Soil protozoan dynamics in a shortgrass prairie

Soil Protozoa (primarily small naked amoebae and flagellates)were counted under control conditions and with stresses of nitrogen, water, or nitrogen and water under field conditions (Ecosystem Stress Area, Pawnee Site. northeastern Colorado, USA)in the summer of 1974. Protozoan populations were also measured in soil cores removed from the field and incubated under a wet-dry cycle. Protozoan numbers were higher in the top I cm of soil and overlying litter than at a depth of 5-6 cm in all treatments. After rainfall totaling 35 mm, the control and fertilized only treatments showed population increases with the fertilized only treatment showing the greater change. Protozoa showed marked responses to the addition of water to soil cores, with the largest numbers appearing after peak CO* evolution, Numbers of active (trophic)forms ranged from about 2O,OOO.g-' dry soil in the control treatment under dry conditions to over 100,COO.g ' dry soil in the irrigated plus fertilized plot. There were few (< 5":,)cystic (dormant)forms in all treatments contrary to earlier studies in mesic climates. A rough estimate of protozoan production is presented.