The source of A-type magmas in two contrasting settings: U–Pb, Lu–Hf and Re–Os isotopic constraints

The sources of post-orogenic A-type magmas from two distinct geodynamic settings are compared. The end of the ca. 514–480 Ma Delamerian Orogeny, southeastern South Australia, was marked by ~ 10 Myr of bimodal A-type magmatism, driven by convective removal of thickened lithosphere. Initial Os and Hf isotope ratios record a heterogeneous lithospheric mantle source, with some input from aesthenospheric mantle. Mafic parental melts fractionated to produce the granites. In contrast, initial Os isotope ratios of the A-type magmas that comprise the ca. 1598–1583 Ma Mesoproterozoic Gawler Felsic Large Igneous Province, central South Australia, record a dominant evolved lower crust component. However, initial Hf isotope ratios from these samples are depleted, indicating a mantle source for lithophile elements. This voluminous, bimodal magmatism lasted for ~ 15 Myr, and ended the Wartakan Orogeny. In both cases the homogenisation of chemical (rheological) heterogeneities, inherited from terrain amalgamation and orogenic thickening, strengthened the lithosphere. The contemporaneous fusion of heterogeneous mantle ± crust may represent a common, stabilising influence on the lithospheric column regardless of tectono-magmatic setting

Management practices to improve soil health and reduce the effects of detrimental soil biota associated with yield decline of sugarcane in Queensland, Australia

Yield decline (YD) of sugarcane is a widespread problem throughout the Australian sugar industry. It is defined as "the loss of productive capacity of sugarcane-growing soil under long-term monoculture". Factors contributing to YD are the monoculture itself, excessive tillage of the soil at planting and severe soil compaction resulting from the use of heavy machinery during the harvesting operation. Collectively, these crop management practices have led to the development of sugarcane-growing soils that are low in organic C and cation exchange capacity, have a high bulk density and have a low microbial biomass. This in turn is associated with a build up of populations of detrimental soil organisms, which affect the growth and health of the sugarcane root system. Significant yield increases have been demonstrated following pasteurization or fumigation of the soil or treatment of the soil with fungicides or nematicides. Several detrimental soil organisms associated with YD have been identified, including a fungal root pathogen (Pachymetra chaunorhiza) and the lesion nematode (Pratylenchus zeae). Experimental evidence, however, suggests there are many other unidentified detrimental soil organisms associated with YD. In order to circumvent YD, major changes to the cane cropping system need to be considered. Different rotation breaks (sown pasture, alternate crops, bare fallow) were evaluated for their impact on soil health and the composition of the community of organisms in soil previously under cane monoculture. Despite the breaks having different effects on populations of beneficial soil biota, all breaks reduced populations of known detrimental soil biota and significantly increased the yield of the following cane crop. A single legume-based break crop appeared to be sufficient to capture the majority of these benefits. Other possible management options including the use of organic amendments and minimum tillage techniques are discussed

Quantification of the effects of rotation breaks on soil biological properties and their impact on yield decline in sugarcane

Three contrasting rotation breaks (sown pasture, alternate crops and bare fallow) were established at five sites in Queensland, Australia, on land that had been under sugarcane monoculture for at least 20 years. The breaks were in place for 30–42 months at four sites and for 12 months at the fifth site. The effects of the breaks on selected soil biological properties were assessed following the removal of the breaks and before the area was re-planted with sugarcane. At the four sites with the long-term breaks, microbial biomass increased under the pasture break, declined under the bare fallow break and did not change significantly under the crop break, compared to microbial biomass under continual sugarcane. At these sites, populations of the root lesion nematode (Pratylenchus zeae) declined under all three breaks whereas populations of free-living nematodes increased under the pasture and crop breaks but declined under the bare fallow break. At the site with the 12 month breaks, a forage legume pasture increased microbial biomass, reduced lesion nematodes and together with the crop break increased populations of free-living nematodes. At the four sites with the long-term breaks there was an increase in the ratio of fungal:bacterial fatty acids and an increase in fatty acid 16:1ω5c (used as a biomarker for mycorrhizal fungi) under the pasture and crop breaks. Also at these sites, the soil microbial community under the pasture, crop and bare fallow breaks, respectively, showed increased, no change or diminished capacity to utilize carbon substrates compared to the soil microbial community under continual sugarcane. The yield of the sugarcane crop following all three breaks was significantly higher than the yield of the crop following continual sugarcane at each of the sites with the long-term breaks. Examination of the longevity of the effect of the rotation breaks on soil biological properties at the sites with the long-term breaks, suggested that effects on some soil organisms (e.g. lesion nematodes) maybe short-lived

Quantification of the effects of rotation breaks on soil biological properties and their impact on yield decline in sugarcane

Three contrasting rotation breaks (sown pasture, alternate crops and bare fallow) were established at five sites in Queensland, Australia, on land that had been under sugarcane monoculture for at least 20 years. The breaks were in place for 30–42 months at four sites and for 12 months at the fifth site. The effects of the breaks on selected soil biological properties were assessed following the removal of the breaks and before the area was re-planted with sugarcane. At the four sites with the long-term breaks, microbial biomass increased under the pasture break, declined under the bare fallow break and did not change significantly under the crop break, compared to microbial biomass under continual sugarcane. At these sites, populations of the root lesion nematode (Pratylenchus zeae) declined under all three breaks whereas populations of free-living nematodes increased under the pasture and crop breaks but declined under the bare fallow break. At the site with the 12 month breaks, a forage legume pasture increased microbial biomass, reduced lesion nematodes and together with the crop break increased populations of free-living nematodes. At the four sites with the long-term breaks there was an increase in the ratio of fungal:bacterial fatty acids and an increase in fatty acid 16:1ω5c (used as a biomarker for mycorrhizal fungi) under the pasture and crop breaks. Also at these sites, the soil microbial community under the pasture, crop and bare fallow breaks, respectively, showed increased, no change or diminished capacity to utilize carbon substrates compared to the soil microbial community under continual sugarcane. The yield of the sugarcane crop following all three breaks was significantly higher than the yield of the crop following continual sugarcane at each of the sites with the long-term breaks. Examination of the longevity of the effect of the rotation breaks on soil biological properties at the sites with the long-term breaks, suggested that effects on some soil organisms (e.g. lesion nematodes) maybe short-lived