Intensive use of N fertilizer, in modern agricultue is motivated by the economic value of high grain yields and is generally perceived to sequester soil organic C by increasing the input of crop residues. This perception is at odds with a century of soil organic C data reported herein for the Morrow Plots, the world's oldest experimental site under continuous corn (Zea mays L.).After 40 to 50 yr of synthetic fertilization that exceeded grain N removal by 60 to 190%, a net decline occurred in soil C despite increasingly massive residue C incorporation, the decline being more extensive for a corn-soybean (Glycine mwv L. Merr.) or corn-oats (Avena sativa L)-hay rotation than for continuous corn and of greater intensity for the profile (0-46 cm) than the surface soil. TIhese findings implicate fertilizer N in promoting the decomposition of crop residues and soil organic matter and are consistent with data from numerous cropping experiments involving synthetic N fertilization in the USA Corn Belt and elsewhere, although not with the interpretation usually provided. There are imporrant implications for soil C sequestration because the yield-based input of fertilizer N has commonly exceeded grain N removal for corn production on fertile soils since the 1960s. To mitigate the ongoing consequences of soil deterioration, atmospheric CO 2 enrichment, and NO,-pollution of ground and surface waters,,N fertilization should be managed by site-specific assessment of soilN availability. Current fertilizer N managemrent practices, if combined with corn stover removal for bioenergy production, exacerbate soil C loss.

Abbreviations: EONR, economically optimum nitrogen rate; FNUE, fertilizer nitrogen uptake effi ciency; HNPK, high nitrogen-phosphorus-potassium fertilizer; ISNT, Illinois soil nitrogen test; SEM, standard error of mean; SOC, soil organic carbon.

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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 (NH4 + amino sugar)‐N as gaseous NH3 The NH3 is collected in H3BO3–indicator solution, and subsequently determined by acidimetric titration. Recovery ranged from 97 to 102% when analyses were performed after treating samples with 15N‐labeled (NH4)2SO4 or glucosamine, but did not exceed 6.5% with labeled glycine and was undetectable with labeled NO3 or NO2 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 NO3 pollution of ground and surface water.

Colorimetric methods based on the Berthelot reaction are used widely for quantitative determination of NH4‐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 NH4‐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 NH4‐N L−1, recovery of NH4‐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.

Most soils contain inorganic nitrogen (N) in the form of ammonium (NHt) and nitrate (NO)"). Nitrite (NO z) also may be present, but the amount is usually too small to warrant its determination, except in cases where NHt or NHt-forming fertilizers are applied to neutral or alkaline soils. Several other forms of inorganic N have been proposed as intermediates during microbial transformations of N in soils, including hydroxylamine (NH 2 0H), hyponitrous acid (H2N20 2), and nitramide (NH 2 N0 2), but these compounds are thermodynamically unstable and have not been detected in soil. Until the 1950s, inorganic N was believed to account for <2% of total soil N, on the assumption that NHt and NO)" are completely recovered by extracting soil with a neutral salt solution. The validity of this assumption was challenged by the finding that some soils contain NHt in a form that is not extracted by exchange with other cations (e.g.

Intensive fertilizer usage of KCl has been inculcated as a prerequisite for maximizing crop yield and quality, and relies on a soil test for exchangeable K in the plow layer to ensure that soil productivity will not be limited by nutrient depletion. The interpretive value of this soil test was rigorously evaluated by: (1) field sampling to quantify biweekly changes and seasonal trends, (2) characterizing the variability induced by air drying and the dynamic nature of soil K reserves and (3) calculating the K balance in numerous cropping experiments. These evaluations leave no alternative but to question the practical utility of soil K testing because test values cannot account for the highly dynamic interchange between exchangeable and non-exchangeable K, exhibit serious temporal instability with or without air drying and do not differentiate soil K buildup from depletion. The need for routine K fertilization should also be questioned, considering the magnitude and inorganic occurrence of profile reserves, the recycling of K in crop residues and the preferential nature of K uptake. An extensive survey of more than 2100 yield response trials confirmed that KCl fertilization is unlikely to increase crop yield. Contrary to the inculcated perception of KCl as a qualitative commodity, more than 1400 field trials predominately documented a detrimental effect of this fertilizer on the quality of major food, feed and fiber crops, with serious implications for soil productivity and human health.

Nitrogen fertilization for corn (Zea mays L.) production has relied extensively on yield-based recommendations that were developed to represent regional averages, yet are routinely applied to individual fields, on the assumption that fertilizer N serves as the major supply for crop N uptake. Using data from 102 on-farm N-response studies, an evaluation was conducted of the Illinois proven-yield (PY) method for accuracy and economic profitability on a site-by-site basis. As additional objectives, the Illinois soil N test (ISNT) was evaluated for detecting whether N fertilization was economical, and for quantifying crop response to N fertilization relative to soil and management factors. For 18% of the site-years studied, N recommendations by the PY method were accurate to within 20 kg ha 21 , whereas 13% were underfertilized by 25 to 129 kg ha 21 (60 kg ha 21 on average) at a current cost of $5 to $170 ha 21 ($75 ha 21 on average), and 69% were overfertilized by 21 to 235 kg ha 21 (103 kg ha 21 on average) at a cost of $12 to $130 ha 21 ($57 ha 21 on average). The latter group included 30 site-years that were completely nonresponsive to N fertilization, all but two of which were predicted by site-average ISNT values assuming a critical test level of 230 mg kg 21. This level was exceeded for 19 of 69 responsive site-years, mostly during 2001-2003 when corn followed soybean (Glycine max L. Merr.) with high plant populations. A higher critical test level would have been required under such conditions, owing to more extensive residue inputs that would promote microbial N immobilization, and increased crop uptake of mineralized soil N. The ISNT was significantly related to crop N requirement, and was the most powerful predictor of error in PY recommendations (P , 0.001).