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.

Carbon and nitrogen dynamics were analyzed during the decomposition of litter and roots of the desert ephemeral pepperweed (Lepidium lasiocarpum). We treated litter bags with the insecticide chlordane and the fungicides benomyl and captan to eliminate or restrict groups of soil biota.The mass losses of buried litter (51, 39, and 25% for untreated, insecticide-treated, and fungicideinsecticide-treated material, respectively) were higher than those of the respective root treatments (35, 18, and 15%) at 96 d. The mass loss of untreated material was correlated with numbers of detritivorousfungivorous microarthropods, and only a small percentage of this loss was as C0 2 : 27 and 42% for litter and roots, respectively. In the absence of microarthropods a higher percentage of mass-loss carbon could be accounted for as C0 2 : 33 and 76% for litter and roots, respectively, indicating that mass loss was due primarily to litter removal by microarthropod activity and not to mineralization. Litter removal by microarthropods was less dependent on abiotic constraints such as soil moisture (r = 0.65, P < .001) than was mass loss when microarthropods were absent (r = 0.79, P < .001). In the absence of microarthropods, mass loss was more closely coupled with biomass of grazers, such as nematodes, which require free water for activity (r = 0.99, P < .0001).Unlike mass loss, carbon mineralization was highest in untreated roots, suggesting a stimulation of microbial activity by microarthropods, while in untreated litter no stimulation was observed when compared to insecticide treatments. This difference was primarily a function of fungivorous microarthropod density, with overgrazing occurring in the untreated litter.Nitrogen budgets indicated the importance of microarthropods in the turnover of root nitrogen. In the presence ofmicroarthropods 132% of the initial root nitrogen could be accounted for after 96 d, while in the absence ofmicroarthropods 270% could be accounted for. This net immobilization of nitrogen was primarily in the soil organic fraction around the roots and was associated with fungal development.Data from this study re-emphasize the importance of microarthropods as regulators of decomposition in deserts and suggest that predation by nematodes or protozoa on bacteria and fungi contributes to rate regulation. Nitrogen flux data suggest that when spring ephemeral plant production is high, decomposition of ephemeral roots with attendant nitrogen immobilization can reduce the nitrogen available to creosotebush, Larrea tridentata, thus reducing shrub production. Higher taxa of soil biota, i.e., nematodes and microarthropods, may thus be important regulators of nitrogen fluxes and of mass loss in decomposition.

We tested the hypotheses that rates of decomposition in a desert should be higher following single large rain events of 25 mm than evenly spaced 6 mm events and that supplemental rainfall should result in higher populations of soil biota. There were no significant differences in mass losses of creosotebush, Larrea tridentata, leaf litter on plots receiving water supplementation and no added water. On some sampling dates, there were higher mass losses in the 6 mm·week treatment. Weekly rainfall produced higher coefficients of variation in mass losses than the other rainfall regimes. A single event pulse compared with weekly pulses of rainfall during the normal "dry" period resulted in no differences in mass losses. Microarthropods and nematodes exhibited numerical responses to supplemental rainfall but the litter microflora did not. These studies provide direct experimental evidence that the conventional wisdom linking decomposition to rainfall in deserts is wrong. The studies also suggest that the effects of litter fauna on surface litter decomposition are minimal; therefore, future studies should focus on activites of the microflora.

No abstract

The effects of soil moisture tension and temperature on the kinetics of the degradation of (2,4‐dichlorophenoxy)acetic acid (2,4‐D) in an Ascalon sandy loam were studied under laboratory conditions to develop a simulation model. Degradation occurred by a slow, first‐order reaction (slow phase) which, under some conditions, was followed by a rapid, first‐order reaction (fast phase). The optimum temperature and moisture tension were 27°C and 0.1 bar, respectively. Degradation rates under optimum conditions were 0.230 and 2.234 µg g−1 soil for the slow and fast phases, respectively. At temperatures above the optimum, no fast phase was observed. The activation energy (EA) values increased from 22.96 to 45.46 kcal mole−1, with increasing soil moisture tension in the range from 0.1 to 1.0 bar. The rate of decomposition of 2,4‐D decreased with increasing soil moisture tension for temperatures between 20 and 35°C. This decrease was a result of the reduced activity of the 2,4‐D‐degrading microorganisms arising from decreased water availability and increased 2,4‐D solution concentration.