Results of micro-morphological studies of rangeland soils at two grazed sites in north-eastern Queensland

[摘要] Soil surface crusts in rangelands and their role in runoff and erosion have long been recognized in tropical Australia. However, these crusts are much less documented than the crusts developed in southern and central arid and semi-arid Australia. The aim of this study was (i) to gain more information about the morphology,genesis and behaviour of soil surface crusts in North-Eastern Australian rangelands, (ii) to test the relevance of the crust typology suggested by Valentin and Bresson (1992), (iii) to investigate the possible relationships between crust type and plot infiltration rate and sediment concentration and (iv) to discuss the relevance of integrating crust typology within a soil surface assessment framework such as suggested by Roth (2004). Two degraded rangeland sites were selected for the micro-morphological studies, about 80 km west of Townsville. Rainfall simulation was used to simultaneously determine rainfall infiltration (as an indicator of runoff incidence) and sediment detachment for 26 mini-plots that reflected a range of soil surface conditions at each site, while at the same time providing rainfall impacted plots for micromorphological sampling. Undisturbed samples of the top 15 cm of the soil were taken from each plot after rainfall simulation. These undisturbed samples were dried prior to impregnation with epoxy resin in which an ultraviolet fluorescent dye was incorporated. One vertical cross section cut from each impregnated block was photographed under UV light using a digital camera. One 6x13 cm vertical thin section was prepared from each impregnated block and observed using a polarizing stereomicroscope. The samples were classified after the typology suggested by Valentin and Bresson (1992, 2002) and Bresson and Moran (2004). Overall, the micromorphology of the soil surface crusts is in good agreement with the typology suggested by Valentin and Bresson (1992, 2002). Structural crusts developed on these soils through compaction by raindrop impact (packing crusts) as well as through vertical sorting of soil particles by winnowing and sieving (sieving crusts and also pavement crusts). Most erosion crusts formed through erosion of the top sandy layer of sieving crusts, but some erosion crusts were transitional with packing crusts, as evidenced by a surface plasmic layer very thin and without any vesicles. Below the crusted layers, the soil matrix showed signs of an intense bioturbation, as evidenced by the abundance of channels, either empty or filled in with pellets and/or microaggregates. However, there appears to be no clear relationship between bioturbation intensity and crust type. Even though there is no clear evidence of the relict or active character of the empty and filled in channels, the high degree of bioturbation points to ability of degraded grazed surfaces to recover and regain hydrological function.Sediment concentration appears to be more related to crust types than infiltration rate, especially when the ground cover was low. In such conditions, erosion crusts generated a sediment concentration much higher than sieving crusts and pavement crusts did. The crust micromorphology is also in good agreement with the soil surface conditions suggested by Roth (2004).

[发布机构] CSIRO

[学科分类] 地球科学(综合)