CHAPTER 14 The Effect of Elevated Atmospheric CO 2 on Grazed Grasslands Paul C.D. Newton, Harry Clark, and Grant R. Edwards CONTENTS Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 Properties of Grazed Pasture Ecosystems. . . . . . . . . . . . . . . . . . . . . . . . . . . . 298 Nutrient Cycling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Physical Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Preference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 INTRODUCTION Grasslands cover about a fifth of the terrestrial surface of the world (Hadley, 1993), and the majority of this area is grazed by animals. The impact of an increasing concentration of CO 2 in the atmosphere on these grasslands has assumed importance, first because of the direct effects on food produc- tion (Gregory et al., 1999), and second because of the influence terrestrial ecosystems can have on the composition of the atmosphere and therefore on our climate (Pielke et al., 1998). In the case of grasslands this includes not only C sequestration, N 2 O release, and CH 4 uptake by soils, but also CH 4 emissions from ruminants. Consequently, many research programs have been developed to explore these impacts, and our knowledge of the likely outcomes is progressing rapidly. However, our understanding is based almost exclusively on cut (as opposed to grazed) grassland (e.g., Wolfenden 297 0-8493-0904-2/01/$0.00+$.50 © 2001 by CRC Press LLC 920103_CRC20_0904_CH14 1/13/01 11:11 AM Page 297 298 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT and Diggle, 1995; Casella et al., 1996a,b; Newton et al., 1996; Clark et al., 1997; Hebeisen et al., 1997; Potvin and Vasseur, 1997; Taylor and Potvin, 1997; Clark et al., 1998; Leadley et al., 1999; Navas et al., 1999, but see Edwards et al., 2000), and grazed swards are very different in their botanical and soil char- acteristics (Watkin and Clements, 1976; Haynes and Williams, 1993). In addi- tion, some of these experiments involved the transfer of previously grazed areas to a cutting management (Newton et al., 1996; Clark et al., 1997; Potvin and Vasseur, 1997; Taylor and Potvin, 1997; Clark et al., 1998; Leadley et al., 1999) and therefore do not necessarily display the responses typical of a cut system but of a system in transition. In these examples it is probable that the change in management resulted in a process of succession, one consequence of which would likely be a loss of early successional species. Clearly, any interpretation of a response to elevated CO 2 in these transitional systems must be made with this background change in mind. Altering the frequency and severity of defoliation can have profound effects on the dynamics of grassland systems (Parsons et al., 1988), and dif- ferences in the soil and plant properties of cut and grazed swards can often (in part) be attributed to difference in the timing and severity of harvesting. However, such comparisons conceal the intrinsic effects introduced by graz- ing animals. Consequently, in this chapter we concentrate on comparing cut- ting with grazing at the same frequency and severity of defoliation. We are concerned with identifying characteristics introduced by grazing animals that have the potential to alter how pastures might respond to elevated CO 2 . While the question of CO 2 ϫ grazing interactions has been raised previously (Wilsey, 1996), we are not aware of any comprehensive treatment of this sub- ject. Without considerations of different responses of cut and grazed swards to elevated CO 2 we are not in a position to extrapolate from the considerable bulk of existing experimental data to grazed grasslands—the predominant pastoral land use. Much of what we present is based on temperate pastures; this does not imply any special importance of this type of grassland but simply reflects that these systems have been more extensively examined in terms of CO 2 effects than any other grassland type, and there is a long and detailed litera- ture on responses of these ecosystems to grazing from which we can draw general principles. PROPERTIES OF GRAZED PASTURE ECOSYSTEMS Despite the common practice of using cutting to simulate grazing by animals, there are clear differences in the ecosystem properties which can be directly related to these managements. The actions that the grazing animal introduces involve nutrient cycling, physical damage to plants and soils, and selective grazing. 920103_CRC20_0904_CH14 1/13/01 11:11 AM Page 298 THE EFFECT OF ELEVATED ATMOSPHERIC CO 2 ON GRAZED GRASSLANDS 299 Table 14.1 Proportion of Nutrients Returned by Milking Cows Grazing at Pasture. Element % Returned % Returned Total in Feces in Urine Returned (%) K11 81 92 Na 30 56 86 Ca 77 3 80 P66 0 66 N26 53 89 From Hutton, J.B., Jury, K.E., and Davies, E.B., NZ. J. Ag. Res., 10:367–388, 1967. Nutrient Cycling Grazing animals return nutrients to the pasture, and it is in the composi- tion and spatial arrangement of these nutrient returns that lies the major dif- ference between cut and grazed systems (Haynes and Williams, 1993). Animals use only a small proportion of the nutrients they ingest; 60–99% are returned to the pasture as dung and urine (Barrow, 1987). The actual amounts returned are dependent on the species of animal and the stage of its devel- opment (Haynes and Williams, 1993). Some typical values for dairy cows are shown in Table 14.1. There are some differences between animal types in the proportion of nutrients returned; for example, sheep return greater amounts of N in the urine than cattle (about 70–75% of the excreted N in urine; Sears et al., 1948). However, as a general principle, the concentration returned depends upon the concentration in the food ingested. In the case of the N in urine and the P in dung, the relationship with the feed composition is linear (Barrow and Lambourne, 1961). Unlike cutting, which removes nutrients from the whole of a paddock and then returns them evenly by fertilizer application, grazing removes nutrients from the whole paddock but returns them heterogeneously in the excreta. A sheep may have 18–20 urinations in a day, each event returning nutrients over an area of 0.03–0.05 m Ϫ2 . A typical value for cattle would be 8–12 urinations, each covering an area 0.16–0.49 m Ϫ2 (Haynes and Williams, 1993). In the excretal areas, the nutrients are at very high concentrations (Table 14.2). There are three consequences of this localized return at high con- centrations. First, the pasture becomes a mosaic of patches ranging from very high to very low nutrient status. The outcome of such a distribution is that pasture growth is very high in a small area of the paddock; for example, Saunders (1984) found that under cattle grazing, 75% of the dry matter was produced from 38% of the pasture area. Second, losses of nutrients through gaseous emissions, leaching, and runoff are all exaggerated in the high nutri- ent patches. Third, plants in the excretal areas can be damaged or killed (e.g., by urine scorch or buried under dung) making immediate recovery of the nutrients less likely (Haynes and Williams, 1993). In addition, animals may 920103_CRC20_0904_CH14 1/13/01 11:11 AM Page 299 300 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Figure 14.1 Potential N response curves at ambient (solid line) and elevated (dashed line) CO 2 . Table 14.2 Typical Application Rates of Nutrients (kg ha ؊1 ) Contained in Single Urination or Defecation Events Sheep Cattle Urination Defecation Urination Defecation N 500 130 1000 1040 S 18 13 35 100 K 450 50 900 400 P — 35 — 280 Data sources and assumptions given in Haynes, R.J. and Williams, P.H., Adv. Agon., 49:119–190, 1993. avoid grazing areas close to excreta (Haynes and Williams, 1993) resulting in ungrazed patches of herbage that may be at ceiling yield interspersed with grazed areas in the early stages of regrowth. The long-term effects of excretal return are to increase organic matter (C and N) storage largely because of the return of organic matter as dung (Carran and Theobald, 1998). This outcome, that grazing management can influence the equilibrium organic matter content (Haynes and Williams, 1993), has important implications for C storage and therefore greenhouse gas emissions from pasture. One negative consequence of grazing is the lower soil Ca and Mg contents due to the high rate of cycling of K through excretal returns (Carran and Theobald, 1998). How might we expect pasture response to elevated CO 2 to be modified by grazing-mediated changes in biogeochemical cycles? The distribution of nutrients into high and low patches is the characteristic that has the greatest potential to interact with CO 2 . Let us consider a hypothetical example of the distribution of N which gives the same average application of 240 kg ha Ϫ1 in both cut and grazed swards, but in the cut sward, the N is distributed evenly and in the grazed sward it is at a rate equivalent to 1000 kg ha Ϫ1 in 20% of the area and at 50 kg ha Ϫ1 in 80% of the area. If plant responses to N are linear at both ambient and elevated CO 2 (Figure 14.1a), then it makes no difference to 920103_CRC20_0904_CH14 1/13/01 11:11 AM Page 300 THE EFFECT OF ELEVATED ATMOSPHERIC CO 2 ON GRAZED GRASSLANDS 301 Figure 14.2 N response curves for Lolium perenne plants grown at 390 or 690 ppm CO 2 . (From Schenk et al., J. Pl. Nutr. 19:1423–1440, 1996.) the response whether the N is distributed homo- or heterogeneously. Consequently, despite a strong response to elevated CO 2 , there is no differ- ence in response between a cut (homogeneous N) and grazed (heterogeneous N) management. If there is no CO 2 effect—despite nonlinear N response curves (Figure 14.1b)—then it makes no difference whether the pastures are cut or grazed. However, if the plant/community responses to N are nonlin- ear, and if they are different between CO 2 levels (Figure 14.1c), then the rela- tive responses to CO 2 will depend on the nutrient distribution; i.e., they will be different depending on whether the pasture is cut or grazed and the man- ner of the difference will depend on the relative shape of the curves. In fact, nonlinear curves of the kind shown in Figure 14.1c are frequently seen in experimental data for a range of variables, such as dry matter (Schenk et al., 1996; Figure 14.2), photosynthesis (Bowler and Press, 1996), and competitive ability (Navas et al., 1999). Obviously the argument made for N can also apply to other nutrients that are returned in high concentration by animals (e.g., P, K, or S) and for which there are likely to be nonlinear response curves and CO 2 ϫ nutrient interactions. Soussana and Hartwig (1996) have described the consequences of ele- vated CO 2 for N cycling in cut systems but were not able to speculate on aboveground transfer of N by grazing animals at elevated CO 2 due to a lack 920103_CRC20_0904_CH14 1/13/01 11:11 AM Page 301 302 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Figure 14.3 Nitrogen content (%) of the leaves of legume species exposed to ele- vated CO 2 for different durations and under different management regimes. Values described as Face are for Trifolium repens and T. sub- terraneum plants sampled after 18 months exposure to ambient (360 ppm) or 475 ppm CO 2 using free air carbon dioxide enrichment; the plants were in an established permanent pasture and were grazed intermittently by sheep (see Edwards et al., 2000). Short term data are for T. repens plants harvested after exposure to 350, 500, 650, or 800 ppm for a period of 4 weeks in controlled environment rooms; the light level was 700 ␮ mol m Ϫ2 s Ϫ1 for the 14 h photoperiod and the day/night temperatures 22/16°C. Long term exposure data are for Lotus uligi- nosus plants growing at different distances from a natural CO 2 spring and presumed to have been exposed to elevated CO 2 for many decades; the CO 2 concentration the plants experienced was estimated as the mean of spot measurements taken at canopy height over a period of three years (see Ross et al., 2000); the plants were subject to intermittent cutting. of experiments under grazing. The arguments for changes in N cycling revolve in part around the well-documented decrease in N content of plant leaves at elevated CO 2 (Poorter et al., 1997) and the increase in the fixed N contribution by legumes (Soussana and Hartwig, 1996). These arguments also apply to grazed swards (although see later section on legume content under grazing), but we must also consider the aboveground return through excreta. The reduction in leaf N content at elevated CO 2 appears to be main- tained over the long term; i.e., over lengths of time during which adaptation could occur (Körner and Miglietta, 1994) and in grazed as well as cut systems (Figure 14.3). Consequently, less N will be returned by animals at elevated 920103_CRC20_0904_CH14 1/13/01 11:12 AM Page 302 THE EFFECT OF ELEVATED ATMOSPHERIC CO 2 ON GRAZED GRASSLANDS 303 CO 2 because the N in the urine is directly proportional to the N in the herbage eaten (Barrow and Lambourne, 1961). The lower N in herbage could be com- pensated for by a greater volume of returns but as dry matter yield (and therefore the potential to increase animal numbers) is not markedly increased at elevated CO 2 (Hebeisen et al., 1997; Edwards et al., 2000) this compensa- tion is not likely. If the N returned in each urination is reduced we might expect lower losses through leaching and volatilization as these are concen- tration dependent (Haynes and Williams, 1993), and greater uptake by plants which are able to use the lower concentrations more effectively. These changes should result in tighter N cycling and greater N efficiency under grazing at elevated CO 2 . Physical Effects By the action of their hooves, animals have the potential to physically alter (usually detrimentally) properties of soils and plants. The hoof of an ungulate exerts a pressure that can be calculated from the area of contact and the mass of the animal. Typical static load values per hoof of domestic ani- mals would be 192 kPa for a cow, 83 kPa for a sheep, and 60 kPa for a goat (Willatt and Pullar, 1983). In practice, the pressure applied is almost always greater than this as the hoof is rarely applied flat to the ground. The result of treading can be seen in soil properties; there is a positive relationship between treading intensity and soil bulk density and a negative relationship with hydraulic conductivity (Willatt and Pullar, 1983). In addition, treading alters surface properties, leaving patches (gaps) of bare ground (Watkin and Clements, 1976; Betteridge et al., 1999). Plants are also susceptible to damage from treading, either by crushing or through cutting of plant parts by sharp hooves. The consequences of these physical aspects of grazing are not always separated from the effects of other grazing influences. However, Edmond conducted a comprehensive series of trials to study the effects of treading alone on pastures (see Brown and Evans, 1973, for a review of this work). Edmond (1970) showed that treading could reduce herbage yields by 30–40%, with the yield reduction being dependent on the plant species pres- ent (Figure 14.4). Lolium perenne is particularly resistant to treading (Edmond, 1964) and is observed to increase in abundance as treading pressure increases. During the process of biting it is not just that leaves are removed—as they would also be under cutting—but there is also the potential for damage to meristems and other plant parts resulting in a loss of function, e.g., photo- synthetic capacity, transport of nutrients, or increased susceptibility to pest and diseases. Part of the reason for this is that the biting process also involves pulling, which lifts plant parts, such as stolons, leaving meristems vulnera- ble to being eaten. Pulling can also lift plant roots from the soil. Typically, 9% of the apical meristems of Trifolium repens are removed during a rotational grazing event (Hay et al., 1991). The consequences of the different mechanical 920103_CRC20_0904_CH14 1/13/01 11:12 AM Page 303 304 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Figure 14.4 Sensitivity of some pasture species to treading by sheep, expressed as yield relative to an untrodden control. (From Edmonds, D.B., N.Z. J. Ag. Res., 7:1–16, 1964.) effects of cutting and grazing are rarely considered and can be compared only at the same defoliation interval, at the same severity of defoliation, and with the same nutrient returns. Sears (1953) conducted a five year study of graz- ing effects on pastures which included a number of subtreatments. From these, we can find a comparison of the mechanical effects of grazing; this does not exclude the treading effects sensu Edmond, but in this case observation showed the most marked effects were through the biting process. In particu- lar there was a sharp decline in Trifolium pratense under grazing because the animals were able to remove plant crowns, whereas the cutting process left them intact (Figure 14.5). We can envisage CO 2 interacting with many of these physical conse- quences of grazing. First, the damaging effects of treading on soil structure— compaction, loss of drainage capacity—may have different effects at elevated CO 2 in which greater allocation of C below ground is frequently observed. Changes in soil biota have also been reported at elevated CO 2 (O’Neill, 1994; Yeates et al., 1999), and these can strongly influence soil structural prop- erties (O’Neill, 1994). Second, the creation of gaps by the grazing animals has important consequences for population processes as these promote both recruitment from seed (Panetta and Wardle, 1992) and vegetative 920103_CRC20_0904_CH14 1/13/01 11:12 AM Page 304 THE EFFECT OF ELEVATED ATMOSPHERIC CO 2 ON GRAZED GRASSLANDS 305 Figure 14.5 Effects of mechanical damage by grazing animals on the botanical composition of a pasture. (From Sears, P.D., N.Z. J. Sci. Tech., 35A:1–29, 1953.) development—e.g., branching and tillering (Arnthórsdóttir, 1994)—and both of these regenerative processes have been shown to be influenced by elevated CO 2 . Many studies have shown elevated CO 2 can alter the number of seeds produced (Lawlor and Mitchell, 1991; Farnsworth and Bazzaz, 1995) which can lead to changes in recruitment in seed-limited species (Edwards et al., 2000). It may also be the case that more seed heads are left intact after defoli- ation by grazing rather than cutting, allowing greater expression of any CO 2 effects on seed characteristics. Other studies have shown changes in seed quality at elevated CO 2 (e.g., in C:N ratio and seed mass) which have the potential to alter germination and establishment rates. If the likelihood exists of different seed behavior in gaps at elevated CO 2 , as shown by Spring et al. (1996), then there is a strong possibility that the increased gap frequency under grazing will result in a grazing management ϫ CO 2 interaction. Increased vegetative propagation has been shown to be an important mech- anism driving changes in species abundance in response to elevated CO 2 in a wide range of situations, such as temperate pasture (Clark et al., 1997) and alpine meadows (Leadley et al., 1999). It should achieve even greater expres- sion in the presence of more gaps (regeneration niches) and be of more criti- cal importance given the losses of meristems experienced in a grazed system. 920103_CRC20_0904_CH14 1/13/01 11:12 AM Page 305 306 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Preference The botanical composition of pastures (species identity and abundance) is determined in part by the method of harvesting. Even a consistent, uniform process, such as cutting, has a selective effect which reflects the vertical dis- tribution of plant species in the canopy. For example, cutting removes pro- portionally more clover (Trifolium repens) laminae than grass (Lolium perenne) laminae in a mixed grass/clover sward because the clover leaves are held higher in the canopy (Woledge et al., 1992). It has been argued that grazing animals such as sheep have a selective effect on sward composition simply by the kind of passive selection imposed by cutting (Milne et al., 1982). In fact, Parsons and co-workers have shown that sheep have a preference (i.e., actively select) for white clover; in this case, it is a partial preference, with sheep preferring a diet of 70% clover and 30% grass (Parsons et al., 1994). Note that this also means that animals might select against clover when the clover percentage in the sward exceeds 70%. As a consequence, the clover removed under grazing is proportionally larger than the clover removed by the passive selection of a lawnmower (Woledge et al., 1992). The outcome for a plant species that is a preferred component of the diet is clearly not favorable in comparison to a nonpreferred species. Indeed, ani- mals may reduce the abundance of their preferred species in the sward until it becomes a small component of their diet (Parsons et al., 1991b)—the “Paradox of Imprudence” (Slobodkin, 1974). The only way in which a plant species can overcome the deleterious consequences of being preferred (in relation to other plant species) is if it holds some advantage in growth over its companion species. By this means, a preferred species can maintain its presence in a grazed sward until a point at which the grazing pressure out- weighs the growth advantage (Parsons et al., 1991b). It has been suggested that the growth advantage held by clover is a greater specific leaf area (Parsons et al., 1991a). At elevated CO 2 , there is strong evidence that legumes are advantaged in comparison to grass species (Newton et al., 1994; Clark et al., 1997; Hebeisen et al., 1997; Leadley et al., 1999); although this evidence comes from cutting experiments, we might anticipate that, in the absence of any change in ani- mal preference, CO 2 would also result in greater legume growth under graz- ing. In a Face experiment grazed by sheep we compared the effects of grazing (Figure 14.6a) and cutting (Figure 14.6b) on pasture responses to elevated CO 2 . The ambient values show that clover was deleteriously affected by graz- ing (compare Figure 14.6a and b); however, CO 2 enrichment gave the clover sufficient advantage to compensate for the grazing effect so that clover growth under grazing at elevated CO 2 (Figure 14.6a) was comparable to clover growth under cutting at ambient CO 2 (Figure 14.6b). In this instance, clover responded positively to elevated CO 2 only when grazed; under cut- ting, there was only a minimal stimulation of clover, suggesting that in this environment factors other than CO 2 set a limit to the growth of clover. 920103_CRC20_0904_CH14 1/13/01 11:12 AM Page 306 [...]... different animals and soils Proc XI Int Grass Cong., 453 –458 Edwards, G.R., Clark, H., and Newton, P.C.D., 2000 Carbon dioxide enrichment affects seedling recruitment in an infertile, permanent grassland grazed by sheep Oecologia, in press Farnsworth, E.J and Bazzaz, F.A., 1995 Inter- and intra-generic differences in growth, reproduction, and fitness of nine herbaceous annual plant species grown in elevated... whole series 920103_CRC20_0904_CH14 308 1/13/01 11:12 AM Page 308 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT CONCLUSIONS We already know that management (fertilizer, cutting frequency) can modify pasture responses to CO2 (Hebeisen et al., 1997) and “no cutting treatment can satisfactorily reproduce the defoliation regime in a grazed pasture” (Watkin and Clements, 1976), there is every... A., and Woledge, J., 1991a Plant-animal interactions in a continuously grazed mixture I Differences in the physiology of leaf expansion and the fate of leaves of grass and clover J Appl Ecol., 28:619–634 Parsons, A.J., Harvey, A., and Johnson, I.R., 1991b Plant-animal interactions in a continuously grazed mixture II The role of differences in the physiology of plant growth and of selective grazing... –218 Navas, M-L., Garnier, E., Austin, M.P., and Gifford, R.M., 1999 Effect of competition on the responses of grasses and legumes to elevated atmospheric CO2 along a nitrogen gradient: differences between isolated plants, monocultures and multispecies mixtures New Phytol., 143 :323 –331 920103_CRC20_0904_CH14 310 1/13/01 11:12 AM Page 310 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT. .. dung and urine NZ J Ag Res., 27:405–412 Schenk, U., Jäger, H-J., and Weigel, H-J., 1996 Nitrogen supply determines responses of yield and biomass partitioning of perennial ryegrass to elevated atmospheric carbon dioxide concentrations J Pl Nutr., 19 :142 3 144 0 Sears, P.D., 1953 Pasture growth and soil fertility I The influence of red and white clovers, superphosphate, lime, and sheep grazing on pasture... Blum, H., and Nösberger, J., 1997 Growth responses of Trifolium repens L and Lolium perenne L as monocultures and bi-species mixture to free air CO2 enrichment and management Global Change Biol., 3 :149 –160 Hutton, J.B., Jury, K.E., and Davies, E.B., 1967 Studies of the nutritive value of New Zealand dairy pastures V The intake and utilisation of potassium, sodium, calcium, phosphorus, and nitrogen in pasture... dominance and succession Ecology, 78:666–667 Ross, D.J., Tate, K.R., Newton, P.C.D., Wilde, R.H., and Clark, H., 2000 Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring Global Change Biol., 6:779–790 Saunders, W.M.H., 1984 Mineral composition of soil and pasture from areas of grazed paddocks, affected and unaffected by dung and. .. elevated CO2 in the same manner as cut pastures In this chapter we have attempted to identify factors and processes that are fundamentally different under grazing and found many that might be expected to interact with elevated CO2 Because of this, and because there is accumulating evidence (e.g., Figure 14. 6) to show that grazing management can modify pasture responses, it is by no means certain that we... studies Plant Cell Env., 14: 807 –818 Leadley, P.W., Niklaus, P.A., Stocker, R., and Körner, C., 1999 A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland Oecologia, 118:39 –49 Milne, J.A., Hodgson, J., Thompson, R., Souter, W.G., and Barthram, G.T., 1982 The diet ingested by sheep grazing swards differing in white clover and perennial ryegrass... –270 Hadley, M., 1993 Grasslands for sustainable ecosystems Proc XVII Int Grass Cong., 21 –28 Hay, M.J.M, Newton, P.C.D., and Thomas, V.J., 1991 Nodal structure and branching of Trifolium repens in pastures under intensive grazing by sheep J Ag Sci., 116:221–228 Haynes, R.J and Williams, P.H., 1993 Nutrient cycling and soil fertility in the grazed pasture ecosystem Adv Agron., 49:119 –190 Hebeisen, T., . grassland (e.g., Wolfenden 297 0-8 49 3-0 90 4-2 /01/$0.00+$.50 © 2001 by CRC Press LLC 920103_CRC20_0904_CH14 1/13/01 11:11 AM Page 297 298 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT and. may 920103_CRC20_0904_CH14 1/13/01 11:11 AM Page 299 300 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT Figure 14. 1 Potential N response curves at ambient (solid line) and elevated (dashed line). of harvesting. However, such comparisons conceal the intrinsic effects introduced by graz- ing animals. Consequently, in this chapter we concentrate on comparing cut- ting with grazing at the