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PURPOSE: We determined the applicability of inferior > superior > nasal > temporal (ISNT) rules on retinal nerve fibre layer (RNFL) thickness and rim area and evaluated the impact of various ocular factors on the performance of the ISNT rules in healthy myopic eyes. METHODS: A total of 138 eyes from 138 healthy myopic subjects were included in this cross-sectional observational study. The peripapillary RNFL and optic disc in each eye were imaged with Cirrus HD optical coherence tomography (OCT) and Heidelberg Retina Tomograph II (HRT2), respectively. The performance of the inferior > superior (IS), inferior > superior > nasal > temporal (IST) and ISNT rules on RNFL thickness and rim area was determined and compared between low-to-moderate myopia and high myopia. The effects of ocular factors [including axial length, disc area, disc tilt, disc torsion, disc-fovea angle (DFA) and retina artery angle] on the performance of ISNT rules were evaluated with logistic regression analysis. RESULTS: The mean axial length and refractive error were 25.57 ± 1.09 mm (range, 22.52-28.77 mm) and -5.12 ± 2.30 D [range, -9.63 to -0.50 dioptres (D)], respectively. Sixty-three per cent of the healthy eyes were compliant with the ISNT rule on rim area, while ISNT rule on RNFL thickness was followed in only 11.6% of the included eyes. For rim area, smaller disc area was significantly associated with increased compliance of the IS rule (odds ratio: 0.46, p = 0.039), IST rule (odds ratio: 0.46, p = 0.037) and ISNT rule (odds ratio: 0.44, p = 0.030). For RNFL thickness, greater DFA was significantly associated with increased compliance of the IS and IST rules (odds ratio: 1.30, p < 0.001; odds ratio: 1.19, p = 0.006, respectively). CONCLUSION: In healthy myopic subjects, 88.4% and 37% of eyes did not comply with the ISNT rule on RNFL thickness and rim area, respectively. Due to significant low compliance in healthy eyes, the ISNT rule and its variants have limited potential utility in diagnosing glaucoma in myopic subjects.
PURPOSE: To determine the factors that influence the satisfaction of the 'ISNT rule' (neural rim width: inferior ≥ superior ≥ nasal ≥ temporal) in normal and glaucomatous eyes. METHODS: The medical records of patients that visited Boramae Medical Center, Seoul, Korea, were reviewed. Each group of normal and glaucomatous eyes was divided into subgroups based on whether or not they satisfied the ISNT rule. ISNT rule assessment was performed by measuring the rim width with stereoscopic optic disc photographs using ImageJ software. Logistic regression analysis was performed to determine the factors that affect ISNT rule satisfaction. RESULTS: Seventy-seven normal eyes and 97 glaucomatous eyes were included in the study. The ISNT rule was intact in 59 (76.6%) of the normal eyes and was violated in 71 (73.2%) of the glaucomatous eyes. Logistic regression analysis revealed a significant influence of axial length in violation of the ISNT rule in the normal eye group, while the mean deviation value was a significant factor for violation of the ISNT rule in the glaucomatous eye group. CONCLUSIONS: The ISNT rule should be cautiously applied when evaluating normal eyes with long axial lengths. In addition, the ISNT rule might not be as effective for detecting early glaucoma.
Purpose:: We determined the accuracy of the inferior > superior > nasal > temporal (ISNT) neuroretinal rim area rule and its variants in adult Asian populations, and evaluated whether disc area impacts its performance characteristics. Methods:: Participants in the Singapore Malay Eye Study (SiMES) and Singapore Indian Eye Study (SINDI) underwent standardized ocular examinations, including optic disc imaging with the Heidelberg retinal tomograph (HRT). Glaucoma was defined using the ISGEO criteria. HRT rim areas in the superior, inferior, nasal, and temporal quadrants were quantified. We determined sensitivity, specificity, and positive (PPV) and negative (NPV) predictive values of violating the ISNT rule and 4 variants (I > S > T, I > S, I > T, and combined I > T and S > T). The influence of disc area was analyzed with multivariate marginal logistic regression. Results:: There were 6112 participants (mean age: 57.6 ± 10.3 years). Glaucoma was present in 194 individuals (3.2%). Among 11,840 eyes, 232 (93.2%) of 249 glaucomatous eyes and 9768 (84.3%) of 11,591 nonglaucomatous eyes, violated the ISNT rule. The ISNT rule had highest sensitivity (93.5%), but lowest specificity (15.7%); I > T had highest specificity (98.2%), but low sensitivity (7.4%). For all variants, PPVs were low (2.1%–8.4%) and NPVs were high (97.9–99.1%). Larger disc area was associated with reduced specificity for the ISNT rule (P < 0.001), and reduced sensitivity (P = 0.01) and increased specificity for I > S > T (P < 0.05). PPV increased (P < 0.05) and NPV decreased (P < 0.001) with increasing disc area. Conclusions:: The ISNT rule based on HRT has high sensitivity, and the I > T, S > T, and combined I > T and S > T variants have high specificity. Disc area influences sensitivity, specificity, PPV, and NPV of the ISNT rule and its variants. Translational Relevance:: The high sensitivity of the ISNT rule, and high specificities of its variants, may have potential utility when used in combination with other HRT algorithms for glaucoma assessment.
PURPOSE: To determine whether the ISNT rule (Inferior>Superior>Nasal>Temporal) or the "IST" rule (Inferior>Superior>Temporal) can be applied to the peripapillary retinal nerve fiber layer (RNFL) thickness as measured using Heidelberg Retinal Tomography (HRT) and Optical Coherence Tomography (OCT). MATERIALS AND METHODS: This was a cross-sectional study of 189 normal and 42 glaucomatous eyes. RNFL thicknesses measured in different quadrants using HRT and OCT were compared to determine the percentage of eyes obeying the ISNT and IST rule. RESULTS: The HRT-measured mean RNFL thickness in normal eyes showed that 25.9% obeyed the ISNT rule and 70.4% conformed to the "IST" rule. The "IST" rule was able to identify normal eyes better (P=0.040), but had a poor sensitivity (45%) and specificity (70%) to diagnose glaucoma. The OCT-measured average RNFL thickness showed that 47.1% of normal eyes obeyed the ISNT rule and 58.7% conformed to the "IST" rule. Exclusion of the nasal sector also increased the number of glaucomatous eyes conforming to the IST rule (31% obeyed the ISNT rule and 50% obeyed the IST rule). Sensitivities and specificities of the ISNT and the IST rules for OCT-quantified RNFL ranged from 42% to 77%. CONCLUSIONS: A larger number of normal eyes obeyed the IST rule compared with the ISNT rule for the RNFL thickness measured by HRT and OCT. Exclusion of the nasal sector from the analysis (IST rule) marginally improved the specificity in diagnosing glaucoma at the cost of the sensitivity, making neither of these parameters (ISNT and IST) likely to be useful clinically.
PURPOSE: We evaluated the applicability of the ISNT rule using Bruch's membrane opening minimum rim width (BMO-MRW) in healthy eyes and eyes with normal tension glaucoma (NTG). MATERIALS AND METHODS: In total, 124 healthy eyes and 136 NTG eyes were analyzed. Using 2-dimensional disc photographs, neuroretinal rim (NRR) thickness was measured at the superior, inferior, nasal, and temporal sides of the optic disc. Using spectral domain-optical coherence tomography, BMO-MRW was measured at the same regions. We compared the applicability of the ISNT rule in healthy and NTG eyes between these 2 methods. If the NRR was not clearly distinguished on disc photographs, the eye was classified into the "indistinguishable NRR" group and we only tested applicability of the ISNT rule using BMO-MRW. RESULTS: The specificity of "violation of the ISNT rule" for the diagnosis of glaucoma was higher when BMO-MRW was used (66.3%) than using disc photographs (42.2%), whereas the sensitivity did not show a significant difference between the 2 methods (91.7% vs. 86.5%). Compared with eyes with distinguishable NRR (179 eyes), eyes with indistinguishable NRR (81 eyes) had higher axial length, more negative refractive error, and higher tilt ratio (P<0.001). The diagnostic ability of "violation of ISNT rule" using BMO-MRW was not significantly different between eyes with indistinguishable NRR and those with distinguishable NRR (P>0.05). CONCLUSIONS: Application of the ISNT rule using BMO-MRW shows superiority in distinguishing between healthy and glaucomatous optic discs compared with disc photographs. The ISNT rule can be applied using BMO-MRW even when NRR is indistinguishable on disc photographs, such as in myopic tilted discs.
OBJECTIVE: To determine whether the ISNT rule (that normal eyes show a characteristic configuration for disc rim thickness of inferior > or = superior > or = nasal > or = temporal), widely used for clinical evaluation of the optic nerve head, can differentiate normal from glaucomatous eyes. METHODS: All subjects underwent complete eye examination, including achromatic automated perimetry, simultaneous stereoscopic disc photography, and confocal scanning laser ophthalmoscopy. Subjects with normal eyes had no evidence of glaucoma or ocular hypertension and had normal perimetry measurements. Subjects with glaucoma had a reproducible visual field defect. One eye from each subject was randomly enrolled. The ISNT rule was assessed by masked evaluation of disc photographs at the 3, 6, 9, and 12 o'clock positions. RESULTS: Sixty-six subjects with normal eyes (33 black and 33 white individuals) and 43 with open-angle glaucoma (15 black and 28 white individuals) were enrolled. The ISNT rule was intact in 52 (79%) of 66 normal eyes and 12 (28%) of 43 glaucomatous eyes (P<.001). Multiple logistic regression indicated that the odds ratio for glaucoma associated with violation of the ISNT rule was 6.04 (95% confidence interval, 1.74-20.95) after adjustment for age; race was not a confounder of this association. CONCLUSION: The ISNT rule is useful in differentiating normal from glaucomatous optic nerves and is unaffected by race.
PURPOSE: To evaluate the efficacy of the ISNT rule in normal eyes of Koreans. METHODS: We retrospectively reviewed medical records of 890 subjects with fundus photographs and evaluated the presence of the cup and ISNT rule satisfaction. If the ISNT rule was violated, the quadrants in which the neuroretinal rim was thinnest and thickest, respectively, were evaluated. RESULTS: Among 890 eyes, 84.7% showed the cup. The subjects without the cup were significantly younger and more hyperopic. Among 754 eyes with the cup, 53.5% showed ISNT rule satisfaction. In 351 eyes violating the ISNT rule, the most common quadrant showing the thickest neuroretinal rim was the inferior (65.5%), and the thinnest, the temporal (98.3%). CONCLUSION: The cup was absent in 15% of the assessed eyes. Only about half of eyes with the cup showed ISNT rule satisfaction. Even in eyes violating ISNT rule, the inferior was the most common quadrant with the thickest neuroretinal rim and the temporal, the thinnest.
BACKGROUND: To report the applicability of ISNT (inferior>superior>nasal>temporal) and IST (inferior>superior>temporal) rules on the retinal nerve fibre layer (RNFL) using spectral domain optical coherence tomography (SD-OCT) for detecting early glaucoma. METHODS: A prospective, cross-sectional study which included 80 eyes of 80 normal subjects and 76 eyes of 76 patients with early glaucoma by Hodapp-Anderson-Parrish classification. All subjects were of age more than 18 years, best corrected visual acuity 20/40 or better and a refractive error within ±5 dioptres (D) sphere and ±3 D cylinder. Control subjects had a normal ocular examination, intraocular pressure <22 mm Hg, no past history of high intraocular pressure, no family history of glaucoma, normal optic disc morphology and visual field. All eyes underwent SD-OCT examination for RNFL analysis. The sensitivity, specificity and likelihood ratio for violation of ISNT and IST rules was calculated for early glaucoma diagnosis. RESULTS: The ISNT rule was followed by 44 (55%) normal and 28 (36.84%) early glaucoma eyes. The IST rule was followed by 48 (60%) normal and 40 (52.63%) early glaucoma eyes. The sensitivity/specificity for violation of ISNT and IST rules for early glaucoma diagnosis was 63.2%/55% and 47.4%/60% respectively. The positive/negative likelihood ratio for ISNT and IST rules was 1.4/0.67 and 1.2/0.88, respectively, for diagnosing early glaucoma. CONCLUSIONS: Even though useful during ophthalmoscopy, ISNT and IST rules by themselves don't clearly distinguish normal eyes from those with glaucoma when applied to the quadrant values on RNFL on SD-OCT examination.
Abstract The power sector in many developing nations faces challenges in meeting consumers’ demands for a reliable electricity supply due to increased load demand, primarily driven by population growth. Urban areas receive 24 h power, while extensive losses within the system limit coverage. Setting up centralised stations offers a temporary solution, but the capacity of the ageing transmission lines is uncertain. This study proposes integrating distributed generators (DG) into the power system, using Inherent Structural Network Topology (ISNT) for DG siting and a modified Forward-Backward Sweep model for sizing. Voltage Stability Index (VPI) assesses network stability. The model considers voltage profile and line losses, optimizing DG sites and sizes. Results demonstrate the model’s efficacy, offering insights for optimal DG planning to minimize losses and enhance voltage profiles. The study informs power system engineers for future planning, aiding decisions on DG location and size, potentially reducing line losses and improving voltage profiles, thus assisting in network upgrades or expansions.
AIMS: To evaluate the accuracy of the ISNT rule (I=inferior, S=superior, N=nasal, T=temporal) and its variants with neuroretinal rim width and retinal nerve fibre layer (RNFL) thickness measurements differentiating normal from glaucomatous eyes. METHODS: The diagnosis accuracy of the ISNT rule and its variants was evaluated in a population-based study. Neuroretinal rim widths were measured on monoscopic optic disc photographs with an image-processing program. RNFL thickness measurements were obtained with spectral-domain optical coherence tomography (SD-OCT). RESULTS: In this study including 940 normal subjects and 93 patients with glaucoma, the sensitivity of the ISNT rule with optic disc photographs was 94.1% (95% CI 90.2 to 98.1), whereas its specificity was 49.2% (46.9 to 51.6). When using the IST rule, the sensitivity decreased to 69.9% (62.1 to 77.6) with a higher specificity, 87.0% (85.3 to 88.6). All the diagnosis indicators were somewhat lower for the different rules using RNFL thickness: the sensitivity of the ISNT rule was 79.4% (72.6 to 86.2) and its specificity was 34.1% (31.9 to 36.4). With the IST rule, the sensitivity decreased to 50.0% (41.6 to 58.4) while the specificity increased to 64.9% (62.7 to 67.2). CONCLUSIONS: The ISNT and IST rules applied to neuroretinal rim width measurement by optic disc photographs are useful and simple tools for differentiating normal from glaucomatous eyes. The translation of these rules to RNFL thickness by SD-OCT is of limited value.
The purpose this study was to investigate the association between intraocular pressure (IOP) and the outcome of ISNT rule assessment of the optic disc in healthy patients seen in mainstream optometric practice. Consecutive patients over the age of 40 years and with no known general- or ocular pathology were enrolled from 4 optometric practices in Norway and Sweden during a period of 9 months. The sample was stratified by age group (40-49, 50-59, 60-69 and 70-79 years) and ISNT rule outcome (obeyed or broken). Subjects underwent a full optometric examination including stereoscopic assessment of the optic disc with the use of the ISNT rule, followed by measurements of intraocular pressure (IOP), central corneal thickness (CCT) and vertical size of the optic disc. Odds ratio (OR) was used to assess the association between IOP and the outcome measure. Stratified and multivariable analyses were employed to investigate the effect of age, gender, CCT, refractive error and disc size on the association. IOP was dichotomized at a cut-off corresponding to the 75th percentile (18 mmHg), whereas numeric covariables were categorized at cut-offs corresponding to the 33rd (ONH 1.6 mm, CCT 545 μm) and 66th percentile (ONH 1.8 mm, CCT 571 μm), respectively. A total of 177 patients (F: 57%, age range 40-79 years) were included. IOP was significantly higher in patients with broken ISNT rule (16.6 vs. 15.5 mmHg, p = 0.046), corresponding to a crude OR of 2.0 (CI 0.95-4.24). In stratified and multivariable analyses the association between IOP and outcome of the ISNT rule was strengthened in males (OR 4.8) and in subjects with small discs (OR 2.2), however only the effect modification by gender reached significance. Further research is necessary to understand the possible implications of these findings on the assessment of optic discs with the use of the ISNT rule.
PURPOSE: To determine whether nonglaucomatous optic disc cupping in children violates the ISNT rule (which states that for normal optic discs the neuroretinal rim width is greatest in the order inferior >or= superior >or= nasal >or= temporal). METHODS: Digital ocular fundus photographs from a random cohort of children with large optic disc cups of nonglaucomatous origin were analyzed in masked fashion by using computer graphic software. The diameter and perimeter of each optic disc and optic cup and the width of the neuroretinal rim were drawn and measured. Measurements were compared to a random cohort of normal pediatric optic discs. RESULTS: The ISNT rule was intact in 9 (16%) of 55 eyes of nonpremature children with nonglaucomatous cupping, in 6 (21%) of 28 eyes of children with a history of prematurity and nonglaucomatous cupping, and in 35 (73%) of 48 eyes with normal discs. CONCLUSIONS: Violation of the ISNT rule occurs with greater frequency in the pediatric population with large optic disc cups of nonglaucomatous origin, compared with the pediatric population with normal optic discs. In discs with small cups, neuroretinal rim width conforms to the overall oval shape of the disc, which is usually greatest in vertical dimension, whereas discs with large cups possess greater variability of relative neuroretinal rim width around the disc, greater relative vertical cup/disc ratio versus horizontal cup/disc ratio, and lower predictability of the ISNT rule.
PURPOSE: The study investigated the applicability of the inferior≥superior≥nasal≥temporal neuroretinal rim thickness (ISNT) rule to rim thickness measured using Cirrus high-definition spectral-domain optical coherence tomography (Cirrus HD-OCT) in healthy eyes and eyes with glaucoma. METHODS: Rim thicknesses of 4 quadrants were investigated using the rim thickness curve of Cirrus HD-OCT for 40 eyes with mild glaucoma (mean deviation ≥ -6 dB), 40 eyes with moderate to advanced glaucoma (mean deviation < -6 dB), and 80 healthy eyes (control group). The relative amount of rim loss in each quadrant in the glaucoma group compared with the control group was calculated. The ISNT rule was considered intact if there was a gradual decrease in rim thickness in the order of inferior≥superior≥nasal≥temporal quadrants. RESULTS: The ISNT rule was intact in 13%, 18%, and 8% of eyes in the control, mild, and moderate to advanced glaucoma groups, respectively, and the distribution was not significantly different among the 3 groups (P>0.05). Relative rim loss in the inferior quadrant of the moderate to advanced glaucoma group (63.8%) was greater than that of the mild glaucoma group (53.6%; P=0.010), whereas rim loss in other quadrants was not significantly different between the 2 groups (P>0.05). CONCLUSIONS: The ISNT rule with rim thickness measured using Cirrus HD-OCT has limited clinical value in the diagnosis of glaucoma. There is a topographic difference in inferior rim loss according to the stages of glaucomatous damage.
PURPOSE: To evaluate the ability of the neuroretinal rim (NRR) rules determined using spectral domain optical coherence tomography (SDOCT) in diagnosing glaucoma and to study the effect of optic disc size and disease severity on the diagnostic ability of these rules. METHODS: In a cross-sectional study, 125 eyes of 96 glaucoma patients and 96 eyes of 72 control subjects underwent optic nerve head (ONH) imaging with SDOCT. Inferior (I), superior (S), nasal (N) and temporal (T) NRR areas were automatically determined by the sdoct software. Diagnostic abilities of ISNT (I > S > N > T), IT (I > T) and ST (S > T) rules in glaucoma were evaluated using sensitivity, specificity and likelihood ratios (LR). Effect of optic disc size and disease severity [based on mean deviation (MD) on visual fields] on the diagnostic ability of the NRR rules was evaluated using regression models. RESULTS: Sensitivities of ISNT, IT and ST rules were 80.8%, 60.0% and 29.6%, respectively, and the specificities were 32.3%, 84.4% and 93.8%, respectively. Positive LRs of ISNT, IT and ST rules were 1.19, 3.84 and 4.74, respectively, and negative LRs were 0.60, 0.47 and 0.75, respectively. Sensitivities of ISNT (coefficient: -1.06, p = 0.02) and IT (-0.71, 0.05) rules decreased with increasing disc size. Positive LR of IT rule increased significantly (-0.01, 0.04) with decreasing MD, and negative LR of IT rule decreased (got better) significantly (0.26, 0.05) with decreasing disc size. CONCLUSIONS: Neuroretinal rim rules, as determined by SDOCT, do not allow robust differentiation of glaucomatous from non-glaucomatous discs.
Retinal fundus image processing paves the way for diagnosis of many eye disorders. The fundus image features such as optic disc, optic cup, optic rim and blood vessels help to analyse the pathological condition of eye. The analysis involves calculation of ocular parameters such as Rim to Disc Ratio (RDR) and Inferior Superior Nasal Temporal (ISNT) Ratio and verification of ISNT Rule. For this analysis the fundus image features have to be segmented into ISNT quadrants. This paper presents a novel approach towards mask image generation for segmenting fundus image feature into ISNT quadrants using array centroid method. The existing method uses a predefined mask image which is not accurate and the proposed array centroid method overcomes the limitations of existing method. The method involves centroid calculation, array initialization; mask image generation and, mask-feature image multiplication. The proposed method is applied to the fundus images collected from a reputed eye hospital.
Glaucoma is a common eye disease that damages an optic nerve due to abnormally high pressure inside the eye. Glaucoma can cause visual impairments and eventually lead to blindness. There is no appropriate treatment to prevent blindness when the optic nerve is damaged. Therefore, an early diagnosis is important to prevent vision loss from glaucoma. An automated framework for glaucoma screening from fundus images is advantageous. It can facilitate the ophthalmologist in the diagnosis and prevent blindness. Many glaucoma screening algorithms have been developed based on a clinical indicator, the cup-to-disc ratio (CDR). However, these algorithms have some limitations for myopia and genetically large optic cup eyes. Therefore, this paper proposes a framework for glaucoma screening that can be applied even in myopia. The 2 clinical indicators, cup-to-disc ratio (CDR) and neuroretinal rim area rule (inferior > superior > nasal > temporal (ISNT)), are applied in the proposed screening algorithm for accurate glaucoma assessment. Moreover, the automatic classification of glaucoma or non-glaucoma from fundus images is performed by a support vector machine (SVM). Therefore, the experimental results show that the proposed screening algorithm can accurately classify glaucoma to normal eyes or myopic eyes.
 HIGHLIGHTS
 
 A framework for glaucoma screening is proposed to classify myopia and genetically large optic cup eyes
 Two clinical indicators, cup-to-disc ratio (CDR) and neuroretinal rim area rule (inferior > superior > nasal > temporal (ISNT)), are applied in the proposed screening algorithm for accurate glaucoma assessment
 Automatic classification of glaucoma or non-glaucoma from fundus images is performed by a support vector machine (SVM)
 The proposed framework can provide less time consuming, cost-effective and physician independent system for glaucoma screening
 
 GRAPHICAL ABSTRACT
We investigated apoptosis in tumor-infiltrating lymphocytes (TILs) obtained from 41 colorectal carcinomas by in situ nick translation (ISNT). When the ISNT labeling index (LI) was determined as the number of positive nuclei per 1000 nuclei of TIL in tissue sections, the median LI was 12.0 (range, 2-30). The ISNT LI of colorectal carcinoma with lymph node metastasis was higher than that of colorectal carcinoma without metastasis. The cases with a high LI of 212.0 had a significantly poorer prognosis than those with a low LI. We also confirmed immunohistochemically that a part of the TILs expressed Fas using the sections adjacent to what contained abundant ISNT-positive TILs. Moreover, Fas ligand (FasL) expression was detected on the cell surface as well as the cytoplasm of colorectal cancer cells in 61% of cases. Apoptosis in TILs was consistently seen more frequently in FasL-positive cases than in FasL-negative ones. These findings indicate that the FasL expressed in colorectal carcinoma cells may kill the Fas-positive immune effective TILs by means of a Fas-FasL system termed Fas counterattack. This tumor immune evasion induced by FasL may therefore affect the malignant potential of human colorectal carcinoma.
The Illinois soil nitrogen test (ISNT) has shown promise as a predictor of corn ( Zea mays L.) response to N fertilization in several field trials, but controversy has arisen regarding whether the test detects a particular class of organic N compounds or a constant fraction of the organic N in agricultural soils (SON). The goal of this study is to elucidate the chemical properties of the ISNT‐labile N and its relationship to microbial growth in agricultural soils. In the ISNT, NH 3 generated during alkaline hydrolysis of whole soil samples is separated from the hydrolysate by diffusion and quantified by trapping and acidimetric titration. Analyses of pure organic N compounds confirmed that the ISNT detects the N in some components of microbial biomass, such as the monomeric amino sugars (AS) of bacterial cell walls (95% recovery) and amides (AM, 55% of monomers), but not α amino acids (αAA) or chitin (CTN), a refractory AS polymer. Because the ISNT detects different proportions of these N containing compounds than any other conventional acid hydrolyzable SON fraction, ISNT‐N can be considered a fifth fraction that together with the others permits one to distinguish AS of bacterial and fungal origin. Using this approach, the average proportions of SON components observed in 10 manured (and 16 non‐manured) agricultural soils from Illinois are 18% (10%) CTN, 11% (5%) bacterial amino sugars (BAS), 10% (17%) AM, 29% (25%) αAA, and 15% (20%) unknown. The reason that the sum of CTN and BAS is 50% greater than the conventional AS N fraction (AS‐N) of these soils—18% (9.5%) of SON—is the lack of recovery correction conventionally made to AS‐N data. To investigate N fractions in living biomass, we conducted studies of 15 NH 4 + immobilization in two fresh soils. ISNT‐N was more rapidly labeled than AS‐N, a fact that is indicative of more rapid labeling of bacteria than fungi. Taken together, these findings suggest that the effectiveness of the ISNT for predicting corn N response is due largely to its ability to quantify N in BAS. Because ISNT also detects some AM‐N, the BAS‐N signal may be obscured in soil N datasets where these components are highly variable.
The Illinois Soil Nitrogen Test (ISNT) is a recently developed test that has shown promise for corn ( Zea mays L.) N fertilization. However, among‐field and within‐field spatial variability of this test is unknown and sampling recommendations have not been defined. The objectives of this study were to estimate the spatial variability and structure of the ISNT to provide sampling recommendations and to determine the short‐term effect of N fertilizer on this test. Soil samples were collected on 14 production fields and analyzed for the ISNT. Data was analyzed with traditional statistics and with geostatistical techniques. In the short term, N fertilization did not have a significant effect on the ISNT ( p > 0.3). The ISNT was normally distributed in 12 fields and showed a relatively low coefficient of variation (18% on average). Field mean ISNT ranged from 98 to 255 mg kg −1 and tended to be higher in Mollisols than in Alfisols. On average across fields, at least 10 samples are required to determine the mean ISNT for a given field with a precision of 24 mg kg −1 . The geostatistical analysis revealed that the ISNT has strong spatial structure, as indicated by the bounded variograms, the relatively low nugget/sill ratio (<30% in 10 fields) and the mean range of 150 m. The spatial variability indicates that the ISNT can be mapped with a relatively sparse sampling grid and with a low number of samples, critical characteristics when assessing the economic implication for variable and uniform rate N fertilization.
Recently the Illinois soil nitrogen test (ISNT) was proposed as a means to identify fields where corn ( Zea mays L.) will not respond to additional N fertilizer and which may also be used to predict the economic optimum N rate (EONR). Data from 96 corn N rate response trials across Iowa, Illinois, Michigan, Minnesota, Nebraska, and Wisconsin were compiled to evaluate the usefulness of the ISNT in identifying nonresponsive fields, predicting EONR, and estimating mineralizable N. At each trial site, multiple rates of fertilizer N were applied, including zero N and nonyield limiting rates. Corn was grown following several crops. The ISNT could not accurately predict nonresponsive sites, nor could it reliably estimate EONR. Subsetting the data based on soil drainage class and previous crop did not improve the predictive capability of the ISNT even though ISNT values were significantly different among previous crops and soil drainage classes. The ISNT was strongly correlated to soil organic matter (OM) and was apparently measuring a constant fraction of total soil N (TN). The lack of correlation between the ISNT and relative N uptake (check plot N uptake/N uptake at the maximum N rate) suggests that the ISNT is not measuring the readily mineralizable fraction of soil N. Based on results of this project, the ISNT is not suggested for use in adjusting N rate recommendations for corn in the North Central Region (Corn Belt) of the United States.