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Nitrogen Mineralization in a Desert Soil: Interacting Effects of Soil Moisture and Nitrogen Fertilizer¹

ABSTRACT

The responses of N mineralization to two patterns of supplemental water, N fertilizer, and a drying-wetting episode were examined in order to evaluate the effects of variation in timing and intensity of natural precipitation on N availability. Field plots received either 6 mm water/week or 25 mm water/month with or without 10 g N m^–2. Samples were collected three times from July 1984 to March 1985 and incubated in the lab for 28 d. The effects of drought were simulated by drying soil at 35°C for 28 d followed by 168-d leaching incubations. Supplemental water reduced 28-d mineralization by 22% in soils collected during dry and moderate soil moisture conditions (July 1984, October 1984) but had no effect on soils collected during a moist period (March 1985). Nitrogen fertilizer had no effect on 28-d mineralization in soils from July but increased 28-d mineralization by 58% in soils from October and March. Air-drying increased mineralization rates across all field treatments during the first 14 d of the 168-d leaching incubations. Mineralization rates were lower in soils from watered plots in both the air-dry and field-moist treatments. Air-drying interacted with both the water and N treatments by increasing watering effects and decreasing fertilizer N effects. The observed drying effects appear to be a net result of several processes that, on the whole, tend to increase N availability. Mineralization rates in both experiments were lower in 6 mm/week soils than in 25 mm/month soils which, in turn, were lower than unwatered controls. We hypothesize that increased moisture availability eventually leads to losses of mineralizable N as initially rapid mineralization converts organic N to inorganic forms that are readily lost from the soil.

¹ Contribution from the Dep. of Biology and the Jornada LTER program, New Mexico State Univ., Las Cruces, NM 88003.

² College Assistant Professor, former College Assistant Professor, Research Assistant, and Professor, respectively.

Received for publication October 24, 1986.

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