Land Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Nutrient and carbon cycling
Environmental indicators reported on in this section as originally listed and defined in Hamblin (1998):
| Environmental indicator | |
|---|---|
| L5.1 | Total nutrient export nitrogen, phosphorus and potassium from each AER and drainage basin |
| L5.1A | Rates and distribution of nitrogen, phosphorus and potassium accessions into each AER and drainage basin |
| L5.1B | Sources of phosphorus derived from land activities reaching rivers by catchment |
| L5.2 | a | b | | Terrestrial carbon (organic matter) loss rate by IBRA region |
| L5.5 | Rate of land carbon (organic matter) sequestration by AER and IBRA region |
| L5.7 | a | b | | Proportion of each forestry and farming system with stable nutrient balance by major catchment, AER |
| L5.8 | Estimated success of programs to reduce land carbon loss and increase sequestration by landcover regions |
| L5.9 | Proportion of farmers using soil and plant tissue testing regularly by industry and AER |
Ecologists, agronomists and foresters have studied nutrient cycling in different natural and modified ecosystems for many years, and the relationship to net primary productivity has been established for many systems (Whittaker 1975). Many natural vegetation associations have common attributes that differ from agricultural systems, but all have a set of 'pools' that store carbon, nitrogen, phosphorus, and other nutrients, and set of pathways that cycle the nutrients between them via the transporting media of air and water.
So, what is the most sustainable 'mixed system' that we can hope to design where landscapes are composed of patchworks of land uses that are modified to greater extent (intensive irrigated agriculture) and to lesser extent (conservation parks, native forests and rangelands that are used for sporadic logging or grazing)? The degree of functionality of each component of a system needs to be considered.
Typical patchwork of land uses across mid-rainfall regions of eastern Australia.
Source: Ann Hamblin
Some guidelines for good nutrient cycling in mixed systems are:
- nutrient pools should be adequate for maintenance, reproduction and growth of each component,
- nutrients should not accumulate to excessive degree (potentially toxic) in some areas and be totally depleted from others,
- nutrient leakage from land systems into water systems (surface, aquifers and estuaries) should be similar to geological accretion rates, and
- the scale at which cycling is assessed should be the same as the scale of the constituent elements in the landscape. There is no point in monitoring nutrients in a single irrigation block if a whole irrigation scheme is potentially out of balance, unless the single block is representative of the whole in every respect.
The relationships between the different parts of a mosaic, or patchwork system have recently been studied in both natural and disturbed arid ecosystems (Arnalds and Archer 2000, Ludwig and Tongway 2000).
The increased public attention paid to carbon cycling demonstrates how much knowledge gain of a topic depends upon its status in the public eye. Much more information is now being gathered on various aspects of carbon cycling in Australia, as a result of the Kyoto Agreement, and large gaps in knowledge are also being identified.
The recent Commonwealth Government initiatives to improve our knowledge of continental-scale carbon, water and nutrient systems, through the Australian Greenhouse Office and the National Land and Water Resources Audit, has provided new insights into how these essential processes operate on the island continent.
Until these recent estimates of carbon (C), nitrogen (N) and phosphorus (P) were undertaken, there had been no attempt at a continental budget for N or P since 1990, when McLaughlin et al. estimated fluxes of these elements and sulfur (S) using broad assumptions and available statistics. This earlier work concluded that overall, continental exports of P, S and N have increased dramatically since European occupation, but that even so, inputs generally match or exceed exports of agricultural commodities by 5 to 6 fold (McLaughlin et al. 1990).
The present estimates are still continuing, but preliminary results suggest that application of fertilisers of all forms (including nitrogen fixation from introduced legumes, fertiliser additions, animal manuring, and plant residues) has increased continental nutrient stores by about 15% since European settlement (Raupach et al. 2001).