FIELD SIMULATION OF WET AND DRY YEARS IN THE CHIHUAHUAN DESERT - SOIL-MOISTURE, N-MINERALIZATION AND ION-EXCHANGE RESIN BAGS

Irrigation and rain-out shelters were used to simulate precipitation p atterns of wet and dry years in the northern Chihuahuan Desert. Irriga tion provided approximately double the long-term average monthly preci pitation. Rain was excluded during the wet season, July-October, to si mulate a dry year. N net mineralization in laboratory incubations was undectable at calculated water potentials less than -1 MPa. With incre asing moisture, mineralization gradually rose to the highest observed rates near field capacity. There was no mineralization maximum at mois ture contents below field capacity. Irrigation significantly increased the water potential and rainfall exclusion reduced water potentials t o less than -8 MPa. The general absence of important irrigation effect s may have resulted from the high natural precipitation during the exp eriment or because irrigation inputs were insufficient to increase mic robial activity during very dry periods. Precipitation exclusion reduc ed ion capture during the warm-wet season. After allowing precipitatio n inputs to resume, NH4+-N capture was increased in the cool-dry seaso ns of both 1987-1988 and 1988-1989. NH4+-N capture more than doubled t hat predicted from the overall covariance of moisture input and ion ca pture, suggesting increased availability of N. An unusually hot, dry p eriod in May and June 1989 was followed by a three-to fourfold increas e in the warm-wet season NO3-+NO2-N capture compared to 1988. These da ta suggest that short droughts of about 3 months in length (both simul ated and natural) increased N availability relative to moisture availa bility.