Disintegration of cattle hoof prints in cracking-clay soils of the arid South Australian Stony Plains region during a wet period

Recovery of clay-soil microtopography from trampling by cattle was assessed over 247 days in the Stony Plains region of South Australia during La Niña conditions. Hoof prints took 96 to 247 days to disintegrate. Several prints were still visible nearly seven months after initial measurement. Print volume and area declined more-or-less uniformly over time, but were still considerable for prints present at the end of the study. Rain may facilitate the surface recovery of cracking-clay soils from trampling via shrink-swell processes. In dry years, microtopography might take longer to recover. Considering the threatened and endemic species that these soils support, and their value to the pastoral industry, land managers should consider recovery time from trampling when implementing grazing management strategies.


Introduction
Stony tableland soils of the South Australian Stony Plains region are fine-textured, gypseous, saline, and characterised by numerous stones (gibbers) in the upper soil profile and surface.Gibber plains in this region often show pronounced microrelief in the form of gilgais (Jessup, 1960).'Gilgai' is an indigenous term for the depressions that form in these soils (Paton, 1974).Cracking-clay soils are a common feature of gilgaied gibber plains of the Stony Plains region.They support nationally-threatened species such as plains rats (Pseudomys australis) (Brandle and Moseby, 1999).These soils are also habitat to a diversity of other small animal species, including endemic reptiles such as gibber dragons (Ctenophorus gibba), and little-known species such as Woomera sliders (Lerista elongata) (Waudby, 2013;Petit et al., 2012;Waudby and Petit, 2011).Pastoralists in the region consider these soils to be important opportunistic sources of stock fodder following sufficient rainfall (Waudby et al., 2013b).The integrity of these soil cracks is likely to be of critical importance to biodiversity conservation in the region (Waudby, 2013).Cracking-clay soils may be relatively resilient to trampling by stock since shrink-swell processes (caused by wetting and drying cycles) peculiar to these soil types may allow them to regenerate from mechanical disturbance (Taddese et al., 2002;Drewry, 2006) and since clay soils typically have a greater supporting capacity than other soil types (Lull, 1959).Few studies have examined the physical recovery of these soils from trampling over time.We aimed to quantify the surface recovery of these soils from trampling by cattle during a period of high rainfall, when recovery is expected to be most rapid.

Study site characteristics
The study was undertaken on a 4915 square km cattle lease (Billa Kalina Station;29º55'01.66'' S,136º11'14.45'' E) in the arid South Australian rangelands (northwest pastoral district; Figure 1), during La Niña conditions.Billa Kalina is located in the South Australian Stony Plains region.Stony tableland soils of the Stony Plains region are saline and gypseous with a clay profile, and covered by gibbers (Jessup, 1960), which sometimes form a desert pavement (Bourman and Milnes, 1985).Billa Kalina consists of several land systems-groups of areas with similar patterns of geology, topography, soils, and vegetation.This research took place in the northern section of the property, within the Oodnadatta land system, which is characterised by gibber plains with gilgai microrelief.

Disintegration of hoof prints
Thirteen cattle hoof prints were marked with pin flags and measured.These prints were not part of a cattle pad (trails that cattle form as they travel the same path to water-points).They had been made in the seven days prior to the first measuring period and either during or immediately after a rainfall event.Some had been made by adult cattle and others by calves; some may have been created by the same individuals.Maximum width (diameter) and depth of the print were measured to the nearest 0.5 cm on six occasions, starting on 28 April 2010 and ending on 3 December 2010.The mean area (for a circle: πr 2 ) and volume (for a cylinder: πr 2 h) of soil disturbed in all prints were calculated for all measuring periods.Means were plotted with rainfall, which was measured from the first monitoring session.The approximate number of days until complete disintegration (when the print was no longer visible) was determined for all prints that disappeared during the sampling period.

Climate
Over 150.0 mm fell from April 2010 to December 2010.Mean temperatures ranged from 10.8ºC (± 0.20) in July 2010 to 26.7ºC (± 0.26) in December 2010 as shown in Figure 3.

Disintegration of hoof prints
The hoof print study period spanned 247 days.The maximum number of days until disintegration ranged from 96 to 247 days.Five hoof prints were still clearly visible approximately seven months after their initial measurement, remaining considerable in volume and area.Eight prints had either disappeared or were only a faint imprint that could not be measured by the final (sixth) measuring period.Standard error at each measuring session was smaller for surface area than for volume; mean surface area decreased considerably only at the last sampling date (Figures 4 and 5).

Discussion
Rainfall during the study period exceeded the yearly average for the region.Wetting and drying cycles cause cracking-clay soils to swell and shrink, which may facilitate the recovery of soil microtopography from mechanical disturbance such as trampling (Taddese et al. 2002;Drewry 2006).However, the extent of the influence of rainfall (for example timing, amount, rate, and evaporation) on shrink-swell processes is unclear and warrants further investigation.This study indicates that during rainy   4. Mean volume (cm 3 ± s. e.) of disturbance for each measuring session (cumulative number of days) and rainfall (mm) beginning from the first session.Prints disintegrated over time and could not be measured; n = 13 for sessions 1-3, n = 12 for the fourth session, n = 7 for the fifth session, and n = 5 for the sixth session.The increase in volume during session five occurred because two prints collapsed inwards and subsided Figure 5. Mean area (cm 2 ± s. e.) of disturbance (cumulative number of days) and rainfall (mm) beginning from the first session Prints disintegrated over time and could not be measured; n = 13 for sessions 1-3, n = 12 for the fourth session, n = 7 for the fifth session, and n = 5 for the sixth session.An increase in mean hoof print area during the third session occurred because one print had become a 'pit' and another had seedlings growing inside it (the seedlings may have disturbed the print) conditions in the Stony Plains region, clay-soil recovery can take at least 96 days.Five of the 13 monitored hoof prints were still clearly visible seven months after they were made, and were still large, such as 480.5 cubic cm.Under drier conditions, the rate of recovery may be much longer, particularly at intensively-grazed areas.Hoof prints could have long-term effects on the availability of cracking-clay shelters to wildlife (Waudby, 2013) and on plants for which compaction affects processes important for growth, for example water infiltration (Taddese et al., 2002).Hoof prints may collect moisture (Winkel and Roundy, 1991) and trap seed (Isselin-Nondedeu and Bédécarrats, 2007), facilitating the germination of some plant species.
Land managers seeking to rehabilitate paddocks after grazing or who 'spell' their stock should also consider the time needed for soil to recover.Potential for damage will be

Gilgais hold water after rainfalls and are used as natural watering points by cattle
Typical cracking-clay soils with Helen Waudby's boot for scale Gibber dragons (Ctenophorus gibba) are an agamid species endemic to the southern Lake Eyre Basin Plains rats (Pseudomys australis) likely rely on cracking-clay soils as critical habitat; this juvenile is eating fruit from red spinach (Trianthema triquetra) influenced by other factors than rainfall, including management regimes and stocking rates, herding behaviour, the tendency to follow well-used trails ('pads') to water, soil type, and topographic features, including slope.The mechanisms of soil recovery from trampling in the study area are not well understood as indicated by the paucity of published research.Considering that grazing dominates Australian rangelands (Waudby et al., 2012 and references therein;Waudby et al., 2013a), and that soil properties influence ecological processes (Yates et al., 2000) and rehabilitation efforts, further research is needed on the recovery of soil structure from trampling.

Figure 1 .
Figure 1.Location of the study site on Billa Kalina Station, South Australia

Figure 3 .
Figure 3. Mean temperature (± s. e.) and rainfall during the study period

Figure
Figure 4. Mean volume (cm 3 ± s. e.) of disturbance for each measuring session (cumulative number of days) and rainfall (mm) beginning from the first session.Prints disintegrated over time and could not be measured; n = 13 for sessions 1-3, n = 12 for the fourth session, n = 7 for the fifth session, and n = 5 for the sixth session.The increase in volume during session five occurred because two prints collapsed inwards and subsided