NITROGEN MANAGEMENT AND AGRICULTURAL PRACTICES
Nutrient- and water-management practices are a major challenge for increasing crop production under rainfed dryland soils. The techniques include the use of organic and inorganic fertilizers, recycling crop residues, rotations including legumes, fallow, and soil and water conservation. The adaptation and the degree of technological development of any of these techniques depend on the local physical and socioeconomic context. Many practices have evolved from indigenous knowledge of the farmers, and others have been adopted based on successful experience in similar areas. Nutrient use efficiency is improved with the use of selected cultivars, planting date, weed control, and pest management.
12.4.1 Combined Use of Organic and Inorganic N Sources
Organic matter is considered a good soil conditioner, especially for its favorable impact on water status. Soil humus improves aggregate formation and soil structure. The increase of humus can increase significantly the soil available water capacity (AWC) (Bouyoucos 1939; Russel et al. 1952; Hollis et al. 1977). Hudson (1994) found good relationships between SOM and AWC. He reported that for the example of silt loam soils, the AWC for a soil with 4% OM (by weight) was more than twice that of a soil with 1% OM. The use of manure improves SOM content, which in turn improves soil AWC (Salter and Howarth 1961; Salter and Williams 1963). Bauer (1974) reported that a change of 1% of soil humus from the addition of manure resulted in a change of AWC in the range of 0.06 to 0.11 inch per foot in soils with coarse texture and in the range of 0.01 to 0.03 in soils with moderately fine and fine textures.
Combined use of inorganic and organic fertilizers is adopted to sustain soil fertility and improve crop productivity in drylands (Palm et al. 1997; Sharma et al. 2002; Place et al. 2003; FAO 2004; Hadda and Arora 2006; Bationo et al. 2007; Li et al. 2009; Kihara et al. 2011; Harraq et al. 2016; Calderon et al. 2017). In dryland farming, the combined use of inorganic and organic fertilizer is more beneficial than their separate use. Bouraima et al. (2015) reported that application of manure reduced surface runoff and soil erosion, and N and P loss were reduced by 41% and 33%, respectively, in the case of combined use of manure and chemical fertilizers.
The use of manure is indeed important as an organic amendment for soils in general, and for dryland soils in particular. However, its use can carry some problems depending on the origin of the organic materials (e.g., infested fields or barns). Insects, pathogens, and weed seeds can be propagated through the use of manure. When manure travels from one area to another, problems associated with the use of manure can be disseminated at different spatial scales (Petit et al. 2013). Therefore, proper management of manure is needed to avoid such problems.
12.4.2 Management of Fertilizer Applications
One of the challenges for soil nutrient uptake by plants is soil moisture availability. When soil water is limited, not only is there a water stress for the plant, but also the nutrient movement and absorption by the crops are slowed down or even stopped. Fertilizers can only be applied during the periods of the season that have a good chance of coinciding with rain. This is particularly true for nitrogen, which is split in two or more dressing fractions. For instance, in most
Mediterranean areas, the practice for cereal fertilization is that P, K, and a fraction of N are applied as basal application at sowing. N is usually split in two or three dressing applications that target tillering and heading stages. The risk of a shortage of adequate moisture in midseason can jeopardize crop yields (Nageswara Rao et al. 1985; Matthews et al. 1990; Manyowa 1994; Ben Naceur et al. 1999; Fischer et al. 2003). In many situations, farmers are reluctant to apply nitrogen when there is no upcoming rain, fearing that nitrogen may be wasted and avoiding additional salinity stress to the crops.
IAEA (2005) reported on nutrient and water management project cases in rainfed dominated agriculture (<300 mm) in 11 countries with a w'ide range of cropping systems, characterized by low nutrient and organic matter status. These included w'heat-maize systems on the Loess Plateau of China (Cai et al. 2005), sorghum-castor rotations in Andhra Pradesh of India (Ramana et al. 2005), maize- based systems in the Machakos District of Kenya (Sijali and Kamoni 2005), peanut production in Senegal (Sene and Badiane 2005), and wheat-vetch rotations in the Safi-Abda region of Morocco (El Mejahed and Aouragh 2005). The results from all the studies showed that irrespective of management practices, an important portion (20% to 60%) of applied fertilizer N was lost. These losses were attributed mainly to alkaline soil pH conditions, and occurred essentially at the beginning of the crop vegetative period. On the other hand, the residual value of applied N available to subsequent crops was very low, rarely exceeding 9% of the crop N requirement. Their findings underlined the importance of fertilizer-management practices to minimize losses, especially during the early part of the cropping season. Split application during the dry season allowed an important increase of wheat yields, but the amount of N to be applied at each split needs to be based on the soil N status and crop demand for N. The combined manure with mineral fertilizers in correct proportions could provide 10% to 15% of the crop-N requirement and contribute to increasing observed yields.
Another example of constraining fertilizer use is the case of rainfed fruit tree production. Olive orchards are one of the main crops in many dryland areas, especially the Mediterranean. The cycle of the olive tree starts with blooming, which usually occurs in mid or late spring (the start of the dry season). Most of the active growing cycle occurs in summer and early fall. If P and К can be applied at once early in the season, N splitting is a major problem. Most farmers can apply a first fraction with P and K, but the rest will depend on the rainfall conditions in late spring. The most important vegetative growth and fruit setting occur in summer when no water is available for N mobility in the soil. P and К mobility in this case is even worse.
Methods of application and placement of fertilizers are critical to achieving good distribution of nutrients in the rhizosphere and their uptake by the roots. They should be adapted depending on the planting density and root system (structure and depth). For instance, wheat’s root system is shallow compared to that of maize. The latter is often row planted and tends to have a deep and pivoted root. When soil moisture is deficient, the more the nutrients are away from the root, the less chance there is for their uptake, regardless of the absorption mechanism. Interception is reduced due to limited root growth, and mass flow is hindered by the lack of water needed for the nutrients to be drawn by water movement exerted by roots in response to transpiration.
12.4.3 Tillage and Surface Residues Management
Tillage practices significantly affect soil properties that are in relation to soil moisture, organic matter, and nutrient dynamics in the soil. Under dryland conditions, conventional tillage can expose the soil to high evaporation, rapid loss of organic matter, and potential wind and water erosion. Water use efficiency is generally low with inappropriate tillage practices. Hatfield et al. (2001) reported that it is possible to improve water use efficiency by 25% to 40% through soil management practices that involve tillage. When adequate amounts of crop residues are present, conservation tillage is highly effective for conserving soil and water, achieving favorable crop yields, maintaining soil organic carbon contents, and enhancing soil and water quality (Unger et al. 1997).
Conservation tillage and reduced tillage have gained increasing attention in drylands (Unger 2002; Vere 2005; Cai et al. 2006; Wang 2006; Avci 2011; Mrabet et al. 2012). They are defined as any tillage and planting practices that can leave, respectively, 15% to 30% and 30% or more crop residues on the soils after planting (CTIC 2018). Among the benefits of such a management approach are the maintenance and accumulation of organic matter, as well as water and nutrient use efficiencies in the soil. Results of long-term experiments in drylands of China (Wang 2006) showed that the positive effect of reduced tillage on soil water availability, nutrient balance, and soil fertility indices were strongly improved by use of crop residue, either incorporated or applied as surface mulch. Simulations using the Century model revealed a positive effect on carbon sequestration under reduced tillage. Johnson et al. (2018) reported that conservation tillage systems are much better options for the cultivation of different drought-tolerant common bean varieties in semiarid areas of Kenya due to their soil moisture conservation ability as compared to conventional tillage systems, which are not a sustainable practice in such marginal areas. Sainju et al. (2012) and Sainju et al. (2016) studied N balance in dryland agroecosystems in response to tillage, crop rotations (involving cereals and legumes), and cultural practices (involving planting date, seeding rates and spacing, and application of N fertilization), over several years in the northern Great Plains of the United States. They reported that surface residue N was 30%-34% greater in no-till (NT) than in conventional tillage (CT). They also found that N sequestration rate at 0-20 cm from 2004 to 2011 varied from 29 to 89 kg N ha-1 year1 under CT and NT with spring wheat-pea rotations. Long-term experiments conducted using NT under Mediterranean conditions showed significant increase of SOM and N (Mrabet et al. 2001; Bessam and Mrabet 2003). Lai (2004) suggested that the adoption of conservation tillage practices is among the appropriate management practices that can increase soil carbon sequestration.
Surface residue management is recognized as a water conservation practice and has received important research attention in arid and semiarid regions (Duley and Russel 1939; Van Doren 1978; Unger 1984; Smika and Unger 1986; Bussiere and Cellier 1994; Peng et al. 2014). Surface residues management and residues incorporation greatly affect nutrient cycling, especially that of N (Sauerbeck and Gonzalez 1977; Seligman et al. 1986; Schomberg et al. 1994; Vanlauwe 1996; Rescous 1995; Jensen 1997; Liwang et al. 1999; Schomberg and Steiner 1999a). Mulching, as a surface residue management, is of great importance in water conservation in drylands. Mulch farming, when it is adopted in dryland farming, contributes to modulate soil temperature, reduce water evaporation, protect soil from erosion, and contribute to the sequestration of C (FAO 2004). As stated earlier, any practice that contributes to improving soil moisture will contribute also to better conditions for nutrient bioavailability and uptake.
Several studies conducted in drylands of the Loess Plateau of China reported on the importance of mulching on water and N dynamics. Jin et al. (2008) showed that minimum tillage and mulching over a period of seven years consistently increased the yield of winter wheat, primarily by better water harvest. N uptake by grain and straw, N export, and residual soil N were the highest compared to conventional tillage with no mulching. Bayala et al. (2011) evaluated several studies relative to the effect of several conservation agricultures in drylands of Africa (Burkina Faso, Mali, Niger, and Senegal) for various crops. They also found that mulching was among the best practices for improving soil fertility and crop yields, especially when rainfall is less than 600 mm. Qin et al. (2015) reviewed the effects of mulching on wheat and maize, using data from 74 studies conducted in 19 countries. They indicated that mulching significantly increased yields, water use efficiency, and N use efficiency by up to 60%, compared with no-mulching. Effects were larger for maize than wheat, and larger for plastic mulching than for straw mulching. Plastic mulching performed better at relatively low temperatures, while straw mulching had the opposite trend. The effects of mulching tended to decrease with increasing water input. Mulching effects were not related to SOM content. They concluded that soil mulching can significantly increase maize and wheat yields and improve water and nitrogen use efficiencies, and thereby may contribute to closing the yield gap between attainable and actual yields, especially in dryland and low nutrient input agriculture. Wang and Xing (2016) reported that mulching improved significantly soil water content, soil nitrate-N content, and its vertical distribution in maize root zones. The results of a three-year field experiment by Gao et al. (2009) assessing the effect of mulching with different materials (straw and plastic) showed that N uptake, NUE, and yield of wheat were higher with mulching. In addition, after three years, residual nitrate-N in 0-200 cm soil averaged 170 kg ha-1, which was equivalent to ~40% of the total N uptake by wheat in the three growing seasons.
Although plastic mulching has been evaluated in many studies and proved effective for water and N conservation, organic residues mulching is a much-preferred practice because it is the most appropriate for smallholder farmers and has limited or no environmental impact. Plastic mulching, used for a variety of crops including row vegetables and fruit crops, despite its multiple uses, is slowly degradable and represents a threat to the environment. Organic mulches, on the other hand, provide a source of organic matter and nutrients, and their cycling in the soil contributes to improving soil properties. When previous crop or fallow residues mulching is practiced with conservation tillage in dryland agriculture, the benefits are not only on soil water and nutrients but also on the soil health as a whole (Schomberg et al. 1994; Schomberg and Jones 1999; Fuentes et al. 2003).
12.4.4 Legume Inclusion in Crop Rotations
Monoculture is a common practice in many dryland areas. After harvest, the soil is left bare for several months, and is often subject to grazing for the leftover residues. In summer, wind erosion can be very significant, while water erosion can happen during the fall before the next season’s sowing. Grazing on crop residues causes depletion of organic matter, reduced topsoil protection, and nutrient imbalance. Such a practice can lead to nutrient mining when inputs are lower than outputs.
The N status of soils can be improved by the integration of legumes in crop rotations. This is especially important in drylands where N from previous legume crops is not subject to as much leaching as in humid conditions. Food legumes, such as chickpea, lentils, and faba bean, are commonly used in Mediterranean arid and semiarid regions as alternatives to fallow and continuous cropping (Ryan 2008a; Ryan and Sommers 2010). The adoption of legume crops in the rotations can contribute to increasing organic matter and therefore the potential release of N from mineralization (Ryan 1998; Ryan et al. 2008,2010). When the residues left in the soil are of low C/N ratio, they are susceptible to higher rates of mineralization, and therefore contribute to releasing more N, R and other nutrients in the soil during the cycle of the following crop. Depending on the crop, biological fixation by legume crops can provide variable amounts of mineral nitrogen during a growth cycle (Hardarson et al. 1987). Montanez (2000) reported net nitrogen supplies from legume crops varying from 50 to more than 300 kg N fur'yr1.
The contribution of N from BNF of legumes in a rotation depends on many factors, among which are the availability and compatibility of the rhizobium strain, soil water and temperature conditions, soil aeration, and the amount of initial nitrogen in the soil. In drylands, soil water status is often a determinant factor in the early stages of crop growth, and may also be influenced by the availability of starter nutrients. Molybdenum is a key element for rhizobia N fixation. This element is not very mobile in the soil if soil moisture is limited and can be affected by soil pH conditions as well. In calcareous soils, common in drylands, high pH is not a limiting factor to molybdenum compared to other micronutrients; however, this is not the case in acid soils.
Legume crops are also used in intercropping systems, either with field crops or fruit trees (Vandermeer 1989), mainly in smallholder farming. Intercropping is a common practice in many dryland areas (especially in India and China), and has been reported to improve crop yields and soil nutrient status compared to sole crops (Ramana 2015; Zhang et al. 2015; Rekha et al. 2017; Singh 2017). Legume crops have different BNF capacities that can be taken into consideration for efficient intercropping (Montanez 2000). Using l5N techniques, Bationo et al. (2003) reported that cowpea fixed more than twice the amount of atmospheric N compared with groundnuts, and that the inclusion of cowpea in millet-based cropping systems improved nitrogen use efficiency by about 30%. Ramirez- Garcia (2015) found that barley intercropped with vetch had improved root growth and N uptake.
Song et al. (2007) investigated crop yield and various chemical and microbiological properties in the rhizosphere of wheat, maize, and faba bean grown in the field solely and intercropped (wheat/ faba bean, wheat/maize, and maize/faba bean). They found that intercropping increased crop yield, changed N and P availability, and affected the microbiological properties in the rhizosphere of the three species compared to sole cropping. Bouhafa et al. (2015) and Daoui et al. (2012) reported that legumes improve soil N and P and contribute indirectly to improving soil nutrient status for olive (Olea europea) trees in Mediterranean rainfed olive orchards. The question remains whether intercropping can be adapted in all situations. It may be feasible in smallholder farming systems, but would be of controversial practicability in the more intensive large farms of favorable rainfed areas.
12.4.5 Integrated Soil Fertility Management
Integrated Soil Fertility Management (ISFM) is defined as the set of sound soil fertility management practices that include both the use of fertilizers as well as organic inputs in combination with the knowledge of how to adapt these practices to local conditions, in order to maximize nutrient use efficiency and optimize crop productivity (Vanlauwe et al. 2010, 2015). The approach of ISFM is of particular interest in farming systems that use low chemical fertilizers and rely more on organic sources of nutrients. As discussed earlier, nitrogen dynamics are closely related to organic matter, and therefore are of particular interest in the context of ISFM. Improving SOM content also aims at favoring other soil characteristics, such as water status, which in turn affects nitrogen bioavailability and use efficiency under dry conditions. ISFM is seen also as a soil conservation measure that can contribute to carbon sequestration.
The perception and adoption of soil fertility management in relation to agronomic efficiency need to be considered from multiple socioeconomic angles, with changes that can occur in steps and with increasing knowledge (Figure 12.8) (Vanlauwe et al. 2010, 2015). Innovations adapted to large market-oriented farms in developed countries may not be suitable to smallholders in developing countries where farmers are still struggling with subsistence agriculture. The move through the various steps of the ISFM depends also on the land’s degree of responsiveness. The conditions might be the same, but the concerns, the practices, and the means to face these conditions are widely
FIGURE 12.8 Conceptual relationship between the agronomic efficiency of fertilizers and organic resources and the implementation of various components of ISFM. (From Vanlauwe, B. et al., Outlook Agric., 39, 17-24, 2010.)
different. For instance, land ownership status is a major factor toward investing in the long-term improvement of soil fertility. Itinerant farming as a result of drought and soil degradation causes continuous decline of soil fertility in many countries in the Sahel and sub-Saharan Africa (Reich et al. 2001; Darkoh 2003; FAO 2005a, 2005b).
12.4.6 The "R" Principles for Nitrogen Management in Drylands
FAO (2004) recommended that soil fertility management practices can be grouped according to the movement of nutrients into, within, and out of a system and that the practices can be sorted in four groups as suggested by Hilhorst and Muchena (2000):
- • Adding nutrients to the soil
- • Reducing losses of nutrients from the soil
- • Recycling nutrients
- • Maximizing the efficiency of nutrient uptake
A number of management practices were drawn from several case studies in Senegal and Sudan (FAO 2004). Any soil fertility management approach to be adopted or adapted to drylands needs to comply with the principles of one or more of these categories of practices.
N management needs also to obey to four stewardship principles, commonly referred to as the four Rs: “Right amount,” “Right time,” “Right place,” and “Right form” (Ryan et al. 20П). In the context of drylands, since N availability to crops is highly dependent on several management approaches, especially those related to organic matter and water, we suggest considering an additional, fifth “R,” which is the “Right approach,” as explained below:
Rl: Right amount
- • The amount of N needs to be based on crop requirements, target optimum yields, and soil N balance. The knowledge of initial N stock and the potential of N mineralization from organic matter are key factors in dryland soils. The apparent N use efficiency indices for specific conditions are required to make reasonable estimations of the amount to apply to reach potential yields. Since leaching is generally in rainfed early season conditions, nitrate-N can be used as a good soil test.
- • Since rainfall can change from year to year, N rates should be adjusted to the actual rainfall conditions that coincide with the critical stages of the target crop. A drier year will require reducing the N rates initially planned, while a rainier season would impose increasing the N amounts in order to achieve higher yields and to compensate for the loss by leaching.
- • Volatilization is a common N loss process in dryland soils, especially in calcareous soils with high pH and lime content. Appropriate adjustments based on assessment studies are needed.
- • In dryland agriculture, nitrogen is given more importance by farmers compared to other nutrients. Caution is needed to avoid excess N that may affect crop growth and production.
- • The response of N depends on other nutrients and also influences the uptake of other nutrients. Understanding N interactions with other macro- and micronutrients, especially under water stress conditions, is essential.
R2: Right Time
- • Due to limited rainfall, N applications need to be planned according to the probabilities of the occurrence of rain and should follow the critical growth stages of the crop.
- • Basal N fertilization at the start of the season should depend on the stock of mineral N as well on SOM content. The quality of organic matter (C/N) will determine the importance of N-mineralization versus N-immobilization and the resulting N-balance to be considered for subsequent N-topdressings.
- • N-topdressing is intimately linked to rain occurrence in order to increase the N use efficiency and avoid temporary stress to roots for water and nutrient uptake.
- • Root development is important for water and nutrient uptake, and N is important for root development and for the synergies w'ith other nutrients involved, such as phosphorus.
- • Early available N to the crop for its root development is a prerequisite for water use efficiency in later stages.
R3: Right Place
- • Geographically, N management depends on the degree of aridity from one area to another. Responses to N are higher in the more favorable areas that are less prone to sporadic rainfall occurrence. N rates and time of application will depend ultimately on the amounts and spatial variability of rainfall in a given region.
- • At the parcel level, N is more efficient where roots are dominantly present. Where banding is possible, local applications of N give a better response. The incorporation of N with seeders at sowing is found to trigger better initial growth compared to broadcast application followed by tillage that usually returns N to depths beyond the zone of initial root development.
R4: Right Source
- • The forms of N need to be applied according to the limited soil-water conditions, which determine N mobility, as well as to soil properties, which determine both the mobility as well as the chemistry of N.
- • Different forms of N can be adopted at different growing stages of the crop.
- • Ammonium-N can be subject to important losses under dry conditions, high pH, and high lime content. Nitrification of ammonium can contribute significantly to buffer soil pH.
- • Urea is a dominant N fertilizer. Its use under cool and moist conditions can give good response. When soil moisture is favorable, urea can be transformed rapidly to ammonium, which in turn goes through rapid nitrification (within a few days). However, urea is prone to volatile loss under dry conditions, and can cause injuries to crop leaves when broadcasted as topdressing.
- • Loss by denitrification is very rare in dryland soils.
- • The use of organic matter is a good source of N and a good soil conditioner for N-cycling. The knowledge of N-mineralization rates needs to be taken into consideration in the N-balance.
R5: Right approach
- • Nutrient balance: the right amounts of all essential elements is crucial to guarantee positive interactions for plant growth, including water and nutrient use and efficiency.
- • Surface residues management and mulching play important roles in protecting the soil and improving its fertility, mainly N and C cycling.
- • Integrated soil fertility management is particularly beneficial in dryland soils.
- • A good knowledge of the trends of mineralization versus immobilization of OM residues and amendments (manure, compost, etc.) is necessary to make compromises between nutrient release (mainly N) and SOM buildup. Different strategies are required depending on the nature of the OM used/recycled (mainly the C/N ratio). Mineralization is often sought in low' fertilizer-use systems, w'hich is the dominant situation in drylands. However, the need for increasing SOM in such systems is well justified.
- • Early inorganic N applications help prevent N-immobilization w'hen the C/N ratio of SOM highT
- • Crop rotation with N-fixing legume crops is an ancient practice to be sustained to improve soil conditions, N, and the status of other nutrients. Crop rotation is also a good alternative to fallow' under favorable dryland conditions.
- • Conservation agriculture practices including reduced tillage, no-till, or direct seeding can improve organic matter, water status, and N bioavailability.
- • Weed control reduces competition for water and N, especially under constrained dryland agriculture conditions.