Abstract. Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized >100 preexisting published estimates of BNF to generate biome-and global-level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the model's simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our "best estimate" of potential nitrogen fixation by natural ecosystems is -195 Tg N yr -•, with a range of 100-290 Tg N yr '•. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux.

Native plant communities invaded by cheatgrass (Bromus tectorum L.) are at risk of unnatural high intensity fires and conversion to cheatgrass monocultures. Management strategies that reduce cheatgrass abundance may potentially allow native species to expand and minimize further cheatgrass invasion. We tested whether the selective herbicide imazapic is effective in reducing cheatgrass and ''releasing'' native species in a semiarid grassland and shrub steppe in north-central Oregon. The experiment consisted of a completely randomized design with two treatments (sprayed with 70 g ai ? ha 21 of imazapic and unsprayed) and three replicates of each treatment applied to either 2.5 or 4 ha plots. We repeated this experiment in three different sites dominated by the following native species: 1) bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Lö ve ssp. spicata) and needle and thread (Hesperostipa comata [Trin. & Rupr.] Barkworth), 2) needle and thread and Sandberg bluegrass (Poa secunda J. Presl), and 3) big sagebrush (Artemisia tridentata Nutt.). Nested frequency of all plant species in 1-m 2 quadrats was collected for 1 yr pretreatment and 4 yr posttreatment. In all three sites, cheatgrass frequencies were significantly lower in sprayed plots than unsprayed plots for 3-4 yr posttreatment (P , 0.1). Other annual plant species were also impacted by imazapic, but the effects were highly variable by species and site. Only two native perennial species, hoary tansyaster (Machaeranthera canescens [Pursh] Gray) and big sagebrush, increased in sprayed plots, and increases occurred only at two sites. These results suggest that a short-term reduction in cheatgrass alone is not an effective strategy for increasing the abundance of most native perennial plant species. Resumen Las comunidades de plantas nativas que están invadidas del pasto cheatgrass (Bromus tectorum L.) están en riesgo de fuego intencionales de alta intensidad y convertirse en monocultivos de éste pasto. Estrategias de manejo que reduzcan la abundancia del pasto cheatgrass podrían tener el potencial de permitir que especies nativas se expandan y minimicen la posible invasión por el cheatgrass. Probamos sí el herbicida Imazapicis que es efectivo en reducir el pasto cheatgrass y ''liberar'' especies nativas en pastizales semiáridos y matorral estepario en la parte centro-norte de Oregon. El experimento consistió en un diseñ o completamente al azar con dos tratamientos (asperjar con 70 g ia ? ha 21 de Imazapic y sin asperjar) y tres repeticiones por cada tratamiento aplicado ya sea a parcelas de 2.5 o 4 ha. Repetimos el experimento en tres diferentes sitios dominados por las siguientes especies nativas: 1) (Pseudoroegneria spicata [Pursh] A. Lö ve ssp. spicata) y (Hesperostipa comata [Trin. &Rupr.] Barkworth), 2) (Hesperostipa comata [Trin. &Rupr.] Barkworth) y (Poa secunda J. Presl), y 3) Artemisa (Artemisia tridentata Nutt.). Se recolectaron todas las especies en un cuadrante de 1 m 2 en frecuencia anidada por un añ o como pre-tratamiento y cuatro añ os co...

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