Moisture Controls on Trace Gas Fluxes in Semiarid Riparian Soils

Variability in seasonal soil moisture (SM) and temperature (T) can alter ecosystem/atmosphere exchange of the trace gases carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). This study reports the impact of year-round SM status on trace gas fluxes in three semiarid vegetation zones, mesquite (30 g organic C kg⁻¹ soil), open/forb (6 g organic C kg⁻¹ soil), and sacaton (18 g organic C kg⁻¹ soil) from July 2002–September 2003 in southeastern Arizona. Carbon dioxide and N₂O emissions were highly dependent on available SM and T. During the heavy rains of the 2002 monsoon (238 mm total rainfall), large differences in soil C content did not correlate with variations in CO₂ production, as efflux averaged 235.6 ± 39.5 mg CO₂ m⁻² h⁻¹ over all sites. In 2003, limited monsoon rain (95 mm total rainfall) reduced CO₂ emissions by 19% (mesquite), 40% (open), and 30% (sacaton), compared with 2002. Nitrous oxide emissions averaged 21.1 ± 13.4 (mesquite), 2.1 ± 4.4 (open), and 3.9 ± 5.2 μg N₂O m⁻² h⁻¹ (sacaton) during the 2002 monsoon. Limited monsoon 2003 rainfall reduced N₂O emissions by 47% in the mesquite, but N₂O production increased in the open (55%) and sacaton (5%) sites. Following a dry winter and spring 2002 (15 mm total rainfall), premonsoon CH₄ consumption at all sites was close to zero, but following monsoon moisture input, the CH₄ sink averaged 26.1 ± 6.3 μg CH₄ m⁻² h⁻¹ through April 2003. Laboratory incubations showed potentials for CH₄ oxidation from 0 to 45 cm, suggesting that as the soil surface dried, CH₄ oxidation activity shifted downward in the sandy soils. Predicted climate change shifts in annual precipitation from one dominated by summer monsoon rainfall to one with higher winter precipitation may reduce soil CO₂ and N₂O emissions while promoting CH₄ oxidation rates in semiarid riparian soils of the Southwest, potentially acting as a negative feedback for future global warming.