An evaluation of incubation and chemical methods of obtaining an index of soil N availability showed that incubation procedures involving estimation of the total mineral N produced on incubation of soil under aerobic conditions at 30C for 14 days or estimation of the ammonium N produced on incubation of soil under waterlogged conditions at 40C for 7 days provided a good index of the availability of the N in Iowa soils to ryegrass. It also showed that the results obtained with these soils by a chemical procedure involving estimation of the N extracted by boiling water and potassium sulfate solution were closely related to N uptake by ryegrass and indicated that this procedure deserves consideration as a routine method of obtaining an index of soil N availability. Air‐drying and air‐dry storage of soil samples had marked effects on the results obtained by the incubation methods studied, but had little effect on the results obtained by the chemical methods found to provide a good index of soil N availability.

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Concern for nitrate (NO3‐N) contamination of surface waters in Iowa has brought much speculation with regard to the source of NO3‐N. It is frequently assumed that the large percentage increase in fertilizer N use over the last 50 yr is the primary cause of high concentrations of NO3‐N in surface waters of central Iowa. We performed an analysis of data relating flow and NO3‐N concentration for the Des Moines River to identify the effect of agricultural practices on NO3‐N in the river. Annual and weekly stream flow values for the 1980‐1990 period at a site just below the city of Des Moines were positively related to the NO3‐N concentration in the river water for the 11‐yr period (r2 = 0.60, P < 0.01). From 1980 to 1991 the average annual NO3‐N concentration ranged from 2.0 mg L−1 in the low flow year of 1989 to 9.1 mg L−1 in 1982. Over the 11 yr, the river averaged 5.6 mg L−1 NO3‐N and had an average flow of 733 580 m3 h−1. In 1945, the average flow was 723 710 m3 h−1 and NO3‐N concentrations were 5.0 mg L−1. These values were similar to the 11‐yr average data obtained 35 to 45 yr later. We infer from the data that the intensive agricultural activities in 1945 and 1980 to 1990 are the major source of the NO3‐N to the river rather than solely N fertilizer. Changes in land management, cropping patterns, and land use are therefore required to markedly lower the NO3‐N levels of surface water in this basin.

The high rates of urine‐N deposited during grazing offers the potential for considerable N loss through volatilization, leaching, and denitrification. A field experiment was conducted to evaluate the magnitude and pathways of N transformations and losses under warm, moist conditions. Urine was applied to 3‐m2 plots in a ryegrass (Lolium perenne L.)‐white clover (Trifolium repens L.) pasture at 300 or 600 kg N/ha. These rates simulate the urine affected area of sheep and cattle, respectively. Ammonia volatilization, inorganic N in the soil profile, symbiotic nitrogen fixation (by acetylene reduction assay), and herbage N yields were measured over 53 days. Four days after application from 30 to 35% of the added N could not & accounted for as either soil mineral N or volatilized NH3‐N. Ammonia‐N evolution was rapid only for the first 2 days, and I5 to 18% of the N was volatilized as NH3. Profile NH4‐N was completely nitrified by 21 days. Considerable amounts of the applied N was lost from the 0 to 45 cm profile by 53 days. Since rainfall and irrigation exceeded estimated evapotranspiration, leaching of NO3‐N is the probable loss mechanism. The apparent recovery of N in herbage was 37 and 22% of that added. Acetylene reduction was substantially less in the urine‐treated swards. From 30 to 35% of the urine‐N was not accounted for as inorganic N early in the experiment; immobilization and rapid leaching of urine below 45 cm are probable loss mechanisms. The experiment indicated that in a warm, moist environment, typical of late spring‐early summer in New Zealand, more than half of the N in urine voided by sheep or cattle can be lost, and that an intensively grazed grass‐legume system is far from a closed system, or cycle, with respect to N.

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