Nitrogen cycling and resource patchiness in two mountain big sagebrush (Artemisia tridentata spp. vaseyana) plant communities: Evidence of threshold dynamics?

Background/Question/Methods

There is a widely documented pattern in big sagebrush rangelands of persistent herbaceous understory loss and increasing sagebrush decadence with the prolonged absence of fire or other brush disturbance concomitant with livestock grazing. These diverse and depauperate shrub land communities have been proposed as alternate stable states, but the mechanism by which ecological process feedbacks are altered to perpetuate and maintain understory loss has remained uncertain. Identification of the ecological process dynamics involved in such a switch would provide evidence for existence of functional thresholds in this system and provide key information for potential restoration pathways.

We hypothesize that sagebrush recruitment and growth benefit from expanded soil resource heterogeneity brought about by long-term grazing in the absence of fire and that the growing contribution of nitrogen-poor sagebrush litter to the litter pool decreases litter pool quality, slowing nitrogen cycling rates to the detriment of fast-growing, nitrogen-rich herbaceous species. We conducted a series of three in situ soil incubations and collected litter from five diverse and five depauperate mountain big sagebrush (Artemisia tridentata spp. vaseyana) plant communities in northwestern Colorado.  We compared soil texture, moisture, density, total carbon and nitrogen pools, nitrogen mineralization rates and litter pool size and quality (C:N) across the two site types. 

Results/Conclusions

Preliminary results indicate that nitrogen mineralization differs between the two sites, but only during the first incubation of the growing season (June). Early growing season net nitrogen mineralization rates are lower at depauperate sites, due primarily to slower rates of nitrification. The depauperate and diverse litter pools differ little in total mass, but the depauperate pool is more varied and of poorer quality.  These results suggest a link between nitrogen cycling, resource heterogeneity, and community structure that may be perpetuating understory paucity.  In addition, the diverse sites exhibit greater percent clay, season-long soil moisture and total carbon and nitrogen pools. Thus, soil texture, by affecting available moisture and nutrient holding capacity, is likely also an important control on nitrogen mineralization patterns and may influence the resilience of sagebrush plant communities to alteration of disturbance regimes.