ESTABLISHMENT OF CRITICAL THRESHOLD VALUES OF CHLOROPHYLL METER TO GUIDE NITROGEN FERTILIZATION IN WHEAT

ESTABLISHMENT OF CRITICAL THRESHOLD VALUES OF CHLOROPHYLL METER TO GUIDE NITROGEN FERTILIZATION IN WHEAT

Sherif M. Ibrahim

information Department of Soil Fertility and Microbiology,

Desert Research Center, Cairo, Egypt

Key Words: Chlorophyll meter; Critical values; Nitrogen fertilization; Wheat

ABSTRACT

Rapid acquisition of about nitrogen (N) in-season using gadgets like chlorophyll meters is promising to achieve high N fertilizer use efficiency. Two field experiments were conducted in two successive seasons (2017/2018-2018/2019) on wheat grown in a calcareous soil at West Nile Delta to define and validate critical threshold chlorophyll meter values to guide N fertilization. An increasing rate of N fertilizer was applied in the experiment conducted in the first season to establish plots with different yield potentials and to create variability in chlorophyll meter readings determined at stem elongation growth stage of wheat.The data revealed that an exponential model based on the chlorophyll meter could explain 54% of the variation in the aboveground N uptake. Cate-Nelson graph method was then used to define the critical threshold values of chlorophyll index at 95, 70 and 50 % relative grain yield of wheat. The defined chlorophyll meter values were 58.9, 54.5 and 53.2, respectively. Accordingly, a strategy to refine N application dose was suggested to be applied at stem elongation stage of wheat as guided by the meter in the second season. When appropriate prescriptive N fertilizer was applied (40 kg N fed^-1 in two unequal splits) followed by corrective dose as guided by the chlorophyll meter, the achievedN recovery efficiency was 67% compared with 53.8% by the recommendation rate (100 kg N fed^-1). This study revealed that N fertilizer could be managed more efficiently using rapid and hand-held tools like chlorophyll meter compared with the current general recommendation.

INTRODUCTION

Wheat (Triticum aestivum L.) is an important crop for Egypt’s food security. To support intensive wheat production systems in Egypt, fertilizer nitrogen (N) use has noticeably increased in recent years. In addition,farmers tend to apply N fertilizer even higher than the general recommendation to ensure high yield. In 2014-2015, 22.8 % of the total N fertilizer consumed in Egypt was applied to wheat only(Heffer et al., 2017). In some newly reclaimed lands of Egypt, N recovery efficiency has been found to be 35.6 to 51.1 % when fertilizer N was applied following the general recommendation (Ali et al., 2017). Apart from high cost, applying doses of N fertilizer higher than the optimum may also adversely affect the environment (Bijay et al., 2003;Fageria and Baligar 2005).In fact, N is well documented as the most difficult plant nutrient to manage because of the large number of transformation and loss pathways. The loss pathways from the soil include denitrification, volatilization, surface runoff and leaching (Raun and Johnson, 1999).

Precision N management strategies that employ the concept of synchronizing N supply with crop N demand hold promise in improving N fertilizer use efficiency and minimizing losses of N from the soil-plant system to the environment (Diacono et al., 2013).However, an important element in precision N fertilizer management strategies is the use of diagnostic tools, which can assess the N need of crop plants in-field. In recent decades, a number of non-destructive methods have been developed, which use leaf greenness, absorbance, and/or reflectance of light by the intact leaf to quantitatively estimate the leaf chlorophyll content.  

There exists a strong correlation between leaf N concentration and chlorophyll content (Schepers et al., 1992 and Schlemmer et al., 2005). The hand-held chlorophyll meters  provide a quick and non-destructive method for estimating leaf chlorophyll content in the field. The meter can be used to assess N status of the crop, to make N fertilizer recommendations and diagnose N distribution problems (Noulas et al., 2018). The chlorophyll meter has been used in managing N fertilizer following two approaches. The first approach is the sufficiency index (ratio of the meter value of the as-needed treatment and the well fertilized treatment), and when its value falls below a certain set (mostly 0.95 or 0.90) a prescribed dose of N fertilizer is applied (Varvel et al., 1997; Hussain et al., 2000; Francis and Piekielek, 2019). The second approach consists of applying a N fertilizer dose whenever chlorophyll meter value is less than a critical threshold value (Peng et al., 1996;Bijay et al., 2002;Ali et al., 2015).

The objectives of this study were i) to predict N uptake at stem elongation growth stage of wheat usingchlorophyll meter, and ii) to define and validate critical threshold chlorophyll meter values to guide N management in order to obtain high yield levels along with increased use efficiency of N fertilizer.

MATERIALS AND METHODS

The experimental site

Field experimentwas conducted on wheat (Triticum aestivum L.) variety Giza 171 at Mariout Research Station, Desert Research Center in two successive seasons (2017/2018-2018/2019). This area has relatively moderate temperatures, with 31 °C as average in the summer. Only 100–200 mm of rain falls during an average year, and most of this falls in the winter months. The maximum average of 34 °C is recorded as the hottest temperatures in July and August. Winter temperatures are normally in the range of 9 °C at night to 19 °C at day. Initial soil samples collected from the experiments were mixed, air dried, ground, sieved and analyzed for physical and chemical characteristics as reported in Table (1).

Table 1. Some physical and chemical properties of the topsoil (0-30 cm) of the experimental sites.

^* pH in saturated soil paste.

^** Electrical conductivity in saturated soil paste extract.

The analyses were done using the procedures outlined by Page et al. (1982) as: Soil texture using the pipette method, pH in saturated soil paste, EC in soil paste extract, CaCO₃ using calcimeter, Organic matter using the procedure of Walkely and Black, Available N was extracted by 2 M KCl solution according to Dahnke and Johnson (1984) and N in the extract was estimated by digestion and Kjeldahl method, Available P and K were extracted by 1 M NH₄HCO₃ in 0.005 M DTPA adjusted to a pH of 7.6 (Soltanpour, 1991) and then determined using spectrophotometer and flamephotometer, respectively.

Experimental design and treatments

Two field experiments were established for the purpose of this study. The first season experiment consisted of N fertilizer levels of 0, 20, 40, 60, 80, 100, 120, 140, kg fed^-1 applied as ammonium nitrate (33.5 % N) in three equal split doses at sowing, 30 days after sowing (DAS) and 55 DAS for Giza 171 wheat cultivar. The experiment were laid out in a randomized complete block design with four replications. The purpose of applying a range of N fertilizer rates was to establish plots with large variability in N uptake and yield of wheat. Wheat was sown in early-mid November by hand. Phosphorus (as single superphosphate 15.5 % P₂O₅) and potassium (as potassium sulfate) were applied at sowing following the general recommendation.

The second season was designed in order to validate the critical threshold values of chlorophyll meter for fine tuning N fertilizer application. The treatments consisted on applying different scenarios of prescriptive N application at sowing and 30 DAS, following by corrective dose as guided by the chlorophyll meter. The experiment was conducted in randomized complete block design with three replications.

Chlorophyll meter measurement

Chlorophyll meter (atLeaf model) was used to assess chlorophyll index for the purpose of this study. The chlorophyll meter readings were obtained by inserting the middle portion of the index leaf in the slit of the meter. The stunted, diseased or insect affected plants were avoided for measurements. Readings from ten randomly selected plants was collected from each plot and the average was recorded.

Plant sampling and analysis

Aboveground plant samples were collected at stem elongation growth stage from an area of 1 m² from each plot after avoiding the border rows. At maturity, wheat crop was harvested manually. Grain and straw samples collected from each plot were dried in hot air oven at 70º C and ground. Grain yields were adjusted to 14% moisture for reporting. The samples were digested in H₂SO₄ – H₂O₂ mixture and total N was determined by micro-Kjeldahl method (Kalra, 1997).

Calculations and statistical analysis

Regression analysis was performed using Statistical Product and Service Solutions (SPSS 18.0). The analysis of variance (ANOVA) was used to determine the effects of N treatments on the generated data. Duncan’s multiple range test (DMRT) was used to test the differences between means at p < 0.05 as described by Gomez and Gomez (1984).The Cate-Nelson graphic method (Cate and Nelson, 1965) was used to define threshold values of the chlorophyll meter reading. The graph was used to define the critical threshold values at 95, 70 and 50% relative grain yield. The recovery efficiency of N (RE_N) as described by Cassman et al. (1998)was computed as:

RESULTS AND DISCUSSION

Wheat grain yield as affected by N fertilizer rates

Wheat grain yield data from the first season were averaged and plotted against the increasing fertilizer N application rates (Fig.1). As shown from the curve, the relation exhibited a quadratic response function. Analysis of the function revealed that grain yield reached a plateau at about 3500 kg fed^-1 by fertilizer N application rate of 120 kg fed^-1. The N fertilizer rate required to obtain economic grain yield, defined as 95 % of the maximum yield of 3300 kg fed^-1was computed to be around 90 kg fed^-1. The general recommendation of N fertilizer for wheat grown in the study area is 100 kg fed^-1. Nevertheless, in the quest of obtaining high yield levels, farmers in this area generally apply N fertilizer levels higher than the general recommendation, which implies that unnecessary amounts of N fertilizer are being applied leading to low N fertilizer use efficiency. Apart from susceptibility of the excess N fertilizer to be lost from the soil-plant system to the environment where it is not required it can also lead to deterioration of soil health (Bijay, 2018). The data from the field experiments suggest that there is need to develop and follow site-specific management strategies based on synchronizing N fertilizer supply with N demand of the crop and supply of N from the soil from all sources other than fertilizer.

Figure 1. Response of wheat to the increasing rate of N fertilizer fitted to quadratic function.

Prediction of N uptake using chlorophyll meter

Rapid prediction of N uptake where plant can response to N inputs prior to harvest is important for developing successful N fertilizer management strategy. Application of an increasing rate of N fertilizer in the first season created variability in N accumulated and biomass at stem elongation stage. These variabilities were reflected in increments in chlorophyll meter values. Different regression models were tested, and the highest R² value was found for the exponential model (Fig. 2). The R² value for the relationship between N uptake at stem elongation stage and chlorophyll meter was 0.54. This relation was found to be highly significant (P> 0.001) and 54 % of the variation in N uptake at stem elongation growth stageof wheat can be then explained by the chlorophyll meter. The N uptake at this stage can be estimated using the equation: N uptake (kg fed^-1) =

In-season monitoring of N uptake is essential for setting up strategies to optimize N fertilization and reduce the environmental risks associated with fertilizer N use. Inadequate prediction of N uptake may result in over- or under-recommendation of N fertilizer than the required (Yao et al., 2012). Several studies have shown that N status and grain yield of many crops can be estimated from spectral measurements of leaves (Varvel et al., 1997;Raun et al., 2001;Ali et al., 2014). In fact, portable sensors like chlorophyll meter have opened up a new approach to acquire crop growth information in rapidly and in a non-invasive manner to make in-season decisions.

Figure 2. Relationship between chlorophyll meter values and total N uptake at stem elongation growth stage of wheat fitted to power function.

Establishment of threshold values of chlorophyll meter

The Cate-Nelson graph method was used to define the critical chlorophyll index value at relative grain yield of 95, 70 and 50 % (Fig. 3). These value were suggested to apply an increament doses of N fertilizer at stem elongation stage. For instance, when predicting 95% of maximum grain yield, there is no need to apply N fertilizer and so on. Subsequently, applying medium and high doses of N fertilizer at 70 and 50% relative grain yield, respectively. The graph suggested that the critical values of chlorophyll index at 95, 70 and 50 % relative grain yield of wheat were 58.9, 54.5 and 53.2, respectively. Accordingly, a strategy to refine N application dose was suggested to be applied at stem elongation stage of wheat as guided by the chlorophyll meter (Table 2). The suggested strategy are applying 0, 40, 60 or 80 kg N fed^-1 corresponding to chlorophyll index values of more than 58.9, 58.9-54.5, 54.5-53.2 and less than 53.2. These ranges of N application doses were suggested to challenge the current blinded general recommendation that does not account for the variation in field-to-field soil properties and other management practices that effect the N fertilizer need. 

Figure 3. The Cate-Nelson graph of the relation between chlorophyll meter values and 95, 70 and 50 % relative grain yield of wheat.

Table 2. Chlorophyll meter index values as defined by the Cate-Nelson graph method and the suggested corrective doses of N fertilizer.

The chlorophyll meter readings at stem elongation of wheat in this study matched with Feekes 6 growth stage (around 50 days from sowing) and it is considered to be the appropriate stage to obtain information and make in-season corrective N fertilizer management decisions. For example, Raun et al. (2001) reported that relationships between optical sensors readings and grain yield of wheat were the highest between Feekes 4 and 6 stages. Also, Zhang et al. (2019) reported that leaf dry matter in wheat during Feekes stages 4 to 7 are more variable than other stages and agronomic information can be reliably obtained. Also, Bijay et al. (2011)and Varinderpal et al. (2017) reported that Feekes 6 stage of wheat in northwestern India is the appropriate stage to decide the amount of site-specific N fertilizer to be applied as topdressing.

Validation of the established critical values of chlorophyll meter

The experiment conducted in the second season was used to evaluate the performance of the critical threshold values of chlorophyll meter defined from the first season data. To create plant variability in biomass and N uptake at stem elongation stage of wheat, different doses and timings of N fertilizer were practiced before applying the corrective dose as guided by the chlorophyll meter.

The data listed in Table (3) show that grain yields obtained as guided by the chlorophyll meter is statistically higher than the other treatments. The meter-based N management overcame the heterogeneity in wheat growth caused by different prescriptive N management, but with using less N fertilizer amounts. For instance, applying a total of 40 kg N fed^-1 as prescriptive in two doses (15 and 25 kg fed^-1) then applying a corrective dose as guided by the chlorophyll meter at stem elongation growth stage resulted in grain yield of 3447 kg fed^-1. Data pertaining to N recovery efficiency show that the meter-guided N treatments results in higher use efficiency as compared to the general recommendation. When appropriate prescriptive N fertilizer was applied (40 kg N fed^-1 in two splits) followed by corrective dose, an average increase of 12.5 % RE_N compared with the general recommendation. Therefore, using the chlorophyll meter in guiding N management could effectively manage N fertilizer in order to obtain higher yield along with high N use efficiency.

Table 3. Wheat grain yields, total N uptake and N recovery efficiency as influenced by different N fertilizer treatments as guided by the chlorophyll meter.

^* RE_N = Recovery efficiency of nitrogen fertilizer

Means followed by the same letter are not significantly different within the same column at the 0.05 level of probability by Duncan’s multiple range test (DMRT).

CONCLUSIONS

The chlorophyll meter is rapid and reliable gadget for predicting N uptake of wheat from measurements made at stem elongation growth stage. By following exponential regression model, chlorophyll meter can explain 54% variability in N uptake at stem elongation growth stage of wheat. Thus, this gadget can be used reliably for managing N fertilizer in wheat on a field specific manner. The critical threshold values of chlorophyll index at 95, 70 and 50 % relative grain yield of wheat were 58.9, 54.5 and 53.2, respectively. Accordingly, a strategy to refine N application dose was suggested to be applied as guided by the meter. The suggested strategy efficiently used in managing N fertilizer, compared with the general recommendation, and resulted in an increased N recovery efficiency level.

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تأسیس القیم الحرجة لمقیاس الکلوروفیل کدلیل للتسمید النیتروجینی فی القمح

شریف محمود إبراهیم

قسم خصوبة ومیکروبیولوجیا الاراضی ، مرکز بحوث الصحراء ، القاهرة

الحصول على معلومات بشکل سریع عن حالة النیتروجین خلال الموسم بإستخدام بعض الادوات مثل مقیاس الکلوروفیل یعتبر منظور واعد للحصول على کفاءة عالیة من الاسمدة النیتروجینیة. تجربتین حقلیتین تم إجرائهم خلال موسمین متتابعین (2017/2018-2018/2019) على القمح النامی فی أرض جیریة بمنطقة غرب الدلتا بمصر لتحدید الحدود الحرجة من مقیاس الکلوروفیل لإرشاد التسمید النیتروجینی. تم إضافة معدلات متزایدة من النیتروجین فی الموسم الأول لتأسیس قطع تجریبیة ذات معدلات نمو مختلفة وذلک لعمل إختلافات فی قراءات مقیاس الکلوروفیل المأخوذة فی مرحلة الاستطالة للقمح. أوضحت النتائج ان المعادلة الأسیة استطاعت تفسیر 54% من التغیرات فی الکمیة الممتصة من النیتروجین. تم إستخدام طریقة الرسم البیانی لکات-نیلسون لتحدید الحدود الحرجة لمقیاس الکلوروفیل 95 ، 70 ، 50 % من المحصول النسبی للقمح. القیم التی تم تحدیدها هی 58.9 ، 54.9 ، 53.2 على التوالی. وبالتالی تم إقتراح إستراتیجیة لإضافة جرعة تصحیحیة من النیتروجین عند مرحلة الاستطالة فی الموسم الثانی. فعند إضافة 40 کجم فدان^-1 على دفعتین تتبعهم جرعة تصحیحیة وفقا لقراءة مقیاس الکلوروفیل، أمکن الحصول على کفاءة استرداد للنیتروجین بقیمة 67% بالمقارنة بقیمة 53.8% نتیجة للمعدل الموصى به (100 کجم ن فدان^-1). لهذا کشفت تلک الدراسة ان السماد النیتروجینی یمکن إدارته بشکل أفضل بإستخدام الأدوات المحمولة والتی تعمل بشکل سریع مثل مقیاس الکلوروفیل بالمقارنة بالموصى به.