughness across the landscape can contribute to spatial variations in the composition of atmospheric 18 O-CO2 and that temporal trends in wind circulation on land can play a role in the interannual variability of atmospheric 18 O-CO2. In comparison, air turbulence has a limited effect on the isoforcing of 13 C-CO2.

Influence of phenology and land management on biosphere-atmosphere isotopic CO2 exchange

Stable isotope and micrometeorological techniques have long been used to study carbon cycle dynamics at a variety of spatial and temporal scales. Combination of these techniques provide a powerful tool for gaining greater process information at the ecosystem and regional scales and can provide a meaningful way to scale processes from leaf to region. In this chapter we review the recent literature and examine the key processes influencing biosphere–atmosphere 13CO2 exchange. These processes are examined from the perspective of agricultural land management and rapid seasonal changes in phenology. Novel measurement techniques are introduced that can be used to better quantify the 13CO2 exchange between the biosphere and atmosphere to determine how ecosystem processes, land use modifications, and phenology impact the isotopic composition of the atmosphere (i.e. the atmospheric isotopic forcing associated with land surface processes). High temporal resolution isotope mixing ratio and flux measurements, based on tunable diode laser absorption spectroscopy, are presented. The results demonstrate that the isotopic composition of respiration at the ecosystem scale is strongly linked to plant assimilated carbon, which is dependent on plant metabolic physiology and growth phase. We review how this strong isotopic coupling between ecosystem respiration and photosynthesis can impact isotope-based flux partitioning of net ecosystem CO2 exchange, the variation in the canopy isotopic discrimination parameter, and the resulting isotopic forcing on the atmosphere.

Technical Report: Investigation of Carbon Cycle Processes within a Managed Landscape: An Ecosystem Manipulation and Isotope Tracer Approach

The goal of this research is to provide a better scientific understanding of carbon cycle processes within an agricultural landscape characteristic of the Upper Midwest. This project recognizes the need to study processes at multiple spatial and temporal scales to reduce uncertainty in ecosystem and landscape-scale carbon budgets to provide a sound basis for shaping future policy related to carbon management. Specifically, this project has attempted to answer the following questions: 1. Would the use of cover crops result in a shift from carbon neutral to significant carbon gain in corn-soybean rotation ecosystems of the Upper Midwest? 2. Can stable carbon isotope analyses be used to partition ecosystem respiration into its autotrophic and heterotrophic components? 3. Can this partitioning be used to better understand the fate of crop residues to project changes in the soil carbon reservoir? 4. Are agricultural ecosystems of the Upper Midwest carbon neutral, sinks, or sources? Can the proposed measurement and modeling framework help address landscape-scale carbon budget uncertainties and help guide future carbon management policy?