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 soil), open/ forb (6 g organic C kg soil), and sacaton (18 g organic C kg 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 6 39.5 mg CO2 m h 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 21.16 13.4 (mesquite), 2.16 4.4 (open), and 3.96 5.2mg N2Om h (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 6.3 mg CH4 m h 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. ARID AND SEMIARID LANDS cover as much as 30% of the earth’s surface (Potter et al., 1996) and may be increasing in response to global change (Emanuel et al., 1985). Although researchers have estimated the contribution of world soils to the atmospheric budgets of CO2, N2O, and CH4, flux data for semiarid life zones are extremely sparse. Semiarid ecosystems are characterized by frequent drought stress, daily and seasonal T extremes, low organic matter content, and low nutrient reserves (Sküjins, 1981; Lal, 2001), leading researchers to believe that semiarid soils are not significant consumers or producers of trace gases. As a result, arid and semiarid life zones were, until recently, largely overlooked in greenhouse gas (GHG) inventories (Bowden, 1986; Potter et al., 1996). Increasing atmospheric concentrations of the trace gases CO2, N2O, and CH4 are a major focus of current studies on global climate change. Atmospheric CO2 has risen from 280 to 370 mL L since the beginning of the Industrial Revolution and in the same time period atmospheric CH4 has risen from 0.75 to 1.8 mL L (Khalil, 1999). Atmospheric CH4 has a lifetime of 12 to 17 yr and is responsible for approximately 25% of anticipated global warming (Intergovernmental Panel on Climate Change IPCC, 1995). Preindustrial N2O levels in the atmosphere were 285 nL L, and have risen to 311 nL L today (Khalil, 1999). With an atmospheric lifetime of 120 yr, the increase in atmospheric concentrations of N2O could have significant implications for future climate change (Schlesinger, 1997). Atmospheric concentrations of CO2, N2O, and CH4 are each affected by biological transformations of C and N in soils. Carbon dioxide efflux is strongly correlated with SM, litter quality, and T in semiarid soils, where low rainfall acts as a strong constraint on biological activities (Conant et al., 2000, 2004). Studies have also shown that N2O production (Virginia et al., 1982) and CH4 consumption (Striegl et al., 1992) in arid soils may be concentrated in brief periods following wetting events, but the mechanisms influencing fluxes of these two trace gases are not well defined. Nitrous oxide production in semiarid soils is reported to result from respiratory denitrification, a process whereby nitrate (NO3) is used as a terminal electron acceptor by denitrifying bacteria in anoxic soils (Virginia et al., 1982). However, recent studies by McLain and Martens found that N2O fluxes from semiarid soils were strongly correlated to soil C mineralization rates (McLain and Martens, 2005a) and that N2O fluxes were significantly reduced by additions of an inhibitor of eukaryotic activity (cycloheximide) (McLain and Martens, 2005b). These results, together with the low soil C content and lack of persistent low moisture potentials common to semiarid soils, suggest that respiratory denitrification is limited in these life zones, and indicate that much of the N2O production in semiarid soils may result from the activity of aerobic heterotrophic fungi. Consumption of atmospheric CH4 by methanotrophic microorganisms is ubiquitous in aerobic soils of temperate, tropical, boreal, grassland, and forest systems (Mosier et al., 1991; Keller et al., 1993; Topp and Pattey, 1997). The CH4 sink in arid soils has been reported to develop in response to SM inputs (Striegl et al., 1992) but is thought to be of short duration because methanotrophs have not been found to be xerotolerant in laboratory incubations (Schnell and King, 1996). J.E.T. McLain, USDA-ARS, U.S. Water Conservation Lab., 4331 East Broadway Rd., Phoenix, AZ 85040; Dean A. Martens (deceased), USDA-ARS, Southwest Watershed Research Center, 2000 East Allen Rd., Tucson, AZ 85719. Received 4 Apr. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:367–377 (2006). Soil Biology and Biochemistry and Forest, Range & Wildland Soils doi:10.2136/sssaj2005.0105 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: GHG, green house gas; IPCC, Intergovernmental Panel on Climate Change. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 367 Published online February 2, 2006