PLANT EFFECTS ON SPATIAL AND TEMPORAL PATTERNS OF NITROGEN CYCLING INSHORTGRASS STEPPE

Because of the water-limited nature and discontinuous plant cover of s hortgrass steppe, spatial patterns in ecosystem properties are influen ced more by the presence or absence of plants than by plant type. Howe ver, plant type may influence temporal patterns of nutrient cycling be tween plant and soil. Plants having the carbon-3 (C-3) or carbon-4 (C- 4) photosynthetic pathway differ in phenology as well as other attribu tes that affect nitrogen (N) cycling. We estimated net N mineralizatio n rates and traced nitrogen-15 (N-15) additions among plant and soil c omponents during May, July, and September of 1995 in native plots of C -3 plants, C-4 plants, or mixtures of C-3 and C-4. Net N mineralizatio n was significantly greater in C-3 plots than in C-4 plots during both July and September. C-3 plots retained significantly more N-15 in May than did mixed and C-4 plots; these differences in N-15 retention wer e due to greater N-15 uptake by C-3 plants than by C-4 plants during M ay. There were no significant differences in total N-15 retention amon g plant communities for July and September. Soil N-15 was influenced m ore by presence or absence of plants than by type of plant; greater qu antities of N-15 remained in soil interspaces between plants than in s oil directly under plants for July and September. Our results indicate that plant functional type (C-3 versus C-4) can affect both the spati al and the temporal patterns of N cycling in shortgrass steppe. Furthe r research is necessary to determine how these intraseasonal differenc es translate to longer-term and coarser-scale effects of plants on N c ycling, retention, and storage.