Chapter 6Soil Air and Heat Regime

Section 1 Soil air

一、 Composition of soil air

Atmosphere O2 CO2 N2 Other gases

Atmosphere near soil surface

20.94 0.03 78.05 0.95

Soil air 18.0-20.03

0.15-0.65

78.8-80.24

－

Compare soil air with atmosphere （ Volume %）

Characteristics of soil air

1 ． The level of CO2 in soil is about eight times higher than the normal atmospheric level.

2． The O2 level in soil air is slightly lower than it is in atmospheric air.

3． Soil air usually is much higher in water vapor than is the atmosphere.

4． Also, under waterlogged conditions, the concentrations of gases such as methane （ CH4 ） and hydrogen sulfide (H2S) are notably higher in soil air.

 Depth

/cm

Plastic mulches Bare soil

05-01 07-29 05-01 07-29

CO2 O2 CO2 O2 CO2 O2 CO2 O2

0 － － 0.915 － － 0.056 0.056 －

5 0.158 20.497 1.006 20.439 0.70 20.649 0.211 20.653

10 0.420 20.397 1.060 20.275 0.104 20.513 0.279 20.668

15 0.250 20.486 0.865 19.953 0.134 20.857 0.385 20.506

20 0.483 20.478 1.348 20.060 0.150 20.121 0.406 20.634

30 0.573 19.865 1.159 20.005 0.313 20.181 1.157 20.362

50 0.922 19.929 1.520 19.698 0.402 20.198 1.281 19.873

Average 0.615 20.124 1.268 19.953 0.269 20.329 0.847 20.022

The soil air contents of the mulches and bare cotton farmland in different growth stages (%)

二、 The movement of soil air（一）、 Convection (Mass Flow)

Convection –The soil air is transferred from higher pressure area to lower pressure area.

Where qv is the volume convective flux of air; k is the permeability of the air-filled pore space; is the viscosity of soil air; is the pressure gradient in soil air.

pkqv )/(

（二）、 Soil air diffusion

• The process of diffusion can be represented by a flow equation in which the driving force is the gas concentration gradient (Fick’s Law).

dx

dcDqd

•where qd is the diffusive flux (mass diffusing across a unit area per unit time), D is the gas diffusion coefficient, and dc/dx is the concentration gradient of the gas.

•A difference in the concentration of a particular gas in one zone compared with any other zone in the soil can result in the movement of that gas by a process called diffusion.

Soil respiration:

Section 2 Soil heat

一、 The source of soil heat

（一） Solar radiation

Solar radiation is the primary source of energy to heat soils. But clouds ad dust particles intercept the sun’s rays and absorb, scatter, or reflect most of the energy.

H I

E

a

（二） Biological heat

Organic matter is decomposed by microorganism.

（三） Heat inside the earth

二、 The radiation balance and affect the factors in soil surface

（一） The radiation balance in soil surface

Little of the solar energy reaching the earth actually results in soil warming. The energy is used primarily to evaporate water from the soil or leaf surfaces, or is radiated or reflected back to sky.

Only about 10% is absorbed by the soil and can be used to warm soil.

• The radiation balance of a soil surface can be written as:

• R = [(I+H) – (I + H)×α] + (G – E)

• = (I + H) (1 - α) – r• where R is the net radiation received per unit area. I is

the sun directly radiation, H is atmospheric radiation; αis the albedo and represents the fraction of R which is reflected by the soil surface, G is the incoming longwave radiation, E is the longwave emission of the surface.

（二） The affect factors of the radiation in balance soil surface

1、 Solar radiation capacity2、 Albedo in soil surface3、 Available radiation in soil surface（ 1） Cloud, fog、water vapor and wind etc.

(2) Elevation and inclination of angle of surface toward the sun (3) Soil surface roughness (4) Mulches of soil surface

三、Heat balance of soil

The heat balance of the soil can be described by:

S = Q P LE + R

where S is to obtain heat or to lose heat in unit time;Q is balance of radiation; LE is transpiration of water by the vegetative cover; P is heating of the atmospheric air by conduction at the soil surface; R is heating of the soil.

一、 Soil heat capacity (soil thermal capacity)

Soil heat capacity-The amount of heat energy required to raise the temperature of 1 g or 1 cm3 of the soil. C is mass heat capacity (Jg-1℃-1) ; Cv is volume heat capacity（ Jcm-3℃-1）。

The volume heat capacity of soil（ Cv） can be described by following equation:

Cv = mCv·Vm + oCv·Vo ＋ wCv·Vw+aCv·Va

As volume heat capacity of air is very small, it will be omitted . therefore ,the above equation will be becomed: Cv = 1.9Vm + 2.5Vo + 4.2Vw (Jcm-3 ℃-1)

Section 3 Thermal properties of soils

Table 6-3 Soil heat capacity of different soil composes

二、 Soil heat conductivity

Heat moves through soil mainly by a process called conduction.Heat conductivity( thermal conductivity) The heat conductivity, λ, of a soil is defined as the heat flux density by conduction through the soil divided by the temperature gradient. (在单位厚度（ 1厘米）土层，温差为 1℃时，每秒钟经单位断面（ 1厘米2）通过的热量焦耳数（）。其单位是 J.cm-2.s-1.℃-

1。 )

)(/)(

/

2121 ttAT

Qdor

dtt

ATQ

＝

Table 6-4 Soil heat conductivity of different soil composes

三、 Soil thermal diffusivity

The soil thermal diffusivity represents the ratio of the thermal conductivity () to the volumetric heat capacity of the soil.(土壤热扩散率 是指在标准状况下，在土层垂直方向上每厘米距离内， 1℃的温度梯度下，每秒流入 1cm2土壤断面面积的热量，使单位体积（ 1cm3）土壤所发生的温度变化。其大小等于土壤导热率 /容积热容量之比值。 ))/( 2 scm

CvD

Where is the thermal

conductivity, and Cv。

Section 4 Soil temperature

一、Month temperature changes of the soil

Season temperature changes

二、Daily variations

三、 Influence of topography, landform and soil nature in soil

temperature

(一） Elevation

（二） Slope and slope direction （三） The soil constitutes and soil natures

土壤空气氧浓度临界值 (Vol%)

作 物 O2临界值 作 者

大 麦 7%~10% Geisler

玉 米 14% Geisler

豌 豆 20% AmmoreGeisler

棉 花 10% Tacket

谷类胚芽

10% Gill

1 、土壤含水量愈低，其热容量越小，导热率愈低土壤升温越快( )

2 、灌溉可降低土壤表层土的昼夜温差。 ( )

3、中耕松土，可降低土壤表层土的热容量，而促进表土升温。 ( )

4 、一日之内土壤上下各层温度变化是同步的 ( )

5、土壤热量全部来源是太阳辐射。 ( )

6、土温的变化与土壤肥力的高低无关。 ( )

7、土壤热容量随土壤容重和含水量的增加而增大 ( )。

8 、土壤有机质含量越高，其热容量越大 ( )

9 、土壤热扩散率随含水量的增加而增加，随热容量的增大而减小 ( )。

10、热扩散率表示土壤升温的难易，导热率表示是热传导的快慢。 ( )

——课堂测验 快速判断！！

本章小结：一、概念

土壤呼吸 呼吸商 气体扩散 Eh 土壤通气性

土壤热容量 土壤导热率 土壤热扩散率 土壤热 状况

二问答题

1、土壤空气的组成有何缺点 ?

2 、土壤通气 性对土壤肥力有何影响 ?

3、土壤 Eh的意义是什么 ?

4、如何调节土壤的通气性 ?

The End