Characterisation of hydrogen isotope profiles in an agroforestry system: implications for tracing water sources of trees

Tracing sources of water utilised by plants is important to understand species interactions in intercropping/agroforestry systems, particularly where species vary greatly in life-form. Isotopic techniques are an increasingly common means to trace water sources. The distribution of stable isotopes of water within the soil–plant–atmosphere continuum is indicative of a range of hydrologic processes and plant functions. Before we can infer plant or other biological effects on the distribution of isotopes we require a thorough characterisation of the environmental isotope distribution as well as an understanding of the physical processes that determine this distribution. Unless distinct features are recognisable in the isotopic ‘landscape’ surrounding a plant, links cannot be made between plant function and environment. As a means to trace water acquisition by four Eucalyptus species in an agroforestry planting, we tested a simple ‘end member’ approach and a more extensive characterisation of hydrogen and deuterium distribution profiles within an agricultural soil in south-western Australia. Hydrogen isotope distribution within the soil was highly uniform; an apparent result of climatic factors, soil physical properties and the ability of tree roots to redistribute soil water. Such findings have important implications for measurement strategies and experimental design when attempting to quantify plant use of water from differing soil sources. In the absence of large isotopic discrimination among water sources, simple ‘end member’ models are likely to be misleading and isotopic labelling techniques may be more appropriate in some environments.