Some Aspects of Nitrogen Management and Its Real Time Application in Direct Seeded Rice Using Leaf Color Chart

Proper application of nitrogen (N) fertilizer is vital to improve the growth and grain yield of rice crop. As there prevails more aerobic period in direct seeded rice, nitrogen loss is generally more in such environment. Therefore, nitrogen recommendation for direct seeded rice is slightly higher (22.5-30 Kg ha) than that under the transplanted rice. Insufficient and/or inappropriate nitrogen fertilizer application is highly critical to the crops. Optimal nitrogen management strategies aim at matching the nitrogen fertilizer supply to the actual crop demand. Leaf color is generally used as a visual and subjective indicator of the rice crop need for nitrogen fertilizer. The Leaf Color Chart is a simple and inexpensive tool for real time nitrogen management in rice. It helps farmers to improve their decision-making process in nitrogen management. It provides the idea of when and how much nitrogen fertilizer to apply based on relative greenness of the rice leaf. In overall, LCC based nitrogen management improves productivity and profitability of the rice crop by nitrogen saving and ensuring its higher use efficiency.


Introduction
Rice (Oryza sativa L.) is one of the most popular cereal crops in the world. It is a staple food for nearly half of the world's population. More than 90% of this rice is produced and consumed in Asia . Total global rice production was 472.96 million mt. from the area 159.44 million ha with the productivity of 4.4 mt ha -1 (USDA, 2018).
Puddled rice transplanting is the burdensome and time consuming crop establishment method with more labor and water requirement which are becoming scarce too. It destroys the soil physical properties by dismantling the soil aggregates and ultimately affects the growth and productivity of succeeding wheat crop (Bhurer et al., 2013). Pathak et al. (2011) reported that climate change may cause the variability of monsoon rainfall and also the risks of early season drought. So, direct seeded rice (DSR) can be an alternative method to conventional transplanted rice as it avoids puddling and transplanting of young seedlings and requires less water, labor, time, drudgery and cultivation cost (Ali et al., 2012). The direct seeded rice matures 7-10 days earlier than the transplanted rice and thus allows timely sowing of succeeding wheat crop which assists to improve the system productivity (Giri, 1988). Moreover, the grain yield obtained from the DSR is found comparable to that of Mini Review P. Subedi and S. Panta (2018) Int. J. Appl. Sci. Biotechnol. Vol 6(2): 81-86 This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT the transplanted rice if managed properly (Gill et al., 2014). Some of the major challenges found in dry direct seeded rice are high weed infestation (Joshi et al., 2013) and more nitrogen (N) loss through denitrification, volatilization, leaching and runoff than in conventional transplanted rice (Kumar and Ladha, 2011). Farmers generally do blanket application of nitrogen fertilizer in splits, which may produce optimum yields, but cannot help increase N use efficiency beyond a limit. Large field-to-field variability of soil nitrogen supply, agro-climatic conditions and varietal differences further lowers fertilizer N use efficiencies when broad-based blanket recommendations are used (Singh et al., 2010). Therefore, nitrogen management is considered as one of the most challenging parts of the direct seeded rice to achieve higher grain yield and nitrogen use efficiency (Ali et al., 2012).
Real time (also called need based) nitrogen management requires periodic assessment of nitrogen status in standing crop and its application according to the need of the crop (Witt et al., 2004). In this context, Chlorophyll meter (SPAD) or Leaf Color Chart (LCC) can be used to assess the actual plant nitrogen status (Balasubramanian et al., 1999). LCC provides the guideline for effective nitrogen management by giving the idea of when and how much nitrogen fertilizer to apply for maintaining and optimizing nitrogen status in rice plants to ensure high grain yield (Sathiya and Ramesh, 2009). So, this review article is prepared based on research findings related to need based N management using LCC in direct seeded rice.

Nitrogen Requirement for Rice Crop
Nitrogen is an essential nutrient for rice production and plays an important role in sustaining high yields (Liu et al., 2007). Nitrogen is required for chlorophyll formation which gives the leaves green color and enables the plants to gain energy for nutrient uptake and growth. The chlorophyll content of leaf determines the ability of plant to utilize solar radiation and is directly related to leaf nitrogen content, which depends on nitrogen uptake (Richardson et al., 2002). As there prevails more aerobic period in DSR, Nitrogen loss is generally more in such environment. Therefore, generally nitrogen recommendation for DSR is slightly higher (i.e 22.5-30 Kg ha -1 ) than that under the transplanting method although recommended P and K are same under both methods Kumar and Ladha, 2011;Gathala et al., 2011).

Nitrogen Loss from Rice Fields
The main nitrogen fertilizer used for rice in Asia is urea but is not used efficiently and the rice crop recovers only about 40% of applied nitrogen (De Datta, 1986). The unique condition of the rice fields can promote nitrogen losses through nitrification, denitrification, ammonia volatilization and leaching and that not only lead to decrease in nitrogen use efficiency but also to soil, water and atmospheric pollution (Zhao et al., 2010). The losses of NH3 from different upland and lowland cropping systems found to be from negligible amount to 50 % of total nitrogen applied depending upon the method of its application and environmental conditions (Keller and Mengel, 1986). Yu et al. (2013) found that 23.9 % of total applied nitrogen through urea lost by volatilization from rice field. Sahu and Samant (2006) reported 10-40% of applied nitrogen lost through denitrification whereas Russo (1996) found 15-45% of surface applied ammonium fertilizer loss through nitrification-denitrification process in rice soils. Similarly, Reddy and Patrick (1975) reported that more frequency of alternate wetting and drying (2 days interval) of soil causes higher N loss (24.3%) as compared to less frequency of alternated wetting and drying (64 days interval, 12.9% loss) due to nitrification-denitrification which is the main feature of DSR.

Time of Nitrogen Application in Rice Crop
The demand for nitrogen by a growing crop is not constant through the growing season, being highest uptake associated with the period of most rapid growth stage. The main concern is that the timing of nitrogen fertilizer application so that it is taken up by the plant when it is really needed. Plants with deficiency of nitrogen during a high demand period may not produce full yield potential even with high nitrogen rates applied too late. Therefore, application timing can play a critical role in optimizing crop response, higher use efficiency and reducing the fertilizer cost (Pagani et al., 2013;Hossain and Islam, 1986). Direct seeded rice requires very little or no supply of nitrogen fertilizer during first 3-4 weeks. Much of the nitrogen fertilizer applied as basal dose is not utilized by the crop and lost from the root zone. Therefore, to obtain high efficiency of nitrogen fertilizer, the major part of total nitrogen needs to be applied at the stage of 3-4 weeks after germination and rest at PI stage (Krishnaiah, 1998). Mahajan and Chauhan (2011) mentioned that nitrogen requirement of DSR at different growth stages can be met by increasing the number of splits and doses of nitrogen.

Real Time Nitrogen Management
In real time nitrogen management, the timing of nitrogen fertilizer applications is determined through periodic monitoring of crop nitrogen status (Witt et al., 2004). Cassman et al. (1994) reported that monitoring of plant N status is important in improving the balance between crop N demand and N supply from soil and applied fertilizer. Real-time N management was developed based on the physiology of rice leaf photosynthesis, tillering and leaf area growth to optimize canopy development and increase biomass accumulation and yield formation (Peng et al., 2003).
Crop-demand based nitrogen application is one of the important options to reduce nitrogen loss and increase nitrogen use efficiency of a crop. Leaf color chart can be used for adjustment of fertilizer nitrogen application based on plant nitrogen status (Balasubramanian et al., 1999). It is quick and non-destructive method for estimating leaf N status (IRRI, 1996). LCC measures leaf greenness and the associated leaf N by visually comparing light reflection from the surface of leaves and the LCC.

Leaf Color Chart (LCC)
A leaf color chart is a simple and inexpensive diagnostic tool used to measure green color intensity of rice leaves to assess the nitrogen requirements by non-destructive method (Nchimuthu et al., 2007). It is an important tool that enables farmer to adjust the nitrogen fertilizer application based on crop demand (Ali et al., 2012;Witt et al., 2005). It was jointly developed by the International Rice Research Institute (IRRI) and the Philippine Rice Research Institute (PhilRice) from a Japanese prototype in late 1990s (Shukla et al., 2004). It is made of high quality plastic material (8×3 inches) (Singh, 2008). It consist of six color shades ranging from light yellowish green (No 1) to dark green (No: 6) color strips fabricated with veins resembling those of rice leaves (Sathiya and Ramesh, 2009;Nchimuthu et al., 2007). The LCC used in Asia are typically a durable plastic strip about 7 cm wide and 13 to 20 cm long, containing four to six panels that range in color from yellowish green to dark green as in Fig 1 (Hushmandfar and Kimar, 2011).

Guidelines for Using the Leaf Color Chart
 Take LCC readings once every 7 to 10 days, starting from 14 days after transplanting (DAT) for transplanted rice and from 21 days after sowing to flowering for DSR.  Choose the topmost fully expanded leaf color measurement because it reflects the N status of rice plants. The leaf color is measured by placing the middle part of the leaf on LCC and comparing the leaf's color with its color.  During measurement, provide shade with your body, because leaf color reading is affected by Sun's angle and sunlight intensity. If possible, the same person should take LCC readings at the same time of the day.  Take readings of ten leaves at randomly chosen in a plot. Alternately, if more than five leaves show reading below the set critical value, top dress N fertilizer to correct N deficiency (Ramanathan et al., 2003).  Generally, critical value for semi dwarf high yielding varieties 4.0. If the average value fall below 4.0, top dress N fertilizer (20-30 Kg ha -1 ) to correct N deficiency (Yoseftabar, 2013).

Grain Yield and Nitrogen Saving In LCC Based Management
It is important to manage nitrogen fertilizer more efficiently by making its application based on actual needs of rice plant so that a large fraction of applied nitrogen is translocated into grains (Singh and Singh, 2003). When N application is non-synchronized with crop demand, nitrogen losses from the soil-plant system are large, resulting in low N fertilizer use efficiency (Follett and Follett, 1992). Nitrogen management based on LCC cv. 4 helped to avoid excess application of nitrogen to rice and reduced nitrogen requirement from 12.5 to 25% without causing yield reduction (Bajpai et al., 2002). Likewise, some research findings related to yield and nitrogen saving in LCC based nitrogen management is presented in the Table 1.

Conclusion
It is concluded that leaf color chart (LCC) is an easy to use and cost effective tool for monitoring chlorophyll of rice leaf. LCC based nitrogen management helps farmers to estimate the real time plant nitrogen demand of the crop and assures nitrogen saving without compromising their yields.