Abstract

Nitrogen and potassium are major nutrients in crop fertilization. The monovalent cation K⁺ is the ionic form of K in soil and is taken up as such by plants. For N there are several ionic and non-ionic forms in soil but plant uptake by non-legumes is usually restricted to the two main monovalent ionic forms: NH₄⁺, a cation and NO₃^-, an anion. The source of N available to plants and N transformations within the soil, influence K reactions in soil, K absorption by roots, short and longdistance transport within the plant, as well as K demand by crops. NH₄⁺ and K⁺ share similar valence and size properties, and consequently compete for the same exchangeable and nonexchangeable sites of soil particles. The form of N fertilizer can thus affect K availability in both the short and longterm. In the short-term, K concentration in the soil solution may increase following NH₄⁺ fertilization, whereas long-term NH₄⁺ fertilization has been reported to deplete exchangeable and non-exchangeable K in soil. The available N form affects K adsorption to root surfaces and plant membranes. Transport of K⁺ through plant membranes is affected both by NH₄⁺ and NO₃^- by direct competition between K⁺ and NH₄⁺ for transporters, as well as by indirect effects of electrical balance following NH₄⁺ assimilation. K⁺ transport to plant shoots decreases with an increasing NH₄⁺/NO₃^- ratio in the root zone; the long distance transport of NO₃^- in plants is related to K⁺ recycling within the plant. The effects of K⁺ on fruit quality have been shown to be dependent on the N source. Consequently, the interactions of K⁺ with N forms have practical implications for fertilization of crops.

Introduction

Nitrogen and potassium are major crop nutrients and fertilizer components. Römheld and Kirkby (2010) recently reviewing the subject of K in agriculture considered topics including soil K availability, K uptake by plants, as well as physiological and practical aspects of plant K status. General interactions between N and K in crop fertilization have been described by Milford and Johnston (2007). The main focus of this work was to draw attention to the requirement for adequate K supply to crops in order to optimize yield response to N fertilization. Potassium status of the soil was shown to exert a considerable influence on crop uptake and response to N, as a consequence of the physiological role of K in plants. Recently, the K nutrition of crops under varied regimes of nitrogen supply was reviewed by Zhang et al. (2010). The present minireview focuses on the effect of N form on crop response to K in agriculture. The paper specifically discusses the interactions of NH₄⁺ and NO₃^- with K⁺ in the entire soil-plant system. It includes the following areas of interest: the influence of N source and the effects of N transformations within the soil on soil K reactions; K absorption by roots; short and long-distance transport of K in plants; and whole plant response to K supply and K demand.

Ammonium and potassium interactions in soil

The main broad differences between K and N properties in relation to their chemical reactions in soil, uptake by plants and their roles in plant physiology are presented in Table 1. The exclusive monovalent ionic form of K in the soil contrasts to the several ionic and non-ionic forms of N which are present. This means that potassium is taken up by plants as K⁺ whereas nitrogen can be absorbed as NH₄⁺, a cation and as NO₃^-, an anion. Nitrogen is a major constituent of organic molecules in plants which include amino-acids, proteins and nucleic acids. On the other hand, K remains exclusively as an inorganic cation, with one of its major physiological roles as an osmotic-regulator in plant tissues. This difference between plant N and K raises the question of possible interactions or competition between these two nutrients. NH₄⁺ and K⁺ are both univalent cations and have a similar ionic radius (2.8A0). Consequently, they compete for the same exchangeable and nonexchangeable sites of soil particles. The form of N fertilizer can thus affect K availability both in the short- and long-term.

Potassium availability to plants in soil is governed by the transfer between four main pools in the soil: structural, fixed (non-exchangeable), exchangeable and soluble (Römheld and Kirkby, 2010). The soluble and exchangeable phases exist in all soils, the latter providing negative charge sites on clay mineral surfaces and organic matter. The fixed or non-exchangeable phase exists only in micaceous type clays (2:1 layers like illite, vermiculite and other clays from this group) (Römheld and Kirkby, 2010; Zhang et al., 2010). In soils, equilibrium exists between these different pools and the relationship between them is presented in Fig. 1. The size of the soil solution pool is very small, about 5 percent of total crop demand at any given time (Mclean and Watson, 1985), and 0.1-0.2 percent of the total soil K. However, it plays a crucial role in the K cycle as can be seen in Fig. 1. It is the only pool that is available for plants and it is in equilibrium with exchangeable K (1-2% of the total soil K) and fixed K (1-10% of the total soil K). These pools are the main contributors to K supply to plants because of the very rapid rate of exchange of soluble ions with exchangeable ions, occurring in less than seconds (Eick et al., 1990; Bar-Tal et al., 1995; Sparks, 2003). When K⁺ is removed from the solution by plant uptake or leaching, the soil solution is rapidly replenished by K⁺ from exchangeable sites, whereas the fixed clay replenishes the exchangeable sites slowly by a diffusion controlled process. The main K pool in soil (90-98%) is the structural K in minerals like micas and feldspars, but the release of this form to the soil solution during the process of weathering is very slow. Microbial biomass provides an additional K pool in soil which contains from 2.1-25.4 μg K g-1 soil or 0.1 percent of the NH₄⁺NO₃^- extracted K (Khan et al., 2009). This pool and the role of micro-organisms is not crucial for the K cycle, as it is in the N cycle, because the main form of K in living tissues is the ion K⁺ which is readily transferred to the soil solution.

Soil K status influences K uptake by plant roots. The amount of K in the soil depends on soil type, production level, retention or removal of crop residues and the input of K as fertilizer or as a component of irrigation water. Imbalance in fertilizer application, especially N fertilizers with no K, is very common. This leads to a deficiency in K due to the continuous removal of K during crop uptake that alters the amount and status of remaining K (Römheld and Kirkby, 2010). Release and fixation rates of K in soil are highly dependent on soil K balance, confirming that these are reversible processes that depend on plant uptake and fertilizer inputs (Bar-Tal et al., 1991; Simonsson et al., 2007). In a greenhouse experiment, where corn was grown in pots (3 kg soil) for one season, the control treatment (nil K input) resulted in the release of 2.6 and 1.1 mmol kg^-1 from loess and sandy loam soils, respectively (Bar-Tal et al., 1991). In the same experiment, application of 30 mmol K per pot resulted in fixation of 1.33 and 1.20 mmol kg^-1 to a loess and a sandy loam soil, respectively. These changes in non-exchangeable K are quite considerable when compared to the initial levels of 7.6 and 1.9 mmol kg^-1 in the loess and the sandy loam soils, respectively. This may be explained by the use of small volume pots which enhanced the rate of changes in the soil. Under field conditions, however, the same processes would be expected to induce these changes at a much slower rate; taking several years to obtain similar results. Crop uptake of K under a negative soil K balance due to imbalanced NPK fertilization in the field is mainly met through K released from non-exchangeable sources (Römheld and Kirkby, 2010; Lal et al., 2007). On the other hand, a considerable increase in fixed K is observed in permanent plot experiments in high K input treatments (Benbi and Biswas, 1999) and in pot experiments (Bar-Tal et al., 1991).

The NH₄⁺ ion is also present in soil solution and in exchangeable and fixed soil pools. There is also equilibrium between these forms as was described for K. The main chemical difference between K⁺ and NH₄⁺ is that the latter can also be oxidized to nitrite and NO₃^-, mainly through microbial activity. Only the fixed phase NH₄⁺ is relatively protected from the oxidation process and can be retained in the clay interlayers for long periods. The similar properties of the K⁺ and NH₄⁺ ions leads to strong interactions between them in the soil. Both ions are held by the same non-exchangeable sites in the interlayers and edges of interlayers of the 2:1 clay minerals. Consequently, a simple competition where application of one ion should displace the other and increase its fraction in soil solution is to be expected. However, the reality is more complex and application of NH₄⁺ or K⁺ to soil may lead to increases as well as decreases in the non-exchangeable pool of the counter ion. Some examples and the possible explanations are presented below.

Simultaneous injection of anhydrous ammonia (AA) and a KCl solution in a silty clay loam soil decreased K⁺ fixation, whereas the concentrations of the exchangeable and soluble K⁺ increased (Stehouwer and Johnson, 1991). The decrease in K⁺ fixation was attributed to preferential NH₄⁺ fixation blocking K⁺ fixation and the increased exchangeable K⁺ explained by a decrease in K⁺ fixation (Fig. 2) and to pH-induced increases in cation-exchange capacity (primarily in the organic fraction) (Stehouwer and Johnson, 1991). Liu et al. (1997) reported that in field experiments, the soil fixation capacities for both NH₄⁺ and K were significantly reduced by sustained high rates of K fertilization, but not by N fertilization. Kenan et al. (1999) also found that the fixation of NH₄⁺ was reduced by K addition before NH₄⁺ was added, and that the reduction was proportional to the amount of K previously fixed. Enhanced K⁺ fixation with increasing K application rates and reduction with increasing NH₄⁺ application rates was reported by Chen and MacKenzie (1992) and Du et al. (2007). The sequence of NH₄⁺ and K⁺ application may influence K fixation. Chen et al. (2007) reported that NH₄⁺ at high rates of application before K fertilization to rice plants resulted in poor growth and reduced K uptake compared to NH₄⁺ application after K fertilization. However, when NO₃^- was used as the N source, plant growth was not affected by the order in which N and K were applied. The explanation for the negative effect of the early NH₄⁺ application on K availability was that the NH₄⁺ blocked the non-exchangeable sites, thus reducing the capacity of the soil to store K to meet the continuous consumption by the plants. Evangelou and Lumbanraja (2002) reported that the surface of vermiculite clay exhibits high specificity for K⁺ at low-K fractional loads which cause the clay interlayer to collapse and thus a large proportion of interlayer NH₄⁺ most likely becomes fixed. The available exchange sites appear to have relatively lower affinity for NH₄⁺. Therefore, the data indicates that availability of applied NH₄⁺ in vermiculitic soils would depend on the presence or absence of applied K⁺.

Clay minerals of the smectite type have a unique structure that enables a high exchangeable cation capacity in the interlayers. The size of the hydrated exchangeable cation affects the interlayer distance. Changes in the composition of the exchangeable cations affect the interlayer distance or swelling of the clay. However, the high affinity of K⁺ and NH₄⁺ ions to exchangeable sites leads to their gradual fixation, a process which results in transformation of smectite to illite. The transformation from smectite to illite is a pedological or geological process that takes thousands of years to complete. When one considers transformations on a geological scale, the effect of temperature on this transformation was found to be critical, starting at 50 to 60°C. In the past few decades, new observations have indicated that the transformation of smectite to illite occurs also under ambient air temperature conditions with the relatively short time scales associated with soil cultivation. Such fast transformation has been shown in laboratory conditions under cycles of wetting and drying (Eberl et al., 1986 and 1993). Sandler and Harlavan (2006) suggested that high K concentrations may lead to this transformation without wetting and drying cycles. In vitro experiments of K⁺ and NH₄⁺ fixation under wetting and drying cycles were conducted and the illitization of the original smectite by both ions was similar (Miklos and Cicel, 1993). The formation of illite from smectite in experimental field plots (Versailles) fertilized by inorganic K fertilizer and manure have been reported by Pernes-Debuyser et al. (2003). Additional laboratory experiments showed that the changes over one growing season were measureable (Barré et al., 2007a, 2007b and 2008). The opposite process of fast release of interlayer K and decomposition of illite clay due to agricultural activity has also been reported. The most extreme cases were observed in a rice field where wetting and drying cycles are common and the potassium is easily leached, leading to measureable changes over a relatively short time of 30 years (Li et al., 2003). Continuous maize cropping over several decades resulted in decomposition of soil illite, whereas no change was obtained under a corn-oat-hay rotation with the same fertilization treatment (Velde and Peck, 2002).

Numerous published data indicate that K⁺ and NH₄⁺ are attracted by the same exchange sites with similar affinity. A slight preference for K⁺ over NH₄⁺ has been demonstrated in laboratory exchange isotherms (Fig. 3, Chung and Zasoski, 1994). They also reported that similar exchange isotherms were obtained in the bulk solution and rhizosphere, independent of the solution pH and ionic strength. Consequently, the concentration of NH₄⁺ in the soil solution has a strong and direct impact on the distribution of K⁺ between the soil solution and the exchange complex and vice versa. The situation becomes more complex when the soil contains additional cations. In a laboratory study, Evangelou and Lumbanraja (2002) investigated the exchange of K⁺ - NH₄⁺ -Ca+2 on vermiculite and hydroxy-aluminium vermiculite. They found that in the case of vermiculite, binary exchange data alone may not be able to predict ternary exchange data. However, in the case of hydroxy-aluminium interlayered vermiculite, binary data may be able to predict ternary data as long as the third cation’s distribution between exchange and solution phases remains constant across the isotherm. Liu et al. (1997) found that K⁺ application enhanced NH₄⁺ fixation but reduced the amount of exchangeable NH₄⁺ on the clay surface. Wang et al. (2010) reported that the application of the fertilizer ammonium sulfate to different soil types from China in incubation experiments significantly altered the distribution between pools of native K and added K in the soils. Addition of ammonium sulfate significantly increased water-soluble K⁺ and decreased exchangeable K⁺ in almost all the soils except the paddy soil that contained considerable amounts of 2:1 type clay minerals with K⁺ added. Added ammonium sulfate also reduced the formation of fixed K⁺ in the soils with K⁺ added and suppressed the release of fixed K⁺ in the three studied soils without K⁺ added.

The nitrogen source may significantly affect rhizosphere pH (Nye, 1981; Marschner, 1995; Marschner and Römheld, 1996; Bar-Yosef, 1999; Bloom et al., 2003). The common N fertilizer sources in soil systems are urea, NH₄⁺ and NO₃^-. Urea is the cheapest N source (per N unit) and is the most concentrated N fertilizer (46%); it is highly soluble, moves easily in irrigation water and is therefore widely used in agriculture. The hydrolysis process of urea yields the derivative NH₄⁺ ions - and thus urea is a source of the NH₄⁺ form for plant nutrition. Rapid adsorption on the surface sites of the solid phases, and nitrification reactions diminish the NH₄⁺ content in soils and media, and hence, rhizosphere NH₄⁺ concentrations are commonly low even under irrigation with a high NH₄⁺-N/NO₃^--N ratio.

In the field, the transformation of NH₄⁺ to NO₃^- is usually a rapid process, between days to weeks, depending on temperature, soil moisture and pH. Therefore, in most fertilizer application methods the form of N should not have a strong impact on K distribution between the exchangeable and solution phases. However, when continuous fertigation is used it is expected that the N source will effect K concentration in the solution in the wetted soil, and several studies have validated this assumption. For example, during the irrigation season, K⁺ concentration in the soil solution of a citrus orchard increased when N was applied as ammonium sulfate, as opposed to ammonium nitrate (Fig. 4, Erner et al., 2011, submitted). The application of a nitrification inhibitor enhanced the effect of the ammonium sulfate on soil solution K⁺ concentration, indicating that the effect of the fertilizer on K concentration was via the NH₄⁺ - K⁺ exchange process.

Another mechanism by which the N source influences K availability and leaching is unique to calcareous soils common in arid and semi-arid regions. In carbonate-bearing soil, the acid produced by nitrification of NH₄⁺ gives rise to an increase in the concentrations of Ca²⁺ and Mg²⁺ in the soil solution, which can exchange with K⁺, leading to higher K⁺ solution (unpublished data of the author, Kolahchi and Jalali, 2007).

Potassium and nitrogen interactions in plants

Nitrogen is a unique nutrient that can be absorbed either as the cation NH₄⁺or the anion NO₃^- (Marschner, 1995). This characteristic of nitrogen influences plant nutrition in general and the uptake of other elements that are taken up as ions as well as numerous physiological processes in plants (Marschner, 1995; Forde and Clarkson, 1999; Mengel and Kirkby, 2001; Epstein and Bloom, 2005). The cation-anion balance in plant tissues is maintained by diffusible and non-diffusible organic and inorganic ions, and has been found to be notably affected by the sources of N nutrition (Marschner, 1995; Mengel and Kirkby, 2001; Epstein and Bloom, 2005). It has been widely reported that NH₄⁺ nutrition depressed the uptake of cations, especially in leaves and petioles, and that NO₃^- nutrition depressed that of anions (Kirkby and Mengel, 1967).

The nitrogen source has a strong impact on soil rhizosphere and the rhizosphere pH via three mechanisms (Marschner, 1995; Marschner and Römheld, 1996; Bar-Yosef, 1999): (i) displacement of H⁺/OH^- adsorbed on the solid phase; (ii) nitrification/denitrification reactions; and (iii) release or uptake of H⁺ by roots in response to NH₄⁺ or NO₃^- uptake, respectively. Mechanisms (i) and (ii) are not associated with any plant activity, and affect the whole volume of the fertigated substrate, whereas mechanism (iii) is directly related to the uptake of nutritional elements, and may be very effective in changing the pH because it affects a limited volume in the immediate vicinity of the roots (Moorby et al., 1984; Gahoonia and Nielsen, 1992; Gahoonia et al., 1992; Marschner and Römheld, 1996; Taylor and Bloom, 1998; Bloom et al., 2003). The extent of the pH changes caused by the three mechanisms described above depends on soil properties, plant activity, and all the environmental factors that affect nitrification rate.

Transport through membranes and long-distance transport in plant

The selective transport of nutrients through plant membranes is mainly regulated by transporters (Hirsch et al., 1998). Two distinct membrane transport systems for K uptake by plants have been described: high and low affinity transport systems (HATS and LATS); (Maathuis, 2007; Maathuis and Sanders, 1997; Nieves-Cordones et al., 2007). At low external K⁺concentrations (<1 mM), HATS are dominant; while at higher external K⁺ concentrations (>1 mM) LATS become dominant, mostly via ion channels (Maathuis and Sanders, 1997; Hirsch et al., 1998). Over the past two decades, the complex system of K⁺ transporters and the genes encoding them have been identified (Szczerba et al., 2009).

NH₄⁺ inhibits HATS which operate primarily at low external K⁺ concentrations (<1 mM), while LATS, which dominates at higher K⁺ external concentrations, are relatively NH₄⁺ insensitive (Hirsch et al., 1998; Nieves-Cordones et al., 2007 and others) (Fig. 5). On the other hand, NO₃^--grown plants regulate cytosolic K⁺ concentrations at high K⁺ external concentration better than NH₄⁺-grown plants by potassium efflux through LATS (Szczerba et al., 2006).

NO₃^- is a univalent anion that can serve as a counter ion to K⁺ (Abdolzadeh et al., 2008; Guo et al., 2007; Jarvis et al., 1990; Lu et al., 2005; Mathuis, 2007; Pettersson, 1984) and vice versa. Ivasshikina and Feyziev (1998) reported that the rate of NO₃^- uptake by maize seedlings was faster when K⁺ served as the accompanying counter ion, rather than the other main cations, Ca²⁺, Mg²⁺ and Na⁺ (Fig. 6). It was found that growing sugarcane seedlings in a K⁺ depleted nutrient solution for a 3 1/2-month period reduced the roots’ capacity to absorb NO₃^- (Subasinghe, 2006). On the other hand, growing sugarcane seedling in an N depleted nutrient solution reduced the initial K⁺ uptake rates and decreased the affinity of roots for K⁺.

The potassium ion is easily mobilized in the whole plant because it is not assimilated in organic compounds. Like other cations, it is transported from the root system upward to the canopy via the xylem, but it also has the property of high phloem mobility and, as a result, a high degree of re-translocation via the phloem (Marschner, 1995). K uptake and re-translocation play an important role in NO₃^- transport from roots to shoots as a counter ion and assimilate loading in the phloem (Maathuis, 2007). A conceptual model of K circulation and NO₃^- uptake was suggested by Ben Zioni et al. (1971). According to this model, NO₃^- is absorbed by roots and transported via the xylem to the shoots with K+ as the counter ion. In the shoot, NO₃^- is reduced and assimilated into organic compounds, whereas K+ is transported downward to the roots with malate via the phloem (Fig. 7). Then the root takes up more NO₃^- and exudes HCO₃^- to maintain electro-neutrality or takes up more K⁺, according to the external and internal K concentrations and the N form. The rate of uptake and partitioning of K in plants can be modified depending on the N forms supplied. NH₄⁺-N as the sole N source caused a decrease in potassium uptake relative to the NO₃^--N supply in tobacco (Lu et al., 2005) and (Ganmore- Neumann and Kafkafi, 1980) tomato plants. In tobacco plants, using NH₄⁺ as the sole N source resulted in more K translocated to leaves than NO₃^- in terms of the amounts of xylem-transported K (Lu et al., 2005). In contrast, Ganmore- Neumann and Kafkafi (1980) reported that increasing the NO₃^-/NH₄⁺ ratio in the solution resulted in higher K concentrations both in roots and leaves of tomato plants (Fig. 8), in agreement with other reports that roots treated with high levels of NO₃^- absorbed and translocated more K⁺ (⁸⁶Rb) than seedlings treated with low levels of NO₃^- (Pettersson, 1984; Zsoldos et al., 1990). Ganmore-Neumann and Kafkafi (1980) showed that temperature elevation from 8 to 34°C resulted in a gradual decrease and increase of K content in roots and shoots, respectively, due to enhanced transport from increased temperatures (Fig. 8). Engels and Kirkby (2001) reported that cycling and recycling of K⁺ increased with increasing shoot growth rate, which is in accordance with the suggested model by Ben Zioni et al. (1971). Cytosolic K⁺ concen-trations vary between 40 and 200 mM, depending on K⁺ supply and nitrogen form (NO₃^- or NH₄⁺) demonstrating the strong impact of N form on potassium dynamics in plants (Szczerba et al., 2006).

Effects on whole plant, crop yield and quality

K⁺ depletion of the nutrient solution enhances the absorption of NH₄⁺-N, but in contrast suppresses the absorption, translocation, and assimilation of NO₃^--N, while simultaneously lowering leaf nitrate reductase activity (NRA). This behavior suggests that plants require an adequate supply of K for absorbing NO₃^--N and maintaining high levels of NRA as compared with the assimilation of NH₄⁺-N (Ali et al., 1991). K⁺ activates plant enzymes functioning in NH₄⁺ assimilation and transport of amino acids (Hagin et al., 1990). Therefore, an adequate supply of K⁺ enhances NH₄⁺ utilization and thus improves yield when both N forms are applied together. Potassium has been shown to strongly stimulate carboxylic acid accumulation in fruits (Erner et al., 1993). This is because the high rate of K⁺ uptake increases the cation-anion balance in the plant tissue and, in order to maintain electro- neutrality, carboxylates are synthesized (Marschner, 1995). NO₃^- uptake also stimulates carboxylation in plant tissue in order to remove hydroxyls produced in the NO₃^- reduction process and to balance the excess accumulation of cations over anions (Marschner, 1995). Thus, both K⁺ and NO₃^- have similar effects on organic acids production in plants leaves and fruits (Serna et al., 1996).

The NH₄⁺:NO₃^- ratio is an important N-management tool, particularly in soil-less culture (Silber and Bar-Tal, 2010) and in soil under fertigation (Bar-Yosef, 1999). For a given N dose and irrigation regime, the N-NO₃^-:N-NH₄⁺ ratio affects Ca, Mg, and K uptake (Bar-Tal et al., 2001a, b; Kirkby and Mengel, 1967; Neilsen et al., 1995), and carboxylic acid biosynthesis (Kirkby and Mengel, 1967; Mengel and Kirkby, 2001). However, in a recent experiment investigating the effect of N source on grapefruit, no effect on fruit quality and acid concentration in the fruit was found (Erner et al., 2011, submitted). A possible explanation for this result is: in soil grown plants, two confounding effects of NH₄⁺ take part, on the one hand a higher NH₄⁺ concentration increases K⁺ concentration in the soil solution (Fig. 4) and on the other hand the competition between NH₄⁺ and K⁺ on root uptake reduced the uptake of K⁺ by the trees.

Conclusions

The overall interactions of N form and K⁺ application on whole plant response and crop yield are a product of the above described effects.

Proper K⁺ application can improve N fertilizing efficiency, increase yield and reduce environmental pollution.

Further studies on the mechanisms of K⁺ and NH₄⁺ fixation in soil clay minerals are required for better understanding of long-term effects on their availability to plants.

Further studies on the interactions of K⁺, NH₄⁺ and NO₃^- at the molecular and whole plant levels are required for improving N and K fertilization.

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