Influence of burn intensity and vegetation type on recovery of soil nitrogen cycling

Background/Question/Methods

Fire frequency and spatial extent are increasing dramatically in the Western United States and represent an important critical zone loss term at larger spatial and longer temporal scales for net ecosystem carbon and nitrogen (N) balance. Fire strongly affects N cycling and losses through volatilization, leaching and erosion. Because N is often a limiting nutrient in terrestrial ecosystems, more intense fires may exacerbate N limitation and thus play an important role in post-fire recovery of ecosystem functions. We examined the recovery of soil N cycling and retention processes in response to varying burn intensities in different vegetation types in the Valles Caldera, NM. We measured soil pH, moisture, N pools and process rates three weeks, three months, 9 months and 1 year after the Las Conchas Fire in NM. 

Results/Conclusions

Preliminary data show that the N pools and rates of net N mineralization differed dramatically among vegetation types and burn intensities. Three weeks after the fire, severely burned sites in mixed conifer stands had higher rates of net N mineralization than unburned sites. Within the same time frame, inorganic N was immobilized more rapidly in severely burned sites than unburned sites in ponderosa pine stands. Soil ammonium pools declined with time whereas nitrate remained high one year after the fire. Repeated measures ANOVA tests indicated significant burn intensity, vegetation, and time effects on soil N pools and process rates. Findings from our study suggest differences in fire intensity and vegetation can have profound effects on the spatial and temporal distribution of soil N pools and processes.