Nitrogen Behaviour Under Dry Soil Conditions Abstract Introduction Moisture Affects Crop Yield and Nitrogen Demand

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1 Nitrogen Behaviour Under Dry Soil Conditions Cynthia Grant and Fernando Selles, Agriculture and Agri-Food Canada, Brandon Research Centre, Brandon, MB R7A 5Y3 Abstract Efficient N fertilization will improve profitability, especially with high fertilizer prices. On the prairies, moisture usually determines yield potential, nitrogen requirements and best fertilizer management practices. With dry conditions, moisture conservation is critical in the soil profile and the seed bed for yield and crop emergence, so management should optimise both N efficiency and moisture conservation. Furthermore, the interaction of water availability and nitrogen requirements works both ways; under conditions of insufficient N availability, the crop does not use water efficiently. Therefore, even under conditions of water stress it is important to manage N requirements properly to ensure optimal use of the fertilizer and water resources. Losses from fall-banded N are low under dry conditions and inhibitors or controlled release products are not likely needed to improve efficiency. However, if the soil is bare over much of the winter, loss of standing stubble and disruption of the soil surface with fall banding reduces snow trapping and increases evaporation over the fall and early spring, reducing moisture supply. In contrast, if a good snow cover is retained over winter, banding in fall rather than spring avoids drying the seed bed and can improve crop emergence and early growth. A one-pass reduced tillage system improves moisture conservation over a separate banding operation. With dry conditions and low yield potential, the N rate for optimum economic yield is low. Seed-placed application may be suitable, if the seed-bed utilization relative to application rate is sufficient to avoid seedling damage. At higher rates, side-banding, Agrotain or controlled release products can increase safety, although some side-banding systems may compromise seed-bed integrity and moisture conservation. In-crop N applications under dry conditions can help manage risk. If a low N rate is applied at seeding, N can be applied in-season if improved conditions cause N deficiencies. Surface N applications are generally less efficient than in-soil applications, due to volatilization and immobilization losses and the need for moisture to carry the fertilizer to the roots. Introduction The two major factors affecting crop yield on the prairies are normally nitrogen supply and available moisture. When moisture is limited, N management practices need to be selected not only to improve N use efficiency, but also to optimise the amount of moisture that will be available to the growing crop. The balance in relative importance between N efficiency and moisture conservation will vary depending on the amount and distribution of available moisture. Moisture Affects Crop Yield and Nitrogen Demand Nitrogen fertilization should make up the difference between the demands of the plant and the supply from the soil. Nitrogen demand of the plant is mainly influenced by the crop yield potential and protein content. Throughout the prairies, the yield potential and therefore the amount of nitrogen required to attain the optimum yield is generally determined by the amount and timing of moisture. Figure 1 shows the long-term trend in wheat grain yield on the prairies since the mid-1920 s. Although there is a steady increase in crop yield over that period of about 0.27 bushel per acre per year, there is a very large amount of variability from year to year. About 2/3 of that variability can be explained by moisture stress, particularly during July. 1

2 Grain Yield (bu/ac) Yield trend = 0.27 bu/ac/yr Year Yield Trend Running mean Figure 1: Average wheat grain yield on the prairies has been gradually increasing over time. Large year to year variation occurs, primarily due to available moisture. Under dry conditions, any moisture either trapped or conserved will generally lead to an increase in crop yield potential. Therefore, a primary objective of nutrient management under dry conditions is to apply fertilizers in an efficiency way with minimal loss of moisture. The physical effects of nutrient management on moisture trapping and conservation become very important under dry conditions. Also, the efficiency with which a crop converts water into grain, or water use efficiency is affected by the nutritional status of the crop. A recent study at Swift Current, SK, showed that the average water use efficiency for hard red spring wheat was 1.1 bu/ac per inch of available water under low N fertility conditions and 4.2 bu/ac per inch of water under high N fertility. This shows that in dry years it is important to ensure the crop has an adequate supply of nitrogen to use the scarce water more efficiently (Selles et al. 2006). As crop yield increases, the amount of nitrogen required to attain that yield increases as well. So, nitrogen requirements are much higher under moist conditions where yield potential is high than under dry conditions where yield potential is low (Figure 2). With very low N requirements under dry conditions, investment in N fertilizer is lower, and the amount of potential N loss from fertilizer application is lower. Therefore, the drier the conditions, the more important moisture conservation becomes relative to enhancing nitrogen use efficiency. 2

3 1.0 Probability of achieving maximum yield Very Dry 5.7 inches Dry 9.4 inches Moist 11.2 inches Wet 13.5 inches Available N (kg ha -1 ) Figure 2: Nitrogen required to achieve maximum yield increases with increasing moisture. Moisture Supply and Nitrogen Losses Nitrogen losses are also determined in large part by the moisture supply; therefore both the crop demand and N that is available from the soil and from fertilizer applications will be driven by moisture. Nitrogen is lost from the soil by the following pathways, all of which are affected by moisture. 1) Volatilization is the loss of N to the atmosphere as ammonia gas. Ammonium and ammoniumproducing sources, such as urea, are readily lost by volatilization when left on the soil surface, while nitrate sources are not. Volatilization losses tend to be higher under dry condition. Factors that increase water evaporation from the soil, such as warm air and soil temperatures and high winds increase the volatile losses of ammonia, especially when the fertilizers are broadcast without incorporation or are dribble banded. Applying fertilizer when temperatures are cool, winds are light and there is a good likelihood of receiving rain in the near future help to reduce volatilization losses. Volatilization can be high if fertilizers are not incorporated and there is no significant rainfall for several days after fertilizer application. 2) Immobilization refers to the "tie-up" of N in the soil microorganisms as they use the N for their growth and reproduction. Both ammonium and nitrate can be used by microorganisms and lost through immobilization. This is a temporary loss, since the N will recycle when the microorganisms die and decompose, but it restricts N availability in the year of application. If fertilizers are in contact with crop residue and conditions are moist and warm, there can be a large amount of N tied up through immobilization. 3) Denitrification is the conversion of nitrate-n to gaseous forms of N, which can be lost to the atmosphere. Denitrification occurs when available oxygen in the soil is limited. This can occur under flooded conditions or when moist soils are very compacted. Losses are therefore higher on fine-textured soils and on soils subject to water-logging, such as bottom-slope positions. Even when the soil is not completely flooded, there will be microsites in the soil where oxygen availability is limited and 3

4 denitrification can occur. Rate of denitrification will be faster when soil temperatures are warm, because the activity of the microorganisms that cause denitrification increases with increasing soil temperature. Topography can have a large effect of denitrification losses, since low areas is the field may be saturated while the higher portions of a field may be well-drained. Therefore, denitrification losses will be affected by both the general weather conditions and by site-specific drainage conditions in the field. 4) Leaching is the movement of N in the soil water through the soil profile beyond the rooting zone. When the N moves below the rooting depth, the plants can no longer reach the N, so it is lost for crop use. Ammonium-N is normally bound to soil particles and so protected from leaching losses. Therefore, N in the nitrate form is much more susceptible to leaching losses than is the ammonium form. Leaching will increase with increasing precipitation and is higher on light-textured soils with lower water holding capacity. As with denitrification, the topography of a field can have a large effect on leaching losses, since nitrate will move with the water movement in a field, following the drainage patterns. When selecting a fertilizer management program, the soil and environmental conditions affect the relative risk of losses by volatilization, immobilization, denitrification and leaching. Therefore, the potential benefits of different N management practices designed to minimise these losses, such as banding, timing of application, or use of enhanced efficiency fertilizers, will depend on the environmental conditions driving the amount and pathways of loss. If fertilizers are surface broadcast or dribble-banded, volatilization and immobilization will be major paths of loss. Because of the high potential for volatilization and immobilization losses, surface applications of N tend to be less efficient than in-soil banded applications. While broadcast and incorporated N applications tend to prevent volatile N losses, they can also lead to major losses in moisture and drying of the seed-bed. Therefore, this is not a desirable method of application under dry conditions. If N is broadcast and left on the soil surface, volatilization can continue until the N moves into the soil, particularly if the soil surface stays moist because of dew or light showers. Since rainfall shortly after fertilizer application will move the fertilizer into the soil and reduce losses, the efficiency of surface applications tends to improve in higher rainfall areas, where precipitation is more likely occur soon after fertilizer application N Lost (mg N) Control Urea Urea + Agrotain UAN UAN + Agrotain UAN + ATS Days After Application Figure 3: Volatilization losses are lower with UAN or when Agrotain is used than with untreated urea (Grant et al. 1996). 4

5 % N Lost Control Agrotain Irrigation Irrigation + Agrotain Days After Application Figure 4: Applying 2.0 cm of water 4 days after urea application (indicated by arrow) reduced volatilization loss, but did not eliminate it. Application of an addition 2.0 cm of water on day 7 (indicated by arrow) stopped volatilization. Use of Agrotain delayed volatilization losses, but water was needed to carry the fertilizer into the soil, or volatilization began to increase as the Agrotain broke down (Rawluk et al. 2001). If surface-applied fertilizers are used without incorporation, for example in winter wheat, pastures or for in-crop N applications, and rainfall is not expected within a day or two, dribble-banding urea ammonium nitrate can reduce volatilization losses (Figure 3). Treatment of urea with Agrotain can slow the conversion of the urea to the volatile ammonia and delay losses, allowing more time for rain to carry the fertilizer into the soil (Figures 3 and 4). This becomes more important when applications are made during warm weather, since volatilization increases with temperature. So the benefit from either dribble-banding UAN or treatment of urea with Agrotain is likely to be greater when applications are made later in the spring when temperatures are increasing and conditions are drier than it will be with applications made under cold conditions in early spring. 5

6 N mineralized (kg/ha) Rainfall increased microbial activity and mineralization Microbial activity was low under dry conditions Sampling date Durum Stubble Pea Stubble Figure 5: Microbial activity slows under dry conditions in the summer, reducing both N tie-up through immobilization and N release through mineralization. Rainfall moistens the soil, increasing microbial activity and residue decomposition. This enhances immobilization with a high C:N ratio residue or mineralization with a low C:N ratio residue. If fertilizer is managed to avoid volatilization, losses by denitrification and leaching, the other major pathways for N loss, are very small under dry conditions. Under dry conditions, microbial activity is low and immobilization and mineralization slows down (Figure 5). Conversion of ammonium to nitrate also slows under dry conditions. The nitrate portions of ammonium nitrate and urea ammonium nitrate are susceptible to leaching and denitrification losses as soon as they enter the soil. However, if an ammonium or ammonium producing source of N is used (i.e. anhydrous ammonia or urea), the ammonium must convert to nitrate before significant losses occur. This conversion occurs under saturated soil conditions. The warmer the soil, the more rapid is the conversion from ammonium to nitrate. So, warm wet conditions lead to the greatest loss. Under dry conditions, fertilizer bands can remain in the soil for a relatively long time without significant losses, while under wet conditions, losses may be considerable. 6

7 120 Grain yield (% of spring) r = 0.11 ns r = 0.66* Sep 20-Sep 30-Sep 10-Oct 20-Oct 30-Oct Date of application Highs Lows Linear (Lows) Linear (Highs) Figure 6: Grain yield as a percentage of the yield attained with spring banded N as a function of timing of fall application and position in the field (high = upper slope and low = lower slope) (Tiessen et al. 2005). This becomes especially apparent when considering fall applications of fertilizer. Fall banding is attractive to producers because it moves a portion of the workload away from the busy spring season and may allow earlier seeding. Fertilizer prices may be lower in the fall than the spring and there may be tax benefits of buying fertilizer before the year-end. If a producer does not use a one-pass seeding and fertilizing system, banding in the fall in a dry area may be a good management practice, because it can avoid the moisture loss and seed-bed disruption that can occur from banding or incorporation of fertilizer in the spring. However, under wet conditions or on heavy clay soils, losses from fall-applied fertilizer can be high although if soils are dry, losses may be relatively low, even if the fertilizer is applied early in the fall. In studies conducted by Tiessen et al. (2005) in Manitoba, fall-banded urea produced yields as high as spring banded urea in well-drained portions of the field, even if applied in early September (Figure 6). However, yields with the early fall-banded fertilizer were more than 15% lower in the poorly-drained portions of the field. The earlier the fertilizer was applied, the greater the losses in the depressional areas. So, if considering fall fertilizer application, either for spring-seeded crops or at time of seeding for winter wheat, the moisture conditions and drainage patterns in the field should be considered. If conditions are dry and the field is well-drained, fall-banded applications of ammonium fertilizers may perform very well. If the field is wet, compacted or poorly drained, N loses may be high, particularly if the N is applied early in the fall, when the soil is still warm. Banding N fertilizer tends to reduce N losses and increase nitrogen use efficiency. Under moist to moderately dry conditions, banding tends to provide improved crop yield and is a widely recommended practice. However, using a separate banding action to place N fertilizer away from the seed also tends to increase moisture loss. Fall banding may be particularly problematic under very dry conditions where snow-cover is limited. Loss of standing stubble from the fall banding action reduces snow-trap on the field and can leave the field susceptible to evaporative losses. Therefore, fall banding can greatly reduce available moisture for the following crop. However, in areas where snow cover is more reliable, fall banding can provide better moisture than systems such as spring banding. Spring banding can loosen and dry the seed-bed, reducing available moisture and seed-bed quality. A low-disturbance one-pass seeding 7

8 and fertilizing system is likely to provide the greatest moisture conservation and seed-bed quality under dry conditions. It is important to look at the opening system to determine the degree of disturbance near the seed-row. If the opener dries and loosens the soil near the seed-row, it can reduce emergence. Figure 7: The amount of available moisture after seeding has a strong effect on the N fertilizer that can safely be applied with the seed. 350 May water stress (mm) % loss > 20% loss Urea-N seed placed (kg/ha) Under very dry conditions, rates of N application are low and seed-placed N application with a lowdisturbance opener may be a good option. If the fertilizer is placed directly with the seed, it eliminates the extra expense, draft requirements and soil disturbance required to side-band the fertilizer requirements. Seed-row placement is a form of banding, so is efficient in terms of reducing N losses. However, applying excess nitrogen with the seed can lead to seedling damage, reduced crop yields, reduced response to nitrogen fertilizer and reduced nitrogen use efficiency. The amount of N that can be placed with the seeding increases with increasing available moisture between seeding and crop emergence (Figure 7). The amount of seed-placed fertilizer that can be safely applied depends on a number of factors including environmental conditions, crop grown, soil type, width of the seed/fertilizer band, row spacing and fertilizer source. Use of air seeders with wide sweeps allows for increased levels of seed-placed fertilizer, since the concentration of fertilizer in contact with the seed is reduced as the seed and fertilizer are spread over a wider zone. However, wide sweeps can disrupt the seed-bed and lead to moisture loss. Rate applied with the seed must be decreased with light textured soils, low soil organic matter, cool growing conditions, low soil moisture, in the presence of salts or free lime, or with the use of wide row spacing. Small seeded crops such as flax or canola are more sensitive to seedling damage than crops such as wheat or barley. Use of urease inhibitors or polymer coated urea may increase the level of urea that can safely be applied with the seed. Since urease inhibitors will slow the conversion of urea to ammonium/ammonia, the concentration of toxic salts and ammonia in contact with the seedling will be reduced and the urea will have a greater opportunity to diffuse away from the seed before causing injury. The coated products 8

9 release the fertilizer into the soil slowly over the season, reducing the fertilizer concentration near the germinating seedling. Field studies have shown that these products can increase the amount of fertilizer that can safely be placed with the seed (Figure 8) Stand Density (Plant/m2) Urea Agrotain CRU Nitrogen Rate (kg/ha) Figure 8: Effect of seed placed urea, urea with Agrotain, or polymer coated urea (CRU) on stand density of durum wheat (Malhi et al. 2003). Nitrogen losses may occur between time of application of N at seeding in the spring and the time when the crop takes up the N from the soil solution. Controlled release fertilizers or split applications can reduce the time that the fertilizer is present in the soil solution prior to crop uptake, thus reducing the risk of N losses. The wetter the conditions, the greater the losses and the greater the potential benefit from use of split application or enhanced efficiency fertilizers. This is again because both application rates and N losses tend to be greater under wet conditions. Use of a product that slows the release of the fertilizer into the soil solution, or slows the conversion of ammonium sources to nitrate, will minimise the likelihood of losses and can improve nutrient use efficiency. Similarly, use of split application may be of greater benefit under wet conditions, both because of the higher potential losses and because in wetter environments, rainfall is more likely to occur and carry the fertilizer into the rooting zone. However, under dry conditions, it is unlikely that either split applications or controlled release fertilizers will increase crop yield. In research that we conducted at nine sites across Canada over three years to evaluate the benefits from split applications of N and the used of the controlled release ESN product, neither split applications or use of ESN as a banded application at the time of seeding increased yield under dry conditions. However, under moist conditions, yield was increased by using either split applications or ESN as compared to where the same rate of uncoated urea was used. Use of the ESN was also beneficial when applied as a fall-band in wetter areas. 9

10 Table 1: Effect of split applications of N fertilizer on wheat yield (Selles) 100% at Seeding/flag leaf split Seeding/rain split Type of year Spring/growing Seeding 75/25 50/50 75/25 50/50 season Yield (bu/ac) Yield relative to 100% N applied at seeding Dry/Normal Normal/dry Moist/moist Table 2: Effect of split applications of N fertilizer on wheat protein content. Type of year 100% at Seeding/flag leaf split Seeding/rain split Spring/growing Seeding 75/25 50/50 75/25 50/50 season Protein (%) Protein relative to 100% N applied at seeding Dry/Normal Normal/dry Moist/moist Studies conducted by Selles at Swift Current also showed that split applications of N fertilizer did not produce yield losses when the moisture conditions in the spring were normal to moist, regardless of whether 75% or 50% of the N required was applied at seeding and the rest at a later date. However, in dry springs splitting N produced yield losses between 11 and 17% when the remainder of the N was applied at the flag leaf stage (Table 1). Furthermore, split applications of N did not produce higher yields than placing the entire N application at seeding, regardless of the weather conditions. It appears that in dry springs, adequate N fertility is required early in the growing season to increase the vigour of the seedlings and develop a larger root system that can explore a larger volume of soil to increase water uptake, and hence crop growth early in the growing season. Split applications only rarely increased grain protein content relative to where all N was applied at seeding (Table 2). However, split applications can be used as a risk management tool. If conditions are dry, a producer may reduce the amount of N applied at seeding to reduce the investment in fertilizer on a potentially low yielding crop. If weather conditions improve and the crop appears N-deficient, extra N can be applied to meet the crop N demand. Use of a Spad meter, Green-Seeker or other tool for evaluating crop N status may help in determining if the crop actually needs more N. Summary Under dry conditions, crop yield potential is generally restricted by available moisture and N losses from immobilization, denitrification and leaching can be relatively low. Therefore, for dry situations, it is very important to select management practices that conserve soil moisture. The benefits that a producer will obtain from practices such as fall fertilization, N banding, split applications or use of enhanced efficiency products such as ESN or Agrotain will depend on the overall weather conditions and on the microenvironment within the field and will be much lower under dry conditions than in wet areas. Producers must be aware that under dry conditions, failure to obtain a response to added N does not mean that the applied fertilizer N is lost. In fact, most of the N applied will be available to crops in the 10

11 following growing season, because under dry conditions N losses are minimal if the fertilizer was managed properly. This can be seen as a postponement of the return on the fertilizer investment rather than a loss of the investment, References Grant, C.A., Jia, S., Brown, K.R. and Bailey, L.D Volatile losses of NH 3 from surface applied urea and urea ammonium nitrate with and without the urease inhibitor NBPT. Can. J. Soil Sci. 76: Malhi S. S., Oliver E., Mayerle G., Kruger G. and Gill K. S Improving effectiveness of seedrow-placed urea with urease inhibitor and polymer coating for durum wheat and canola. Commun. Soil Sci. Plant Anal. 34: Rawluk, C. D. L., Grant, C. A. and Racz, G. J Ammonia volatilization from soils fertilized with urea and varying rates of urease inhibitor NBPT. Can. J. Soil Sci. 81: Selles, F., Clarke, J.M. and DePauw, R.M Quantification of the yield and protein response to N and water availability by two wheat classes in the semiarid prairies. Can. J. Plant Sci. 86: Tiessen, K. H. D., Flaten, D. N., Grant, C. A., Karamanos, R. E., Entz, M. H Efficiency of fall-banded urea for spring wheat production in Manitoba: Influence of application date, landscape position and fertilizer additives. Can. J. Soil Sci. 85: