GRAZING IMPACTS ON THE SPATIAL DISTRIBUTION OF SOIL MICROBIAL BIOMASS AROUND TUSSOCK GRASSES IN A TROPICAL GRASSLAND

Authors: Northup, Brian; BROWN, JOEL - USDA-NRCS-FORMERLY CSIRO; HOLT, JOHN - SCIRO

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/1999
Publication Date: N/A
Citation: NORTHUP, B.K., BROWN, J.R., HOLT, J.A. 1999. GRAZING IMPACTS ON THE SPATIAL DISTRIBUTION OF SOIL MICROBIAL BIOMASS AROUND TUSSOCK GRASSES IN A TROPICAL GRASSLAND. APPLIED SOIL ECOLOGY. v. 13(3). p. 259-270.

Interpretive Summary: Soil microbes (including bacteria, fungi and single-celled animals) are important to the cycling of nutrients, structure and health of soils, particularly in the semi-arid tropics. The amount of microbes in soil has normally been measured by weight (biomass). We undertook an experiment to study the effects of native grasses and grazing practices on the amount and ddistribution of microbial biomass in the soil of a seasonally dry tropical eucalypt woodland in north Queensland, Australia. Soil samples were collected at 7 locations along short transects (28 inches long) that ran through the centers of 3 species of native grass and bare spaces (areas without plants), parallel to the slope, in pastures under 5 different grazing practices. Amount of microbial biomass, and levels of carbon, nitrogen and organic materials in the samples were determined. Tests were then conducted to describe differences in levels of microbial biomass noted din the samples around different grasses, and different grazing practices. The highest amounts of microbial biomass were noted close (+/- 8 to 12 inches from plants). Heavy grazing pressure also caused reductions in the amount of organic carbon, nitrogen and organic material in the soil. Results of this study indicate that the level of microbial biomass in the soils of eucalypt woodlands of north Australia are naturally variable and changes in plant community composition in response to heavy grazing pressure reduces the productivity of landscapes, limits the incorporation of resources into the soil, and ultimately leads to the loss of soil health.

Technical Abstract: The role of grass tussocks in supporting soil microbial biomass (SMB) in grazed ecosystems is not fully understood, nor is the spatial distribution of SMB in response to different grass species. We undertook a study in 1997 that examined fine-scale distributions of SMB in grazed experimental paddocks located in the eucalypt woodlands of northern Queensland, Australia. SMB was determined on soil samples collected at seven location along 60 cm transects in the vicinity of three grass species (Bothriochloa ewartiana, Chrysopogon fallax, Heteropogon contortus) and bare spaces, in replicate paddocks under five different grazing regimes. Data (n=280) were analyzed as a split-split plot in a randomized complete block. Paddock management, micro-patch, and location effects were significantly (p < 0.05) different, as were management x location and micro-patch x location interactions. The highest SMB levels were recorded at tussock centres on ungrazed (control) and lightly grazed paddocks, with lower levels recorded on degrading (15-27% of controls) sites. Successively lower levels were noted from tussock centres outwards to the most distant locations (+30, -30 cm) with level of paddock degradation. High levels of SMB were noted around tussocks of B. ewartiana and C. fallax, while the lowest were recorded across bare patches (59% of levels for the above species). Heavy grazing reduced inputs of organic materials and carbon into the soil, thereby limiting resources available for microbial growth. Fine-scale monitoring of the plant-microbe-soil interface should be combined with large-scale measures of landscape response to properly describe degradations and recovery processes.