Non-symbiotic nitrogen (N 2 ) fixation by diazotrophic bacteria is a potential source for biological N inputs in non-leguminous crops and pastures. Perennial grasses generally add larger quantities of above-and belowground plant residues to soil, and so can support higher levels of soil biological activity than annual crops. In this study, the hypothesis is tested that summer-active perennial grasses can provide suitable microsites with the required carbon supply for N 2 fixation by diazotrophs, in particular during summer, through their rhizosphere contribution. In a field experiment on a Calcarosol at Karoonda, South Australia, during summer 2011, we measured populations of N 2 -fixing bacteria by nif H-PCR quantification and the amount of 15 N 2 fixed in the rhizosphere and roots of summer-active perennial grasses. Diazotrophic N 2 fixation estimates for the grass roots ranged between 0.92 and 2.35 mg 15 N kg -1 root day -1 . Potential rates of N 2 fixation for the rhizosphere soils were 0.84-1.4 mg 15 N kg -1 soil day -1 whereas the amount of N 2 fixation in the bulk soil was 0.1-0.58 mg 15 N kg -1 soil day -1 . Populations of diazotrophic bacteria in the grass rhizosphere soils (2.45 Â 10 6 nif H gene copies g -1 soil) were similar to populations in the roots (2.20 Â 10 6 nif H gene copies g -1 roots) but the diversity of diazotrophic bacteria was significantly higher in the rhizosphere than the roots. Different grass species promoted the abundance of specific members of the nif H community, suggesting a plant-based selection from the rhizosphere microbial community. The results show that rhizosphere and root environments of summer-active perennial grasses support significant amounts of non-symbiotic N 2 fixation during summer compared with cropping soils, thus contributing to biological N inputs into the soil N cycle. Some pasture species also maintained N 2 fixation in October (spring), when the grasses were dormant, similar to that found in soils under a cereal crop. Surface soils in the rainfed cropping regions of southern Australia are generally low in soil organic matter and thus have lower N-supply capacity. The greater volume of rhizosphere soil under perennial grasses and carbon inputs belowground can potentially change the balance between N immobilisation and mineralisation processes in the surface soils in favour of immobilisation, which in turn contributes to reduced N losses from leaching.

Continuous-cropping systems based on no-till and crop residue retention have been widely adopted across the low-rainfall cereal belt in southern Australia in the last decade to manage climate risk and wind erosion. This paper reports on two long-term field experiments that were established in the late 1990s on texturally different soil types at a time of uncertainty about the profitability of continuous-cropping rotations in low-rainfall environments. Continuous-cereal systems significantly outyielded the traditional pasture-wheat systems in five of the 11 seasons at Waikerie (light-textured soil), resulting in a cumulative gross margin of AU$1600 ha -1 after the initial eight seasons, almost double that of the other treatments. All rotation systems at Kerribee (loam-textured soil) performed poorly, with only the 2003 season producing yields close to 3 t ha -1 and no profit achieved in the years 2004-08. For low-rainfall environments, the success of a higher input cropping system largely depends on the ability to offset the losses in poor seasons by capturing greater benefits from good seasons; therefore, strategies to manage climatic risk are paramount. Fallow efficiency, or the efficiency with which rainfall was stored during the period between crops, averaged 17% at Kerribee and 30% at Waikerie, also indicating that soil texture strongly influences soil evaporation. A 'responsive' strategy of continuous cereal with the occasional, highvalue 'break crop' when seasonal conditions are optimal is considered superior to fixed or pasture-fallow rotations for controlling grass, disease or nutritional issues.

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In low-rainfall environments, a high frequency of cereal crops has been favoured for optimising productivity and risk. However, cereals at high intensity often lead to declining water-use efficiency and increasing inputs to cope with emergent nutritional, disease and weed problems. The value of including breaks in the cropping sequence can involve a high level of uncertainty in low-rainfall areas where non-cereal crops are more risky and profitability is largely determined by the subsequent benefit to cereal productivity. In this study, we aimed to improve understanding of the magnitude and primary source of break benefits such as nutrition, water and disease management in a low-rainfall environment where a high level of within-field soil variability can also contribute to uncertainty about the value of breaks. In on-farm field experiments near Karoonda in the South Australian Mallee, breaks were grown in 2009 or 2010 on four distinct soil types across a dune–swale catena. The effect of these breaks on subsequent cereal production was measured for up to 3 years. In addition, the effect of breaks on nutrition and water available, along with disease infection in subsequent cereal crops, was explored and actual yields were compared with nitrogen and water-limited potential yields. Consistent cumulative benefits to subsequent cereal crops of at least 1 t ha–1 after 3 years accrue from breaks grown on the different soil types. The inclusion of breaks had beneficial effects on the cycling and supply of nutrients along with some short-term impacts on infection by Rhizoctonia solani AG8 in subsequent cereals, whereas there were no conclusive effects of breaks on the supply of water to subsequent crops. This study suggests that the inclusion of both legume and brassica breaks is likely to be beneficial to subsequent cereal production where nitrogen is a factor limiting productivity in low-rainfall, semi-arid environments.