This presentation will illustrate the potential benefits and limitations of using stable isotopes to study nutrient flow and food web dynamics in agro-ecosystems, drawing examples from a three year study of rice production systems in California’s Sacramento Valley.
Following legislated reductions in the burning of straw, rice growers have adopted alternative methods of straw disposal. Most have turned to straw incorporation, often in conjunction with winter flooding in an effort to increase the rate of straw decomposition and reduce the potential for pathogens and diseases to over-winter. However, results from the long-term Rice Straw Management Project at Maxwell, and other sites suggest it has also altered both nutrient availability and the population dynamics of pest invertebrates like rice water weevil (Lissorhoptrus oryzophilus) and crayfish (Procambarus clarkii), as well as non-pest species.
Using stable isotopes, we were able to link changes in straw management with increased nutrient availability and early season plant vigor both under winter flooding and straw incorporation (P=0.0007 and P=0.0305, respectively). The 13C isotope signature of the invertebrates was not affected by straw management practices. However, increased soil organic N availability resulted in a 2 per mil enrichment in delta 15N values for rice grown in straw incorporated plots. A similar increase occurred in the delta 15N values of the invertebrates. Coupling the isotope data with population counts and gut content analysis, we were able to follow crayfish populations and quantify the impact on crayfish feeding habits.
Keywords: stable isotopes, rice
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