Agriculture, Ecosystems and Environment 114 (2006) 159–169 www.elsevier.com/locate/agee
Review
Natural recovery of soil physical properties from treading damage of pastoral soils in New Zealand and Australia: A review J.J. Drewry * Land and Environmental Management, AgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand Received 24 August 2005; received in revised form 28 November 2005; accepted 29 November 2005 Available online 26 January 2006
Abstract This paper reviews natural recovery of deteriorated soil physical condition under animal treading in grazed pastoral systems, particularly in New Zealand and Australia. While much research has focused on soil compaction and physical deterioration from animal treading, there has been much less focus on natural recovery of soil physical properties after treading damage has occurred. Natural recovery of deteriorated soil physical condition improves soil properties including hydraulic conductivity, macropore volume and bulk density. Soil physical condition naturally recovers when animals are partially or completely excluded from pasture, although improvements are likely to be limited to no deeper than 10–15 cm soil depth, under common grazing practice or animal exclusion. However, the physical deterioration and natural recovery processes are linked in a cycle. Natural recovery of soil physical condition in this cycle is therefore important when evaluating management practices affecting soil deterioration on-farm, field trial interpretation, and ungrazed riparian zone soil structure. This review also discusses directions of future research to enhance soil management, including quantifying and evaluating soil physical deterioration and natural recovery. Several knowledge gaps relating to pastoral agriculture in New Zealand and Australia, particularly under rotational grazing management on intensive dairy farms are discussed. Further research is required into the consequences of farm management practices that enhance natural rejuvenation of degraded soils. Consequently, integration of both deterioration and natural recovery of soil physical condition in the soil compaction and recovery cycle is needed to improve farm system evaluation and management. Natural recovery of soil condition when animals are partially or fully excluded from grazing is therefore important in management and modelling of pastoral and ungrazed riparian soil, and subsequent environmental impacts. # 2005 Elsevier B.V. All rights reserved. Keywords: Compaction; Pugging; Recovery; Amelioration; Macroporosity
Contents 1. 2. 3.
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recovery of soil physical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Short-term recovery of soil physical properties – grazing interval duration or exclusion for 1 year 3.1. Dairy cattle treading management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Intensive cattle treading causing puddling or poaching . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Extensive and hill-country treading management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recovery of soil physical properties after exclusion of grazing animals for up to 3 years . . . . . . . 4.1. Animal treading versus animal exclusion treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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* Present address: Integrated Catchment Assessment and Management Centre, School of Resources, Environment and Society, and the Cooperative Research Centre for Landscape Environments and Mineral Exploration, Building 48A Linnaeus Way, The Australian National University, Canberra ACT 0200, Australia. Tel.: +61 2 6125 4670; fax: +61 2 6125 8395. E-mail address:
[email protected]. 0167-8809/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2005.11.028
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4.2. Treading intensity treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recovery of soil physical properties after exclusion of grazing animals for greater than 3 years 5.1. Animal grazing versus animal exclusion areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Recovery of soil physical properties under fencelines . . . . . . . . . . . . . . . . . . . . . . . . . Overview of mechanisms for natural recovery of soil physical deterioration . . . . . . . . . . . . . . 6.1. Effect of earthworms on soil physical condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Animal treading can result in degradation of soil physical quality through hoof action of grazing animals (Betteridge et al., 1999, 2002; Pande et al., 2000; Ward and Greenwood, 2002). Deterioration in soil physical quality from trampling causes soil deformation through (i) soil compaction and (ii) soil homogenisation through shearing or pugging and poaching. Such terms, however, require careful use and defining. Soil compaction has been traditionally described as ‘‘the compression of an unsaturated soil body resulting in a reduction of the fractional air volume’’ (Hillel, 1980). The effect of soil compaction is to decrease soil porosity, particularly the volume of the large inter-aggregate pores (macropores). Once the air volume is reduced, or the soil is saturated, then the term consolidation can be used (Hillel, 1980). Consolidation is the compression of a saturated soil by squeezing out water. Consolidation is a more gradual process than compaction, as the viscosity of water is much greater than air. Poaching or puddling in contrast to compaction, are terms used for slurry-induced soil conditions on very wet soil when trampled by stock. Pugging in wet soft soil causes deep hoof imprints and is often associated with considerable pasture damage. In contrast to soil physical deterioration caused by machinery, soil physical deterioration by grazing animals is likely to be more widespread within paddocks particularly in wet conditions, than for example, under tracks of wheeled implements. Such treading-induced damage described above includes reduced soil permeability through reduced pore space and continuity and disrupted soil pore networks, and increased bulk density (Drewry and Paton, 2000; Menneer et al., 2001). Indicators of soil physical health or condition are becoming an increasingly important area of research and for environmental reporting for government agencies (Sparling et al., 2004). Indicators of soil physical properties commonly include bulk density (dry soil mass per unit volume); a measure of large soil pore volume, for example macroporosity, (the volumetric percentage of soil drainage and aeration pores, commonly >30 mm diameter); and saturated hydraulic conductivity (ability of the soil to transmit water). Although macropore volume or macroporosity has been found to be a sensitive indicator, its definition varies. Macroporosity describes the volumetric percentage of pores
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greater than 30 mm diameter (McLaren and Cameron, 1996). It is primarily soil macropores that are responsible for adequate soil aeration and rapid drainage of water and solutes through soil (McLaren and Cameron, 1996). However, macropores have also been defined in other studies with a range of different equivalent pore diameters, which must be kept in mind when comparing studies. For example, macropore diameter has been defined as being >50 mm (Carter, 1988), and >195 mm (Koppi et al., 1992). Soil compaction by treading and subsequent natural recovery of soil physical properties has been shown to be cyclical (Drewry et al., 2004), but few studies have integrated these components in pastoral systems. Processes contributing to natural recovery of physically degraded soil include wetting and drying cycles, subsequent soil cracking, earthworm burrowing and root penetration and decay, and freeze and thaw cycles during winter (Greenland, 1981; Hodgson and Chan, 1984; Dexter, 1991; Greenwood and McKenzie, 2001). Physical deterioration of soil from the surface to about 5 cm deep, for example, can be naturally ameliorated quite rapidly by the burrowing activities of macro-invertebrates associated with dung deposition. For example, air-filled porosity and infiltration rate increased, and soil bulk density declined in the top 3 cm of soil under cattle dung pats (Herrick and Lal, 1995). However, in contrast, physical deterioration of soil from depths below 15 cm are likely to be naturally rejuvenated much less slowly, if at all. Indeed, while soil physical deterioration is often visually evident on surface soil, or at 0–5 cm soil depth, deterioration of macropore structure commonly occurs at 5–10 or 10–15 cm depth under cattle treading (Drewry et al., 2004; Drewry and Paton, 2000). Macropore structure is often reduced particularly at 5–10 cm under dairy cow treading, but in contrast, may also be less damaged beneath 10 cm (Drewry, 2003; Singleton and Addison, 1999). Cattle exert greater static pressure (160–192 kPa) on soil than sheep (83 kPa), although these pressures are known to at least double when animals are walking (Willatt and Pullar, 1983). However, even though dairy farms are often situated on well-structured soils, soils on New Zealand dairy farms have been shown to be more compact than similar soils on sheep farms (Drewry et al., 2000). Consequently, farm management strategies to reduce or prevent treading-induced soil deterioration have been devised. Treading management
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strategies such as reducing treading intensity (Drewry, 2003; Drewry and Paton, 2000), and animal exclusion (Greenwood et al., 1997, 1998; Stephenson and Veigel, 1987) enhances the rate of soil physical improvement after treading damage has occurred. However, little research has been undertaken in New Zealand or Australia on the extent to which physically degraded soil can naturally recover after treading damage, particularly for farm and grazing management options available for pastoral sheep, beef or dairy farming. Similarly, there is also a paucity of international literature covering this subject area. This review focuses on the extent of natural recovery of soil physical condition under pastoral grazing management practices in New Zealand and Australia, and discusses the need for land management strategies to assist natural recovery of physically degraded soils.
2. Recovery of soil physical properties The time taken for natural recovery of soil physical condition may take weeks, months, or even years. Most of the few studies on short-term recovery of soil physical condition were conducted within a rotational grazing system, where animals may re-graze pastures within weeks or several months. Other studies have investigated shortterm soil physical recovery after complete exclusion of animals. Consequently, the review is structured in the following sections for appropriate treading management and duration of soil recovery. Studies reporting natural recovery of soil physical condition for periods less than 1 year are summarised in Table 1. Studies reporting natural recovery of soil physical condition for periods 1 year or greater are summarised in Table 2.
3. Short-term recovery of soil physical properties – grazing interval duration or exclusion for 1 year 3.1. Dairy cattle treading management The extent of natural recovery of soil physical condition depends on grazing management (and hence treading damage) regimes and climatic conditions. For example, much of the soil structural damage on dairy farms in southern regions of New Zealand is likely to occur in spring or autumn when cows are grazed while the soil is moist or wet. In southern regions, soil compaction and pugging damage are likely to be greatest in spring. A rotational grazing system, with 14–21 days between paddock grazing, is common on southern New Zealand dairy farms from September to May (i.e., during lactation), giving a grazing density of 70–90 cows ha 1 for 24 h (Drewry and Paton, 2000; Monaghan et al., 2005). In contrast, during winter in southern regions of New Zealand, dairy cattle are normally removed from the farm milking area, so pasture is not grazed
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or treaded (Drewry et al., 2004; Monaghan et al., 2005). Cattle are commonly winter-grazed on different farms, or on Brassica forage crops. In contrast, winter treading damage by dairy cows is often reported in trials in the North Island of New Zealand as dairy cows are more likely to be grazed on-farm over winter at higher densities than during lactation (Ledgard et al., 1996; Singleton et al., 2000; Drewry et al., 2003). For example, in the North Island Waikato region of New Zealand, nonlactating dairy cows are grazed on strips at stock densities that can reach 300–600 cows ha 1, often in wet conditions, with resulting treading damage and pugging likely (Singleton and Addison, 1999). On farms where stock are excluded from pasture, by being placed and fed on a standoff feed pad during the susceptible wet winter period, soil physical deterioration is avoided in paddocks, while previous treading damage could also be naturally rejuvenated. It is generally thought that compacted soils can be rejuvenated by natural processes over the winter period, where, under some farm management, dairy cows are moved off-farm. However, Drewry et al. (2004) quantified the extent of soil compaction and natural recovery of soil physical condition during a 3-year trial on a southern New Zealand dairy farm, where cows were removed during winter. The silt loam soil (Aeric Fragiaquept) was regarded as susceptible to physical degradation by treading, particularly in wet conditions. Soil physical properties including macroporosity (pore diameter >30 mm), bulk density and saturated hydraulic conductivity were consistent with significant soil physical deterioration that occurred through the wet spring period as a result of dairy cattle grazing. Recovery of soil physical condition within 0–5 cm and 5–10 cm soil depths occurred over summer and autumn, but for many soil properties, recovery over winter was much less than during summer and autumn (Drewry et al., 2004). Improvements for many soil physical properties were greater at 0–5 cm than at 5–10 cm. An example of the mean recovery of macroporosity over the range of grazed and ungrazed treatments from the end of spring to autumn is shown in Table 1. Similarly, a cyclical pattern of spring soil compaction by dairy cows followed by natural recovery was shown by Monaghan et al., (2005). Natural improvements to soil bulk density and saturated hydraulic conductivity over the summer to end of winter period was evident in the 0–5 cm soil layer within rotationally grazed treatments (Monaghan et al., 2005). However, in contrast to the study of Drewry et al., (2004), the cyclical pattern was less obvious at 5–10 cm and 10– 20 cm soil depths. Factors that may have contributed to this could have been the more physically resilient soil (Dystrochrept) structure, and the greater organic carbon (5.5–6.2%), than in the study of Drewry et al. (2004). Hydraulic conductivity was also observed by Elliott et al. (2002) to be greater in summer than in the winter following a treading event.
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Table 1 Extent of natural recovery of soil physical properties, for periods under 1 year Management
Grazed/treated
Ungrazed/excluded
Improvement (%)
Post-damage interval
Soil depth cm
Soil type and texture
Classification
Organic carbon (%)
References
Air permeability (10 12 m2) Ksat (mm h 1) Kunsat (mm h 1) Bulk density (Mg m 3) Infiltration rate (mm h 1)
Sheep grazed vs. ungrazed
31.2
43
38
8 months
0–5
NA
Typic Dystochrept
5.4% at 0–7.5 cm
Nie et al. (1997)
383 91 0.72
483 152 0.65
26 67 11
111
124
11
56–74 days
Surface
Silty clay
Lithic Haplustoll
OM 5.8–6.3%
Warren et al. (1986b)
8.4
15.8
88
4 months (year 1)
0–5
Silt loam Silt 34–40%, Clay 16%
Dystochrept and Aeric Fragiaquept
6.4% at 0–7.5 cm
Drewry and Paton (2000)
7.4
10.9
47
5–10
8.5
15
76
5.5
12.5
127
12.5
18
44
4 months (year 1) 4 months (year 2) 4 months (year 2) 5 months
0–5
Silt loam Silt 65%, Clay 32%
Aeric Fragiaquept
4.5% at 0–7.5 cm
Drewry et al. (2004)
8.5 34
10.7 71 under dung
26 108
5 months 140 days
5–10 Surface
Silt 42%, Clay 20%
Typic Argiustoll
1.05
0.93 under dung
11
13
>21 under dung
61
Macroporosity (>30 mm) %
Cattle grazed & measured prior to regrazing Cattle grazed vs. ungrazed
Macroporosity (>30 mm) %
Macroporosity (>30 mm) %
Various managements
Infiltration rate (mm h 1) Bulk density (Mg m 3) Air-filled porosity (%)
Under dung vs. control
0–5 5–10
Herrick and Lal (1995)
J.J. Drewry / Agriculture, Ecosystems and Environment 114 (2006) 159–169
Soil property
8–16 years c. 0.5 c. 0.9
40
0–20 0–5 6 years 45 years 379 0.9–1.0
Ksat (mm h 1) Bulk density (Mg m 3) Penetration resistance (MPa)
Grazed vs. ungrazed Sheep grazed vs. excluded
259 1.1
46 10
0–1
Sandy loam. Silt 11–16%, clay 25–36%
Paleustalf
1.7–2.4%
Basher and Lynn (1996) Braunack and Walker (1985) 6.7–8% Dystochrept
Francis et al. (1999) Udic Dystochrept Silt loam. Silt 67%, clay 27% 0–5.1 0–30 16 months 1 year c1.5–1.58 51 Ksat (mm h 1)
Sheep grazed vs. mown pasture
c. 1.38 25
91 104
Loam 0–5.1 12 months 32–58 c1.41–1.48 c. 1.37 Cattle grazed vs. ungrazed
0–5 4 years 14.% 10.9
37
15.3 11.1
38
2 years
0–5
Silt loam. Clay 18%, silt 57%
NA
Mollisol
OM 5%
(0–10 cm) 29.7 t/ha
Stephenson and Veigel (1987)
Greenwood et al. (1998) Orr (1975) Haplustalf Clay 19–30% 0–4 2.5 years 1.17 1.186 mean
Bulk density (Mg m 3) Macropore volume (>50 mm) Macropore volume (>50 mm) Bulk density (Mg m 3)
Sheep grazed vs. ungrazed Cattle grazed vs. ungrazed
13
Improvement Ungrazed/Stock excluded Grazed/treated Management Soil property
Table 2 Extent of natural recovery of soil physical properties, for periods 1 year or greater
Post-damage interval
Soil depth cm
Soil type or texture
Classification
Organic carbon (%)
References
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Removal of cows for several rotations following soil damage was suggested by Drewry and Paton (2000) as a practical on-farm management tool to improve the physical structure in the 0–5 cm and 5–10 cm soil depths with pasture being conserved as silage or balage. However, this grazing management is likely to be insufficient where physical damage below 10 or 15 cm has occurred, as short-term natural recovery of soil physical condition commonly occurs only in surface layers of soil. During year 1, increases in macroporosity (pores >30 mm) over 4 months in an ungrazed treatment at 0–5 cm were greater than at 5– 10 cm depth (Table 1). Similar increases at 0–5 cm depth were also reported for the treatment which simulated the removal of conserved pasture. There are few other published studies evaluating the effects of on-farm management strategies on recovery of soil physical condition. Soil physical condition naturally recovers to the greatest extent when animals are completely excluded from pasture. Inclusion of appropriate soil physical measurements in field trials where stock are excluded from field areas, will help evaluate the dynamics and magnitude of natural soil physical changes. Such natural soil physical changes would be useful in field trials where soil pore volume and continuity measurements are important, and as this review discusses, are likely to change over time after stock has been excluded. Similarly, such soil physical changes under ‘‘cut-and-carry’’ or nil-grazing pastoral systems would also be valuable to assess soil physical changes under non-grazed systems. These systems include housing or standing cattle off the pasture, and therefore harvesting pasture by mechanical means rather than by in situ grazing. Although vehicles can result in some soil physical deterioration, this system is likely to result in less soil physical damage than under grazing. However, in New Zealand and Australian dairy grazing conditions, such a strategy is likely to be logistically and economically practical only under intensive agricultural systems where extensive soil physical damage would otherwise occur, requiring further field or modelling evaluation, particularly the potential for natural improvements in soil physical condition as it affects nutrient cycling and pasture growth. However, such systems, with even applications of housed animal effluent, are estimated to contribute to reduced nitrogen leaching and greater pasture production, although viability is greatly dependent on capital costs (de Klein, 2001). 3.2. Intensive cattle treading causing puddling or poaching Grazing intensity can determine whether there is less likely to be soil puddling due to less intensive grazing pressure during lactation, compared with greater grazing intensity under equivalent soil and weather conditions. For example, during high-density winter grazing in Waikato, a high incidence of visual puddling is common. Visual
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pugging damage following intense grazing management by cattle in a Waikato, New Zealand trial took up to 29 weeks to recover to the level of untreaded plots, whereas the effect of pugging on hydraulic conductivity lasted only 1–3 months (Zegwaard et al., 2000). This study also reported a reduction in pasture production of 51% in a 24-h duration grazing treatment. Due to favourable growing conditions following damage, full recovery of pasture growth was reached after the 14th week. Also in the Waikato region, natural recovery of soil physical condition at 2, 6, 10, 14 and 21 weeks after a winter pugging event on wet Te Kowhai silt loam Typic Orthic Gley (Typic Ochraqualf) and Kereone loam Typic Orthic Allophanic (Eutric Hapludand) soils was reported by Drewry et al. (2003). That study indicated that of the soil physical properties, soil macroporosity (pores >30 mm) and saturated hydraulic conductivity showed the greatest recovery after the winter pugging event, compared with bulk density. Saturated hydraulic conductivity across all sites at 0– 5 cm depth almost fully recovered within 6 weeks of the winter grazing event (Drewry et al., 2003), a similar trend to that shown by Zegwaard et al. (2000). For example, at the Typic Ochraqualf site, saturated hydraulic conductivity at 0–5 cm in the pugged treatment improved from 60 to 208 mm 1 2 weeks after the pugging event, and to 647 mm 1 6 weeks after the pugging event (Drewry et al., 2003, and unpublished data). At the Typic Ochraqualf site, at 5–10 cm, saturated hydraulic conductivity 14 weeks after the pugging event improved from 20 to 467 mm 1 (Drewry et al., 2003, and unpublished data), showing the slower natural response rate for 5–10 cm compared with the 0–5 cm depths. The time taken for visual hoof damage from soil puddling and deformation following a 3-day beef cattle treading event to reduce to half of the initial value, ranged from 87 to 165 days depending on the characteristics of the soil (Elliott et al., 2002). For example, the longest recovery time (165 days) occurred in a clay loam topsoil (Umbric Dystrochrept), where even after 1 year there was still 21% of the initial damage present. In contrast, the shortest recovery time (87 days) occurred in a clay loam (Ochreptic Hapludult) topsoil with moderately to strongly developed structure. The organic matter content (0–7.5 cm) of these hill and steep land soils was very high, and typically ranged from 15–20%. In contrast, the time taken for the percentage of bare ground to decrease to 50% of initial value was only about 50 days for all 3 soils. Additionally, the infiltration recovery rate following cattle treading damage was strongly correlated to the degree of treading damage and soil physical characteristics including land micro-topography, within the same site (Tian et al., 1998). While there has been a number of studies evaluating soil conditions under treaded versus non-treaded treatments, research is now required to evaluate management that can be applied on-farm to maximise the rate of natural soil physical recovery. The mechanisms of the recovery of soil physical
condition could be a result of earthworm and insect burrowing, and root penetration and decay, although these were not reported in these studies. Further research into the mechanisms of recovery of soil physical condition will help better formulate practical management strategies for restoring physically damaged soils. 3.3. Extensive and hill-country treading management Thirty days was found to be insufficient to allow full recovery of infiltration during rotational grazing of extensive rangeland agriculture, even though some recovery of infiltration of the soil (Petrocalcic Calciustoll) trampled by heifers at a 30-day rotation interval occurred (Warren et al., 1986a). During the dormant seasons (winter and drought) mean infiltration rate was found to have improved by mid-way through the rest period to a level significantly greater than the post-graze condition (Table 1), but remained lower than the pre-graze condition (Warren et al., 1986b). Conversely, recovery of soil physical properties can occur within 6 months. In a study of a cattle treading event in New Zealand hill country, Nguyen et al. (1998) found that infiltration rate, water contamination and nutrient runoff in plots that had been grazed for up to 3 days, had returned to similar levels to undamaged control plots, when measured 6 months after grazing. As the interval between treading and recovery measurements was 6 months, they suggested further study at shorter post-grazing intervals is needed to investigate shorter-term soil recovery. Consequently, to determine how quickly soil physical recovery occurs, the choice of sampling interval is important. In contrast, soil physical properties at 0–5 cm depth from very compact soil (Typic Fragiudalf) in tracks from deer fenceline pacing, showed little recovery 6 weeks after a treading event (McDowell et al., 2004). However, saturated hydraulic conductivity and macroporosity were much greater away from tracks, although there was little improvement 6 weeks after grazing. Consequently, there is increased likelihood of overland flow containing soil contaminants on physically degraded soils, even after periods of natural recovery (Nguyen et al., 1998; McDowell et al., 2004). Improvements in soil physical structure also occurred under pastoral fallowing during an 8-month period (Nie et al., 1997). Following winter damage, they showed improvements in hydraulic conductivity, air permeability, and bulk density in pasture fallowed (ungrazed) over the growing season (Table 1). Most of the limited number of studies of short-term recovery of soil physical condition have generally been conducted on one soil type, and have often not included different management strategies. Some studies have evaluated soil recovery over several soil types, but often each in different years. Conducting concurrent trials on contrasting soil types would help enhance our understanding of the dynamics of soil physical condition.
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4. Recovery of soil physical properties after exclusion of grazing animals for up to 3 years 4.1. Animal treading versus animal exclusion treatments Improvements in soil physical condition has also been shown to occur when stock were excluded for one to several years (Table 2). For example, a greater percentage of pores as macropores (>50 mm) occurred where livestock were excluded from previously heavily grazed areas (Orr, 1975). There were greater macropore volumes at 0–5 cm in the second growing season after exclusion, than non-exclusion, while at 5–10 cm and 10–15 cm improvements in macropores did not become apparent until the third year of the study. The author also noted that freezing conditions were more prevalent in the first year than in subsequent years. Orr (1975) concluded that more than a year of stock exclusion was needed for significant soil recovery, and that recovery continued up to 4 years. Improvements in bulk density and unsaturated hydraulic conductivity were found by Greenwood et al. (1997, 1998) for ungrazed plots when sheep were excluded for 2.5 years, compared with plots receiving sheep grazing. However, improvements in soil physical condition were largely limited to the 0–4 cm depth of soil, with the greatest differences occurring at 0–2 cm depth (Table 2). Clay content of these soils varied from 19–30% (Table 2). In contrast, Drewry and Paton (2000) reported improvements in soil physical condition down to 10 cm depth at their site, perhaps indicating the greater impact of previous common-practice dairy cattle treading. The extent of natural recovery of soil physical condition for plots protected from grazing that were previously grazed by cattle was also reported by Stephenson and Veigel (1987; Table 2). About 12 months after protection from trampling and grazing, mean soil bulk density values, to 5.1 cm depth, for the 10 cattle ha 1 treatment, recovered to 58% of the non-compacted, ungrazed plots (Table 2), with other improvements summarised in Table 2. Recovery of the degraded soils was nearly complete after two growing seasons. 4.2. Treading intensity treatments Improvement in soil physical condition following several grazing management treatments in the Waikato, was reported by Drewry (2003). For example, macroporosity (pores >30 mm) at 0–5 cm was greater in a never-grazed (21.6%), never-pugged (21.3%) and 3-h grazing (16.3%) treatments compared with conventional grazing control (12.8%). In the never-grazed treatment, pasture was trimmed and clippings returned to simulate return of nutrients. However, the first sampling for the never-grazed treatment was 14 months after dairy cows were excluded. At 5–10 cm, macroporosity for the never-grazed treatment was greater than the normal grazing treatment. Natural soil rejuvenation processes are the likely cause of the improved soil quality in
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the never-grazed treatment. The mechanisms of pastoral soil recovery under different grazing managements, particularly where visible surface pugging does not occur, should be more clearly defined. However, both natural rejuvenation from earthworm burrowing, wetting and drying cycles and plant root penetration, and prevention of physical deterioration are likely causes for the improvements shown in the never-pugged and the 3-h grazing treatments.
5. Recovery of soil physical properties after exclusion of grazing animals for greater than 3 years 5.1. Animal grazing versus animal exclusion areas Several livestock exclusion studies have shown improvements in soil bulk density over long periods. For example, Basher and Lynn (1996) reported that bulk density improved by about 10% at 0–5 cm depth, in New Zealand high country where stock has been excluded for 45 years (Table 2). Recovery of damaged soil surfaces has also been shown after removal of sheep in semi-arid woodland improved soil porosity (Braunack and Walker, 1985). Porosity in areas ungrazed for 8 and 16 years was greater than in regularly grazed soil. The ungrazed areas showed an uneven surface and longer large pores compared with grazed areas. They suggested that recovery of soil physical condition was incomplete even after 16 years, and although not specific, that recovery depended mainly on soil type and bioclimatic conditions. Similarly, soil (Kandiustalf; 15% silt, 63% clay) bulk density under pasture was less in plots ungrazed for 5 years, compared with cattle grazing (Bell et al., 1997). For example, bulk density at 5–10 cm was 1.14 Mg m 3 and 1.24 Mg m 3 for ungrazed and grazed areas, respectively. Bulk density at 10–15 cm depth was 1.22 Mg m 3 and 1.28 Mg m 3 for ungrazed and grazed areas, respectively. These soils had organic carbon 30 mm) in a nevertrodden treatment ranged from 18.7 to 23.6% at 0–5 cm, indicating a probable upper limit for macropore (>30 mm) volumes, compared with 7.5–13.3% in the ‘‘normally trodden’’ regime (Singleton and Addison, 1999). Few differences in soil physical state beneath 10 cm indicated that soil at this depth was beyond the zone of major hoof compaction (Singleton et al., 2000). This could also indicate that there was little soil physical recovery beyond that depth. Zegwaard (2000) showed that the typical rise in soil surface height under a fenceline was 40–80 mm higher than the level in treaded areas, where trampling or remoulding, or erosion, processes may be present in trampled areas. Alternatively, it could also be due to improved soil structure under the fencelines from soil recovery, or a combination of some or many factors. Indeed, greater topsoil depth in exclusion areas was also reported by Basher and Lynn (1996), who suggested that soil rejuvenation was the likely cause. This review has indicated that natural recovery of soil physical condition under compacted, pastoral conditions is likely to be confined within the top 15 cm soil depth (Bell et al., 1997; Drewry and Paton, 2000; Drewry et al., 2004; Greenwood et al., 1997; Herrick and Lal, 1995). Such surveys indicate recovery is common following soil physical damage within paddocks, and that the likely maximum values of soil physical condition are found under fencelines (or long-term exclusion areas) where stock cannot walk. It is likely that soil physical condition under fencelines is a combination of soil recovery processes in the absence of soil compaction, but separation of the two processes is difficult.
6. Overview of mechanisms for natural recovery of soil physical deterioration This section outlines some of the mechanisms for natural recovery of soil physical condition. It is not intended as an in-depth review of such processes, as this is beyond the intended scope of this paper. As previously stated, there are many natural processes and soil factors affecting soil physical deterioration and rejuvenation. These include soil shrinkage and swelling, clay or texture characteristics, cracking or micro-cracking from rapid wetting to reduce soil tensile strength, the wetting and drying process, freezing and thawing, frost penetration, plant root penetration, earthworm burrowing and plant root penetration and decay, and management practices (Larson and Allmaras, 1971; Greenland, 1981; Hodgson and Chan, 1984; Grant and Dexter, 1989; Dexter, 1991; Francis and Fraser, 1998; Greenwood and McKenzie, 2001). Soil moisture is important, as rewetting soil has been shown to increase the relief of soil surface roughness, which can improve plant root penetration (Grant and Dexter, 1989). It is also likely that natural rejuvenation of soil physical condition by pasture roots is most likely to occur in about the top 10 cm for many soils. The majority of pasture roots and earthworms are within this zone (Springett, 1985; Williams et al., 1989). Some pasture species, for example kikuyu (Pennisetum clandestinum), has been shown to be more beneficial than other species for improving aggregate stability and soil physical degradation (Bell et al., 1997). 6.1. Effect of earthworms on soil physical condition Compacted surface soils can be ameliorated by the activities and processes of macro-invertebrates (Herrick and Lal, 1995). However, many studies have reported the detrimental effects of trampling on earthworm and invertebrate numbers, species distribution and earthworm population density (Hitchinson and King, 1980; Piearce, 1984; Cluzeau et al., 1992; Lobry de Bruyn and Kingston, 1997). Factors considered to be important include direct trampling, impedance of earthworm and insect locomotion, changes in the amount and composition of vegetative feed available and reduction of air and water movement though reduced soil porosity (Piearce, 1984; Lobry de Bruyn and Kingston, 1997). While studies have shown structural and chemical improvements in degraded arable soil due to earthworms and plants (e.g., Fraser et al., 2003), few studies however, have researched the recovery of pastoral soil physical condition due to earthworms or the related mechanisms of fauna-induced soil recovery. Earthworm casting and movement were suggested by Piearce (1984) as mechanisms for increasing overall soil porosity. Although earthworms (Aporrectodea caliginosa and Lumbricus rubellus) are most active in the top 10 cm of soil, others (Aporrectodea longa and Lumbricus terrestris) have been found to be active deeper in the profile occupying a
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particular ecological niche and contributing to improved soil structure (Springett, 1985; Springett and Gray, 1997; Francis and Fraser, 1998). Although no relationship was found between the number of recovered earthworms and saturated hydraulic conductivity in the study of Francis and Fraser (1998), this may have been because the soil had been sieved and therefore disturbed, with natural drainage pores disrupted. Earthworm population density was, however, related to macropore number, but not to total or mean macropore area (Francis and Fraser, 1998). In contrast, soil in earthworm burrow linings has been shown to be more compact than the surrounding soil matrix, resulting in reduced hydrology (Bastardie et al., 2005). Reuse of burrows by earthworms makes them more stable than unused burrows, as earthworms maintain the burrow structure over several years (Bastardie et al., 2005). Continuous pores and increased aggregate stability have also been associated with benefits of ungrazed pasture leys (Bell et al., 1997; Connolly et al., 1998). Large juvenile, adult earthworms, and deep burrowing species have been found to be the most resistant to the effects of soil compaction (Piearce, 1984; Cluzeau et al., 1992). A shift in the earthworm population composition from Aporrectodea caliginosa to Lumbricus rubellus as a result of cattle trampling under irrigation was shown by Lobry de Bruyn and Kingston (1997). Summer irrigation also led to a deterioration in soil structure compared with dryland plots, suggesting that once structural damage occurs, it may take several years for soil physical structure to naturally improve. The dynamics of the soil physical deterioration and natural recovery cycle warrant further investigation, particularly under increasing stock density and expansion of dairying and irrigation on soils not traditionally used for intensive farming.
7. Conclusions This review has indicated that natural recovery of deteriorated soil physical condition under pastoral farming is likely to be limited to 10 cm or at most 15 cm depth, under common grazing practice or after animal exclusion. Degraded soil physical condition naturally recovers to the greatest extent when animals are completely excluded from pasture. Complete animal exclusion for months or years, as discussed in this review, is likely to result in not only less soil physical damage than under intensive grazing in wet conditions, but also natural improvements in degraded soil structure when animals are removed from pastoral soil. However, such a practice is generally not practical under intensive New Zealand and Australian grazing conditions unless cut-and-carry management strategies are used. Such systems require further field or modelling evaluation, particularly the potential for natural improvements in soil physical condition as it affects nutrient cycling and pasture growth. In addition, however, complete exclusion of animals in areas such as ungrazed riparian buffer zones is becoming
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more common on farms. Consequently, after these areas are fenced off and stock excluded, soil physical condition is expected to improve naturally, with subsequent increased infiltration and macropore volume expected. As has been shown in this review, physical improvements to degraded soil structure also occur when stock is partially excluded from pasture. In a practical sense, this will be where soil physical deterioration on-farm is avoided by restricting the duration of grazing in moist and wet paddocks for several hours, or removing animals to hard stand-off areas when soils are very wet. This practice has benefits to soil physical structure when the soil is wet. Although research has been conducted intentionally or co-incidentally on recovery of soil physical condition after treading damage has occurred, there are a number of knowledge gaps relating to pastoral agriculture, particularly under rotational grazing management, as used on intensive dairy farms. Further research is therefore required into the consequences of farm management practices that enhance natural rejuvenation of degraded soils. Further research needs also include natural recovery of soil physical condition over ‘short’ time periods, such as soil recovery within rotational grazing management timeframes. However, the potential for recovery of soil physical condition needs to be balanced with possible further soil physical deterioration when re-grazed. In addition, natural recovery of soil physical condition over longer periods within whole-farm systems will help assess the components of the ‘soil compaction and recovery cycle’, as such integrated assessment has generally not been attempted. Consequently, integration of both deterioration and natural recovery of soil physical condition in the soil compaction and recovery cycle is needed to improve farm system evaluation and management. Natural recovery of soil condition when animals are partially or fully excluded from grazing is therefore important in management and modelling of pastoral and ungrazed riparian zone soil, and subsequent environmental impacts. Acknowledgments The early draft of this work was funded by the New Zealand Foundation for Research, Science and Technology (contract C10X0017). The author also acknowledges assistance from The Australian National University and the Cooperative Research Centre for Landscape Environments and Mineral Exploration while completing the manuscript. The author thanks the reviewers for helpful comments on the manuscript. References Basher, L.R., Lynn, I.H., 1996. Soil changes associated with cessation of sheep grazing in the Canterbury high country, New Zealand. NZ. J. Ecol. 20, 179–189.
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