GRAZING IMPACTS ON SPATIAL DISTRIBUTION OF SOIL AND HERBACEOUS CHARACTERISTICS IN AN AUSTRALIAN TROPICAL WOODLAND

Authors: Northup, Brian; BROWN, JOEL - USDA-NRCS; ASH, ANDREW - CSIRO (AUSTRALIA)

Submitted to: Agroforestry Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2004
Publication Date: 5/1/2005
Citation: Northup, B.K., Brown, J.R., Ash, A.J. 2005. Grazing impacts on spatial distribution of soil and herbaceous characteristics in an Australian tropical woodland. Agroforestry Systems. 65(2):137-150.

Interpretive Summary: Grazing lands in northern Australia can be easily harmed by disturbances like heavy grazing, or grazing and drought. These areas require the use of grazing systems that allow grasses to survive, and do not over-use available forages or soils. However, there is little information available on what the best level of use is, or how plants and soils will respond to different grazing pressures. In this study, we tested how six different grazing regimes (based on timing and intensity of grazing) applied over five or 15 years affected native grasses and soil properties in areas close to native grass plants, in pastures of eucalypt woodland in northern Queensland. We found that grass plants influenced soil carbon and nitrogen in small areas (< 12 inches) around the plants. Amounts of nitrogen and carbon were highest at locations close to plants, and declined under the heavier grazing pressures. Areas with high amounts of carbon, nitrogen, and plant roots and litter were also smaller and widely scattered under heavier grazing. Heavily grazed pastures produced less forage, had smaller grass plants, and these plants were more widely spread in the pasture than in less-heavily grazed pastures. Pastures in northern Australia do not resist disturbance strongly, so condition can quickly change from good to poor. Nor do they recover quickly following disturbance. The effects of land management in these areas should be checked with combinations of plant and soil properties that change quickly with disturbance, to detect problems as quickly as possible.

Technical Abstract: Land condition in northern Australia can be easily degraded by grazing or grazing in combination with drought. These areas require the use of grazing systems that allow persistence of native tussock grasses, and maintenance of soil condition. However, there is little information available on best level of utilization, or how plants and soils will respond to grazing and drought. This study examined effects of six different grazing regimes (based on timing and grazing intensity), applied over five (n=4) and 15 (n=2) years, on herbaceous vegetation (standing crop, basal area, size and spacing of grass tussocks) and soil properties (soil C, total N, total P, and soil-borne plant matter [roots and surface litter]) around grass tussocks in replicate (n=2) paddocks of eucalypt woodland in northern Queensland. Grass tussocks influenced soil C and total N at small (<30 cm) spatial scales, and applied grazing pressures significantly (P<0.05) affected all soil properties except total P. Concentrations of N and C were highest at locations in close proximity to plants, and levels declined under heavier grazing pressures. These paddocks also had less standing crop, smaller tussocks, and more widely dispersed surviving plants. Further, areas with high amounts of soil C, N and roots-litter were smaller and more widely dispersed under heavy grazing. North Australian landscapes have limited resistance to disturbance, or resilience available for recovery. As such, the effects of disturbance should be monitored by a combination of plant (basal area, plant spacing) and soil characteristics (total N, soil borne plant matter) that can detect the degradation process at the earliest stage possible.