No abstract

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.

The roles of plant-based systems and plant-based research and development for management of dryland salinity in southern Australia vary over a range of biophysical and socio-economic conditions, and differ according to the resources at risk (protection of water resources, biodiversity, infrastructure, dispersed assets such as agricultural land, and salt-affected land). Recommended responses are sensitive to a range of biophysical and socio-economic conditions. Extension and incentives to promote currently available perennials or salt-tolerant plants are only appropriate as the main policy response in a minority of cases. Regulation or permits to limit planting of perennials can be justified in certain areas of high-water-yielding catchments. For the majority of agricultural land that is at risk or is contributing to dryland salinity, the most logical policy response is to invest in development to improve salinity management technologies, including research and development into new plant-based systems. Situations where plant-based R&D for profitable farming systems is the best option include: (i) to reduce salinity impacts on water resources where groundwater systems are responsive and the dependence on fresh runoff for consumptive use is low; (ii) to protect infrastructure and biodiversity where there is relatively high responsiveness of groundwater and the urgency of response is low; (iii) to protect dispersed assets (e.g. agricultural land, most remnant vegetation on farms, flood risk mitigation) where profitable perennial plant options are lacking; and (iv) for land that is already salt affected.

Dryland salinity, caused largely by insufficient water use of annual crops and pastures, is increasing in southern Australia. A field experiment in north-eastern Victoria (average annual rainfall 600 mm) assessed the potential for lucerne grown in rotation with crops to reduce the losses of deep drainage compared with annual crops and pasture. Soil under lucerne could store 228 mm of water to 1.8 m depth. This compared with 84 mm under continuous crop (to 1.8 m depth), except in 1997–98 where crop dried soil by 162 mm. Between 1.8 and 3.25 m depth lucerne was able to create a soil water deficit of 78 mm. The extra water storage capacity was due to both the increased rooting depth and increased drying abiliy of lucerne within the root-zone of the annual species. Large drainage losses occurred under annuals in 1996 and small losses were calculated in 1997 and 1999, with no loss in 1998. Averaged over 1996–1999, drainage under annual crops was 49 mm/year (maximum 143 mm) and under annual pastures 35 mm/year (maximum 108 mm). When the extra soil water storage under lucerne was accounted for, no drainage was measured under this treatment in any year. Following 2 years of lucerne, drainage under subsequent crops could occur in the second crop. However, with 3 or 4 years of lucerne, 3–4 crops were grown before drainage loss was likely. Our calculations suggest that in this environment drainage losses are likely to occur under annual species in 55% of years compared with 6% of years under lucerne. In wet years water use of lucerne was higher than for crops due to lucerne’s ability to use summer rainfall and dry soil over the summer–autumn period. During the autumn–winter period crop water use was generally higher than under lucerne. The major period of increased soil water extraction under lucerne was from late spring to midsummer, with additional drying from deeper layers until autumn. Under both lucerne and crops, soil dried progressively from upper to lower soil layers. Short rotations of crops and lucerne currently offer the most practical promise for farmers in cropping areas in southern Australia to restore the water balance to a level which reduces the risk of secondary salinity.

Water and nutrient losses, pasture and animal production were measured for a prime lamb enterprise at Maindample (rainfall 750 mm/year) and a wool enterprise at Ruffy (rainfall 671 mm/year) in north-east Victoria from 1998 to 2000. Each site comprised 3 paddock-scale treatments: control, unsown pasture receiving about 5�kg�phosphorus (P)/ha.year; medium input, sown pasture (about 10 kg P/ha.year); and high input, sown pasture (≥22 kg P/ha.year). Sown pastures were based on phalaris (Phalaris aquatica L.) at Maindample and cocksfoot (Dactylis glomerata L.) at Ruffy, and sheep were set stocked. Pastures at Maindample created a larger soil water deficit (commonly about 120 mm) compared with Ruffy (about 70 mm) in summer. Maximum soil water deficit at Maindample occurred under the high input pasture and was about 30 mm greater than the medium or control treatments. Phalaris content ranged between about 200 and 1300 kg DM/ha, between 10 and 70% of total composition (commonly about 20%). In contrast, at Ruffy the control, which had 20–40% native grasses (Austrodanthonia and Microlaena) achieved the greatest soil water deficit, about 25 mm greater than for sown pastures. Cocksfoot persisted poorly at the acid soil at Ruffy. Small differences in green leaf area over summer (about 200 kg DM/ha) between treatments could generate the soil water deficit differences over the summer.The most striking result was the markedly different pathways of water loss between surface losses and deep drainage at the 2 sites, which was of greater consequence than the effect of pasture type. At Maindample, on average, 166 mm of water was lost annually (22% of rainfall) with 110 mm of this as surface runoff. At Ruffy, annual water loss was 128 mm (19% of rainfall) with 110 mm of this total water loss estimated as loss through deep drainage.Phosphorus losses were low in all treatments (≤1 kg P/ha.year), and nitrate-N (NO3-N) losses (7–12 kg N/ha.year) were comparable with previous work. Concentrations of P in water were highest from the high treatments at both sites, averaging 0.91 and 0.83 mg P/L in surface runoff from Maindample high and Ruffy high treatments, respectively. Average soil water NO3-N concentrations ranged from 3 to 26 mg N/L. Both P and N concentrations were higher than acceptable for aquatic health.Environmental risks in terms of water and nutrient losses could be either higher or lower for sown than unsown pastures, depending upon soil type, botanical stability, persistence and the soil water extracting ability of the pasture. Results indicated that better environmental outcomes could be achieved if soil types were targeted for particular land uses. High management skills are needed if grazing enterprises are to be both profitable and have lower off-site impacts.