土壤学第九章 土壤养分(英文版)

全文电子教材
土壤与土壤资源学
（上篇：土壤学）
林学专业
2 O 2
SO
2
H 2O
O 2
Mineral
Nutrients
英文版—土壤养分
Chapter 9. Soil Nutrients
Soil nutrient availability is one of the factors that often limit tree growth and soil productivity. Other factors commonly limiting for tree growth can include soil moisture availability, climate (such as temperature and precipitation), soil physical properties (such as drainage and soil compaction), or a combination of the above factors. N is often a nutrient that is most deficient for plant growth. Nitrogen deficiency can be caused by low N content in the soil or by the slow release rate in ecosystems such as the boreal forests or peatlands where low temperature or poor aeration encourages accumulation of organic matter and reduces N mineralization rates. Phosphorus is also frequently deficient in soils where there is very little P in the parent material or where most of the P has been lost through weathering during the soil formation processes, such as in the tropics.
There are 16 elements that are considered essential for plant growth. Lack of any of those essential nutrients will hinder the proper growth and functioning of the plants and will prevent the plants from completing their life cycle. Among those 16 essential nutrients, C, H, and O come from the air and water and are usually not deficient, although recent climate change studies using CO2 enriched air showed that increasing atmosphere CO2 concentration can significantly increase forest productivity; however, plants usually acquire the other essential nutrients from the soil. Among the macronutrients (N, P, K, Ca, Mg, and S), Mg and S can also sometimes be deficient for tree growth. Potassium and calcium deficiencies in forests are very rare. In terms of micronutrients (Mn, Zn, Cu, Fe, Mo, B, and Cl), B, Zn, Cu, and Fe deficiencies, especially B deficiency, are most frequently reported. These nutrients are called micronutrients because they usually exist on the earth and are required by plants in very small quantities. In addition to those 16 essential nutrients, cobalt (Co), vanadium (Va), nickel (Ni), silicon (Si), and sodium (Na) have been found to be essential to some plants. For example, nickel has been found to be essential for soybeans and Si for rice. In this chapter, we will discuss the importance of soil nutrients in tree growth, discuss the macronutrients and micronutrients, describe the cycling of nutrients in the soil, and provide an introduction to the mechanisms of plant nutrient uptake.
9.1 Nutrients: available forms, availability and functionality
The interaction of numerous physical, chemical, and biological properties in soils controls the availability of soil nutrients for plant uptake. Understanding these processes will enable us to manage selected soil properties to optimize nutrient availability and soil productivity. To understand these interacting processes will require us to have a good knowledge of the soil properties and processes covered in the earlier chapters. Not all nutrients present in the soil are available for plant uptake and different nutrients have different available forms.
a) Forms of nutrients plant can uptake
Details of available nutrient forms will be discussed in the next section where the individual macro- and micronutrients are presented. The forms of the essential nutrients that plants can uptake, along with their functionality and normal amounts in plants, are listed in Table 11.1. One thing common to all nutrients is that plants acquire most of their needed nutrients from the soil solution and mostly in the inorganic form. Some acquisition of nutrients through the gaseous form is possible. For example, plants can absorb NH3 and SO2 in the air through the stomata. Nitrogen cycling is one of the most complex as compared with the cycling of the other essential nutrients. One of the important mechanisms for increasing plant N availability is through symbiotic N-fixation. With this mechanism, most of the N the host plant uptake comes from the bacterial that can fix N2 in the air. There have been reports to indicate that trees sometimes can take up organic N in the form of simple amino acids and proteins. The uptake of organic form of N has been found to be mostly assisted by mycorrhizas and this uptake mechanism is very important in soils with low fertility and for nutrients with low mobility in the soil. A few species of plants are able to use animal proteins as an N source directly. These carnivorous plants, such as the common bladderwort (Utricularia vulgaris) and the sundew (Drosera rotundifolia), have special adaptations that are used to lure and trap insects and other very small animals. The plants digest the trapped organisms, absorbing the nitrogenous compounds the organisms contain as well as other compounds and minerals, such as potassium and phosphate. Most of the carnivores of the plant world are found in bogs, a habitat that is usually quite acidic and thus not favorable for the growth of nitrifying bacteria.
b) Nutrient availability
Nutrient availability is an important area of interest in soil nutrient management. Nutrient availability falls into the soil science discipline of soil fertility. Soil fertility is narrowly defined as “the status of a soil with respect to the amount and availability to plants of elements necess ary for plant growth”. Of all soil properties, fertility is the one with which man is most involved; it is the property that can be readily changed by man in his exploitation or management of the land. In intensively managed forest systems, such as in plantations, soil nutrient availability can be altered and managed through silvicultural techniques such as site preparation, weed control, thinning, and fertilization. Even in natural forests, where there is very little human control of processes, soil nutrient availability is not a completely stable factor but changes with stage of forest succession, natural disturbance regimes, and with soil profile development. Occurrence of fire and extensive wind throw can result in sudden dramatic changes in soil nutrient availability. A soil, particularly one with the heterogeneity of many forest soils, cannot be considered to have a unique single, static level of soil fertility.
Since plants take up most of their needed nutrients from the soil solution, nutrient availability is controlled by the interaction of numerous physical, chemical, and biological properties in soils. The basic relationship between the various components of the dynamic soil system is depicted in Figure 11.1. In reactions 1 and 2, plants absorb nutrients (cations and anions) from the soil solution and release small quantities of ions such as H+ (to balance the charge in soil solution, if
cations are absorbed by plants), or OH- and HCO3- (if anions are absorbed). In reactions 3 and 4, changes in ion c oncentrations in soil solution are “buffered” by ions adsorbed on the surface of soil minerals. Ion removal from solution causes partial desorption of the same ions from these surfaces. In reactions 5 and 6, minerals contained in the soil can dissolve to re-supply soil solution with many ions; likewise, increases in ion concentration in soil solution resulting from fertilization or other inputs can cause some minerals to precipitate. In reactions 7 and 8, soil microorganisms can remove ions from soil solution and incorporate them into microbial tissues, and conversely, when microbes or other organisms die, they release nutrients to the soil solution. Microbial activity produces and decomposes organic matter or humus in soils. These dynamic processes are very dependent on adequate energy supply from organic C, inorganic ion availability, and numerous environmental conditions. In reactions 9 and 10, plant roots and soil organisms utilize O2 and respire CO2 through metabolic activities. As a result, CO2 concentration in the soil air is greater than in the atmosphere. Diffusion of gases in soil decreases dramatically with increasing soil water content and soil depth. In reactions 11 and 12, numerous environmental factors and human activities can influence ion concentration in soil solution, which reacts with the mineral and biological processes in soil. For example, adding ammonium fertilizer to soil can increase the N concentration in the soil solution, but over time, N concentration in the soil solution will decrease due to plant uptake, volatilization losses, transformation of ammonium into nitrate through the nitrification process, and immobilization of ammonium by microorganisms and fixation by clays and organic matter through inorganic reactions.
All of these processes and reactions are important to the availability of plant nutrients; however, depending on the specific nutrient, some processes are more important than others. For example, microbial processes are more important to N and S availability than mineral surface exchange reactions, whereas the opposite is true for K, Ca, and Mg.
c) Functions of inorganic nutrients in plants
Table 9.1 lists some of the functions of nutrients in plant growth and physiology. Inorganic ions affect osmosis and thus help to regulate water balance in plants. Several inorganic ions can serve interchangeably in this role, in many plants this particular requirement is described as non-specific. On the other hand, an inorganic nutrient may function as part of an essential biological molecule; in this case the requirement is highly specific. An example of a specific function is the presence of magnesium in the chlorophyll molecule. Some of the common functions of mineral nutrients are discussed below.
Catalysts: A key role of the inorganic nutrients is their participation in some of the enzymatic reactions of the plant cell. In some cases, they are essential structural parts (a “prosthetic group”) of the enzyme. In other cases, they serve as activators or regulators of certain enzymes. Potassium, for instance, which probably affects 50-60 enzymes, is believed to regulate the conformation of some proteins. Changing the shape of an enzyme could, for example, expose or obstruct reaction sites.
Electron transport:Many of the biochemical activities of cells, including photosynthesis and respiration, are oxidation-reduction reactions. In such reactions, electrons are transferred to or from a molecule that functions as an electron acceptor or donor. The cytochromes, which contain iron, are involved in electron transfer.
Structural and molecular components:Some mineral elements serve as structural components of cells, either as part of a physical structure or as part of the molecules involved in cellular metabolism. Calcium combines with pectic acid in the middle lamella of the plant cell wall. Phosphorus occurs in the sugar-phosphate backbone of DNA and RNA and in the phospholipids of the cellular membranes. Nitrogen is an essential component of amino acids, chlorophylls, and nucleotides. Sulphur is found in two amino acids that form a component of proteins.
Osmosis:The movement of water into and out of plant cells is largely dependent on the concentration of solute in the cells and in the surrounding medium. The uptake of ions by a plant cell thus may result in the entry of water into the cell. The increased turgor pressure results in expansion of the immature cell, which is the chief cause of cellular growth, and in the maintenance of turgor in the mature cell. This is an example of conversion of energy from one form to another by a living system; the chemical energy (ATP) expended in the active uptake of ions by the plant cell is translated into the physical energy of water movement.
Effects of cell permeability: Calcium has a direct effect on the physical properties of cellular membranes. When there is a calcium deficiency, membranes seem to lose their integrity, and solutes within the membranes or cells leak out.
9.2 Macronutriens: N, P, K, Ca, Mg, and S
9.2.1 Nitrogen
a) Origin and distribution of N
The N in soil is derived from the earth’s atmosphere. The N content of surface mineral soils typically ranges from 0.02 to 0.5%. About 98% of the earth’s N is contained in the igneous rocks deep under the planet’s crust, where it i s effectively out of contact with the soil-plant-air-water environment in which we live. Therefore, we must concentrate our discussion of N cycling on the remaining 2% that cycles in the biosphere. Most of the N found in the soil comes from biological N fixation. The atmosphere contains a large amount of N2 (78% of the atmosphere is N2 gas). Some 75,000 Mg of N is found in the air above 1 ha of the land surface. However, the very strong triple bond between two nitrogen atoms makes this gas quite inert and not directly usable by plants or animals. Were it not for the ability of certain microorganisms to break this triple bond to form nitrogen compounds, vegetation in the terrestrial ecosystems around the world would be rather sparse, and little N would be found in soils.
Most of the N in terrestrial ecosystems is found in the soil. The soil contains 10 to 20 times as much N as does the standing vegetation (including roots) of forest ecosystems. Most soil N occurs as part of organic molecules. Soil organic matter typically contains about 5% N; therefore, the distribution of soil N closely parallels that of soil organic matter. Except where large amounts of chemical fertilizers have been applied, inorganic N (NH4+ and NO3-) seldom accounts for more than 1 to 2% of the total N in the soil. Unlike most of the organic N, the mineral forms of N are mostly quite soluble in water and may be easily lost from soils through leaching and volatilization.
b) Forms of N in the soil
The different forms of N that can be found in the soil can be divided into two categories: inorganic and organic forms of N. As discussed above, most of the soil N exists in the organic form.
Inorganic N: Inorganic forms of N include ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), nitrous oxide (N2O), nitric oxide (NO), and the nitrogen gas (N2). Trace amounts of nitrite may be present in the soil. Nitrite is toxic to plants and is generally quickly converted to nitrate in the nitrification processes. Therefore, nitrite usually does not accumulate in the soil. N2O, NO, and N2 are the products of dinitrification or contained in the air trapped in the soil pores. As will be discussed below, conditions in forest soils generally favor the formation of ammonium and plants are adapted to this dominant form of N as a N source. Ammonium is the product of mineralization of organic N. Nitrate is formed through the nitrification process. There is usually abundant nitrate accumulation in the soil where conditions favor nitritication. The inorganic N content in soils is very dynamics as its concentration is affected by a large number of factors, including temperature, moisture content, plant uptake, microbial population, organic matter content, and so on. There are distinct seasonal and diurnal changes in soil inorganic N contents in the soil.
Both inorganic N forms are soluble in water. Ammonium is mainly present in the soil on exchangeable sites and the positively charged ammonium can be attracted on to the negatively charged surfaces of clay and organic particles. This mechanism presents NH4+ from being easily lost from the soil solution. NH4+ can also be fixed in the clay structure, making it unavailable for plant uptake as well as from being lost through leaching. On the other hand, most of the NO3-, if present, will be found in the soil solution and is much more proven to be lost through leaching.
Organic N: Organic N usually represent greater than 95% of the total soil N. Organic N occurs as proteins, amino acids, and other complex N compounds. Organic N can be separated into three types based on their solubility and how easy they can be hydrolyzed: a) soluble organic N: usually less than 5% of the total soil N content. Some of the soluble organic N (such as simple amino acids) can be take up directly by plants, especially with the assistance of mycorrhizas. This fraction of the organic N can be easily hydrolyzed to release NH4+ for plant uptake; b) hydrolyzable organic N. This fraction of organic N can be hydrolyzed to simpler soluble organic N when treated with acids or alkalis; and c) non-hydrolyzeable organic N. The content of this fraction can be as high as 50% of the total N in the soil. This is the most stable fraction of the soil organic N and the nature of this fraction of N is still not very clear. Much of the organic N forms organo-mineral complexes. Organic N in these complexes are much more stable than the non-complexed organic N in the soil.
c) N cycling processes
The processes of N cycling are presented in Figure 11.2. The main N cycling processes are discussed below.
Biological N fixation:Through biological N-fixation, certain organisms convert the inert dinitrogen gas of the atmosphere to N-containing organic compounds that become available to all form of life through the N cycle. Terrestrial ecosystems have been estimated to fix 130 to 180
million Mg of N, about twice as much as is industrially fixed in the manufacturing of fertilizers.
Symbiotic bacteria (Rhizobia) fix N2 in nodules present on the roots of legumes. This fixed N may be utilized by the host plant, excreted from the nodule into the soil and be used by other nearby plants, or released as nodules or legume residues decompose after the plant dies or is incorporated into the soil. Other microorganisms that are also capable of fixing N include Actinomycetes and Frankia that fix N in symbiosis with non-legume tree species such as alders, Myrica, and Casuarina; Azotobacter and Azospirillum are heterotrophic free-living fixers; and blue-green algae and Anabaena are autotrophic free-living fixers.
Regardless of the organisms involved, the key to biological N fixation is the enzyme Nitrogenase, which catalyzes the following reaction:
(Nitrogenase)
N2 + 8H+ + 6e- ® —————————→2NH3 + H2
(Fe, Mo)
The nitrogenase are proteins that contain Fe and Mo. The nitrogen fixation process requires a great deal of energy. The energy either comes from the host plant for organisms that form symbiosis, or from the soil organic matter for the heterotrophic free-living bacteria, or from the sun light for the autotrophic free-living organisms. The accumulation of ammonia will inhibit N fixation and too much nitrate in the soil will inhibit the formation of nodules. In addition to Fe and Mo, N-fixing organisms also require high amounts of P and S as these nutrients are either part of the nitrogenase molecule or are needed for its synthesis and use.
The production of N by industrial fixation is based on the Haber-Bosch process, in which H2 and N2 gases react to form NH3, under high temperature and pressure:
Catalyst
3H2 + N2 ® ——————→NH3
1,200 °C, 500 atm
Immobilization and mineralization: The majority (95-99%) of the soil N is in organic compounds that protect it from being lost but this also leaves it largely unavailable to higher plants. The quantities of NH4+ and NO3- available to plants depend largely on the amounts applied as N fertilizers and mineralized from organic N in soil. Much of the organic N is present as amine groups (R-NH2), largely in proteins or as part of humic compounds. When soil microbes attack these compounds, simple amino compounds (R-NH2), such as lysine (CH2NH2COOH) and alanine (CH3CHNH2COOH), are formed. Then the amine groups are hydrolyzed, and the N is released as ammonium ions (NH4+), which can be oxidized to the nitrate form. This enzymatic process is termed mineralization, that includes the ammonification (from simple amino compounds to NH4+) and nitrification (from NH4+ to NO3-) processes. A specific term called aminization describes the process from the amine groups and proteins to simple amino compounds:
H2O
Proteins ® RCHNH2COOH + R-NH2 + CO(NH2)2 + CO2 + energy
Bacteria, fungi
Using an amino compound (R-NH2) as an example of the organic N source, the mineralization process can be indicated as follows:
+2H2O +O2 +1/2O2
R-NH2 ⇌OH- + R-OH + NH4+ ⇌4H+ + energy + NO2- ⇌energy + NO3-
-2H2O -O2 -1/2O2
The opposite of the mineralization process is immobilization, the conversion of inorganic N ions (NH4+ and NO3-) into organic forms. Immobilization can take place by both biological and non-biological (abiotic) processes, the latter being of considerable importance in forest soils. Through the biological processes, as microorganisms decompose carbonaceous organic residues in the soil, they may require more N than is contained in the residues themselves and thus may immobilize NH4+ and NO3- in the soil solution. The microbes need N to maintain a C:N ratio of about 8:1. The microorganisms incorporate mineral N ions into their cellular components, such as proteins, leaving the solution essentially void of NO3- and NH4+ ions. During the immobilization process, microorganisms can compete very effectively with plants for NH4+ or NO3-. When the organisms die, some of the organic N in their cells may be converted into forms that make up the humus complex, and some may be released as NH4+ and NO3- ions. During the decomposition of nitrogenous compounds, microorganisms incorporate the N into amino acids and proteins (as part of the microbial biomass) and release excess N in the form of ammonium ions. In alkaline media, the N may be converted to ammonia (NH3), but this conversion usually occurs only during the decomposition of large amounts of N-rich material, as in the mature pile or a compost heap that has contact with the atmosphere. Within soil, the ammonia produced by ammonification is dissolved in the soil water, where it combines with protons to form the ammonium ions. Mineralization and immobilization occur simultaneously in the soil; whether the net effect is an increase or decrease in the amount of mineral N available in the soil depends primarily on the ratio of C to N in the organic residues undergoing decomposition.
The amount of plant available N released from organic N depends on many factors affecting N mineralization, immobilization, and losses of NH4+ and NO3- from the soil. Mineralization being a microbial process will increase with a rise in temperature and is enhanced by adequate, although not excessive, soil moisture and a good supply of O2. Maximum aerobic activity and N mineralization occur between 50 and 80% water-filled pore space. Optimum temperature for N mineralization ranges between 25 and 35 °C.
One of the factors affecting N mineralization and immobilization is the C:N ratio of the decomposing material. The N content of humus or stable soil organic matter ranges from 5 to 6%, whereas C ranges from 50 to 60%, giving a C:N ratio ranging between 8 and 12. When fresh organic material is added to the soil, there is a rapid increase in the number of heterotrophic organisms, accompanied by the evolution of large amounts of CO2, during the initial stage of decomposition. If the C:N ratio of the initial material is greater than 30:1, N immobilization occurs. As decay proceeds, the C:N ratio of the residue narrows and energy supply diminishes.
Some of the microbial population dies because of the decreased food supply, and ultimately a new equilibrium is reached, accompanied by the mineralization of N. Generally speaking, when organic substances with C:N ratios between 20 and 30 are added to the soil, there may be neither immobilization nor release of mineral N. For organic materials with C:N ratio less than 20, there is usually a release of mineral N early in the decomposition process.
In the organic matter mineralization processes, bacteria dominate the breakdown of proteins in neutral and alkaline environments, with some involvement of fungi, while fungi predominate under acidic environments (and most forest soils are acidic).
Many studies have shown that only about 1 to 4% of the organic N of a soil mineralizes annually. Even so, the rate of mineralization provides sufficient mineral N for normal growth of natural vegetation (such as forests) in almost all soils except those with low organic matter, such as the soils of deserts and sandy areas. Mineralized soil N constitutes a major part of the N taken up by plants.
Nitrification: Several species of bacteria common in soils are able to oxidize ammonia or ammonium ions in a process called nitrification. This is an energy yielding process, and the energy released in the process is used by these bacteria to reduce CO2 in much the same way that photosynthetic autotrophs use light energy in the reduction of CO2. Such organisms are known as chemosynthetic autotrophs (as distinct from photosynthetic autotrophs). The chemosynthetic nitrifying bacterium Nitrosomonas is primarily responsible for oxidation of ammonium to nitrite ions (NO2-).
Nitrosomonas
2NH4+ + 3O2 ® 2NO2- + 4H+ + 2H2O + energy
bacteria
Nitrite is toxic to plants, but it rarely accumulates in the soil. Nitrobacter, another genus of bacteria, oxidizes the nitrite to form nitrate ions (NO3-), again, with a release of energy:
Nitrobacter
2NO- + O2 ® 2NO3- + energy
bacteria
Once nitrate is formed and if it is not quickly taken up by plants or microbial organisms (in the process of microbial immobilization), it can be lost from the soil through leaching, when there is water percolating through the soil profile, and denitrification under anaerobic conditions. Nitrification will significantly increase soil acidity by producing H+ ions. Nitrification requires NH4+ ions, but excess NH4+ is toxic to Nitrobacter and must be avoided. The nitrifying organisms, being aerobic, require O2 to make NO2- and NO3- ions, and are therefore favored in well-drained soils.
In forest soils, fortunately, nitrification rates are very low and most of the available form of N is present in the ammonium ion form. There are several possibilities that nitrification rates are low in forest soils. One possibility is that nitrification rates are inhibited by the low soil pH as forest soils are usually acidic. A second possibility is that nitrifying bacteria population is very low (that itself may be related to the inhibition by the acidic condition and other limiting factors) in forest
soils. Under prolonged incubations in the lab, nitrification eventually develops, although this may take as long as one year under optimum conditions. Another possibility is that microbial populations in forest soils have a very strong ability to immobilize the nitrate produced from nitrification. Therefore, under such a scenario, as soon as the nitrate is formed the microbial populations take it up. Recent gross N mineralization studies using 15N-labeled fertilizers confirmed such cases.
Nitrification is also a microbial process and is thus affected by soil environmental factors. Nitrification is affected by 1) soil NH4+ content, 2) population of nitrifying organisms, 3) soil pH, 4) soil aeration, 5) soil moisture, and 6) temperature. If there is no NH4+ in the soil solution, nitrification does not occur. Variation in populations of nitrifiers results in differences in the lag time between the addition of the NH4+ and the buildup of NO3-. Because of the tendency of microbial populations to multiply rapidly in the presence of an adequate supply of C, the total amount of nitrification is not affected by the number of organisms initially present, provided that temperature and moisture conditions are favorable for sustained nitrification.
Nitrification takes place over a wide range in pH (4.5 to 10), with an optimum pH of 8.5. Nitrifying bacterial need an adequate supply of Ca2+, H2PO4-, and a proper balance of micronutrients. Nitrifying bacteria are aerobes and maximum nitrification occurs at the same O2 concentration in the aboveground atmosphere. Nitrification rates are generally highest in soil water contents at field capacity or 1/3 bar water potential (80% of total pore space filled with water). In terms of temperature, the temperature coefficient, Q10, is 2 over the range 5 to 35 °C. Thus, a twofold change in the nitrification rate is associated with a shift of 10 °C within this temperature range. Optimum soil temperature for nitrification is 25 to 35 °C.
Nitrate leaching:Nitrate ions are not adsorbed by the negatively charged colloids that dominate most soils. Therefore, nitrate ions move down easily with drainage water and are thus readily leached from the soil. This process constitutes a loss of N from the soil system for plant uptake and also causes several serious environmental problems. Leaching of nitrate from acidic sources (nitrification or acid rain) also facilitates the loss of Ca and Mg and other nutrient cations. Much of the nitrate mineralized in certain highly weathered, acid, tropical Oxisols and Ultisols leach below the root zone before annuals can take it up. It has been found that some of this leached nitrate is not lost to groundwater, but is stored several meters deep in the profile where the highly weathered clay have adsorbed it on their anion exchange sites. Deep-rooted trees are capable of taking up this deep subsoil nitrate and subsequently using it to enrich the surface soil when they shed their leaves. Trees such as Sesbania, grown in rotation with annual food crops, can make this pool of leached N available for food production and prevent its further movement to ground water. Agroforestry practices such as this have the potential to make a significant contribution to both crop production and environmental quality in the humid tropics.
Ammonium fixation: Ammonium ions carry positive charges and thus can be attracted to the negatively charged surfaces of clay and humus, where they are held in exchangeable form, available for plant uptake, but partially protected from leaching. However, because of the particular size of the ammonium (and potassium) ion, it can become entrapped within cavities in the crystal structure of certain clays. Several 2:1 type clay minerals, especially vermiculites, have the capacity to fix both ammonium and potassium ions in this manner. Vermiculite has the greatest capacity, followed by fine-grained micas and some smectites, to fix ammonium and potassium in this manner. Ammonium and potassium ions fixed in the rigid part of a crystal structure are held in。