An understanding of N cycling in the soil‐plant‐atmosphere components of wheat (Triticum aestivum L.) production systems is necessary to maximize yield and quality. The objectives of this study were to examine N cycling and observe the effects of N surplus and deficit on N absorption/desorption in the soil and atmosphere and to evaluate translocation within the plant. Soil, plant, and microclimate measurements were taken concurrently, and soil, plant, and atmospheric ammonia (NH3) transport determined. During the early vegetative phase, plant N concentration reached a maximum; however, during the remaining growth periods, N concentration decreased even though N uptake from the soil continued until plant maturity. More total N was translocated to grain from leaves than stems, and translocation from the leaves began earlier than that from stems. Isotope and total N studies showed that after anthesis about half of the grain N came from remobilization from leaves and stems and the other half directly from the soil. A progressively larger percentage of N came from mineralized organic matter as the season progressed. Nitrogen was lost as volatile NH3 from the plant after fertilizer application and during the senescence period. Prior to anthesis, atmospheric NH3 absorption was observed during a period when soil N was temporarily unavailable. About 21% equivalent of the applied fertilizer was lost as volatilized NH3. During the period of soil unavailability an amount equivalent to about 1% equivalent of the applied fertilizer was gained from atmospheric NH3 by plant absorption.

The N required by soybeans [Glycine max L.) Merr.] is furnished by soil and by symbiotically fixed N. The latter is associated with available photosynthate. A determinate soybean cultivar, ‘Bragg’, was grown in sand in the greenhouse in 1974 and outdoors in 1975 to further evaluate the effect of growth stages and NH4+‐N and NO3‐‐N on 14C translocation, N fixation as measured by acetylene reduction by intact nodules, and accumulation of dry matter and nitrogen.Plants exported very little 14C to the nodules from photosynthesis of 14CO2, and acetylene reduction was very low when significant sink demand by pods became evident. Inorganic N supplied throughout the season or 10 days prior to sampling reduced 14C in nodules and acetylene reduction by nodules. NO3‐‐N generally decreased acetylene reduction more than NH4+‐N. 14C transport to plant parts other than nodules was not influenced by inorganic N. There is indication that in the determinate cultivar of soybeans used, more photosynthate was transferred to nodules during the vegetative and early reproductive stages than that reported previously for indeterminate cultivars. The data further emphasize the high demand for N at the pod‐fill stage where 333 mg N per plant (23% of total) was required during a 20‐day period, mid‐pod to late pod fill stage, and at the same time N fixation was declining.

Because large amounts of poultry wastes are often applied to hilly land in the southeastern United States, information is needed on the environmental hazards of this practice. A rainfall simulator was used to study the effect of application of poultry litter (manure plus wood residues) on runoff water quality and soil loss, on moderately sloping (7%) land. Increasing rates of litter were surface-applied on fallow soil and grassland and also incorporated in the fallow soil. Runoff and soil loss were drastically decreased by litter application on fallow soil, and runoff was reduced on the grassed soil. The grassed soil had little soil loss with or without litter application. The coliform bacterial content of runoff water from plots receiving the higher application rates of surface-applied litter was appreciable afterward. Incorporating litter into soil generally reduced coliforms during the later stages of runoff. Moderate applications of poultry manure to sloping land (especially grassland) should not create a major water quality problem, unless excessive rainfall occurs.

The understanding of nitrogen (N) cycling in the soil‐plant‐atmosphere is necessary to maximize N use efficiency and to develop N budgets for wheat (Triticum aestivum L.) production. The objectives of this study were to determine the relative uptake rates of residual soil N and fertilizer N in conservation tilled winter wheat. These data were combined with soil mineralization and aerial NH3 flux data to present a N budget for the soil‐plant‐atmosphere system. Fertilizer N uptake was determined using ammonium nitrate (15NH415NO3) tagged with 3.78 atomic % 15N. The buried polyethylene bag technique was used to determine N mineralization. Fertilizer N uptake and N mineralization rates were determined four times during the spring growing season. Amounts of NH4 and NO3 in the top 0‐ to 300‐mm soil layers were determined biweekly. Fertilizer N levels in the surface to 75‐mm soil layers decreased rapidly due to plant uptake and immobilization. Of the N fertilizer utilized by the plants, 61% was absorbed in the first 28 d after application. During early vegetative growth stages (Feekes stages 3–5), fertilizer N uptake was 1.33 kg ha−1 d−1. During the elongation stage (Feekes stages 5–8), however, fertilizer N was immobilized and uptake of fertilizer N ceased. This resulted in a period of soil N insufficiency which was associated with atmospheric NH3 influx to the plants. After 4 to 5 wk, mineralization of fertilizer N became apparent and fertilizer N uptake rates increased until harvest. Influx of atmospheric N was small but total NH3 efflux from the soil‐plant system was 15.5 kg N ha−1. About 21% of the spring applied fertilizer N was lost through NH3 volatilization and losses of this magnitude need to be considered in N balance studies. Placement of fertilizer N below the surface soil layer may decrease immobilization and increase plant uptake of N.