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Over the last century, proliferation of velvet mesquite (Prosopis velutina) in Southwestern grasslands has led to mesquite removal to increase livestock forage, but the effects of mesquite on the fluxes of C- and N-based gases are not well understood. We report soil surface fluxes and porespace concentrations of carbon dioxide (CO(2)), nitrous oxide (N(2)O), and methane (CH(4)) over two monsoon seasons beneath mesquite canopy (LIVE), in open grassland (OPEN), and under a mesquite skeleton (DEAD) in Southeastern Arizona. In 2003, CO(2) production was not different among sites, but on three 2004 measurement dates, COZ production was greatest from LIVE. In both years, N(2)O production was highest in LIVE in early monsoon, while CH(4) production dominated in OPEN and DEAD sites. CH(4) consumption increased in all sites as the summer progressed, resulting in net monsoon CH(4) fluxes near zero. Over both years, temperature and precipitation accounted for significant variability in CO(2) flux and precipitation correlated with N(2)O production. Soil moisture accounted for significant variability in CH(4) flux in 2003. Observed variations in trace gas dynamics suggest that several years of measurements would be required to accurately predict mesquite management effects on trace gas flux in Southwestern rangelands. (C) 2008 Elsevier Ltd. All rights reserved.