Abstract The use of 15 N‐labeled dinitrogen ( 15 N 2 ) affords the only direct means of measuring free‐living nitrogen fixation (FLNF); however, progress in utilizing this approach has been impeded by methodological limitations arising from the presence of nitrogenous contaminants, a lack of atmospheric uniformity, and incomplete description of procedural details. Such constraints are eliminated with an ex situ technique comprehensively described herein, which involves circulating 15 N 2 generated by hypobromite oxidation through a closed system that includes chemical (sulfuric acid–potassium permanganate) and cryogenic (isopentane–liquid N 2 ) traps for atmospheric purification and an incubation chamber consisting of a desiccator equipped with a pressure gauge. Studies to evaluate the circulation system described showed that a uniform atmosphere was readily achieved with a 10‐L desiccator by pumping for 30 min, and that both chemical and cryogenic traps were necessary to ensure complete (98.8%–99.6%) removal of gaseous contaminants subject to physicochemical retention by sterilized soil samples. The method proposed was successfully demonstrated in detecting the stimulatory effect of organic carbon (C) on FLNF in active soils, and can be further utilized to improve the reliability of ex situ assessment of FLNF in relation to soil processing and storage, climatic conditions, microbial dynamics, and land management practices.
Increased fertilizer N uptake efficiency (FNUE) leads to more economical corn ( Zea mays L.) production and lower environmental impact. Excessive N application reduces FNUE and may affect subsequent crop response through its influence on NO 3 –N carryover and the amount of readily mineralizable organic N in the soil. Our objective was to determine how prior fertilizer N application rate affects (i) grain yield and agronomic optimum N rate, (ii) contributions of fertilizer‐ and soil‐derived N to N uptake, and (iii) FNUE. Labeled 15 NH 4 15 NO 3 was applied at 0, 67, 134, 201, or 268 kg N ha −1 to subplots within a continuous corn long‐term N rate study. Estimates of FNUE were higher by the difference method (49–69%) than with the isotope ( 15 N) method (31–37%), and different trends were observed with each method as N application rate increased. The disparity between methods is consistent with a differential effect of long‐term N application rate on mineralization–immobilization. Recovery of labeled N from the plant–soil system ranged from 71% at the 67 kg ha −1 N application rate to 64% at the 201 kg ha −1 application rate. Fertilizer N accounted for an increasing proportion of crop N uptake as the N rate was increased, but soil N uptake was always more extensive, accounting for 54 to 83% of total plant N. Crop uptake of labeled N during the second growing season after 15 N application ranged from 2.2 kg ha −1 with the lowest N rate to 7.8 kg ha −1 with the two highest rates.
Abstract Colorimetric methods based on the Berthelot reaction are used widely for quantitative determination of NH 4 ‐N in biological and environmental samples. Studies to evaluate phenol and salicylate, the most commonly used chromogenic substrates, revealed minor interferences by metallic cations, whereas up to a threefold shift in absorbance was observed with 38 diverse N‐containing organic compounds. Interferences differed markedly between phenol and salicylate. The possibility of a simple correction was precluded by the fact that interferences were both positive and negative, and depended on the temperature during color development and the concentration of NH 4 ‐N. Fourteen compounds were evaluated as alternatives to phenol and salicylate, of which the Na salt of 2‐phenylphenol (PPS) proved to be the most promising. Using PPS, macro‐ and microscale batch methods and an automated flow‐injection method were developed. These methods are simple, convenient, and sensitive. Using the PPS microscale method, for which the limit of detection is 0.17 mg NH 4 ‐N L −1 , recovery of NH 4 ‐N added to soil extracts ranged from 98 to 104%, with a coefficient of variation of 1.4 to 2.7%. As with phenol and salicylate, precipitation of metal hydroxides was observed. Precipitation was controlled by chelation with citrate rather than ethylenediaminetetraacetic acid (EDTA), which suppressed color development by preventing monochloramine formation. Compared with Berthelot methods that use phenol or salicylate, interference by amino acids was decreased by up to 10‐fold. Interference by other organic N compounds was virtually eliminated.
ABSTRACT Turfgrass managers frequently apply N as a foliar spray when low application rates are desired. This practice is believed to promote foliar N uptake that benefits turf; however, very little information is available concerning the quantity of N absorbed by turfgrass foliage or the effect of various spray parameters on foliar N uptake under field conditions. This research was conducted to evaluate fertilizer N uptake efficiency of foliarly applied 15 N to creeping bentgrass [ Agrostis stolonifera var. L. palustris (Huds.) Farw. ‘Pennlinks’] under field conditions. The effects of spray volume, N carrier, adjuvant addition, and tank mixing with commonly applied turf care products (e.g. chlorothalonil) on foliar N uptake were conducted to evaluate foliar uptake of fertilizer 15 N by creeping bentgrass. From 6 to 34% of foliar‐applied N was taken up mostly within 2 h and completely by 4 to 6 h after fertilizer application. Uptake efficiency increased significantly when spray volume was decreased but was unaffected by N carrier, adjuvant addition, or tank mixing.
Abstract A common strategy for improving fertilizer N uptake efficiency by corn ( Zea mays L.) is to synchronize application with crop N demand during the growing season, which can be done using the Y Drop system recently developed for surface dribble placement. A 2‐yr field study was conducted using dual‐labeled urea‐ammonium nitrate (UAN) solution to compare fertilizer 15 N uptake efficiency (F 15 NUE) for surface and subsurface sidedress applications to soils of contrasting fertility under either second‐year corn or a corn–soybean [ Glycine max (L.) Merr.] rotation. Besides placement, treatments were designed to allow comparison of 15 N uptake from UAN applications made at planting vs. at the V9 growth stage. The importance of soil N supply was demonstrated by estimates of N derived from fertilizer that ranged from 17 to 47% in total aboveground biomass and never exceeded N derived from soil. Of the fertilizer N recovered in the crop at harvest, the majority originated from the 15 N applied at sidedressing rather than at planting. The range in F 15 NUE was from 12 to 42% (26% on average) for grain and from 14 to 51% (31% on average) for total aboveground biomass, the only significant difference occurring when the subsurface treatment outperformed Y Drop placement under conditions conducive to urea N loss through NH 3 volatilization. Both methods of application offer the same fundamental advantage, in that fertilizer N is supplied during the period of maximal crop uptake.
Abstract A recent development in 15 N analysis of inorganic N in soil extracts involves diffusion of NH 3 into acidified glass‐fiber disks in disposable specimen containers. A simple and convenient technique was developed for isotope‐ratio analysis of the NH 4 in these disks using a mass spectrometer equipped with an automated Rittenberg apparatus (ARA‐MS). For collection of diffused NH 3 , the disks were treated with 10 µL of 1 M H 2 SO 4 . Following diffusion for 6 d, the disks, containing 50 to 150 µg of N, were transferred to a disposable plastic sample tray (96 wells per tray) and treated with 10 µL of 28.9 M (concentrated) HF, which digested the disks without affecting the tray. The tray was placed in a desiccator containing anhydrous CaSO 4 , for drying of the digested disks in 1 to 3 d. Good agreement was obtained between 15 N analyses of diffused and nondiffused NH 4 ‐N when correction was made for isotopic dilution by NH 4 in the reagents used in diffusion. Trays containing the processed disks can be conveniently shipped to a service laboratory for analysis by ARA‐MS.
Recent work indicates that accumulation of amino sugar N in soil reduces the yield response of corn ( Zea mays L.) to N fertilization, and that nonresponsive sites are detectable by determination of amino sugar N in soil hydrolysates. Unfortunately, the hydrolysis process is too complicated and time‐consuming for use in routine soil testing. A much simpler technique was developed to estimate amino sugar N without the need for acid hydrolysis. In this test, 1 g of air‐dried soil is treated with 10 mL of 2 M NaOH in a 473‐mL (1‐pint) wide‐mouth Mason jar, and the sample is heated for 5 h at 48 to 50°C on a hot plate to liberate (NH 4 + amino sugar)‐N as gaseous NH 3 The NH 3 is collected in H 3 BO 3 –indicator solution, and subsequently determined by acidimetric titration. Recovery ranged from 97 to 102% when analyses were performed after treating samples with 15 N‐labeled (NH 4 ) 2 SO 4 or glucosamine, but did not exceed 6.5% with labeled glycine and was undetectable with labeled NO 3 or NO 2 Comparative studies using 12 nonresponsive and 13 responsive soils showed a very high correlation between soil‐test N and hydrolyzable amino sugar N ( r = 0.90***). Test values were significantly higher ( P < 0.001) for nonresponsive (237–435 mg N kg −1 ) than for responsive (72–223 mg N kg −1 ) soils. The soil test described has important economic implications for production agriculture, and also should be of value for controlling NO 3 pollution of ground and surface water.
