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Title: GRAZING IMPACTS ON SOIL MICROBIAL BIOMASS AND A TUSSOCK GRASS SPECIES IN A DRY TROPICAL WOODLAND, AUSTRALIA

Author
item Northup, Brian
item BROWN, JOEL - USDA-NRCS
item HOLT, JOHN - CSIRO LAND & WATER
item GRICE, ANTHONY - CSIRO TROPICAL AGRI.

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Soil microbes are important to the maintenance of soil condition in the nutrient-poor, semi-arid tropical woodlands of Australia. A series of experiments were used to examine how grazing management affected the distribution of soil microbial biomass in pastures receiving different utilization levels of current year's herbage (1993-1998) and long-term grazing histories (1984-1993). Experiments considered how management affected the distribution of microbial biomass at different depths of the soil profile and locations between neighboring desert bluegrass (Bothriochloa ewartiana) plants, and different seasons of the year. Greater amounts of microbial biomass were noted near grass plants than locations between plants, and heavy grazing pressure reduced biomass by 25 to 38% across the sampled areas. Wet season microbial biomass was 19% higher than dry season biomass. Heavy grazing impacted microbial biomass through its negative effects on grass tussocks which limited inputs of organic matter derived form roots and surface litter, and further reduced levels of available soil resources. The grasses, microbes and soil condition should be monitored by a system focused on integrated responses within Australian landscapes to better define the impacts of management, rather than isolated responses.

Technical Abstract: Soil microbes are involved in the development of soil condition in the Australian semi-arid tropics, are heterogeneously distributed at fine spatial scales, and susceptible to disturbance. In 1997-1998, distribution of soil microbial biomass between neighboring Bothriochloa ewartiana (Domin) C.E. Hubb tussocks in paddocks receiving different utilization levels of current year's herbage during 1993-1998, and long-term grazing histories (1984-1993) were examined. Two experiments considered: 1) spatial distribution of microbial biomass by depth (0-75 mm and 75-150 mm) and location (nine points along transacts) between neighboring tussocks; and 2) impacts of wet and dry seasons. Areas close to tussocks (+ 15 cm) had significantly (P<0.05) higher microbial biomass, compared to areas midway between tussocks. Heavily grazed paddocks had 25 to 38% less microbial biomass across the sampled locations. Wet season microbial biomass was 19% higher than during the dry season, across paddocks and transect locations. Heavy grazing inhibited microbial biomass through its impact on the grass tussocks, which limited inputs of organic matter derived from surface litter and roots, and further reduced levels of available soil resources. The grasses, microbes and soil condition within Australian landscapes should be monitored as an integrated system to better define the impacts of management, rather than using isolated responses.