HR: 0800h
AN: B21A-0856 [Abstracts]
TI: N$_{2}$O Production and Consumption in Semi-Arid Soils
AU: * van Haren, J
EM: jvanharen@tucson.ars.ag.gov
AF: USDA-ARS SWRC, 2000 E. Allen Rd, Tucson, AZ 85719
United States
AU: McLain, J
EM: jmclain@tucson.ars.ag.gov
AF: USDA-ARS SWRC, 2000 E. Allen Rd, Tucson, AZ 85719
United States
AU: Martens, D
EM: dmartens@tucson.ars.ag.gov
AF: USDA-ARS SWRC, 2000 E. Allen Rd, Tucson, AZ 85719
United States
AB:
We investigated how summer monsoon precipitation affects surface and subsurface N$_{2}$O production and consumption in
semi-arid rangeland soils, southern Arizona, USA. Surface fluxes and profiles up to 2 m deep were sampled for N$_{2}$O
concentration, $\delta$$^{15}$N and $\delta$$^{18}$O. Soil samples for incubations and C and N stable isotope composition
were taken at the surface through 50 cm depth.
Pre-monsoon soils were dry with small N$_{2}$O fluxes (1.6$\pm$0.6 $\mu$g/m$^{2}$/hr). Initial precipitation events wetted
the surface 10 cm and triggered much larger N$_{2}$O fluxes (11.7$\pm$6.7 $\mu$g/m$^{2}$/hr). $\delta$$^{15}$N values of
N$_{2}$O surface fluxes averaged -9.7$\pm$1.9$\permil$ after precipitation and increased as the soil surface dried
(-6.5$\pm$2.1$\permil$). Soil profile $\delta$$^{15}$N values after rainfall ranged from -3.8 to -1.5$\permil$, with one
outlier at 8.6$\permil$. $\delta$$^{18}$O values of N$_{2}$O fluxes following rainfall were highly variable
(29$\pm$32$\permil$), but were more consistent (43.4$\pm$0.1$\permil$) when the soil surface dried. Soil profile
$\delta$$^{18}$O values following rainfall averaged 44.1$\pm$1.0$\permil$, with one outlier 55.2$\permil$. Analysis of
N$_{2}$O trapped from aerobic laboratory incubations of surface soil had $\delta$$^{15}$N (-10.8$\permil$) and
$\delta$$^{18}$O (22.0$\permil$) values indicative of nitrification processes. Assuming that incubation isotope values
represent production, we calculated that 40% of N$_{2}$O was consumed before leaving the soil.
Our results suggest that after precipitation, surface produced N$_{2}$O preserved isotopic signal of production but as the
surface dried, flux and soil profile N and O isotope values reflected considerable N$_{2}$O consumption. Isotopic variability
of fluxes following rainfall indicates that several distinct surface production mechanisms.
DE: 4805 Biogeochemical cycles (1615)
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting