No abstract

Fertilizer and irrigation must be managed for efficient resource use and preservation of environmental quality as well as maximum production. Two furrow irrigation experiments growing broccoli (Brassica oleracea) were conducted. The first consisted of three N rates (90, 180, and 270 kg/ha), two irrigation treatments (replenishment of water lost by evapotranspiration and evapotranspiration plus 30%), and two N application procedures (application to the soil and in the irrigation water) on a sandy loam soil (coarse‐loamy, mixed, thermic Typic Xerofluvent). Plant growth increased with increasing N application. For a given N application, there was higher average production with the lesser amount of water application. The method of N application had very little effect on production under the lower irrigation treatment, but production was consistently higher for N application with water rather than to the soil when the higher water application treatment was used. The ratio of N in the plant to N applied decreased with increasing N application, decreased with higher water application, and decreased with application in the irrigation water as compared to soil application. The second experiment consisted of two N rates (115 and 225 kg/ba) and two N application procedures (application to soil and in the irrigation water) on a loam soil (fine‐loamy mixed, thermic Calcic Haploxeroll). Yield and N uptake were significantly higher with 225 as compared to 115 kg/ha of N. Conventional preplant and side‐dress N application to the soil resulted in significantly higher broccoli head yield than injecting N in the irrigation water. Leaching of N was not a factor under the experimental conditions.

Nitrous oxide evolution from soils may contribute to partial destruction of the ozone layer in the stratosphere. Knowledge on factors affecting the N2O/N2 evolution ratio from soils is important in properly accessing the hazard associated with N2O evolution. Soils were wet to various air‐filled porosities with 400 ppm 15N‐labeled NaNO3 solution or to 5% air‐filled porosity with various nitrate concentrations. The treated soils were incubated in closed containers in the laboratory and periodically checked for N2O and N2 evolution. The incubation flasks were opened and brought to equilibrium with the atmosphere after each gas sampling. All samples which were sufficiently wet to cause denitrification had an initial high N2O evolution rate which decreased and approached zero with time. Dinitrogen gas evolution rate was initially low but increased after 3 to 4 days. The N2O/N2 ratio was initially high (infinity in some cases) and decreased rapidly with time. Except for the three lowest initial nitrate treatments, there was relatively high nitrate concentration throughout the incubation period; thus the decrease in the ratio could not be attributed to low nitrate concentration during the latter part of incubation. At a given time during the early stages of denitrification, the N2O/N2 evolution ratio increased with increased initial nitrate concentration. The ratio eventually became zero even under highest nitrate treatment. It is proposed that the enzyme dissimilatory nitrate reductase develops rapidly after anoxic conditions are initiated, and that the enzyme dissimilatory nitrous oxide reductase develops only after a period of time following anoxic conditions.

The practice of applying N fertilizer through trickle irrigation systems is rapidly increasing in popularity. Fertilizer N requirements are usually divided into small increments and then applied at regular intervals during the season. This field study, conducted on an Arlington loam (coarseloamy, mixed, thermic Haplic Durixeralf), was designed to investigate the uptake and partitioning of N by trickle‐irrigated tomatoes (Lycopersicum esculentum Mill.) and the potential for denitrification when N is continuously metered into the irrigation water. Solutions containing 1.8, 3.6, or 5.4 mol N m−3 were applied when the soil matric potential at the 25 cm depth reached either −10 or −30 kPa. The resulting N application totals ranged from 120 to 585 kg N ha−1. Early in the season, large differences in petiole NO3‐N concentrations were observed which appeared to be directly related to the rate of N supply. However, these differences gradually decreased as the season progressed. Total N uptake, which was linearly related to N application, was less than the amount applied at N rates greater than 300 kg ha−1. As the N application rate increased, an increasingly greater proportion of total plant N was partitioned into the vines. Denitrification N losses, as indicated by direct field measurements of N flux during a 4‐day period, were very small in the two lowest N treatments, ranging from 0.3 to 2.0 μg m−2 s−1. These low rates of denitrification were presumably due to competition between roots and denitrifiers. However, in the high N treatments, where N supply exceeded uptake, N fluxes ranged from 2.0 to 17.5 μg m−2 s−1. Aside from its influence on the rate of N supply, irrigation frequency had little effect on denitrification and N uptake. These results indicate that adequate N can be applied to tomatoes using high‐frequency N fertilization without large denitrification N losses.