Soil factors affecting the sustainability and productivity of perennial and annual pastures in the high rainfall zone of south-eastern Australia
Authors: White R. E. , Helyar K. R. , Ridley A. M. , Chen D. , Heng L. K. , Evans J. , Fisher R. , Hirth J. R. , Mele P. M. , Morrison G. R. , Cresswell H. P. , Paydar Z. , Dunin F. X. , Dove H. , Simpson R. J. 
Published: September 2002

A field study was carried out in the high
rainfall zone (HRZ, >600 mm p.a.) of southern Australia from March 1994 to
August 1997 to test the hypothesis that sown perennial grasses and liming
could make the existing pastures more sustainable through better use of water
and nitrogen. The site, on an acid duplex soil at Book Book near Wagga Wagga
in southern New South Wales, was typical of much of the HRZ grazing country in
southern New South Wales and north-east Victoria. The experiment consisted of
4 replicate paddocks (each 0.135 ha) of 4 treatments: annual pasture (mainly
ryegrass Lolium rigidum, silver grass
Vulpia spp., subterranean clover
Trifolium subterraneum and broadleaf weeds) without
lime, annual pasture with lime, perennial pasture (phalaris
Phalaris aquatica, cocksfoot
Dactylis glomerata and subterranean clover
T. subterraneum) without lime, and perennial pasture
with lime. Soil pH (0–10 cm) in the limed treatments was maintained at
5.5 (0.01 mol/L CaCl2), compared to 4.1 in the
unlimed treatments. The pastures were rotationally grazed with Merino ewe or
wether hoggets at a stocking rate which varied with the season, but was
10–25% higher on the limed pastures [14.8–17.3 dry
sheep equivalent (dse)/ha] than the unlimed pastures. One replicate
set of pasture treatments was intensively monitored for surface runoff,
subsurface flow (at the top of the B horizon), water potential gradients and
ammonium volatilisation. Other measurements of nitrogen inputs,
transformations and losses were made on all paddocks.
In a normal to wet year, surface runoff, subsurface flow and deep drainage
(>180 cm depth) were about 40 mm less from the perennial than the annual
pastures. The reduction in deep drainage under the perennials was about
one-third to one-half (20–29 mm/year). The smaller loss of solution
NO3– from the perennial pastures (up to 12 kg
N/ha.year) suggested soil acidification under perennials was reduced by
about 1 kmol H+/ha.year. Denitrification and
volatilisation losses of N were small (1–12 kg N/ha.year). Nitrogen
fixed by subterranean clover (above ground parts) ranged from 2–8 kg
N/ha in the drought of 1994–95 to 128 kg N/ha in a normal year
(1996). The soil-pasture nitrogen balance was positive for all treatments and
averaged 76 kg N/ha.year over 2 years.
The abundance of introduced and native earthworms increased from 85 to
250/m2 in the limed pastures between 1994 and 1997.
Introduced species, such as Aporrectodea trapezoides,
were especially responsive to lime. Animal production per hectare was
10–25% higher on pastures with lime. Critical gross margins per
dse were lowest ($16/ha) for a long-lived perennial pasture (>15
years), and highest ($20/ha) for a short-lived perennial (5 years).
Overall, there were substantial benefits in animal production, improved soil
quality and water use from establishing perennial grass pastures with lime on
these strongly acid soils.