Tải bản đầy đủ - 0 (trang)
IV. Soil Processes and Pasture Response in Excreta-Affected Areas

IV. Soil Processes and Pasture Response in Excreta-Affected Areas

Tải bản đầy đủ - 0trang

145



NUTRIENT CYCLING UNDER GRAZED PASTURE

Table VI

Typical Chemical Composition of Feces from Major Types of Farm Animals'



Feces source



Neutral

detergent

soluble



Nitrogen



Hemicellulose



Cellulose



Lignin



Ash



Dairy cattle (lactating)

Cattle (fattening)

Sheep (forage fed)



41

53

45



2.0

3.0

2.5



20

22

15



28

17

28



20

8

15



12

7

13



~~



~



~



~



~



Data from Smith (1973). Values given as percentage of dry matter.



water-soluble metabolic products of both endogenous and microbial origin

(Mason et al., 198 1 ).

The consistency of feces varies greatly with diet and is affected mainly by

the water and structural carbohydrate content of the herbage. Lush spring

grass, which has a high moisture and low structural carbohydrate content

(and high digestibility), can result in rather liquid feces. During and Weeda

( I973), for instance, observed that in spring and autumn, periods of highest

pasture production, cattle dung was liquid and covered much greater

surface areas than in the summer, when feces were firm.The ash content of

cattle feces can be in the range of 20-40% (Healy, 1968; Underhay and

Dickinson, 1978) and a considerable portion of this can be silica arising

from ingestion of soil (Healy, 1968).

The nutrient content of feces was discussed in detail in Sections II1,A

and II1,B. The total nutrient content of cattle feces is well documented, but

that of sheep is less well known. However, observations (R. J. Haynes and

P. H. Williams, unpublished observations, 1989) have indicated that for

sheep and cattle grazing the same pasture, nutrient concentrations in feces

are very similar. Typical application rates of the major nutrients applied to

the dung patch of sheep and cattle are shown in Table VII along with the

annual output per cattle beast and per hectare of farm. It is evident that

very large amounts of organic matter and N, Ca, Mg, P, and K are

deposited within the dung patch area.

2. Feces Degradation



Two major processes contribute to dung degradation and thus the release of nutrients. These are (1) physical breakdown, which is caused

mainly by raindrop impact (and treading) and (2) biological degradation,



146



R. J. HAYNES AND P. H. WILLIAMS

Table VII



Typical Application Rates of Major Nutrients Applied to Sheep and Cattle Dung Patches

and Annual Output per Cattle Beast



Typical dung

concentration



Application

rate per

dung patch

(kg ha-1)"



Parameter



(%I



Sheep



Cattle



Organic matter

N

P

S

K

Ca



80

2.6

0.70

0.25

1 .o

2.0

0.66



4000

130

35

13

50



32000

1040

280

100



100



800

264



Mg



33



400



Annual

Annual output

output per

from cattle per

cattle beastb

hectare of farm'

(kg animal-' yr') (kg ha-I yrl)

699

23

6.1

2.1

8.8

17

5.8



1748

57

15



5.3

22

43

15



a Assuming that for sheep and cattle a dung patch consists of 0.01 and 0.20 kg (dry weight)

of dung and the area covered is 0.02 and 0.05 m2, respectively.

Assuming cattle defecate 12 times per day.

Assuming 2.5 cattle per hectare.



which is brought about by biota such as fungi, bacteria, beetles, and

earthworms. The initial release of nutrients is very dependent on factors

affecting the physical breakdown of dung deposits (Underhay and Dickinson, 1978; Rowarth et af., 1985).

a. Physical Breakdown

Physical breakdown is influenced greatly by climate and the initial

consistency of the feces (Weeda, 1967). When dry weather follows dung

deposition, a hard crust forms on the dung patch (Weeda, 1967; Underhay

and Dickinson, 1978). The formation of such a crust partially protects the

pat from the eroding effect of raindrop impact and it also inhibits rain from

penetrating and rewetting the pat. The weather immediately after dung

deposition is therefore a major factor affecting degradation. As a consequence, it has often been noted that in temperate climates the disappearance of dung is less rapid during summer (when a crust forms) than in

winter (Weeda, 1967; MacDiarmid and Watkin, 1972b; Rowarth et al,,

1985). Under temperate conditions (annual rainfall of 1500 mm), Rowarth

et af. (1985) observed a rapid exponential breakdown of fresh sheep feces

(Fig. 6). Fecal samples had completely degraded within 17 days in winter

but lasted over 100 days in summer. By contrast, research under dryland

conditions (300- 550 mm annual rainfall) using air-dried fecal material



NUTRIENT CYCLING UNDER GRAZED PASTURE



=

m



.-



100



8



r



0.



C



.=



80



0



*-\*



\



60-



.-Em

3



2



147



40



\.



-



Winter



Time after dung deposition (days)

Figure 6. Relationship between the decrease in the dry weight of sheep dung pads and

time under dry summer and wet winter conditions. (Redrawn from Rowarth et al., 1985.)



(Bromfield and Jones, 1970; Rixon and Zorin, 1978) has shown that

decomposition is slow and occurs at a more or less linear rate. Bromfield

and Jones (1970) found a 40% weight loss from fecal samples under field

conditions over a 2-year period.

The consistency of the excreta is another important factor influencing

degradation rate. Under moist conditions (e.g., winter) the more liquid

dung patches disappear more rapidly than the firmer ones (Weeda, 1967).

In contrast, under drying conditions, during which a hard crust forms,

initial consistency is unimportant. These effects are shown in Table VIII;

in summer and spring, initial consistency had no effect on degradation

whereas in autumn and winter the liquid dung broke down more rapidly.

An additional interacting factor influencing the rate of degradation can

be the physical form of fecal material. Sheep, for example, can excrete

dung in the form of firm pellets or larger more liquid pads; the former have

a much greater surface area per unit weight of material excreted. The larger

surface area makes pellets more susceptible to raindrop impact and physical degradation and as a consequence pellets normally degrade more rapidly than pads (Rowarth et al., 1985).

b. Biological Degradation

Microbial decomposition of dung material is essential in order to release

most of the fecal N and S that are present in organic combination. This



R. J. HAYNES AND P. H. WILLIAMS



148



Table VIII

Effect of Dung Consistency and Rainfall on the Number of Months Required for the Major

Part of the Dung to Disappeaf

~



~~



~~



~~~~



Rainfall means after

Season in which

dung was deposited

Summer

Autumn

Winter

Spring



deposition of dung

(mm)



Dung consistencyb

2



3



3.5

1.5



3

3



2

5



6.5



3.5

4

6

6.5



1



6



5



lweek



2weeks



3.5

4.5



8.5



-



3.8

0.3



7



6.5



1.8

1.o



8.4

2.0

2.3

4.3



4

4

4



Data from Weeda (1967).

Consistency scale: 1, very liquid; 5 , very firm.



may occur during the degradation of the dung pat and/or after the pat has

been dispersed and dung particles have been washed into the surface soil by

rainfall.

Coprophagous invertebrates, notably dung beetles, dipterous larvae, and

earthworms, play an important role in dung deposition through promoting

aeration and microbial activity and incorporating dung into the soil

(Curry, 1987a,b). In some localities, dung beetles (especially Geotups spp.

and Aphodius spp.) remove large quantities of dung and permeate the pats

with burrows, thus increasing aeration (Bornemissza, 1970; Holter, 1979).

In the African savanna, termites are the main agents of degradation in the

dry season (Omaliko, 1981). Dung flies and other macroarthropods are

commonly active in burrowing and in the physical degradation of pats.

Earthworms are also active in burrowing, ingestion, and mixing of dung

with the soil. Indeed, dung is actively sought out by lumbricoid earthworms and the presence of dung enhances growth and reproduction of

earthworms in agricultural soils (Watkin and Wheeler, 1966; Lofs-Holmin,

1983).

Holter (1979) found that earthworms accounted for 50% of the &sap

pearance of cattle dung pats from the soil surface in Denmark and beetle

larvae accounted for another 14-20%. In some localities the absence of

coprophagous fauna adapted to cope with large quantities of animal dung

can result in the accumulation of dung at the soil surface and sward

deterioration on intensively grazed pastures (Gillard, 1967).

Rainfall is important for biological degradation because it can maintain

dung moisture levels suitable for microbial activity. Thus, the formation of



NUTRIENT CYCLING UNDER GRAZED PASTURE



149



a dry crust over the pat, which inhibits rewetting, not only hinders physical

degradation but also inhibits microbial decomposition. Temperature and

humidity also influence the activity of microbial populations in dung.

Losses of CO, and NH, to the atmosphere are most rapid in the first few

weeks after dung deposition (MacDiarmid and Watkin, 1972a; Anderson

and Coe, 1974). The loss of NH, occurs through volatilization, whereas the

loss of CO, is mainly due to microbial respiration. Underhay and Dickinson (1978) observed that despite the presence of a vigourous coprophilous

microflora, loss of organic matter from cattle dung was only about 15%

over a 2-month period and the calorific value of the dung decreased by

18%over the same period. This slow decomposition has been attributed to

the fact that fungal activity is initially confined to the surface layers of the

dung pats (Dickinson and Underhay, 1977). The cattle dung pat has a

nonporous matrix and the initial moisture content is often 400-700% of

the dry matter content. Hence O2may well become limiting in the center

of the pat and this restricts the spread of hyphae into the center.

Loss of organic matter from dung pats, through CO, evolution, can

occur more rapidly than the loss of nutrients. Thus, there can be an

increase in the concentration of elements such as Ca, Mg, Fe, P, and N in

dung during its decomposition (Dickinson and Underhay, 1977; Omaliko,

1984). The processes involved in the release of the major nutrients from

feces are outlined below.

3. Release of Nutrients



a. Nitrogen

Because the bulk of the N in feces is in organic forms it must first

undergo microbial mineralization before it is released as mineral forms.

The amount of N mineralized from feces is closely related to the total N

content of feces, but N mineralization is slower from feces than that from

the plant materials they were derived from (Barrow, 1961;Floate, 1970a,b)

(Fig. 7). The slower mineralization is not due to a difference in C :N ratios

because Floate (1 970a) found that the C: N ratio of sheep dung (22 : 1 to

27 : 1) was similar to that of the ingested herbage. However, a large proportion of the C content of feces consists of undigested fibrous material

(cellulose, hemicellulose, and lignin), which degrades only slowly. The slow

degradation of fecal material apparently results in a slow release of other

nutrients present in organic form (N and S) (Barrow, 1960). Despite this,

under dryland conditions Rixon and Zorin ( 1978) observed that retention

of N in feces was proportionally greater than the retention of the bulk of

the fecal material, with the result that increases in the total N concentration in sheep feces were observed during decomposition. Studying the



1so



R. J. HAYNES AND P. H. WILLIAMS



Y



E



-aE



Y



Plant material



0



I



m



0



Feces



0



-



*



I



l



l



I



I



1



1



2



3



6



9



12



Incubation period (weeks)



Figure 7. Net total mineral N production as a percentage of original total N from

Agrostis-Festucu plant material and sheep feces incubated for 12 weeks at 30'C. (Redrawn

from Floate, 1970b, with permission from Pergamon Press PLC.)



decomposition of cattle dung, Underhay and Dickinson (1978) observed a

decrease in N concentrations during the first 35 days (indicating preferential loss of N), but this was followed by an increase in N concentrations

during a subsequent 35-day period of decomposition.

As noted previously (Section IV,A,2), NH, is lost from decomposing

dung, particularly during the first week after deposition (MacDiarmid and

Watkin, 1972a). Over the first 13 days of decomposition of cattle dung,

MacDiarmid and Watkin (1972a) measured a loss of 4.7% of the dung N

and Ryden et al. (1987) measured losses of 1.2 and 12.0%,respectively for

cattle and sheep dung over a 2-week period following deposition.

The balance between C and N mineralization in feces is likely to be

greatly influenced by environmental factors as well as the amount and

composition of the C and N content of feces. Floate (1970c), for example,

observed that as the incubation temperature was reduced from 30" to 10°C

and then 5"C, the losses of C as CO, from sheep feces over a 12-week

period were 16,4, and 2%, respectively. In contrast, the largest amounts of

mineral N were produced at 10°C. Floate (1970d) found that mineralization of N was greatest at 100%moisture holding capacity (MHC) and was

almost completely inhibited at 25% MHC. However CO, production was

greater at 25% MHC than at either 50 or 100%MHC.

The release of mineral N from feces results in elevated concentrations of

mineral N in the soil below the dung patch. The high concentrations of



NUTRIENT CYCLING UNDER GRAZED PASTURE



151



NO; that can accumulate (e.g., 90 to 130 bg N g-') (Ryden, 1986) suggest

that the dung patch (like the urine patch) can be a significant source of

both NOT leaching and gaseous losses of N,O and N, (through denitrification/nitrification) from grazed pastures (see Section IV,C,3).

b. Sulfur

The release of S from animal excreta has received little attention. Barrow

(1 96 1) reported that the amount of S mineralized from fecal pellets was

closely related to the initial S content of the feces. Mineralization of feces

released proportionately less S than did plant litter of the same S content

(Barrow, 196 1 ; Boswell, 1983).

Boswell ( 1 983) observed two distinct processes in the release of S from

sheep feces. An initial rapid process in which soluble and more labile S was

released was followed by a protracted slower process in which more resistant material was mineralized. During the initial period the loss of fecal S is

more rapid than the loss of dry matter (Kennedy and Till, 198 1 ; Boswell,

1983), suggesting preferential mineralization of S and/or leaching of S

from fecal pellets. The half-life of fecal S was found by Boswell(l983) to be

1 54 days under controlled environmental conditions with adequate moisture.

Fecal S does not appear to be rapidly available to pasture plants. Under

controlled environmental conditions Boswell ( 1983) found that ryegrass

plants recovered about 5% of the applied 35Slabel from sheep feces. Over a

12-month period under field conditions, 16% of the radioactivity in applied 35S-labeledfeces was recovered by Kennedy and Till ( 198 1) in aboveground pasture herbage.

c. Phosphorus

Under dryland conditions, the availability of P from dung is initially the

consequence of leaching of water-soluble inorganic P (Bromfield, 196 1 ;

Rixon and Zorin, 1978). Rixon and Zorin (1978) measured a 50%decrease

in P concentration in fecal samples placed under bushes in the saltbush

rangeland for 20 months. An 80%decrease in P concentration was measured over the same time period for fecal samples placed on irrigated

pasture.

Under temperate conditions, wherein physical breakdown of dung is

rapid, leaching of P from fecal material is relatively unimportant. Rowarth

et al. (1989, for instance, observed that the concentration of both total P

and water-extractable P from dung samples remained relatively constant

with time and that the major mechanism controlling movement of P from

feces into the soil was the rate of physical breakdown.

The availability of fecal P to plants has been investigated in several



152



R. J. HAYNES A N D P. H. WILLIAMS



short-term greenhouse experiments (McAuliffe et al., 1949; Bromfield,

1961;Gunary, 1968). In these studies ground feces were incorporated into

the soil and the inorganic fecal P content was found to be as effective as a P

source as readily soluble fertilizer P. The organic P content did not, however, appear to be readily available, at least in the short term.

In short-term field trials (8 and 17 weeks, respectively, in spring and

autumn), Rowarth et al. ( 1990) found that P uptake from fecal inorganic P

was less than that for monocalcium phosphate. Nonetheless, under field

conditions, During and Weeda (1973) observed that herbage yields and P

uptake were greater from cattle dung than superphosphate. Yield response

of herbage to dung and superphosphate persisted for 2 and If years,

respectively. Similarly, McAuliffe and Bradfield (1955) found that the

availability of P from superphosphate and feces was similar for a first cut of

grass, but by the third cut the availability of P from feces exceeded that

from superphosphate.

The superiority of dung as a P source was partially ascribed by During

and Weeda (1973) to the large quantity of N (and other nutrients), which

stimulated initial pasture yields more than superphosphate, thereby increasing P uptake and usage. In addition, dung applications may have

decreased phosphate adsorption by the soil (see Section IV,A,4), thus

increasing phosphate availability.

d. Other Nutrients

Release of K and Na from feces is rapid because the bulk of these

elements is in water-soluble form. Weeda ( I 977) observed that in the soil

below dung pats, peak levels of exchangeable K were reached 1 month after

application (Fig. 8). In contrast, the release of Ca and Mg was much slower

(Fig. 8) and peak levels were not reached until 4 months after application.

The slower release of Ca and Mg is expected due to the lower proportion of

water-soluble Ca and Mg present in dung (Section III,B,4). Underhay and

Dickinson (1978) observed that Ca was leached from cattle feces more

rapidly than Mg, but the reason for this is unclear.

The release of micronutrients from feces has received little attention,

although Barrow (1987) pointed out that the alkaline conditions in the

dung are likely to limit the solubility of nutrients such as Fe, Mn, Zn, and

Cu and thus retard their release.

4. Effect on Soil Properties



Increases in extractable P, exchangeable Ca and Mg, and sometimes

exchangeable K commonly occur in the surface soil 2.5 to 5 cm below

dung patches (Davies et al., 1962; MacDiarmid and Watkin, 1972b; Dur-



153



NUTRIENT CYCLING UNDER GRAZED PASTURE



Figure 8. Changes in exchangeable cation levels in the surface 3.8 cm of soil with time

below cattle dung patches (values represent differences between levels under dung pats and

those in the surrounding soil). (Redrawnfrom Weeda, 1977.)



ing et al., 1973; Weeda, 1977). Increased levels of exchangeable Ca, Mg,

and K in the soil (0-2.5 cm) below cattle dung patches 3 years after

deposition are shown in Table IX. Over a 15-month period following cattle

dung deposition, Weeda (1 977) observed a sustained rise in Truog P levels.

Mean levels over that period in control and dung-covered plots were 17

and 22 pg P g-*, respectively, in the 0- to 3.8-cm soil layer and 10 and 13pg

P g-l, respectively, in the 3.8- to 7.6-cm layer.

Due to the high pH and high CaCO, content of feces (Section III,B,4), an

increase in soil pH below the dung patch is a common phenomenon

(Davies et al., 1962; During et al., 1973; Omaliko, 1984). Davies et al.

(1962) recorded a soil pH increase from 6.1 to 6.7 below dairy cow dung



Table IX

Effect on Soil Properties of Cattle Dung Additiona

Exchangeable cations



Control

Dung



9.4

10.9



0.76

0.89



5.2

5.6



12.4

17.7



11

15



1.5

2.6



'Data from During er al. (1973). Effects are evaluated 0-2.5 cm below the area of

addition after a 3-year period.



154



R. J. HAYNES AND P. H. WILLIAMS



pats 10 months after application and During et al. (1973) measured a pH

increase from 5.2 to 5.6 in the 0- to 2.5-cm increment 3 years after

application (see Table IX). In the soil/pasture/animal system dung appears

to be the main natural agent that maintains soil pH or arrests decreases in

pH (During et al., 1973).

Soil organic matter content (organic C and total N) was shown by

During et al. (1973) to be significantly increased below dung patches even

on a soil with an initially high soil organic matter level (Table IX). This

increase is attributable to the very large inputs of organic matter that are

deposited in the dung patch (e.g., equivalent to 20-50 tonnes per hectare

(During and Weeda, 1973) (Table MI). Dung deposition plays a major

role in the buildup in soil organic matter that often occurs under improved

pasture (see Section II,C, 1).

In the surface soil below the dung patch there is a decrease in both

phosphate and sulfate adsorption capacity (During and Weeda, 1973;

During et al., 1973).This is thought to be due to the significant increases in

soil pH (During et al., 1973). The increased soil organic matter content

may also contribute to this effect because soluble organic materials are

known to be able to block some of the adsorption sites on hydrous oxide

surfaces and thus decrease phosphate adsorption (Sanchez and Uehara,

1980; Yuan, 1980).



B. RESPONSE

OF PASTURE

INTHE FECALPATCH

1. Direct Adverse Affect



Dung pats can cover the soil surface and exclude light from the sward for

several months, leading to the death of plants that are covered. MacDiarmid and Watkin (197 1) observed that if cattle dung patches remained for

more than 15 days on the pasture, there was little regrowth from plants

underneath the pat because pasture herbage, particularly clovers, decayed

rapidly. Under solid dung pats pasture often dies, but Weeda (1967) noted

that under very liquid dung patches pasture regrowth is fairly rapid because

these pats disperse and disintegrate rapidly. Where plants were killed below

the patch, Weeda (1967) found that although the outside 2.5 cm of the

patch area was rapidly covered by tillers of surrounding grasses, the central

area remained sparsely covered for 6 to 12 months and in some cases for

up to 2 years.

Recolonization of the bare area may come from the surrounding herbage



NUTRIENT CYCLING UNDER GRAZED PASTURE



155



or from seeds of pasture and weed species in the soil and/or dung. While

recolonization is occumng, pasture production is lost from the area.

Weeda ( 1967) found that in grass/clover pastures white clover was usually

the first species to recolonize these areas and it tended to remain dominant

for 12- 18 months.

In sheep pastures, bare areas of land due to dung deposition are rarely

seen. There are two important factors that contribute to this difference.

First, the weight of dung produced per sheep per defecation is much less

than that per cattle beast (0.0 1 versus 0.20 kg, respectively; see Table VII).

Second, sheep feces are often produced as small pellets that are scattered

over the pasture rather than being deposited as one large pat, as is the case

with cattle.

2. Positive Pasture Response



Even though there is often a depression in pasture growth immediately

below the dung pat, the pasture adjacent to the pat often shows an appreciable positive dry matter response. Cattle dung patches affect growth of

herbage over an area five to six times that actually covered (Norman and

Green, 1958; MacLusky, 1960; MacDiarmid and Watkin, 1972b; During

and Weeda, 1973; Weeda, 1977). The positive effects of dung on

surrounding pasture growth are often long-lived and may last up to 2 years

(Weeda, 1967, 1977; During and Weeda, 1973; Richards and Wolton,

1976). The increased growth is usually related to increased grass growth

(Norman and Green, 1958; MacDiarmid and Watkin, 1971; Weeda,

1977).

Weeda (1977) found that herbage in the zone of dung application was

reduced during the first If months after application due to pasture death.

However, in the adjacent zone (a band 12.7 cm wide around the pat)

pasture growth was stronger than in the control and this compensated for

the depression in the center. In the following spring (approximately 1 year

after application) herbage yields in both the patch area and the adjacent

zone were significantly superior to those in an area unaffected by dung.

When herbage around dung patches was cut to simulate grazing (taking

account of reduced herbage utilization around dung patches; see Section

IV,B,4), Weeda ( 1967) found that there was no dry matter yield increase in

the patch area but there was a 38% increase in yields in the adjacent zone.

Overall, there was a 28% increase in dry matter due to dung deposition

over a l)-year period. In similar experiments During and Weecia (1973)

measured overall dry matter increases of 27 and 30%, respectively over a

3-year period for late winter and summer dung depositions.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

IV. Soil Processes and Pasture Response in Excreta-Affected Areas

Tải bản đầy đủ ngay(0 tr)

×