How do prescribed burns influence nitrogen availability and cycling rates in a low-nutrient system?

Background/Question/Methods

Nitrogen pulses are particularly important in nitrogen-limited systems because they can deliver a large amount of nitrogen in a short period of time. Previous work in a pine forest found that changes in relative availability of nitrate and ammonium following fire are short-lived, but it remains to be understood how burns influence total short-term nitrogen availability. Fire may reduce soil nitrogen if the nitrogen is volatized, or increase it if ash is returned to the system and mineralized. Thus, litter characteristics may interact with fire to influence its effect on nitrogen cycling. To understand the role of fire in pulsed nutrient availability, we asked the following questions: How does fire influence short-term nitrogen cycling rates in a longleaf pine forest? Does leaf litter influence the effects of fire on nitrogen cycling?

We measured nitrogen cycling rates and availability of inorganic nitrogen forms in burned and unburned sites at Fort Bragg, NC. Soils were incubated in situ, during which time treatment sites experienced prescribed burns. To examine how litter modulates the effect of fire, we removed surface litter in some plots within all sites. Soil cores were taken from incubated soils. Gross mineralization, net mineralization, and net nitrification rates were calculated using the change in nitrogen availability at each plot. 

Results/Conclusions

In pre-burn soils, there was more nitrate available than ammonium. Post-burn soils had higher overall total inorganic nitrogen (TIN) availability, and more ammonium available than nitrate. Nitrogen cycling rates were generally higher in burned sites, but due to high variability between sites, differences were not significant. Litter removal had a significant positive influence on net mineralization and a marginally significant influence on net nitrification rate.

These findings highlight the importance of burns for short-term nitrogen availability in this system. Burning resulted in a TIN pulse nearly 1.5x that of in unburned sites, and the dominant inorganic nitrogen form shifted from nitrate to ammonium. If plants vary in their ability to utilize nitrate and ammonium, a change in the frequency of burning might influence community composition. Cycling rates were greatest in the absence of litter, perhaps because litter in this system is highly recalcitrant. The effects of burning on nitrogen cycling may be best understood in terms of changes to other soil properties: pH, organic matter content, and soil moisture were all significant predictors of cycling rates. Unmeasured variables, like community composition, may help explain variability in inter-cite cycling rates.