Factor influencing nutrient use efficiency (NUE)

Factor influencing nutrient use efficiency (NUE):-

A. Environmental Factors:-

1. Temperature:-

> Temperature fluctuation not only affects the N cycle but also affects the plants. 

> The temperature rise affects root growth, stresses the nutrients, and water uptake by the plants, and reduces crop production. 

> An increase in temperature not only increases the urea hydrolysis rate but also increases the pH; the rise will boost the rate of ammonium conversion to ammonia; therefore, ammonia volatilization will occur. 

> Warm soil water cannot hold much ammonia gas. As the temperature cools down, volatilization will not stop, but we can see a clear difference between the volatilization rate at low and high temperatures.

2. Rainfall and Irrigation:-

> Rainfall and irrigation affect nutrient leaching, runoff, and denitrification. 

> When we apply irrigation or unwanted rainfall event that occurs after urea application increases the urea dissolution and hydrolysis rate, this will increase the ammonia concentration in the soil solution,

making it susceptible to leaching, runoff, and volatilization. 

> Volatilization loss can be reduced if ammonia leaches down within the root zone, as the surface concentration is reduced to be available for volatilization. 

> When the moisture content is low in the soil, the nitrification rate is high; when it is high, this will reduce nitrification.

> Excessive rainfall and irrigation are the primary reasons for nitrate leaching and denitrification, and the leaching rate is high in highrainfall areas (humid and subhumid areas).

3. Fires:-

> Forest fires, prescribed fires, and crop residue burning affect soil's chemical, physical, biological,

and mineralogical properties.

> Annually, 800 to 1200 million tons of plant residues are disposed of by burning. 

> Low-intensity fires are beneficial due to the deposition of nutrients into the soil and increase nutrient availability and pH. 

> However, low fire intensity also has disadvantages; it develops hydrophobicity in the soil because it soaks less water. Even in low-intensity fires, thirty-six months are not enough to cancel out the effects of fire; it will still affect the chemical properties of soil.

B. Managemental Factors:-

1. Nitrogen management:-

> When we talk about nitrogen management in crops and NUE, both includes the time of application, application rate, source of N, and method / place of application.

> When we talk about the time of application of N, we have to consider that we have to apply N close to the rapid growth stage of plant life when N uptake is maximum; otherwise, if we will apply N

when uptake is minimum, this will increase the N losses. 

> Instead of a one-time application split application of N fertilizer is recommended to reduce N losses. 

> Application of N at the heading stage is more beneficial because it will improve N uptake and increase yield. 

> Recovery of N applied at planting ranged from 30 to 55% while that applied at anthesis ranged from 55% to 80%. 

> We must apply N by considering the plant growth stage and climate conditions. 

> Nitrogen application when soil is moist will increase the chances of N leaching down. Before applying N, we must monitor rain and irrigation or soil moisture. 

> In sandy soils, the application of N at the sowing time will face N leaching due to rainfall and irrigation.

2. Leaching:-

> Leaching is the downflow of the water and nutrients from the root zone. From the active forms of N, nitrate is more susceptible to leaching, one of the main pathways of nitrogen loss from the soil solution. 

> Leaching losses range from 10% to 20% of the amount of N applied. 

> The negative charge on it makes it susceptible to leaching because the soil also has a negative charge; that is why soil cannot adsorb nitrate and ultimately leach down. 

> Besides this, nitrate loss also depends on the rainfall and irrigation, nitrification, soil texture, and the amount of nitrogen fertilizer applied.

3. Runoff:-

> Runoff is the water loss from the land's surface over an area. It is not only water; it has dissolved nutrients within it as it flows over the soil's surface. 

> Nitrogen loss through surface runoff is an important pathway of N loss; still, it does not account for a significant fraction of N loss as compared to the other pathways.

> Ammonium NH4+ is the main form of N that is lost through the runoff. 

> Nitrogen loss through the surface runoff is 13% of the applied N fertilizer. 

> Nitrogen loss through the runoff depends on the application rates of the N, the place where N is applied as if it is surface applied or not, soil surface conditions, and frequency of rain events.

4. Tillage:-

> Tillage influences the NUE by affecting crop N uptake. 

> The manner in which soil is tilled can have a direct impact on several soil properties, including soil aeration, decomposition rate of residues, soil temperature, structure, microbial activity, N mineralization, and moisture. These properties are responsible for the increased production of N2O. 

> There are varying responses in N2O emissions to reduced tillage (RT)/no-till (NT) methods based on the soil type, region, and management of crop residue, as evidenced in the literature.

5. Weeds, Pests, and Diseases:-

> Weeds, pests, and diseases not only influence the product quality but also affect the NUE by affecting the demand for N. 

> Excessive N affects the plant's resistance to insects and diseases. 

> Pests rely on plants with sufficient N supply instead of low nitrogen; this will reduce N recovery efficiency. 

> Weeds compete with plants for space, air, light, and nutrients, especially N. 

> Some weeds are more responsive to N than crops, and the result of this competition is yield loss; ultimately, NUE drops down. 

C. Soil-Based Factors:-

1. Soil pH:-

> When discussing crop production and N losses, soil pH plays an important role. 

> As the pH drops, soil acidification will occur; this will reduce the uptake of N by the plant and decrease the nitrogen use efficiency. 

> Soil pH is also responsible for ammonia volatilization; when urea is applied, it causes an increase in pH; although it is temporary, this increase in soil pH increases the rate of conversion of ammonium to ammonia, this will increase the concentration of ammonia available for volatilization. 

> However, this change is temporary; as the nitrification rate increases, the rate of ammonia volatilization will reduce as the pH is reduced because of nitrification, and the optimum pH for nitrification is in the range of 4.5-7.5.

> Soils with high buffering capacity (capacity of soil to resist pH change) will decrease volatilization

losses. 

> Soil pH also influences both nitrification and denitrification; as a result, it can influence the emission of N2O and N2.

> Denitrification and soil pH has a direct relationship with each other. When the pH is acidic, the

denitrification rate is low; it will increase with the increase of soil pH. This behavior will influence the

emission of N2O and N2.

2. Soil Texture:-

> Nitrogen use efficiency is also influenced by soil texture, whether the soil is fine-textured or coarse-textured.

> Low water and nutrient holding capacity, poor soil texture, high bulk density, soil surface sealing and crusting, poor aeration, and water lodging will reduce the NUE. 

> If we talk about the N loss through leaching, it depends explicitly on soil texture that is higher in coarse-textured soils as compared to fine-textured soils because the drainage is easy in coarse-textured

soils due to the presence of macropores.

> On the other hand, denitrification losses are significant in fine-textured soils compared to coarse-textured soils because fine-textured soils have fewer macropores and a higher number of micropores. These micropores get clogged with the high irrigation and rainfall; ultimately, the aeration rate drops,

increasing denitrification and ceasing nitrification. That is why denitrification and N2O emissions

depend on soil texture also.

3. Soil Salinity:-

> Soil salinity is a threat to food security. 

> Soil salinity inhibits vegetative and reproductive growth, making plants suffer from ionic and osmotic stress, reducing the nitrogen and water use efficiency by reducing the uptake of N and water, suppressing the photosynthetic activity due to the toxicity because of salts accumulation in plants, and

influence the stomata opening and shoot growth. 

4. Plant Based Factors:-

1. Plant Nitrogen Losses:-

> Plants are also responsible for the loss of N from the soil-plant system to the atmosphere, mostly in

the form of ammonia, mainly during the reproductive stage or after anthesis. 

> This N loss varies from crop to crop or from one variety to another. Gaseous plant N loss in excess of 45 kg N ha-1 yr-1 has also been documented in soybean.

2. Crop Rotation:-

> Crop rotation is the sequence in which crops are grown in an area. 

> Effective utilization of soil N through crop rotation can significantly decrease the possibility of nitrate leaching. 

> In addition, crop rotation can also have an impact on the rate of N mineralization by altering factors such as soil moisture, temperature, pH, plant residue, and tillage practices.

3. Crop Nutrition / Balance Diet:-

> Another reason for the low NUE is the imbalance of crop nutrition.

> Various studies prove that the deficiencies of micronutrients and macronutrients can reduce the

NUE. 

> Nitrogen uptake increases with the increasing phosphorous level. 

> Studies have shown that the nitrogen-phosphorous combination of fertilizer application has a 14% higher NUE than the nitrogen application alone.

4. Crop Varieties:-

> Crop varieties / cultivars influence the NUE because every genotype affects the uptake and utilization of N. 

> Based on their morphological and functional characteristics, every genotype differs from the other. 

> Hybrid canola has high NUE and productivity compared to parent / traditional canola even under low N input because hybrid canola can remobilize N for seed production and N accumulation in the plant.

> Similarly, hybrid corn performed very well compared to standard corn varieties, and this difference is based on the use of accumulated N before anthesis under low N input.

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