Abiotic processes drive gaseous nitrogen loss from Mojave Desert soils

Background/Question/Methods

In arid ecosystems, gaseous nitrogen fluxes can be a substantial portion of annual nitrogen losses and a significant term in the terrestrial nitrogen cycle. This loss of reactive nitrogen is also an important input to the atmospheric chemistry defining climate and air quality. Our current understanding of the production of nitrogen gases from soils is largely based on the microbial processing of soil nitrogen. However, measurements from the Mojave Desert suggest substantial soil emissions of reactive nitrogen under conditions that are unlikely to support biological activity: extremely high temperature and low moisture conditions. In the field, manipulations of incoming solar radiation show a strong link between abiotic formation of reactive nitrogen gases and surface soil temperatures. To explore the direct effects of soil temperature on reactive nitrogen gas emissions, we manipulated soil temperature in the lab from 15 – 75 ºC and measured reactive nitrogen generation and soil respiration from intact profiles collected from the Mojave Desert. Profiles were collected from interspaces and from beneath the dominant shrub Larrea tridentata. Measurements were made on both dry and wetted soils.  

Results/Conclusions

In dry soils, reactive nitrogen gas fluxes increased with surface soil temperature with losses ranging from 0.05 to 2 ng N m^-2 s^-1. Soil temperatures of ~ 45 ºC appeared to be an important threshold where flux increases were small at temperatures below 45 ˚C and then rose rapidly between 45 and 75 ˚C. Soil respiration was not observed in dry soils, suggesting that nitrogen gas production was driven by abiotic mechanisms. Wet soils showed a similar response, however, fluxes were an order of magnitude higher. Under both moisture conditions cover type affected the magnitude, but not the nature of the temperature response. In wet soils, respiration showed a positive relationship with temperatures up to 40 ºC and then rapidly declined as temperatures exceeded 40 ºC. High soil emissions of reactive nitrogen at temperatures above 40 ˚C and the absence of soil respiration at these temperatures supports the hypothesis that nitrogen gas emissions are often driven by non-biological processes. These temperature dependent abiotic mechanisms are likely to dominate reactive nitrogen gas loss in the Mojave Desert where ~ 75% of estimated annual reactive nitrogen gas emissions occur in the summer when soil temperatures are >40 ºC.