Chapter 4 Biological Nutrient Availability in Soil
[目的与要求]
1．掌握土壤养分（化学、空间、生物）有效性的概念和特点；
2．掌握植物根际的性质；
3. 熟悉根系生长与养分有效性的关系；
[重点]
1 养分迁移到根表的方式；
2 根际营养的特性和植物的适应机制；
3 VAM菌根的重要性；
[难点]
1．土壤养分有效性的相对性；
2．根际特性对养分活化、迁移和吸收的影响。
[课堂组织]
讲述、生活实例和多媒体教具结合
[教学内容]
nonavailable
Soil Nutrient
available
Characteristics of biological nutrient availability :
1 以矿质态养分为主；
2 位置接近植物根表或短期内可以迁移到根表的有效养分。
土壤养分生物有效性的含义：
1 指土壤中矿质态养分的浓度、容量与动态变化；
2 指根对养分的获得与养分向根表迁移的方式和速度；
3 是在根系生长与吸收的作用下，土壤中养分的有效化过程以及环境因素对养分有效化的影响。
4.1 Chemical availability of Soil Nutrient
4.1.1 Available Nutrient Extracted by Chemicals
化学有效养分指采用化学方法从土壤中提取出来的有效养分，主要包括可容性的离子态与简单分子态养分，易分解态和交换吸附态养分以及某些气态养分。
Extraction of chemical available nutrients
Agents: A whole range of conventional extraction methods involving different forms of dilute acids, salts, or complexing agents depending on nutrient forms and soil sorts.
Methods: Chemical,
Physical ----Electric Superfilter
Table 4—1 Mean Content of Readily Soluble Phosphorus in 15 Soils
Extracted with Various Solutions
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Extraction solution Readily soluble Phosphorus(mg/kg)
NaHCO3 24.9
(NH4)2CO3 23.9
NaOH+Na2C2O4 30.4
H2SO4+HCl 29.4
HAc+NH4NO3+NH4F+HNO3+EDTA 70.1
NH4HCO3+DTPA 14.8
NH4HCO3+ NH4C2O4 46.9
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Different goal and different soil choose different methods!!
Relativity!!
Experiments on oat using 28 sorts of soils for 7 years made by Burd and Hoagland.
Error in Relationship between chemical available nutrient and uptake quantity and growth by plant
Rapid and inexpensive method for obtaining information on nutrient availability in soils——used in recommending fertilizer application for many years with relative success.
4.1.2 Intensity and Capacity of Nutrients
4.1.2.1 Intensity factor (强度因素)
Intensity factor: the nutrient concentration in the soil solution (Schofield 1955)
4.1.2.2 Capacity factor（容量因素）
Capacity factor: the quantity absorbed to the solid phase or bound to labile organic compounds----a pool providing nutrient for intensity factor
4.1.2.3 Buffer factor (缓冲因素)
B= △Q/△I
Buffer factor: the ability of the soil to maintain nutrient intensity, which indicated how intensity varies with quantity.
Mini-review:
1.Only the potential capacity of a soil to supply nutrients to plants;
2.Not sufficiently characterize the mobility of the nutrients in soil;
3.Not provide any information on the plant factors.
4.2 Space Availability of Soil Nutrients
4.2.1 Nutrient Position and Availability
Root system soil has only 3% of the topsoil volume.
4.2.2 Movement of Nutrients to the Root Surface
Principles of nutrient movement to root surface was firstly stressed by Barber（1962）：
A. Root interception
B. Mass flow
C. Diffusion
4.2.2.1 Root interception（截获）
Definition: root directly absorbs nutrients from soil surface occupied by root system, which there is no transport process of nutrients.
① Root Interception ② Mass Flow ③ Diffusion
图4-1 植物根获取土壤养分的模式
4.2.2.2 Mass Flow（质流）
Definition: movement of water and dissolved nutrients to the root surface driven by transpiration of plants.
Nutrient conc.×the amount of water transpiration
Nutrient quantity absorbed by mass flow (%) = ×100%
Total amount of nutrient absorbe
The distance of nutrient movement by mass flow is longer than diffusion.
4.2.2.3 Diffusion（扩散）
Definition: movement of nutrients to root surface along a nutrient gradient.
Quantity is based on: diffusion coefficient.
Character:
Slower speed;
Shorter distance (0.1~15mm).
4.2.2.4 Contribution
Nutrients absorbed by root interception is a few. Most depends mass flow and diffusion.
The distance of nutrient movement by mass flow is longer than diffusion.
Table 4—2 Estimated Amounts of Mineral Nutrients Supplied to Maize Roots in a Fertile Silt Loam Soil by Root Interception, Mass Flow, and Diffusion
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Amount
“available”in Total uptake Supply (kg/ha) by
the topsoil by crops
Nutrient (kg/ha) (kg/ha) Interception Mass flow Diffusion
Calcium 4000 45 40 90 -
Magnesiu 800 35 8 75 -
Potassium 300 110 3 12 95
Phosphorus 100 30 1 0.12 28.9
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4.2.3 Factors
4.2.3.1 soil moisture
Importance:
◆ Increase contact between root surface and soil;
◆ Increase diffusion coefficient of ion
4.2.3.2 Fertilization
mass flow
Fertilization nutrition concentration Root interception
Diffusion
4.2.3.3 Absorption and fixation of nutrients
Table 4—3 Amount of P absorbed by oat fro organic and inorganic p
P fertilizer total absorbed P from fertilizer (P mg/pot)
(P mg/pot) absorbed P percentage (%)
Ca(H2PO4)2 12.1 2.3 19
BAPA 16.5 6.4 39
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4.3 Root Growth and Nutrient Availability
4.3.1 Character of Plant Root
4.3.1.1 Morphological structure
Fibrous root system (须根系)
Tap root system (直根系)
4.3.1.2 Root hair
Root hair length about 0.1~1.5 mm, diameter 5~25 µmol/L, 5 ×107~ 5× 108 hairs/cm2 root surface, area of root surface increases 2~20 times
Importance:
Improve nutrient uptake;
Improve nutrient transport
A close positive relation between uptake rate per unit root length and volume of root hair cylinder.
4.3.1.3 Root Depth
4.3.1.4 Root Density
Root density: the ratio of root surface area to unit soil volume
4.3.2 Environment factors affecting root growth
4.3.2.1 Soil physical factors
Bulk density (容重)
More susceptible to drought, require more soluble nutrients in topsoil.
A maximum yield of barley need 18 mg P at a soil of low bulk density, 30 mg P for high bulk density.
Temperature
Low T: nutrient uptake, Enzyme ,hormone……
High T: Protein
4.3.2.2 Mineral Nutrient Supply
The distribution of roots in soil can be modified by the placement of fertilizer.
4.3.2.3 Calcium/Total Cation Ratio and pH
Ca requirement for root growth is not a fixed value relating to pH and other cations.
Table 4—4 Effect of Liming an Acid Subsoil (pH4.6) on the Elongation of Cotton Root
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Percentage of Distance between
Subsoil mass limed layers Relative
limed (cm) root length
Unlimed — 32
10 4.5 38
20 4.0 57
40 3.0 57
60 2.0 70
100 — 100
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4.3.2.4 Organic Matter
Organic matter affects root growth by various ways:
HMW(fulvic acid ) at low concentration enhances root initiation and root elongation;
LMW (phenolic and short-chain fatty acid) at higher concentration inhibited.
4.4 Rhizosphere in Relation to Mineral Nutrition
Rhizosphere (根际）：the soil-root interface (受植物根系活动的影响, 在物理, 化学和生物学性质上不同于土体的那部分微域土区).
Rhizosphere soil: loosely adhering soil (~1-4 mm from the surface of the root axis)
Rhizoplane soil: closely adhering soil (~0-2 mm from the surface of the root axis)
Rhizosphere soil includes Both
4.4.1 Rhizosphere Nutrient
4.4.1.1 Concentration distribution of Rhizosphere Nutrient
① Accumulation ② Depletion ③ Equilibrium
图4-2 不同条件下根际养分浓度变化模式图
4.4.1.1.1 Accumulation
High transpiration rate water uptake >nutrient
4.4.1.1.2 Depletion
Low concentration, low transpiration rate Nutrient uptake > water
4.4.1.1.3 Equilibrium
Nutrient uptake = water transpiration rate
4.4.1.2 Factors affecting concentration distribution of rhizosphere nutrient
4.4.1.2.1 Sorts of nutrients
High conc.-accu.:Ca2+, NO3-, SO4-, Mg2+;
Low conc.-depletion: H2PO4-, NH4+, K+, ----
4.4.1.2.2 Soil Buffer Capacity
4.4.2 Rhizosphere pH
4.4.2.1 the reason for rhizosphere pH change
■The H+ or HCO3- by respiration;
■ The root exudates
4.4.2.2 Factors affecting rhizosphere pH
4.4.2.2.1 Nitrogen form
NO3-—N NH4+—N N2
4.4.2.2.2 Nitrogen fixation
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Fixing N pH and cation-anion ratio
uptake similar to NH4+
4.4.2.2.3 Nutrient Stress
Adaptation capability of plant to nutrient stress :
☆ Exudates and acidification
Proteoid root
4.4.2.2.3 Plant Genetic Characteristic
4.4.2.2.4 Rhizosphere Microorganism
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Respiration CO2 pH
4.4.2.3 Rhizosphere pH change and nutrient availability
Nutrient concentration
Nutrient form
Rhizosphere pH Nutrient transform
Root growth
Root uptake
4.4.2.3.1 Improve activation of phosphorus
4.4.2.3.2 Improve uptake of microelements
4.4.3 Rhizosphere Redox Potentials
In general speaking, redox potential of rhizosphere is lower than that of bulk soil
4.4.4 Root exudates
Root exudates: total amount of organic carbon which growing roots releasing into the rhizosphere.
The amount ranges from 1% to more than 30% of total dry matter production for young plant
The amount is affected by age, various stress (nutrient, water----)
An important factor affecting nutrient availability!!
4.4.4.1 Rhizosphere exudates component
※Secretion （分泌物）
※ Leakage （渗出物）
※ Lysate （粘胶物）
※ Sloughed-off
◆ LMW organic compounds
◆ HMW organic compounds
Soluble exudates constituents: sugars 65%, organic acid 33%, other 2% (aa, fat, vitamin, hormone ---)
4.4.4.2 Factors affecting root exudates
4.4.4.2.1 Nutrient stress
4.4.4.2.2 Rhizosphere microoganism
4.4.4.2.3 Plants
4.4.4.3 Effect of root exudates on nutrient availability
4.4.4.3.1 Increase contact between root and soil
4.4.4.3.2 Chemical activation of nutrient
-----Reduction
------Chelation
4.4.4.3.3 Improve buffer capability of rhizosphere nutrient
4.4.5 Rhizosphere Microoganism
4.4.5.1 Growth and morphology of root
4.4.5.2 Activation and competition of nutrients
4.4.5.3 Change redox potentials
Table 4—1 Potassium Nutritional Status of Wetland Rice Plants, Number of Bacteria, and Oxygen and Iron Concentration in the Nutrient Solution
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Number of O2 Conc. Fe ll Conc.
Supply of K+ bacteria (×106) (mg/liter) (mg/liter)
High K+ 1244 17.0 1.0
Low K+ 1686 8.6 2.4
High K+/no K+b 2036 0.5 10.6
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4.4.5.4 Mycorrhizas and nutrient availability
Mycorrhizas (菌根): 高等植物根系与真菌形成的共生体, 分:
★ 外生菌根(ectomycorrhizas)
★ 内生菌根(endomycorrhizas)
Ectomycorrhizas predominate in tree species of the teperate zone but can also be found in tree and shrub species of semiarid zone
Formation results from penetration of the fungal hyphae into free space to form a net work of fungal mycelium and an interwoven mantle around the root (sheath-forming mycorrhizas), which penetrate the surrounding soil.
Endomycorrhizas grow both inter- and intracellularly in the root cortex.
The most common represent the vesicular-arbuscular mycorrhiza (孢囊-丛枝状菌根 VAM)
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Mechanism for VAM to improve nutrient availability
＊通过外延菌丝大大增加吸磷表面积;
＊降低菌丝际pH值,有利于磷的活化;
＊真菌膜上运载系统与磷的亲和力高于寄主植物根细胞膜;
＊植物所吸收的磷以聚磷酸盐的形式在菌丝中运输效率高.
The extent of infection is controlled not by the phosphorus concentration in the soil solution but the phosphorus content of plant.
In the majority of cases enhanced phosphorus uptake and improved phosphorus nutrition are the primary cause s of growth and yield increase in mycorrhizas plants
VAM can take up several times P----surface area and extension (8 cm).
Outlook
The potential using VAM fungi as “biofertilizers” on a large scale and in agricultural production is fascinating.
Problem:
1 Lack of inoculum
2 Difficulties in the production of pathogen-free inoculum in sufficient quantities
3 Competition with indigenous VAM fungi
[思考题]
1 概念：矿质养分的生物有效性，化学有效性，空间有效性，质流，扩散， 截获，根际，根际分泌物，VAM等
2 如何正确评价化学有效养分的相对性及其在施肥中的应用？
3 土壤养分向根表迁移的方式及其机理？
4 土壤养分不同迁移方式的影响因素及其对养分供应的贡献？
5 根系的不同形态结构与养分有效性的关系？
6 施肥等因素对根系分布的影响及其意义？
7 根际养分的分布特征及其影响因素？
8 根际分泌物及其对植物养分吸收利用的机理和意义？
9 VA菌根及其对植物养分吸收利用的机理和意义？