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V. Organic Matter and Microbial Biomass

V. Organic Matter and Microbial Biomass

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SOIL ORGANIC MATTER IN SEMIARID REGIONS



115



organic matter when a virgin soil is cultivated (Martel and Paul, 1974;

Houghton er al., 1983). Factors that affect crop production ultimately

affect the size and activity of the soil microbial biomass (Biederbeck et al.,

1984).

Net primary production of native grasslands is generally greater than

that of agroecosystems, with the majority of fixed C accumulating in

belowground biomass (Kucera et al., 1967; Allison, 1973; Anderson and

Coleman, 1985). Typically, perennial grassland soils contain about twice

the amount of root biomass as agricultural soils (Yegorov and Dyuryagina,

1973; Sims and Singh, 1978; Sala et al., 1988), and therefore maintain

higher soil organic matter contents. The amount of root biomass is important since it supplies a significant quantity of available C to the microbial

biomass (Lynch and Panting, 1980; McGill et al., 1986). In most cases, the

microbial biomass of grassland soils is about twice that of cultivated soils.

Microbial biomass C ranges from 100 to 600 mg C/kg soil when cropped

to cereals (Insam et al., 1989; Anderson and Domsch, 1989) and can

exceed 1500 mg C/kg soil under native grassland or managed grass pasture

(Ross et al., 1980). Extended cultivation usually reduces soil biomass,

which influences organic matter cycling. Reductions in microbial biomass

result from lower C inputs and environmental stresses created by management. These stresses include increased soil acidity from ammoniacal fertilizer use, soil erosion that decreases C, N , and other essential nutrients,

and increased soil density, which reduces aeration and water availability.

It is generally assumed that microbial biomass C and activity measurements are correlated with soil organic C because soil biomass depends on

the quantity of degradable C sources present in soil (Adams and Laughlin,

1981). Anderson and Domsch (1989) evaluated this relationship over a

range of soils and Insam et al. (1989) evaluated over different climatic

zones. Both found a correlation but the relationship was complicated by a

number of integrative factors, especially in drier regions. Anderson and

Domsch (1989) surveyed 134 plots located on 25 experimental sites within

a narrow temperate climatic zone of central Europe and found a high

correlation between soil microbial biomass C (CM)and soil organic C (CO)

(Fig. 4a). Microbial biomass C averaged 2.3 and 2.9% of organic C for

monocultures and crop rotations, respectively. The difference was primarily due to the type and amount of organic C input; soil physical properties (i.e., texture) were of minor consequence. They suggested that monocultures and rotations were comparable with respect to steady state

conditions, but that no universal equilibrium constant could describe the

increase in microbial C per unit of soil C between cropping systems.

Regression analyses indicated that higher CM: CO ratios were characteristic of soils with regular crop rotation (Fig. 4a). Fallow in the rotation

reduced soil organic matter and subsequently influenced the CM: COratio.



116



PAUL E. RASMUSSEN AND HAROLD P. COLLINS

1000



a



-



-



CROP ROTATIONS



600 -



.$



400 -



(5,



>

24



200



Ir -



-



E



v



z



i!u

Q:



CONTINUOUS MONOCULTURE

Y = 22.4X + 26

R2 = 0.89



0

1000



:



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- CROP ROTATIONS

Y

R’



= 13.0X

= 0.34



+



*



193



R2 = 0.50



01

0



I



10



I



20



I



30



I



40



ORGANIC C (g/kg)

FIG.4. The relationship between microbial biomass and organic C in soil with continuous

monoculture and crop rotations. ( From Anderson and Domsch, 1989 (a); and from Insam et

al., 1989 (b). Reproduced from Soil Biology and Biochemistry, 21, pp. 214-216, 476, by

permission of Pergamon Press PLC. Copyright 0 1989 by Pergamon Press PLC.)



In a similar study of North American soils, Insam et a / .(1989) found low

correlation between microbial C and soil organic C (Fig. 4b), and concluded that the relationship was dependent upon factors other than organic

C. They found that the CM:CO ratio was influenced by macroclimate,

particularly the combined variables of precipitation and evaporation (Fig.

5). Variance across climatic zones was attributed to differences in soil

texture, fertilization, tillage, and crop rotation. Climatic effects on soil

microbial biomass tended to increase as conditions become drier. Insam et

al. (1989) concluded that, since CM: CO is influenced by climatic factors,



117



SOIL ORGANIC MATTER IN SEMIARID REGIONS

50 40 0



0



cn



30



-



Y



\

0



20-



A



mloL

0



I



0.1



0.2



I



0.3



A



I



I



I



0.4



0.5



0.6



I



0.7



:



0.8



0.9



PE

FIG. 5. The effect of climate on microbial biomass C per unit of organic C. Climate is

defined as PE index, or the ratio of precipitation to pan evaporation (in mm). (From Insam et

al., 1989. Reproduced from Soil Biobgy and Biochemistry, 21, p. 217, by permission of

Pergamon Press PLC. Copyright 0 1989 by Pergamon Press PLC.)



the equilibrium constant should be replaced by an equilibrium function

determined from prevailing climatic variables.

Powlson e? af. (1987) proposed that shifts in biomass C measured over

relatively short time periods could indicate changes in soil organic matter

levels long before they could be detected by other methods. Anderson and

Domsch (1989) stated that “soil systems tend towards a state of equilibrium if both the environment and agricultural practices remain constant

over long periods.” They suggested that the microbial biomass responds

more quickly to a change in management than does soil organic matter.

This is not altogether unexpected, since microbial biomass in soil is related

to residue input and residue production in semiarid regions is dependent on

cultural and climatic factors.

Annual cropping, cultivation, residue management and N fertilization

influence the amount and distribution of soil organic matter and ultimately,

microbial biomass (Biederbeck et al., 1984; Coote and Ramsey, 1983;

Rasmussen et af.,1989). Differences in microbial activity were associated

with the distribution of residue, increased moisture, and moderation in soil

temperature. Doran (1987)reported that microbial biomass and potentially

mineraiizable N in no-till soils was 54 and 37% higher, respectively, in the

surface soil of conservation tillage systems than in the surface horizon of

plowed soils. Adoption of conservation tillage practices increased both

biomass C and N , and soil C and N mineralization potentials, but had little



118



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



effect on the soil organic matter level after 16 years (Carter and Rennie,

1982). In a similar study, Campbell et al. (1989) supported these observations, but concluded that reduced tillage had a net positive effect on soil

organic matter accumulationin the top 8 cm of soil. Tillage systems usually

have little effect on soil C below 8 cm (Doran and Smith, 1987).

Potentially mineralizable C and N may provide sensitive parameters to

assess changes in soil organic matter induced by tillage or other soil

management strategies (Carter and Rennie, 1982; Campbell et al., 1989).

High concentrations of active C and N enhance organic matter accumulation. Soil organic matter can be divided into active (labile) and recalcitrant fractions. Schimel et al. (1985) suggested that cultivation reduces

active fractions since they are readily mineralized. Net mineralizable C

released as CO2 from laboratory incubations clearly shows different accumulation of labile C under different management practices (Fig. 6). The

percentage of C mineralized from a grass pasture and cultivated soils under

wheat-pea, continuous wheat, and wheat-fallow rotation were 2.6, 2.0,



600



-



y"



PGP

2.6%



500 -



400 -



\

(5,



------



'f

-



-



w-P

2.0%



W-F

1.6%



DAYS

FIG. 6. Net carbon mineralization from soil during 30-day incubation as affected by

cropping systems; Pendleton, Oregon. PGP, permanent grass pasture; W-P,wheat-pea

rotation; W-W, wheat-wheat rotation; W-F, wheat-fallow rotation. Percentages reflect the

percent of total C mineralized.



SOIL ORGANIC MATTER IN SEMIARID REGIONS



119



1.8, and 1.6%, respectively. Fertilization practices generally increase soil

microbial biomass through increased production of plant material and

greater return of crop residues to soil (Biederbeck et al., 1984). Organic

amendments produce more microbial biomass than inorganic fertilizers

because they increase the proportion of labile C and N, directly stimulating

the activity of the biomass.



VI. MANAGEMENT EFFECTS ON PHYSICAL PROPERTIES

Cultivation of grassland soil usually increases bulk density. An increase

in bulk density is also likely with erosion of topsoil. Changes in bulk

density can have a pronounced effect on the amount of organic C and N in

soil, and if not measured, can lead to inaccurate estimates of nutrient loss

or gain. Voroney et al. (1981) reported that 70 years of cultivation in the

Canadian prairie increased the bulk density of surface soil by 16%. The

increase in North Dakota was 1 1 % (Bauer and Black, 1981). Bulk density

of a cultivated Haplustoll in Agrentina was 13% higher than in its virgin

counterpart (Miglierina et al., 1988). Bulk density increases for six vertisols in Australia ranged from 13 to 28% (Dalal and Mayer, 1986). Residue

addition tends to decrease bulk density. Residue addition in Montana

decreased soil bulk density in the 0-7.5- and 7.5-15-cm soil layers 0.015

and 0.011 g/cm3, respectively, per ton of residue applied (Black, 1973).

In the loess plateau of China, straw addition decreased bulk density

0.020 g/cm3 per ton of residue (Siming et al., 1988).

Incorporation of organic materials into soil promotes the aggregation of

soil particles (Smith and Elliott, 1990). Under native grasslands, stable soil

aggregates can be formed through differential dehydration as a result of

water uptake (Tisdall and Oades, 1982). Root exudates also stimulate

microbial activity and increase production of polysaccharides that promote aggregate stability (Aspiras et d.,1971; Molope etal. 1987). Continuous cultivation of soil generally leads to a reduction in soil organic matter

and an increase in soil erodibility. Studies in Texas (Unger, 1982) and

Canada (Nuttall et al., 1986) show that more intensive tillage decreased

aggregate stability and increased the erodible fraction (<0.84 mm). Tillage

affects C availability to the microbial biomass by disrupting soil structure and exposing protected organic material. After long periods of cultivation, microbial activity may become C limited. This will subsequently reduce the production of aggregating compounds and create a

soil structure that no longer resists soil detachment and transport by

water.



120



PAUL E. RASMUSSEN AND HAROLD P. COLLINS



VII. CULTIVATION AND FUTURE CHANGE IN SOIL

ORGANIC MATTER

While some studies indicate that loss of soil organic matter will cease

after 50 to 60 years of cultivation (Voroney et al., 1981; Bauer and Black,

1981; Unger, 1982), other studies project a continuing decline (Tiessen et

al., 1982; Rasmussen et al., 1989). The difference between studies is likely

related to the rate of residue input in relation to climate and cropping

system. Some systems have reached equilibrium, others have not. The

role of management in determining the equilibrium level is illustrated in

Fig. 2. The range in the organic C after 55 years due to different management practices is nearly as large as the total amount of C lost in the first 50

years of cultivation. This clearly indicates that cropping practices and

residue management have an important role in maintaining organic matter

in soil.

Reducing the frequency of fallow will help to stabilize the system, since

less residue is needed to prevent organic matter loss when soils are

cropped more intensively. Although the mechanisms are not well understood, having a crop on the land has the potential to reduce microbial

activity and slow the turnover of soil organic C (Reid and Goss, 1983;

Sparling et al., 1983). Reid and Goss (1983) suggested that the release of

inhibitory compounds from roots, preferential use of root exudates over

soil organic C, greater microbial predation in the rhizosphere, and plant

competition for organic compounds were likely mechanisms.

Greater use of conservation tillage shows promise for preventing further

decline in organic matter. Technological advances that increase the productive capability of the land also have the potential to increase soil

organic matter content. This potential is tempered by the large fraction of

root and shoot biomass found in noncultivated grasses (Sirns and Singh,

1978). Root and crown biomass undergo rapid oxidation with cultivation

and are almost depleted after the first 20-30 years of farming. Where this

fraction no longer exists, higher production has greater potential for improving the organic matter content of soil. Cereal grain yields have been

steadily increasing for the past 30 years. Although there has been a shift

toward shorter growth habit and higher harvest index, the amount of straw

produced still appears to be greater than prior to the 1960s. Table VIII

shows the change in straw production in the Pacific Northwest, U.S.

brought about by improvements in wheat varieties and crop management.

Straw production the past 20 years is 13 to 66% greater than it was during

the previous 35 years. Aboveground production is presently substantially

greater than the estimated production of native grassland. Thus, it is



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