Assessing belowground plant interference in rice has been difficult in the past because intertwined weed and crop roots cannot be readily separated. A C-13 discrimination method has been developed to assess distribution of intermixed roots of barnyardgrass and rice in field soils, but the suitability of this approach for other rice weeds is not known. C-13 depletion levels in roots and leaves of rice were compared with those of 10 troublesome weed species grown in monoculture in the greenhouse or field. Included were C-4 tropical grasses: barnyardgrass, bearded sprangletop, Amazon sprangletop, broadleaf signalgrass, fall panicum, and large crabgrass; C-4 sedge, yellow nutsedge; and C-3 species: red rice, gooseweed, and redstem. Rice root delta C-13 levels averaged, similar to -28 parts per thousand, indicating that these roots are highly C-13-depleted. Root delta C-13 levels ranged from -12 parts per thousand to -17 parts per thousand among the tropical grasses, and were -10 parts per thousand in yellow nutsedge, indicating that these species were less C-13 depleted than rice, and were C-4 plants suitable for C-13 discrimination studies with rice. Among the C-4 species, bearded sprangletop and yellow nutsedge were most and least C-13 depleted, respectively. delta C-13 levels in shoot and root tissue of pot-grown plants averaged 6% greater for C-4 plants and 9% greater for rice in the field than in the greenhouse. In pots, shoots of rice typically were slightly more C-13 depleted than roots. A reverse trend was seen in most C-4 species, particularly for broadleaf signalgrass and plants sampled from field plots. Corrections derived from inputs including the total mass, carbon mass, carbon fraction, and delta C-13 levels of roots and soil increased greatly the accuracy of root mass estimates and increased slightly the accuracy of root delta C-13 estimates (similar to 0.6 to 0.9%) in samples containing soil. Similar corrective equations were derived for mixtures of rice and C-4 weed roots and soil, and are proposed as a labor-saving option in C-13 discrimination root studies.
