HR: 0800h
AN: H41E-0342    [Abstracts]
TI: Atmospheric Nitrate and Limits on the N Cycle in Atacama Desert Soils
AU: * Ewing, S A
EM: saewing@nature.berkeley.edu
AF: University of California at Berkeley, Ecosystem Science Division 151 Hilgard Hall #3110, Berkeley, CA 94720 United States
AU: Michalski, G
EM: gmichalski
AF: University of California at San Diego, Department of Chemistry and Biochemistry, San Diego, CA 92093 United States
AU: Amundson, R G
EM: earthy@nature.berkeley.edu
AF: University of California at Berkeley, Ecosystem Science Division 151 Hilgard Hall #3110, Berkeley, CA 94720 United States
AU: Wu, J
EM: wu j@berkeley.edu
AF: University of California at Berkeley, Ecosystem Science Division 151 Hilgard Hall #3110, Berkeley, CA 94720 United States
AU: Thiemens, M
EM: mthiemens@ucsd.edu
AF: University of California at San Diego, Department of Chemistry and Biochemistry, San Diego, CA 92093 United States
AU: McKay, C P
EM: cmckay@mail.arc.nasa.gov
AF: NASA, Ames Research Center, Moffett Field, CA 94035 United States
AB: Soils of the hyperarid Atacama Desert in northern Chile are renowned for unusually high levels of naturally occurring nitrate and very low levels of organic carbon (OC). Recent work has indicated that the origin of nitrate in the most enriched deposits is largely atmospheric, but these deposits have not been linked to known soil biogeochemical processes, or placed within a global context. Here we investigate the contribution of biological vs. atmospheric sources of nitrate in three soils along a precipitation gradient (25 mm y$^{-1}$ to $<$2 mm y$^{-1}$) in the Atacama, focusing on the links between C and N cycling. A precipitous decline in biological activity accompanies the transition to extreme hyperaridity. As mean annual precipitation (MAP) decreases from 25 mm y$^{-1}$ to $<$2 mm y$^{-1}$ in well-developed (2 My) soils, soil surface OC levels decrease from 40 to 10 $\mu$mol g$^{-1}$, and steady state OC turnover times increase from $\sim$1 y to $\sim$13,000 y. In the same soils, the total nitrate inventory (to $\sim$2 m depth) increases from 0.1 to 17 kg m$^{-2}$, reflecting decreasing rates of loss (leaching or biological) of atmospherically supplied nitrate and ammonium. Nitrate $\Delta$$^{17}$O values in these soils increase from 9.1 to 17.8$\permil$ with decreasing precipitation, approaching the value of atmospherically-derived nitrate: 23.0$\permil$ determined from nitrate collected in passive deposition traps. These data indicate that the biologically unaltered atmospheric portion of soil nitrate increases from 40$%$ to 80$%$ with increasing aridity. Nitrate $\Delta$$^{17}$O values decrease by about 3$\permil$ with increasing (2 m) depth in the most hyperarid soil, suggesting increasingly biological nitrate, possibly from a previous, wetter climate. In the most humid soil (MAP = 25 mm y$^{-1}$), nitrate $\Delta$$^{17}$O values increase with depth, reflecting increased atmospheric nitrate that has been transported below the overlying zone of biological activity. Taken together, nitrate and OC chemistry in these soils reveals that the pervasive nitrate accumulations of the hyperarid core of the Atacama desert are the result of an incomplete soil N cycle, in which slow but continuous inputs of atmospherically-derived nitrate outpace insignificant hydrological losses and biological additions.
DE: 9360 South America
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting