Tropical Savannas CRC > Research > CRC Research 2001-2008 > Landscape Ecology > Soil biota, nutrients and water in savannas

Soil biota, nutrients and water in savannas

Leaders : Tracy Dawes-Gromadzki, CSIRO Sustainable Ecosystems, Darwin 
Mike Webb, CSIRO Land & Water, Townsville

Full title: Effects of disturbance on biota, and nutrient and water dynamics for landscape function in tropical savannas
Project 1.1.3

Summary | Activity 1: Effects of managed grazing pressure on biophysical processes in the landscape and soil | Activity 2: Effect of grazing on nutrient stocks and fluxes in the landscape | Activity 3: Effects of management on the functional role of macro-invertebrates | Activity 4:The functional role of trees in nutrient dynamics | Outcomes | Outputs | Project team |


Long-term landscape health can only be achieved if the basic ecosystem services of capturing and cycling water and nutrients are maintained. Use of savanna landscapes for pastoral production involves disturbances such as grazing, clearing and fire which can significantly alter the ability of a landscape to retain water, nutrients and carbon. However, we have a poor understanding of how these disturbances affect these key landscape processes at scales ranging from small patches to whole landscapes. Without this knowledge it is difficult to develop sound recommendations to help land managers maintain healthy landscapes.

Maintaining soil biological processes that form the basis of soil health is fundamental to the sustainable management of savannas. These biological processes both depend on, and affect, the availability of nutrients and water. This project will develop a better understanding of those soil biological processes and how they affect, and are affected by, nutrient and water dynamics. It will complement and collaborate with other TS¿CRC projects that focus more on the inputs and outputs from the soil and on the broader consequences of soil health and biodiversity. The better understanding of soil biological processes and nutrient dynamics gained from this project will contribute a robust validation and underpinning for indices of land condition and biodiversity that have been developed within the TS-CRC and will be further developed under the TS¿CRC.

Thus, this understanding of the biophysical aspects of landscape function is critical to developing protocols for sustainable management of Australia¿s tropical savannas and restoring appropriate landscape function to degraded areas.

The project will have a major focus on the Upper Burdekin where it will build on a well-established research infrastructure and knowledge base. Furthermore, there is mounting interest from the industry and public to determine actual contributions of differing land uses in the Burdekin to delivery of nutrients and sediments to the Great Barrier Reef Lagoon (the Burdekin is a focus catchment for the National Action Plan for Salinity and Water Quality). Work will also continue in the Victoria River District and Darwin region where this project can make large contributions to the knowledge of how land management affects the interaction of soil biota and landscape function.

The project is organised around four major activities outlined below.

Activity 1

Effects of managed grazing pressure on biophysical processes in the landscape and soil (Peter O'Reagain)

Climate variability and spatial complexity are possibly the two most important challenges to sustainable grazing management in northern Australia. The Wambiana grazing trial, near Charters Towers in north Queensland, is a substantive, long-term project that aims to evaluate the performance of different methods of managing grazing pressure in a temporally variable and spatially heterogenous environment.

Activity 1 aims to determine how different levels of grazing pressure affect nutrient stocks, flows and losses in a savanna landscape. It will also determine the effect of rainfall and soil characteristics on these processes.

Detailed studies of the impacts of grazing pressure on biophysical processes will be conducted over three seasons on contrasting soil types at the Wambiana grazing trial. These studies will investigate the effects of grazing pressure on firstly, net primary production, litter fall and nutrient uptake by the herbaceous layer. Secondly, nutrient removal and loss through forage consumption, the export of cattle biomass, soil erosion, nutrient loss and runoff across the landscape. And thirdly, nutrient addition and redistribution across the site through the excretion of faeces and urine.

Studies will also be conducted to determine the input of nutrients through litter fall from the dominant Acacia and Eucalyptus species on the site. Studies will also be done on the rate and extent of decomposition of this litter fall, as well as that from the dominant grass species (Bothriochloa ewartiana), on the major soil types of the site.


Activity 1 links directly with the CRC Project 1.1.1. Predicting Outcomes of Savanna Management and will provide important biophysical data for the modelling processes in this project. The activity also links closely with the MLA funded Burdekin Catchment project which is investigating catchment and sub-catchment scale erosion and nutrient loss. Activity 1 will also generate important information for the MLA funded Grazing Land Management Education program.

Activity 2

Effect of grazing on nutrient stocks and fluxes in the landscape (Michael Webb)

Understanding the effects of different grazing management systems on nutrient resources is important to sustain long-term productivity in the tropical savannas and to minimise environmental impacts of potential nutrient losses on downstream ecosystems.

Different grazing management systems affect both nutrient stocks (the total amount of nutrient present at a site and its distribution between different ecosystem pools or compartments) and nutrient fluxes (the pathways and rates at which nutrients move between the different ecosystem pools) in the landscape. Under some grazing systems nutrients may become more vulnerable to loss from the landscape due their redistribution and concentration in pools that are more labile and/or more subject to disturbance processes which increase nutrient mobility and flux rates. The vulnerability of particular nutrients to loss under different grazing regimes will depend on the size and comparative `recalcitrance¿ (i.e. the forms in which nutrients are held and immobilized) of the nutrient pools and on their spatial and temporal distribution across the landscape, the fluxes between these pools, and how all of these are influenced by specific management practices and by climate.

Quantifying net loss of nutrients from the landscape under different grazing management systems is important, but a more detailed understanding of the underlying processes is necessary to develop and adapt management systems to improve nutrient retention capacity and reduce net nutrient losses from the landscape.


Activity 2 links directly with the CRC Project 1.1.1. Predicting Outcomes of Savanna Management and will provide important biophysical and nutrient data for the modelling processes in this project. The activity also links closely with a current MLA proposal Sustainable grazing for a healthy Burdekin catchment project which is investigating catchment and sub-catchment scale hydrology.

Activity 3

Effects of management on the functional role of macro-invertebrates (Garry Cook)

The activities of soil macro-invertebrates are critical to maintain healthy savannas through their role in soil processes such as water redistribution and nutrient cycling, and their function as a food resource to vertebrate fauna. Currently there is little quantitative understanding of this relationship in savannas, or how cattle and fire affects the relationship.

Many studies indicate that this should be a fruitful research direction for better understanding and management of northern Australia¿s savannas. For example, a study on degraded land in West Africa showed that the addition of grass mulch increased soil porosity through increased activity of soil invertebrates, with termites being responsibility for most of that increase. This increased porosity resulted reduced runoff and greater capture of rainfall.

The theoretical framework of our research concerns the pathways by which ecosystems respond to disturbance. Systems may degrade relatively little with increasing disturbance until a threshold is reached, after which degradation occurs quickly. The pathway of recovery may follow a different path, in which systems remain degraded even if the disturbance regime is reduced in severity. We hope that by understanding the role of invertebrates in maintaining landscape functioning to develop ways to hasten recovery of degraded systems and improve the resilience of healthy systems.

Activity 4

The functional role of trees in nutrient dynamics (Susanne Schmidt)

The role of trees for nutrient dynamics in savannas will be assessed to establish how trees and grasses interact to acquire water and nutrients. We are presently analysing data from our pilot project in the Desert Uplands (central Queensland) in cleared and uncleared Eucalyptus melanophloia communities that provides the basis for future work in the Upper Burdekin (north Queensland).

Preliminary data analysis suggests that:

  1. Plant available nitrogen forms are detected in top soil and in deeper soil layers indicating that mobile (nitrate) and less mobile (ammonium) forms of nitrogen leach through the soil profile.
  2. Nitrate was detected in xylem sap of trees indicating that nitrate is taken up by deep-rooted species. The significance of the interception of mobile forms of nitrogen by trees and grasses will be established in the planned project.
  3. Soil and plant d15N values are elevated in cleared areas compared to uncleared remnants suggesting that nitrogen cycling processes are altered in cleared areas.


This project will provide data and relationships for the modelling work being conducted in the Predicting outcomes of savanna management project . This will include the functional relationships between easily measured parameters such as soil surface condition and cover, and consequences such as water infiltration and nutrient stocks and fluxes. The results of the research on the effect of grazing management on water and nutrients in the landscape will support grazing management strategies of Theme 2.

Successful completion of the project will contribute to improved management through:

government agencies with responsibilities in pastoral land management (i.e. QDPI, QEPA, NTDIPE) will have an improved understanding of the how management affects the functional role of vegetation and soil biota in maintaining the processes of water and nutrient dynamics that underpin a healthy landscape. This will be achieved through their collaborative involvement in this project.

The medium and long-term sustainability of land management options will have been investigated in Project 1.1.1 using data provided by this project.

The interaction of the functional significance of soil macroinvertebrates with vertebrate fauna will be taken into account in biodiversity monitoring protocols being developed in the TSCRC. This will assist in robust systems for monitoring pastoral land condition in tropical savannas.


Documentation on:

  • The effect of grazing pressure on nutrient stocks and flows over a range of seasons in contrasting savanna landscapes (Activity 1).
  • The role of trees in maintaining basic biophysical processes in these landscapes (Activity 1 and 4).
  • The rate of recovery of landscape function as a result of reduced grazing pressure (Activity 2)
  • The temporal and spatial changes in sources of nutrients vulnerable to loss (Activity 2)
  • The effects of land condition on the functional role of macroinvertebrates in soil processes (Activity 3)
  • The interaction of the functional roles of macroinvertebrates and vertebrate biodiversity in response to grazing and fire management (Activity 3).
  • Nutrient cycles in relation to nutrient movement through the soil, nutrient uptake by trees and grasses, soil characteristics, and landscape (Activity 4).
  • and
  • Data available for parameterisation of models being developed under project 1.1.1

Project team

T Dawes-Gromadzki, CSIRO SE
M Webb, CLW
S Joyce, CLW
J Kemei, CLW
A Hawdon, CLW
A Spain, CLW
G Cook, CSE
T Gromadzki, CSE
J Ludwig, CSE
A Andersen, CSE
G Wanganeen, CSE
D Williams, CSE
P O'Reagain, QDPI
P Allen, QDPI
S Schmidt, UQ


Dr Garry Cook
Principal Scientist
CSIRO Sustainable Ecosystems
Tel: 08 8944 8427

Fax: 08 8944 8444

PMB 44

Dr Tracy Dawes-Gromadzki
CSIRO Sustainable Ecosystems
Tel: 08 8944 8435

Fax: 08 8944 8444

Tropical Ecosystems Research Centre, PMB 44

Mr Mike Webb
Plant Nutritionist
CSIRO Land & Water
Tel: 07 4753 8562

PMB Post Office