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II. The Pasture System and Its Effect on Soil Properties

II. The Pasture System and Its Effect on Soil Properties

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122



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



B. ROLEOF FERTILIZER

Fertilizer applications have greatly increased pasture production on

many grassland soils that are inherently deficient in nutrients or have

become so following forest clearance. Indeed, the high-yielding, improved

pasture plants used are generally sera1 species that are adapted to highfertility conditions (Tothill, 1978; Snaydon, 1981) and do not perform well

in infertile and/or acid soil conditions. Thus, the application of lime and

fertilizers is necessary during pasture development to boost natural soil

fertility to a level capable of supporting the oversown pasture plants.

During development, sustained heavy grazing pressure has often been

necessary to control the secondary growth and such pressure could only be

achieved with adequate subdivision and highly productive pastures supplied with nutrients (Levy, 1956).

Once an adequate level of fertility is attained, maintenance fertilizer

applications are usually still required in order to sustain a high level of

pasture production. The major reason for this is that nutrients are lost

from the pasture system through the actions of the grazing animals (Williams and Haynes, 1990a). Losses are caused by transfer of nutrients from

major grazing areas through deposition of dung and urine at stock camps

and unproductive farm areas (e.g., raceways and yards). Losses also occur

through concentration of nutrients into small volumes of soil (dung and

urine patches) in quantities greater than the short-term requirements of the

pasture plants in those areas. Nutrients therefore accumulate in patch areas

and are subject to gaseous and leaching losses. In addition, nutrients can be

“lost” from the available soil pool during cycling due to chemical fixation

(e.g., phosphate adsorption) and/or immobilization into organic forms.

In many parts of the world phosphate fertilizers have been required to

boost pasture production, whereas on acid soils lime has also been an

important factor in pasture development. Applications of K and S and

trace elements (e.g., Cu, Co, Mo, and Se) have also been important on

some soils to increase pasture growth and/or improve stock health. In New

Zealand and Australia, phosphate applications have been particularly important in the success of clover-based pastures (Donald, 1965; Levy, 1970).

High levels of available P in soils are required in order to maintain the

presence and N,-fixing activity of the clovers in the pasture and hence

maintain the N input into the grass/legume pasture (Haynes, 1981). N,

fixation rates in the range of 100-300 kg N ha-’ are common for grass/

clover pastures (Hoglund and Brock, 1987). Despite this, in highly productive grass/clover pastures the grass component is often N deficient (Henzell, 1981) and strategic applications of small amounts of N (25 - 50 kg N



NUTRIENT CYCLING UNDER GRAZED PASTURE



123



ha-l) are sometimes applied in early spring and autumn when clover

growth is weak (O'Connor, 1982).

In the grass-based intensive pasture production of Europe and the

United States, N fertilizers are used widely. Pure grass pasture often responds linearly up to 200-400 kg N ha-' yr-' and application rates in this

range are common (Morrison, 1987). Where pastures are cut for conservation, large quantities of nutrients are removed and the optimum N rate can

be greater than that under grazed swards, where N is returned to the

pasture in the form of dung and urine (Baker, 1986). Where high N rates

are used, applications of P and K may also be required (Prins et al., 1986).



C. EFFECTON SOILPROPERTIES

1 . Soil Organic Matter and Nutrient Status



A major effect of improved pastures is often an increase in the organic

matter content of the surface soil. The rate of organic matter accumulation

and the time taken to reach an equilibrium, where organic matter additions are balanced by mineralization and losses, vary considerably with

initial organic matter level, soil type, climate, and management (Whitehead, 1970; Simpson, 1987). Organic C, N, s, and P do not necessarily

accumulate at the same rate at a particular site and neither do they

necessarily reach equilibrium at the same time (Jackman, 1964; Quin and

Rickard, 1981). This is demonstrated by the results shown in Fig. 1. For

Taupo soil the biggest percentage increase during development was for S,

but for Oropi the biggest increase was for P. Percentage increases differed

greatly for C, N, S, and P in both soils. Such differences are attributable to

differences in the individual cycles of the various elements (Stewart, 1984;

Williams and Haynes, 1990b).

In some locations, such as in southern Australia where levels of soil

organic matter were initially very low (e.g., 0.01-O.l%C and 0.0010.0 1% N), large linear increases in organic matter content have been measured for up to 50 years under subterranean clover pastures (Russell,

1986). Accumulations of organic N in the range of 40-80 kg N ha-' yr-I

have been frequently recorded under such pastures (Simpson, 1987). The

initial increase in organic matter occurs in the surface soil, but with time

there is a deepening of accumulation in the profile (Russell, 1986). Increases have been noted to a depth of 20cm. In some situations on

irrigated pastures, organic matter can also accumulate on the surface of the



R. J. HAYNES AND P. H. WILLIAMS



124



0.9



3

5m



Nitrogen



Nitrogen



rn



0.7



v



0.5



c



0.4

0



0.3

,



0.1



I



2I



I



.



1



I



I



I



'0.02



0

0.09

. Phosphorus

0

7



r



;



'



I



g



I



p



,



L



144%



:. 0.05

m



8 0.03

0.01



0.04



0



I



10



20



30



Pasture age (years)



0.02

40

0



rn

rn



10



20



30



40



Pasture age (years)



Figure 1. Accumulation of organic C, total N, organic S, and organic P in two soils

(0-7.5 cm) during pasture development. Percentage increase compared with time zero shown

at 30 and 50 years, respectively, for Taupo and Oropi soils (Data from Jackman, 1964.)



soil as a dense, partly decomposed mat of up to 2.5 cm in thickness

(Kleinig, 1966; Rixon, 1966a).

In other locations, such as most of New Zealand, the soil organic matter

content was initially reasonably high (e.g., 4- 10% organic C), but during

the clearing and burning of forest vegetation (and sometimes subsequent

plowing of the soil) there was a marked decrease in organic matter content

(particularly organic C) in the surface soil (Walker et al., 1959; Jackman,



12s



NUTRIENT CYCLING UNDER GRAZED PASTURE



1960). Such a decrease can be seen in Table I, where organic C content

decreased from 10 to 4.2% following initial development. During ensuing

pasture development there was, however, an increase in organic matter

accumulation and a marked increase in organic C, S, P, and total N

content (Table I).

The major fertilizer applied to New Zealand and Australian pastures is

superphosphate, which contains 9% P and 11% S. The application of

superphosphate results in an increase in the total inorganic P content of the

soil and therefore an increase in the extractable (available) P content

(Rixon, 1966b; Quin and Rickard, 1981; Nguyen et al., 1989). Generally,

the P is accumulated mainly into the alkali-extractable Al-P and Fe-P

fractions (Saunders, 1959; Batten et al., 1979), which represent phosphate

adsorbed to soil colloids. Organic P increases at a slower rate than inorganic P (Saunders, 1959; Walker el al., 1959; Batten et al., 1979). Thus,

Walker et al. (1959) observed that although organic P made up 90% of total

P in soil (0- 10 cm) under undeveloped scrub, following 25 years of pasture development the percentage of total P in an organic form fell to 5596,

even though there was an increase in the actual amount of organic P in the

soil (Table I). The whole of the soil organic P pool does not necessarily

represent a static pool of P unavailable to pasture plants. Tate et al. ( 1991)

observed a temporal pattern in labile (easily extractable) soil organic P

content of pasture soils, suggesting that such a pool may contribute to

pasture P nutrition. They noted net immobilization of P in early winter

and net mineralization in spring concomitant with rapid pasture growth

and P uptake.

Unlike phosphate, sulfate is very mobile in soils and is easily leached,

thus inorganic S does not generally accumulate in the soil. In pastures

fertilized with superphosphate, much of the applied S is, however, accumulated into the organic matter (Walker et al., 1959; Jackman, 1964).

Table I

Changes in Organic Matter Accumulation (0-10 cm) during 25 Years of Pasture

Development with Annual Superphosphate Applicationsu

Stage of

development



OrganicC



TotalN



Totals



(%)



(%)



(%I



(%)



(%)



organic P as

apercentage

oftotalP



Undeveloped scrub

If years of pasture

25 years of pasture



10



0.31



0.036

0.038



0.043

0.052

0.058



0.047

0.070

0.103



90

75

55



a



4.2



6.3



Data from Walker ef al. (1959).



0.20

0.59



0.071



OrganicP TotalP



126



R. J. HAYNES AND P. H. WILLIAMS



In legume-based pastures, the N content of the soil increases rapidly (due

to N, fixation) but the C content often increases more slowly (Walker et

al., 1959; Jackman, 1964). As a consequence, there is a characteristic

gradual decrease in the C :N ratio during pasture development (Walker et

al., 1959; Watson, 1969). Walker et al. (1959) found that the C :N ratio of

an undeveloped scrub soil was 33 : 1, but after 25 years of grass/white

clover management the C :N ratio had decreased to 1 1 : 1. However, to

some extent, this effect is due to the wide initial C :N ratio of the soils

because the C : N ratio of pasture soils usually approaches 1O:l (Whitehead, 1986). For example, for a soil with a low initial C:N ratio (9: I),

Garwood et al. (1977) found the C:N ratio increased slightly to 10: 1 as

organic matter accumulated under a grass/clover pasture.

Changes in management will influence the equilibrium organic matter

level that is reached. Russell and Harvey (1959) showed that after 30 years

of intensive dairy farming on irrigated grass/clover pastures, soils with a

high initial total N content exhibited a decline in N concentration whereas

sites initially low in N exhibited an increase toward the same equilibrium

N level. In New Zealand, intensified use of long-established pastures has

been found to cause a decline in the soil N content (Field and Ball, 1981).

2. SoilpH



Under improved pasture there is a tendency for soil pH to decline over

time (Williams and Donald, 1957; Russell, 1960; Rixon, 1966a; Batten et

al., 1979;Williams, 1980).The increased soil organic matter content in the

surface soil results in an increase in cation exchange capacity (CEC) and an

increase in H+ saturation of the exchange complex (Williams, 1980).

Excretion of H+ in the pasture rhizosphere due to excess cation uptake by

the N,-fixing clover (Haynes, 1983; Jarvis and Hatch, 1985) plus nitrification of ammonium in the urine patch and subsequent nitrate leaching

(Helyar, 1976) are also likely to be major contributors to such acidification.

The rate of decrease in surface soil pH varies in different soils and

environments (Russell, 1986). An example of the decline in pH in the

surface 10 cm under a subterranean clover pasture is shown in Fig. 2. The

rate of decrease in pH was greatest during the first 10 years and over a

period of 50 years the decrease in pH was approximately one unit (Williams, 1980).

Although it may take from 25 to 50 years for the pH to decrease by one

unit (Lee, 1980; Williams, 1980), such a decrease has led to severe reductions in pasture production due to the buildup of phytotoxic levels of

soluble and exchangeable soil Al and Mn (Williams and David, 1976;



NUTRIENT CYCLING UNDER GRAZED PASTURE



127



U



.-0

d

L



Soil pH

Figure 2. Effect of age of subterranean clover pasture on surface soil (0- 10 cm) pH.

(Redrawn from Williams, 1980.)



Cregan et al., 1979; Evans et al., 1988). Such acidity problems have

developed mainly on unlimed pastures in southern Australia on soils with

low initial organic matter contents and low buffering capacities. For New

Zealand pastures it has long been recognized that regular lime applications

(at 2-4 yearly intervals) are required to maintain soil pH at a suitable

value (e.g., 5.6 to 6.2) for optimum pasture production (During, 1984).

3. Biological Activity



The overall biological activity of the soil generally increases under grazed

pasture (Russell, 1986). Associated with the high content of soil organic

matter and dense mass of pasture roots is a large microbial biomass in the

pasture rhizosphere. The microbial biomass is therefore characteristically

large under improved pasture (e.g., 1200,ug C g-I) and it represents a

reasonably large labile pool of nutrients (e.g., commonly 150-225 kg N

ha-' and 10-60 kg P ha-') (Sarathchandra et al., 1984, 1988; Perrott and

Sarathchandra, 1989). Bristow and Jarvis ( 199 1) estimated mean biomass

N levels under grazed pasture of 138-246 kg N ha-' and observed that

biomass N comprised I 1, 3, and 5 times more N at any one time than was



R. J. HAYNES AND P. H. WILLIAMS



128



present in the mineral N component of the soil, the standing crop, or

excretal returns, respectively. Although some seasonal patterns have been

observed in the levels of nutrients present in the microbial biomass under

pasture (Sarathchandra et al., 1988; Tate et al., 1991),it seems likely that it

is the magnitudes of the various nutrient fluxes through the large labile

biomass pool that are of greatest significance to pasture soil fertility.

Jenkinson and Oades (1979) observed that an improved pasture resulted

in a threefold increase in soil ATP content (an index of microbial biomass)

compared with unimproved scrub. Similarly, results presented in Table I1

show that soil microbial biomass was higher under an improved irrigated

grazed pasture than under an unimproved wilderness area. Biomass levels

were further increased under the highly productive, fertilized grazed pasture.

The high organic matter content and high microbial activity under

pasture also results in a high level of activity of soil enzymes, such as

urease, protease, phosphatase, and sulfatase (Ross et al., 1984; Sarathchandra et al., 1984, 1988). Results in Table I1 show that sulfatase and phosphatase activities were greater under improved pasture than under unimproved wilderness and higher under a highly productive pasture than under

an unfertilized pasture. The high levels of enzyme activity reflect a high

rate of N, S, and P turnover through soil organic pools under pasture

conditions.

Native/unimproved pastures harbor complex communities of invertebrate fauna (Curry, 1987a). Management practices greatly influence the



Table I1



Effect of Long-Term Irrigation and Grazing with or without Annual Superphosphate

Additions on Surface Soil (0-4 cm) Properties"

Imgated and grazed



fiOpefiY

~~



~



Organic C (96)

Biomass C @g g-I)

Sulfatase activity (u mol product

g-l h r l )

Phosphatase activity (u mol product

g-' h r ' )



Wilderness area



376 kg superphosphate

per hectare applied

annually



Control



~



~



3.97

809

0.48



4.18

857

1.3



4.30

969

1.5



7.8



10.2



11.0



Data from R. J. Haynes and P. H. Wfiams (unpublished observations).



NUTRIENT CYCLING UNDER GRAZED PASTURE



129



invertebrate communities by affecting sward structure, composition, and

productivity. In intensively managed grasslands, simplified communities

of species develop that are tolerant to disturbance. Some herbivores, such

as earthworms, benefit from increased food supply (e.g., dung pats) in

intensively managed pastures and they increase in numbers, whereas soil

microarthropods, which are favored by surface accumulation of dead plant

material, decline in numbers (Hutchinson and King, 1979; Curry, 1987b).

Pasture improvement has been shown to result in increased earthworm

numbers (Sears and Evans, 1953; Suckling, 1975)and the weight of earthworms per hectare is closely correlated with pasture production. Suckling

(1959) showed that eight years of pasture improvement increased earthworm numbers on grazed hillsides as well as on the more fertile stock camp

areas. Population densities of earthworms in productive, temperate grasslands range typically from 100 to 1000 m-* (Curry, 1987a), and it has been

estimated that on highly productive pastures the weight of earthworms in

the soil approximates with the weight of stock carried on the surface (Sears,

1949; Waters, 1955). Earthworm populations are greater under grass/

clover than comparable all-grass pastures, and applications of N fertilizer

to pastures increase their numbers, suggesting that readily available organic

N is an important factor limiting earthworm populations (Watkin and

Wheeler, 1966). The role of earthworms in the cycling of nutrients under

pasture is substantial (Keogh, 1979; Syers and Springett, 1983) because

they ingest large quantities of pasture litter and return nutrients in the form

of casts. A high proportion of nutrients in casts are plant available and casts

have a high enzyme activity (Syers and Springett, 1983).

4. Soil Physical Properties



Improved pastures are normally associated with improved soil structure

in contrast to arable cropping, wherein structural deterioration often

occurs. Where pasture development results in significant accumulation of

soil organic matter, associated changes that commonly occur include an

increase in aggregate stability (Clement, 1961; Clarke et al., 1967; Haynes

and Swift, 1990),a decrease in bulk density (Russell, 1960; Watson, 1969),

an increase in total porosity and air-filled porosity at field capacity (Garwood et al., 1977), and a greater water retentive capacity over the range

- 10 to - 1500 kPa (Barrow, 1969; Garwood et al., 1977; Haynes and

Swift, 1990).

Soil porosity increases under pasture because of the presence of the

extensive, ramified root system in the surface soil. Root growth and activity increases porosity through movement of existing soil structural units,

enlargement of existing pores, and cracking caused by shrinkage and swell-



130



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



ing of soil due to water extraction. The high-level earthworm activity under

pasture also contributes to increased porosity because the burrowing action

of the worms creates soil pores through a considerable depth of soil (Syers

and Springett, 1983).

Under pasture, not only are soil structural pores created but aggregates

are stabilized through the binding actions of soil organic matter constituents (Tisdall and Oades, 1982; Oades, 1984). Organic stabilizing agents

include mucilaginous glues produced by plant roots and the rhizosphere

microflora; soil humic substances, which form persistent stable complexes

with mineral components of the soil; and fine roots and associated mycorrhizal hyphae, which physically enmesh soil particles (Tisdall and Oades,

1982).



111. NUTRIENT RETURNS IN FECES AND URINE



A. QUANTITIES

RETURNED

The amounts of nutrients returned to the soil in dung and urine vary

widely between farming systems. Rough estimates can usually be made

from a knowledge of the amounts of herbage consumed, its approximate

nutrient composition, and information on animal requirements (Barrow,

1987). The mean nutrient content of urine and feces of dairy cows from

seven farms is shown in Table 111. Some of the nutrients (e.g., K) are

excreted predominantly in urine whereas others (e.g., P, Ca, Mg, Cu, Zn,

Fe, and Mn) are excreted mainly in feces. Other nutrients, such as N, Na,

C1, and S, are excreted in significant proportions in both feces and urine. A

more detailed partitioning of total nutrient intake by dairy cows is shown

in Fig. 3. These data again illustrate the contrast between the partitioning

of K and that of Ca, Mg, and P between urine and dung. It is also evident

that substantial proportions of P and N (26 and 17%, respectively), but

only 5% of K, are removed from the farm as milk.

Nutrient partitioning between dung and urine can vary depending on

the nutrient content of the diet (Barrow, 1987). Similarly, the nutrient

content of dung and urine can vary greatly between individual animals

grazing the same pasture and for individual animals on different days and

at different times of the same day (Hutton et al., 1965, 1967; Paquay et al.,

1970a; Betteridge et a/., 1986; Groenwold and Keuning, 1988). Much of

this variability is related to differences between animals and between days

in the frequency and volume of urine and feces voided (see Section

III,C, I). Such variability is caused by differencesin the intake of individual



NUTRIENT CYCLING UNDER GRAZED PASTURE



131



Table I11

Mean Nutrient Content in Urine and Feces of Lactating Cows on Seven North Carolina

Dairy Farmsa



Parameter



Urine content (g liter')



Feces content (%

fresh weight)



Percentage excreted

in feces



Total solids

Total N

Total P



6.1

11.5

0.2

2.5

7.95

0.17

0.56

1.18

0.00 1

0.002

0.006

0.0002



15.4

2.9

1.2

0.6 1

0.84

1.28

0.63

0.22

0.005

0.02

0.16

0.02



85

48

95

47

28

97

78

41

95

98

99

99



C1



K

Ca

Mg

Na



cu



Zn

Fe

Mn



Data from Safley ef al. ( 1984).



100



-



ao -



Retention



0,



Y



Milk



m



.-cE

m



60 -



Urine



c

0

c



Dung



Lc



0

Q)



40



-



CI)



m



c



E



0,



2



20 -



cr"



n

--



N



(2562)



P



(237)



K



(1720)



Ca



(726)



M9



(222)



Na



(279)



Figure 3. Percentage excretion and retention of nutrient intake in lactating dairy cows.

Nutrient element intake totals (grams per day) are shown in parentheses. (Data from Hutton

etal., 1965, 1967.)



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