Plant effects on spatial and temporal patterns of nitrogen cycling in shortgrass steppe

Because of the water-limited nature and discontinuous plant cover of shortgrass steppe, spatial patterns in ecosystem properties are influenced more by the presence or absence of plants than by plant type. However, plant type may influence temporal patterns of nutrient cycling between plant and soil. Plants having the carbon-3 (C-3) or carbon-4 (C-4) photosynthetic pathway differ in phenology as well as other attributes that affect nitrogen (N) cycling. We estimated net N mineralization rates and traced nitrogen-15 (N-15) additions among plant and soil components 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 mineralization 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 were due to greater N-15 uptake by C-3 plants than by C-4 plants during May. There were no significant differences in total N-15 retention among plant communities for July and September. Soil N-15 was influenced more by presence or absence of plants than by type of plant; greater quantities of N-15 remained in soil interspaces between plants than in soil directly under plants for July and September. Our results indicate that plant functional type (C-3 versus C-4) can affect both the spatial and the temporal patterns of N cycling in shortgrass steppe. Further research is necessary to determine how these intraseasonal differences translate to longer-term and coarser-scale effects of plants on N cycling, retention, and storage.