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United States Department of Agriculture

Agricultural Research Service

Title: A User-Friendly 13C Isotope Discrimination Method for Root Studies with Rice and C4 Weeds in Field Soils

Author
item Gealy, David

Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Proceedings
Publication Acceptance Date: October 8, 2007
Publication Date: February 4, 2008
Citation: Gealy, D.R. 2008. A user-friendly 13c isotope discrimination method for root studies with rice and c4 weeds in field soils. Weed Science Society of America Meeting Abstracts. 1p.

Technical Abstract: Sustainable weed control is a continuing challenge in rice production in the U.S. Some rice cultivars can provide significant levels of barnyardgrass (Echinochloa crus-galli) suppression, much of which is thought to occur below the soil surface. This presentation describes a simple 13C isotope depletion method that can be used to determine the relative biomass of roots of rice (a C3 plant) and a C4 weed species in field soils. Weeds in rice fields that are amenable to this method include barnyardgrass, sprangletop species (Leptochloa spp.), broadleaf signalgrass (Urochloa platyphylla), and palmer amaranth (Amaranthus palmeri), among others. A single weed species should be established in each plot because the isotope method cannot distinguish one C4 species from another. The method is not suitable for red rice (Oryza sativa), which has the same C3 metabolism as rice. Typically, rice plots are divided into weedy sections (established as natural populations or by overseeding the desired C4 weed) and weed-free sections (maintained by herbicide application and hand-weeding). This method is especially useful late in the crop season after the weed and rice have attained maximum biomass accumulation when their roots can become inextricably intertwined. Four soil cores (10 cm diam. by 15-cm deep) are subsampled from the desired location in each weedy plot and combined. A lever-action hole cutter ($150.00) is well-suited for removing cores from moist soil. Roots can be extracted from the soil cores immediately, or the cores can be pre-frozen for later extraction. Rooting distributions of rice and the C4 weed at various soil depths can be determined by subdividing the soil core using a sharp knife. The mixture of weed and rice root tissues is extracted from soil cores in the presence of water using agitation in an electric-powered cement mixer that can be purchased for approx $450. The slurry of soil and roots is then directed through a series of stackable sieves to ensure complete capture of all root material. Root material is dried to constant weight, and undispersed dirt clods and other foreign material are removed by hand so that they will not contribute to the apparent biomass of the sample. The root material is weighed, ground in a Wiley mill (screen with 2 mm openings), and mixed to create a homogeneous powder which is submitted to a service laboratory for analysis of delta 13C depletion levels. 13C depletion values are indicative of the relative biomass of rice and the C4 weed present. The percentage of rice and C4 weed roots present is extrapolated from a 13C depletion regression curve developed by analyzing known proportions of pure root tissue obtained from the two species grown in monoculture under identical conditions. Typically, delta 13C depletion values for pure C4 weed roots and pure rice roots are 12 to 14 and 28, respectively, with standard deviations between 0.1 and 0.3. Total processing time for each sample is 45 minutes. The total laboratory fees for 13C analysis are about $10 per sample.

Last Modified: 12/18/2014