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 (continued)
Much of the research and published literature on nutrient cycling in Australia relates to agricultural systems. There has been a tendency to describe the essential nutrient requirements of agricultural plants applying to all plant systems, so that soils are frequently described as 'infertile' or 'impoverished', when this really refers only to the assessment of their suitability for agricultural production. Many Australian native plants have highly specialised mechanisms for efficient scavenging of sparse nutrients, while others use alternative pathways, symbiotic or parasitic habits, or associations with various micro-organisms, to overcome these deficiencies. Australian perennial species often have extraordinarily deep roots, or two or three separate root systems, which provide the plant with a variety of mechanisms for extracting and cycling water and nutrients through different parts of the regolith and soil horizons (Atwell et al. 1999).
Algal blooms are often a very visible symptom of nutrient imbalance in catchments. An unusually extensive bloom of over 1000 km of the Darling River in 1991 has stimulated a large amount of research in the past decade, together with more attention to reporting on bloom incidence. Although cyanobacteria (especially Anabaena spp.) are toxic, not all blooms are; and blooms can occur in unoccupied as well as heavily used catchments. Figure 60 shows the incidence of major blue-green algal blooms from 1991-1998.
Figure 60: Locations of algal blooms in Australia, 1991-1998.
Source: Richard Davis, CSIRO (unpublished data)
Where does this water pollution come from, and how much of it is anthropogenic? Scientists have over the past decade established different lines of evidence to address these questions.
Special root adaptations to low nutrient environments include fine 'proteoid' roots and symbiotic phosphorus-absorbing fungi in tree roots.
Source: Ann Hamblin
At the time of the Darling episode, much of what was known in Australia about blue-green algae came from overseas (principally US) sources. Phosphorus was considered the limiting nutrient to rapid algal growth, and it was presumed that fertilisers were the principal source of phosphorus. Studies of blooms in the Peel-Harvey and other estuaries in Western Australia during the 1980s tended to confirm this. In the very sandy, rapidly percolating soils of coastal Western Australia, phosphate retention in catchments is low, except where reactive iron, in laterites, adsorbs phosphorus. However, evidence from other parts of Australia has shown that in other environments these assumptions are not always correct (Davis and Hamblin 1998).
More recent work has identified light as the limiting factor in many eastern water bodies, which are often highly turbid with dispersed clay (NLWRA 2001b) that has its origins in the sodic soils that cover so much of Australia (Naidu et al. 1995). When low flow river conditions lead to stratification in the water column, algae access nutrients at the interface between warmer but turbid upper water and colder nutrient-rich lower water, and their numbers increase rapidly . Water management can minimise this risk by maintaining mixing conditions through flow regulation (Banens and Davis 1998). Detailed studies in the Murray-Darling Basin have shown that the origin of stream bed and dam sediments can be traced to calculate the contribution of surface soils since the 1950s. In northern New South Wales rivers, the amount of fertiliser and recent soil contribution to the total in-stream phosphorus load is very small (Martin and McCulloch 1999). In the Darling and its tributaries most of the phosphorus rich sediments have come from earlier accelerated erosion of parent rocks within deep gullies, and not from contemporary erosion of farmland (Olley and Caitcheon 2000).
Although phosphorus has been the principal focus of work seeking to control algal blooms, nitrogen has been the cause for concern in 'leaky' catchments, where water and dissolved nitrogen are rapidly transmitted into rising groundwater and discharge areas, mobilising salt in the process. The environmental consequences of nitrate accumulations in groundwater, water bodies and estuaries are still largely unexplored, but there is no doubt that more nitrate is entering sensitive ecosystems than can be absorbed and utilised by the aquatic biota (NLWRA 2001b). It is not clear what contribution these nitrates make to overall waterway loads of nitrogen, or whether they have in-stream geological consequences.
From the large international literature, and some Australian studies, hydrologists have concluded that land use is the best predictor of phosphorus and nitrogen generation rates. This is used in modelling the expected nutrient loads in streams and the effect of land use change (Young et al. 1996). 'Land use' is here a surrogate for a range of changes that European settlement has introduced, including clearing vegetation, adding fertilisers, and tilling soils. Thus the estimates of nutrient generation can be defined more precisely where detailed information on land management is available.
This finding has been the basis of the Catchment Management Support System model (Davis and Farley 1997) , which has been used to estimate the emissions of total phosphorus (TP) and total nitrogen (TN) from priority catchments in the National Pollutant Inventory. As no measurements of actual export loads have yet been related to these estimates, they should be treated with caution. They serve solely to provide some relative ranking of probable loads entering major rivers and estuaries in the catchments that support most of the population. Additional impacts from point sources of nitrogen and phosphorus cannot be assessed from these data as yet, because not all types of emission facility have started to report, including intensive animal industry installations such as feedlots, poultry farms, dairies and piggeries. Outfall locations of water treatment and sewage works, which have reported, according to scheduled protocols, for one year, are not reported on a geo-locational basis, and cannot be related to the catchment stream values.
|Catchment||Area (km 2 )||Total N (t/year) into water||Total P (t/year) into water|
|Adelaide River||177 468||550||64|
|Botany Bay||110 157||275||48|
|Derwent River||971 070||1 306||342|
|Hawkesbury-Nepean River||2 154 881||5 300||432|
|Hunter River||2 124 138||5 000||463|
|Lake Illawarra||52 395||180||246|
|Perth: Swan River-Kwinana||208 064||260||65|
|Port Jackson||59 512||179||34|
|Port Philip Bay||995 080||2 247||186|
|SE Queensland||3 399 815||10 000||1 511|
|Total||10 252 580||15 297||3 391|
Source: National Pollutant Inventory (2000).
|Land use||Total N (kg/year)||Total P (kg/year)|
|Bushland||2 220 000||150 000|
|Unfertilised grazing||620 000||95 000|
|Fertilised grazing||420 000||65 000|
|Intensive vegetables, orchards, turf||50 000||37 000|
|Urban||110 000||21 000|
|Unsewered urban||6 500||6 900|
|Unsewered peri-urban||290 000||43 000|
|Industrial and commercial||51 000||20 000|
|Disturbed land||19 000||3 800|
|Small subcatchments (aggregated)||92 700||18 400|
|Total||3 879 200||460 100|
Source: National Pollutant Inventory (2000).
An earlier report to the Murray-Darling Basin Commission in 1992 following the major algal bloom outbreak in the Darling River, provided estimates of all point sources of nitrogen and phosphorus entering these river systems (Gutteridge, Haskins and Davey 1992). Table 31 lists the numbers of different types of establishments contributing effluents to the rivers.
|Type of establishment||Number and/or capacity||P effluent load produced (t/year)||N effluent load produced (t/year)||Main locations|
| Animal feedlots||264 feedlots
180 000 cattleB
||80% Qld and northern NSW|
|Fish farms||43 fish farms||120||17||Victoria and southern NSW|
|Piggeries||1 110 000 pigs||8 800||2 530||Darling Downs NSW and Victorian river valleys|
| Sewage treatment works A
(four centres over 60 000)
|38 treatment works
||3 860 (dry year)
5 260 (wet year)
|670 (dry year)
880 (wet year)
Throughout, close to main rivers
| Urban stormwater||24 outfalls
||446 (wet year)
1 012 (dry year)
|55 (dry year)
124 (wet year)
Throughout, close to main rivers
| Irrigation drainage from major irrigation districts||7 major districts
||630 (dry year)
1 480 (wet year)
|110 (dry year)
260 (wet year)
Surface drains: Murray districts,
Shepparton and Kerang, SA Lower Murray
Subsurface drains: Sunraysia, Riverland
A Estimates based on type of treatment plant, size of populations, means of effluent disposal, types of disposal and phosphorus retention in reservoirs.
BFor comparison, in 1999, Australian feedlots held 550 000 head of cattle, which represented only 63% of their capacity. Over 150 000 cattle are held on just 14 feedlots and over 700 hold <1 000 head each.
Source: Gutteridge Haskins and Davey (1992).
This study was particularly concerned to assess the relative contribution of point sources and diffuse sources of nitrogen and phosphorus that could be responsible for major algal bloom outbreaks. Diffuse sources were calculated from descriptions in other studies, from land use areas, climate statistics and modelling for the Murray-Murrumbidgee and Darling basins. The study is particularly instructive in showing the great differences that occur between the various components for wet and dry years. In dry years point sources are the principal contributor, whereas in wet years the very much greater extent and duration of loads derived from runoff ensure that diffuse sources dominate (Figure 61).
Figure 61: Effect of wet and dry seasons on the balance of point sources and diffuse sources of land-derived total phosphorus and nitrogen into the river system of the Murray-Darling Basin.
Source: Gutteridge Haskins and Davey (1992)
The amount of sediment that is being exported from northern catchments in Australia was reported earlier (see Surface soil loss). Apart from the concern over loss of topsoil, which is the critical source of most microbial 'decomposers', clay and organic carbon, the sediments transported in run-off contain much nitrogen and phosphorus. Table 32 summarises data from three climatic regions of Australia.
|Region||Urban||Improved pasture||Unimproved pasture||Cropping||Market gardens||Forests|
|Annual total phosphorus (kg/ha/year)|
|Annual total nitrogen (kg/ha/year)|
nd = no data
Source: Hunter et al. (1997).
There are marked regional differences, in the few studies that have been undertaken, relating to the total nitrogen and phosphorus loads for crops grown in some districts, with high export loads from some tropical crops such as sugarcane and bananas. In general, market gardening (annual horticultural crops) has the highest export rates, followed by other types of cropping and urban land uses. Improved pastures (which have received fertiliser applications and contain more legume) export ten times more phosphorus than unimproved pastures, but nitrogen export is similar from both and similar to losses under forests.
A very comprehensive study of the Johnstone River catchment in north Queensland, was undertaken between 1991-1996 to measure nitrogen and phosphorus from the major types of land use, and the impact that nutrients and sediments might be having on the Great Barrier Reef (Hunter et al. 1996, Prove et al. 1997). The rivers rise on the Atherton Tableland and flow through large expanses of native forest in their midsections, before reaching undulating lowlands and flood plains that are used for grazing pastures, horticulture (bananas), sugarcane and dairying. The CMSS model was used with a 40-year run of rainfall statistics to compute the contribution each land use would have towards sediment, nitrogen and phosphorus in the long term. The relative amounts of phosphorus were closely aligned with the relative amounts of sediment that came from each land use as runoff, as less than 20% of phosphorus was in a soluble form. The amount of bio-available nitrogen, however, showed that nearly 50% of the nitrate (NO3 ) form of nitrogen came from sugarcane, and only 11% from forests, despite the fact that the area of forest is 4 times that of sugarcane. Figure 62 shows the annual export of total nitrogen and phosphorus that came from each type of land use, and the nitrate-nitrogen. The nitrate form of nitrogen is very soluble, entering groundwater, rivers and estuaries.
Source: DNR Qld (1997)
A recent national study on water quality was commissioned by the NLWRA (NLWRA (2001b). In summary, it demonstrates that nutrient and turbidity are the most widespread of all contaminants, followed by salinity, and acidity/alkalinity, and that pesticides and biological contaminants are of much lower occurrence.
Key findings of the study were as follows:
- Australia spends A$142-168 million on water monitoring per year, but much of the information is not publicly available.
- Nutrients are the second-most serious water quality issue in 43 of the 70 assessed river basins that cover the most populated regions of mainland Australia (Figure 63).
- There are more cases of turbidity exceeding water quality guidelines than for either phosphorus or nitrogen, but only 50 basins could be assessed for nitrogen because of difficulties in obtaining data, which was limited by ownership issues.
- Lack of good data and the limited availability of data were major issues throughout this study, and probably seriously underestimate the extent of water quality contamination.
Figure 63: Surface water quality in 2000; significant nutrient exceedances.
Source: NLWRA (2001b)
These studies have two major implications:
- Erosion is still the largest contributor to nutrient pollution in water bodies, as well as to turbidity. Indeed suspended clay carries with it adsorbed pesticides and organic matter, so protection from erosion is by far the most important strategy for improving water quality. This is as important in forests as it is on agricultural land. Unsealed roads or earthworks may be as significant a source as degraded pastures.
- A small area of poorly managed land can have a significant environmental consequence out of proportion to the total area occupied. Keeping surface cover (green manures, trash retention, cover crops) is the only feasible way to reduce the threat in most agricultural lands. In the Accelerated erosion and loss of surface soil section it was shown that over half of the sugar industry practises continuous surface cover, as do all orchards, but vegetable growing, tropical plantations and rain-fed cotton crops are grown with a bare soil surface. Irrigated cotton crops are very carefully managed, and grown in controlled bays, but very heavy storms may cause occasional flooding even in the best-managed systems.