Nematode Density and Biomass in an Annual Grassland Ecosystem

The nematode community structure was examined in grazed and ungrazed annual-plant rangeland on the US/IBP Grassland Biome San Joaquin Site located in the foothill-grasslands of central California. Nematode numbers and biomass were estimated from early growth to mature stages of the annual-plant vegetation. Nematode density was greater on the grazed area, predominately forbs, than on the ungrazed, mainly grass, area. A lower than normal precipitation appeared to be a limiting factor of nematode population density. The nematode trophic structure differed between the two sites, fimgivores and microbivores predominating on the grazed and ungrazed sites, respectively. Indications are that the critical factors controlling nematode density and community structure on this annual grassland are not grazing but soil moisture and temperature. Yuen (1966), Schmitt and Norton (1972), Schmitt (1973), Stanton (1974), Yeates (1974), and Smolik and Rogers (1976) have studied the function of nematodes in belowground grassland ecosystems; however, knowledge of nematodes from the California annual grassland ecosystem is almost nonexistent. Recent investigations in the mixed-grass prairie of South Dakota (Smolik 1974) provided evidence that nematodes constituted a significant pathway of energy flow, and that the application of nematicides significantly reduced nematode populations resulting in large increases in herbage production. Also, phytophagic nematode biomass was significantly higher in an ungrazed pasture than in a grazed pasture. A preliminary study in the shrub-steppe area of south-central Washington (Smolik and Rogers 1976) revealed large numbers of soil-dwelling nematodes but no consistent differences in nematode density or biomass between grazed, ungrazed, and burned areas. In 1972, the annual grassland site of the United States International Biological Program (US/IBP), Grassland Biome, was established at the U.S. Forest Service’s San Joaquin Experimental Range, 40 km northeast of Fresno, California. For a 3-year period, data were collected on abiotic, producer, consumer, and decomposer components of the ecosystem (Duncan 1975). A preliminary soil sampling program was undertaken in 1974 to estimate the importance of nematodes at the San. Joaquin site. Soil samples were taken on two dates (March 13 and May 1, 1974) in the grazed and ungrazed