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Irrigation - Australian Agriculture Assessment 2001 - Landscape balances: water, carbon, nitrogen and phosphorus

SUMMARY

• Agriculture has increased productive capacity of agricultural landscapes. This capacity is determined by changes in water and nutrient availability, assessed through: mapping modelled water, carbon and nutrients balances and distribution of the major stores and fluxes; and determining how stores and fluxes respond to changes in agricultural inputs. This can be used to infer patterns of nutrient balance across Australia at a landscape scale and allows broad rankings of nutrient use efficiency for regional agricultural land uses at river basin scale. It also provides a basis for determining where action is needed to minimise potential off-farm impacts.
• Net primary productivity (a measure of plant biomass gain) is an integrated measure of the coupled water, carbon, nitrogen and phosphorus balances. Distribution broadly follows rainfall patterns and is also influenced by air dryness, light and agricultural inputs. Net primary productivity strongly controls carbon stores in plants, litter and soil.
• Net primary productivity averages 0.96Gt of carbon each year for the Australian continent. Nearly 60Gt of the total continental carbon is stored as plant biomass (45%) and soil carbon (55%).
• Agricultural nutrient inputs have increased continental net primary productivity by 5%; the mineral nitrogen store by 13% and the mineral phosphorus store by 8%. These increases have occurred over less than a quarter of the continent (since more than 75% of Australia is rangelands, national parks, or other largely natural and intact vegetation).
• Addition of nutrients and the use of legumes and irrigation water has increased agricultural productivity, nearly doubling pre-European stores of carbon, organic nitrogen and organic phosphorus. Soil mineral nitrogen, plant-available phosphorus, and nitrogen and phosphorus concentrations in soil water have also increased by up to a factor of five. These increases are concentrated in southern agricultural regions of Australia.
• Local influences of irrigation on net primary productivity and stores of nitrogen and phosphorus are large, but impacts at continental scale are relatively small.
• Harvested product is relatively small component of the continental net primary productivity and landscape stores of carbon, nitrogen and phosphorus. Nutrients applied to agricultural landscapes can exceed those required to achieve optimum production levels and in some regions are approaching diminishing returns. Attention to nutrient balance on farm will lead to more cost-efficient agriculture and fewer off-farm impacts.
• Nutrients leaking from farms can lead to enriched rivers, estuaries and near-shore marine zones. Priority needs to be given to managing farm nutrient inputs more efficiently to counter increasing associated environmental costs.

MASS BALANCES-A BASIS FOR NATURAL RESOURCE ACCOUNTING

The availability of light, water and nutrients determines the capacity of land to produce native vegetation and agricultural yield. In Australia, long-term availability of resources and the consequent potential for generating yield can be assessed by examining the mass balances of the key resources: water and nutrients (in this case nitrogen and phosphorus). Mass balance gives a quantitative picture of:

• resource inflows or sources;
• resource outflows (including both production outputs and unproductive losses or leakages);
• the resource stock (the amount available for use); and
• the way that the stock changes with time in response to the various inflows and outflows.

The mass balances of water and nutrients are linked by carbon (or biomass) since plant biomass is approximately 50% carbon. Balances and cycles of water, nitrogen, phosphorus and carbon interact and constrain each other.

• The rate of plant biomass production determines how much resource is available for harvest and for maintaining natural and farmed animal populations.
• Production of plant biomass is closely linked to plant water use, and is usually a dominant outflow in the landscape water balance.
• Production of plant biomass is linked with uptake of nutrients from soil into plants, after which the nutrients are locally recycled through litter or removed in plant or animal harvest.

The net rate that plants build up carbon from the atmosphere by photosynthesis is known as 'net primary productivity'.

Nutrient balance also depends on several nutrient inflow and outflow processes:

• nitrogen inflows include atmospheric deposition and fixation and fertilisation; outflows include gaseous loss (volatilisation and denitrification), leaching beyond the root zone, export in surface run-off, and removal by harvest;
• phosphorus (taking account only of the part of the total phosphorus in the soil which is chemically available to plants) inflows include physical or biological weathering which releases phosphorus from soil minerals and fertilisation; outflows include leaching, export in run-off, in both dissolved and sediment-bound forms and removal by harvest.

Each of these processes influences nutrient balance, and it is important to determine their relative effects.

Net primary productivity

'Net primary productivity' is the carbon gained over time by plants through photosynthesis, minus the carbon loss over time through plant respiration. It is a fundamental measure of 'landscape yield' and is expressed in carbon units (e.g. tonnes of carbon per hectare per year).

Net primary productivity is not only significant as a measure of landscape yield, but also because plants acquire carbon from the air and nutrients from the soil in tightly constrained ratios, so that it provides information about the plant - soil cycle of nutrients through growth and decay.

A biophysical balance sheet for any landscape has both spatial and change over time variability:

• spatial variation is characterised by mapping the major reserves and flows of resources across the continent;
• a first insight into the changes over time can be gained by comparing the stocks and flows in the mass balances under 'natural' conditions with the corresponding stocks and flows under agricultural land management regimes. Under agriculture, nutrient inputs may be enhanced by fertilisers or legumes; water inputs may be enhanced by irrigation. Change since European settlement gives an initial estimate of the changes in the biophysical balance sheet brought about by agricultural practices.

The biophysical balance sheet can be used to determine:

• nutrient use efficiency from agricultural systems estimates whether nutrient inputs balance outputs or whether there is a net loss or 'mining of available nutrients' or net surplus of nutrients;
• the magnitude of off-farm nutrient fluxes—if there is a surplus of nutrients then transport off farm by leaching, in run-off and through soil erosion, can increase the potential for impacts such as algal blooms in rivers, estuaries and near-shore marine zones.

LANDSCAPE FUNCTION AND MASS BALANCES

a summary of methods

Landscape function: understanding the components and pathways

Flows of water, carbon, nitrogen and phosphorus transfer matter through stores or pools (Figure 2.1). The stores include leaves, wood, roots and soil (including the A and B soil horizons). Pools in the soil are further subdivided for modelling purposes to account for components of the carbon, nitrogen and phosphorus stores with different turnover times or chemical reaction properties, representing the active (metabolic), slow (humic) and passive (inert) components of soil organic matter.

Key flows that exchange matter between the pools include:

• conversion of light into plant biomass by photosynthesis;
• local cycling of water through rainfall, soil evaporation and plant transpiration;
• uptake of nutrients by plants and their return to the soil in decomposed litter;
• grazing and nutrient cycling by animals; and
• harvest of agricultural produce.

Cycling can also be internal—local cycling between the stores in the landscape—or between the landscape and its external environment (e.g. cycling energy through light, heat and evaporation; water through rain and evaporation; carbon through photosynthesis and respiration). Landscapes are also subject to inputs and losses of nutrients through atmospheric deposition, fertilisation, fixation, leakage in run-off and leaching, gaseous losses to the atmosphere, and harvest of product.

Models for landscape balances

Two models were developed for determining landscape balances of carbon, nitrogen and phosphorus and estimate of change in net primary productivity:

• an evolving or time-dependent model (BiosEvolve); and
• an equilibrium or statistically steady state model (BiosEquil).

Predictions of these models are designed to determine large-scale patterns rather than the behaviour of individual farms or paddocks, and should never be
interpreted at single-cell (5 km) scale. Uncertainties even at a large region scale (100km by 100km or greater) for the models are:

• Net primary productivity: 30%
• Organic stores of C, N and P: 50%
• Mineral stores of C, N and P 100%
• Current/pre-agricultural ratios: 50%
• Leaching and drainage fluxes: large uncertainty

Further information on the methods developed for this landscape scale assessment can be found in the project reports on the Australian Natural Resources Atlas.

Mass balance equations

Water

• Total water is equal to plant water plus soil water
• Change in water over time is equal to rainfall minus canopy transpiration, soil evaporation, interception evaporation, runoff and drainage

Carbon

• Total carbon is equal to plant carbon plus litter carbon plus soil carbon
• Change in carbon over time is equal to assimilation minus plant respiration, soil microbial respiration and disturbance
• Change in carbon over time is also equal to net primary production minus heterorespiratioin and disturbance

Nitrogen

• Total nitrogen is equal to plant nitrogen plus litter nitrogen, soil organic nitrogen and soil mineral nitrogen
• Change in nitrogen over time is equal to fertilisation, fixation and deposition all minus volatilisation, leaching, particulate transport, offtake and disturbance

Phosphorus

• Total phosphorus is equal to plant phosphorus plus litter phosphorus, soil organic phosphorus, labile phosphorus and secondary phosphorus
• Change in phosphorus over time is equal to fertilisation plus deposition and weathering minus leaching, particulate transport, occluded phosphorus sink, offtake and disturbance

It is often true that a store on average neither increases nor decreases (though it fluctuates continually about an average value) over a long time. Time-averaged values of change in storage then approach zero as the averaging time increases. This is the 'statistical steady state' condition.

WATER BALANCE

Australia's overall continental water balance is unusual in global terms (Figure 2.2).

Rainfall

Australia is the driest inhabited continent in the world. It receives an average annual rainfall of 465mm compared with a global average annual rainfall of 777mm on land surfaces. Almost all of this is evaporated, leaving only 52 mm of water annually for run-off—much less than the 310 mm of annual run-off averaged over the Earth's land surfaces.

Yearly variation is also very high by global standards and is linked with changing currents and water temperatures in the Pacific, Indian and Southern oceans, with a significant correlation (about -0.5) between annual continental rainfall and the ENSO (El Nino - Southern Oscillation) phenomenon in the Pacific Ocean (Figure 2.3). The ENSO influence is strongest in the north east and weak in south-western Australia.

Distribution of rainfall is strongly non-uniform (Figures 2.4, 2.5). Approximately one third of the continent is classed as arid (receiving less than 250mm average annual rainfall) and another third as semi-arid (250 - 500mm).

Australia's seasonal pattern of rainfall has a 'flip-flop' character—it is wet in the tropics and dry in the south during the southern summer, and the reverse in the southern winter.

• Rainfall pattern in the north is dominated by a humid, monsoonal wet season (October to March) followed by a hot, dry season (April to September).
• In the south west, the pattern is Mediterranean with hot, dry summers and cool, wet winters, watered by frontal rain from the Southern Ocean.
• In the south east, rainfall is more uniformly distributed through the year, under the combined influences of winter rain from the Southern Ocean and summer rain from the Pacific Ocean, brought by weather systems from the north east.

Evaporation and transpiration

Potential evaporation is high and significantly exceeds rainfall in all but the wettest areas (Figures 2.6, 2.7). It is extreme (approaching 10mm/day) in the northern inland in summer and decreases with decreasing solar radiation (i.e. in the south, in winter and near to coasts due to increasing cloudiness).

Actual evaporation (Figures 2.8, 2.9) and transpiration by plants (Figures 2.10, 2.11) are more spatially variable than potential evaporation, reflecting the effects of water limitation in most areas of the continent. Their annual mean and seasonal patterns broadly resemble the patterns for rainfall.

Soil evaporation is the difference between total evaporation and canopy transpiration and is a large part of the total where plant cover is low (e.g. in arid environments). Hence, canopy transpiration maps have more 'contrast' (relative variation) than the total evaporation or rainfall maps (Figures 2.10, 2.11).

Run-off and drainage

Significant run-off is confined to those wet areas where rainfall significantly exceeds potential evaporation. It occurs only in the south east (including Tasmania), over the eastern ranges and in the north (Figure 2.12). Run-off is negligible in the drier divisions.

Drainage has a similar pattern, though with added variability induced through the influence of soil texture (Figure 2.13).

View Drainage Division Map

The broad spatial pattern of the entire steady-state water balance can be seen by plotting its constituent terms as spatial averages across each of Australia's 12 drainage divisions (the largest hydrological units for the Australian continent). At this level of aggregation, the water balance is rainfall = total evaporation + total run-off, where total evaporation includes contributions from both canopy and soil and total run-off includes both surface and subsurface routes. Figure 2.14 shows the rainfall, total evaporation and total run-off for the drainage divisions, along with the canopy transpiration. Canopy transpiration is less than half of the total evaporation in the drier Divisions—Indian Ocean, Lake Eyre, Bulloo-Bancannia and Western Plateau.

Water balance and Australian agriculture

Australian agricultural productivity and sustainability are constrained by rainfall and humidity since water use efficiency of plants decreases as humidity falls. Australian agriculture also has to cope with very high climate variability.

• It needs to be recognised that agriculture is not an option for much of the Australian continent. Therefore, we need to sustainably manage and retain agriculture in good quality, higher rainfall landscapes.
• Australian farming systems need to maximise efficient use of natural rainfall through farming and pasture systems that can respond to and work within the context set by rainfall variability. In areas with waterlogging and dryland salinity, minimising deep drainage beyond the root zone will also be an important consideration. Australian Dryland Salinity Assessment 2000 (NLWRA 2001) outlines the water balance issues involved in salinity management.
• We need to maximise efficient use of irrigation water supplies to provide a more secure (but still variable) source of water for agriculture. Water use efficiency and sustainable practices are imperative for maximising and sustaining irrigation schemes. We need to minimise deep drainage of water beyond the root zone.
• We need to invest in climate forecasting to provide information that supports farmer decisions (e.g. whether to crop or leave fallow, whether to stock or de-stock). Returns on investment in climate forecasting are likely to be high in terms of both increased productivity and reduced reliance on Exceptional Circumstances Drought Relief.

CARBON BALANCE

Landscape yield or net primary productivity

The mean annual net primary productivity, with present agricultural inputs and exports of nutrients off farm together with the increase in production through irrigation, is shown in Figure 2.15. The key role of net primary productivity is determining fluxes and stores in the terrestrial carbon, nitrogen and phosphorus cycles (Figure 2.1). Dominant features of the net primary productivity map are:

• Net primary productivity broadly follows rainfall, but with regional modulation from saturation deficit (or how dry the air is) through its effect on water use efficiency and also, in the case of Tasmania, by light.
• The modulation by saturation deficit implies that there is less net primary productivity per unit rainfall in the north of the continent (where the saturation deficit is high on average, because of high air temperatures) than in the south (where the saturation deficit is lower on average).
• Modulation by light is significant only in Tasmania. Elsewhere, light is not a limiting resource for net primary productivity in Australia.
• The net primary productivity is also strongly modulated in agricultural regions by nutrient inputs (including nitrogen fixation by legumes) and by water inputs through irrigation. Respectively, these inputs remove nutrient and water constraints on plant growth.

• The six drainage divisions with highest net primary productivity are South East Coast, Tasmania, North East Coast, Murray - Darling Basin, South West Coast and South Australian Gulfs. These correlate with the highest areas of agricultural productivity per unit area and are quite different from the ranking for annual rainfall per unit area (Figure 2.16), which for the top six drainage divisions are Tasmania, Timor Sea, South East Coast, North East Coast, Gulf of Carpentaria and Murray - Darling Basin. These changes in ranking occur because all three modulating factors (in addition to rainfall-saturation deficit, light and agricultural inputs) are exerting significant controls on net primary productivity: the strong influence of saturation deficit means that northern regions (Timor Sea and Gulf of Carpentaria) have a low net primary productivity despite their high annual rainfall. This climatic influence cannot be removed by irrigation or nutrient inputs, and is a fundamental limitation on plant growth in northern Australia. However, from this it should not be interpreted that irrigation potential is limited in these regions.
• Net primary productivity of the urban fringes of Australia's capital cities is high—characterised by intensive horticulture and livestock enterprises.
• Limited light means that Tasmania's net primary productivity is lower than other regions in southern Australia, despite these regions having a lower rainfall.
• Net primary productivity in the southern agricultural drainage divisions (Murray - Darling Basin, South West Coast, South East Coast, South Australian Gulfs) is significantly enhanced by nutrient inputs and irrigation despite their relatively low rainfall.

View Drainage Division Map

Carbon stores in biomass and soil

• Carbon stores (figure 2.17) are strongly controlled by net primary productivity (hence also rainfall and saturation deficit), so the distribution for carbon store strongly resemble the distribution for net primary productivity—being higher in southern Australian—the South East Coast, Tasmania, North East Coast, Murray - Darling Basin, South West Coast and South Australian Gulfs drainage divisions.
• The carbon stores are also modulated by temperature because low temperatures slow decay of plant material and high temperatures promote rapid decay.
• The ratio of carbon storage in tropical to temperate regions is lower than the corresponding ratio for net primary productivity, because the tropical carbon stores turn over faster than temperate stores. The coolest parts of Australia (Tasmania and the South East Coast) have a relatively high carbon storage level per unit net primary productivity compared to the warmer tropical regions.

Changes in the carbon balance brought about by European-style agriculture

• Steady-state net primary productivity and carbon stores were compared under two different external scenarios to understand the extent of change on the carbon cycle as a result of the impact of land management changes associated with the introduction of European-style agriculture since 1788:
  • the first corresponds to present climate and agricultural practices (Figures 2.15, 2.16, 2.17);
  • the second with present climate but with no agriculture (i.e. without water input by irrigation, nitrogen input from sown legumes or fertilisation, phosphorus input from fertilisation, and nitrogen and phosphorus exports off farm in agricultural product).

The comparison is between an Australian continent, which is fully equilibrated to current agricultural practices, and a continent that is fully in equilibrium with external forcings in the absence of European-style agriculture, and where climate is assumed to be the same as present climate (Figure 2.18).

Carbon balance (net primary productivity) and Australian agriculture

Water and nutrients have been applied extensively in Australian agriculture.

• Increases in agricultural production will continue to be achieved through nutrient inputs.
• Irrigation and the efficient use of nutrients will continue to contribute to the highest increases in productivity per unit area and returns on agricultural investments. This is explored further from an economic perspective in the Audit's companion report—Australians and Natural Resource Management 2001.
• Australian agriculture needs to ensure a balance between farm nutrient input and export is achieved in order to maximise return on investment and to minimise any off-farm impacts. The farm-gate nutrient balance is discussed in detail in the Nutrient Management section.

Over the bulk of the continent as shown in Figure 2.18 (the grey region) the ratio (or factor by which net primary productivity has increased) is 1. These are the arid and semi-arid rangelands, where agriculture does not exist. However, in agricultural areas, net primary productivity increased locally (at scale of 5 km cells) by up to a factor of two in response to nitrogen and phosphorus fertilisation and nitrogen input from sown legumes. The ratio is higher in irrigation areas.

The largest increases occurred in the cropping zones of Western Australia, South Australia, Victoria and New South Wales. These increases are relative to natural productivity and thus reflects the role of fertilisers and farming systems in improving soil fertility and thereby increasing production above levels produced naturally.

NITROGEN AND PHOSPHORUS BALANCES

Plant available nitrogen stores

• Nitrogen stores (Figures 2.19, 2.20) strongly resemble carbon storage and net primary productivity. Nitrogen stores are coupled to carbon through well-defined, but variable nitrogen - carbon ratios in leaves, wood, roots, litter and soil organic matter.
• In the absence of agricultural inputs of nitrogen, saturation deficit and temperature exert similar controls on nitrogen stores as they do on carbon stores.
• Agricultural nitrogen inputs have a relatively higher impact on stores of soil mineral nitrogen than on net primary productivity and the other stores (carbon, organic nitrogen). This has particular implications for rates of soil acidification, and is discussed further in the Soil Acidification section of this report.

Plant-available phosphorus stores

• Phosphorus is present in every living plant and is vital for harvesting the sun's energy for growth and reproduction.
• Phosphorus stores strongly resemble carbon storage and net primary productivity. Phosphorus stores are coupled to carbon through well-defined, but variable phosphorus - carbon ratios in leaves, wood, roots, litter and soil organic matter. Figures 2.21 and 2.22 show phosphorus stores to be higher in south eastern Australia, particularly in the upper catchments of the Great Dividing Range. High levels of phosphorus cycling through the soil and litter pools in northern Australia significantly reduce nutrient build up.
• Total stores of soil phosphorus encompass several pools, the plant-available store being only a small fraction of the total store held within the landscape. (Figure 2.21 and 2.22). Much of the phosphorus is tightly bound to the soil matrix, attached to organic matter or locked up within soil minerals. The availability of phosphorus to plants in these fractions is comparatively low.

Dissolved nitrogen and phosphorus concentrations in soil water

Dissolved nutrient concentrations were determined to assess the change in nutrient stores principally as a result of introducing agriculture into the landscape. Dissolved nutrient concentrations were modelled and calculated assuming that the plant available pools of mineral nitrogen and labile phosphorus occur in soil solution. The dissolved nitrogen concentration is the ratio of mineral nitrogen store to soil water store. Dissolved phosphorus concentration is the ratio of the labile phosphorus store to the soil water store.

• Distribution of dissolved nitrogen and phosphorus concentrations are quite different to net primary productivity (Figure 2.15) and carbon, nitrogen and phosphorus stores (Figures 2.16, 2.22, 2.23, 2.24, 2.25) because rainfall has comparable influence on both these stores. Hence, rainfall is not a strong modulator of the nitrogen/phosphorus concentration in soil water. However, a north - south influence on distribution can be seen in Figures 2.23 and 2.24. The higher levels of both nitrogen and phosphorus in solution clearly depict the agricultural regions of southern Australia.
• The effect of saturation deficit (mainly controlled by net primary productivity) on the mineral nitrogen store is greater than the effect of saturation deficit on the soil water store (mainly determined by rainfall or energy limitations). It is predicted that nitrogen concentration in soil water decrease as average saturation deficit increases from temperate to semi-arid tropical environments.
  • Modelling also suggests that soil water concentrations of nitrogen and phosphorus may be significant drivers for dissolved nitrogen and phosphorus concentrations in rivers. If so, then climate controls some dissolved riverine concentrations of nitrogen and phosphorus, through saturation deficit, and these riverine concentrations will show a similar south - north gradient to that seen in Figures 2.23 and 2.24. Complicating factors are that:
  • sediment-borne contributions to riverine nitrogen and phosphorus (organic nitrogen, organic phosphorus, sediment-bound phosphorus) will behave differently (see Water-borne soil erosion and Nutrient loads to Australian rivers and estuaries sections);
  • riverine nitrogen and phosphorus sourced from local pollution (effluent, heavily fertilised crops) will not behave similarly;
  • account needs to be made for processes affecting dissolved nitrogen and phosphorus concentrations in subsurface flow from landscapes into rivers (e.g. denitrification in riparian zones leading to loss of nitrogen to the atmosphere, and sorption of dissolved labile phosphorus onto soil particles as secondary or occluded phosphorus that effectively act as a phosphorus sink); and
  • account needs to be made for processes acting on the dissolved nitrogen and phosphorus concentrations in surface flow from landscapes to rivers (e.g. water involved in rapid surface run-off is unlikely to equilibrate its nitrogen and phosphorus concentrations with soil water except in a very shallow layer through which surface run-off and soil water are mixed).

Changes in nitrogen and phosphorus stores brought about by European-style agriculture

• Compared to the pre-European settlement era, the advent of agriculture has increased the concentrations of mineral nitrogen, labile phosphorus and the nitrogen and phosphorus concentrations in soil water by a factor of up to 5 (Figures 2.25, 2.26, 2.27). The largest proportional increases were predicted for southern Australian—in particular the south-east region of South Australia and south-west of Western Australia where nutrient levels were naturally low.
• By comparison, agriculture increased plant available nitrogen and phosphorus by up to two times that reached before settlement and show similar regional patterns.

View Drainage Division Map

The largest increases in the soil - water nitrogen and phosphorus concentrations occur in the South West Coast, the Murray - Darling Basin and the South Australian Gulfs (Figure 2.28).

Nitrogen fluxes and total nitrogen balance

Fluxes contributing to the total landscape store of nitrogen (including nitrogen in plant, litter, soil and mineral pools) for total nitrogen balance are:

• nitrogen input from applied nitrogenous fertilisers (Figure 2.29a);
• input from nitrogen fixation by legumes, including native legumes and sown crop and pasture legumes;
• input of nitrogen from the atmosphere by dry (particulates and gases) and wet deposition (rainfall) (Figure 2.29b);
• loss of nitrogen from the landscape to the atmosphere as nitrogenous gases (volatilisation), including nitrous oxide and others (Figure 2.29c);
• loss of nitrogen from the plant-available mineral pool by transport in dissolved form (leaching), mainly through deep drainage of water (Figure 2.29d);
• horizontal transport of nitrogen in particulate form by water or wind erosion (operating as either a sink or source depending on whether the net erosion process is depleting or depositing particulate material);
• net removal (off-farm export) of nitrogen in harvested plant or animal product, (can be negative if harvested product from elsewhere is used as an agricultural input, as in use of off-site hay for stock feed) (Figure 2.29e);
• disturbance fluxes: fire and grazing. Fire releases nitrogen to the atmosphere as biomass is burned. The effect of grazing (other than that accounted for in animal production exports off farm) is to accelerate loss of nitrogen to the atmosphere as plant nitrogen is excreted by animals.

Most of these (with the exceptions of disturbance and particulate transport) have been estimated explicitly as part of this assessment. The sum of the disturbance and particulate transport fluxes appears as a residual in the closed, steady-state total landscape nitrogen balance, which requires that all the above fluxes sum to zero in the long-term average.

Summary of nitrogen fluxes

Pre-European settlement:

• Before European-style agriculture was introduced, the nitrogen balance was dominated by input of nitrogen from natural fixation. Contribution from atmospheric nitrogen deposition as an input flux was (and remains) small.
• Pre-agricultural losses of nitrogen occurred through a mixture of volatilisation, leaching and disturbance (grazing and fire). Modelled estimates indicate that leaching caused the largest loss, but substantial uncertainty remains about the magnitudes of each process.
• Spatial distributions of all major nitrogen fluxes prior to European-style agriculture were closely connected with the distribution of net primary productivity.

Present day:

• Introduction of European-style agriculture substantially changed the nitrogen budget—on the input side nitrogen fixing remained the largest term, but this has been greatly enhanced in agricultural areas by sown crop and pasture legumes.
• Nitrogen input from fertilisers was a much smaller contributor to continental nitrogen balance though locally significant in areas of high nitrogen-based fertiliser use; nitrogen inputs from sown legumes exceeded those from fertilisers by a factor of seven.
• Losses also occurred through disturbance, leaching and volatilisation in the current nitrogen budget. The magnitude of disturbance has increased dramatically in comparison with the pre-agricultural budget. We attribute this mainly accelerated nitrogen loss to the atmosphere through volatilisation of nitrogen from excreta by grazing stock.
• A significant proportion of the nitrogen being applied agriculturally (either through sown legumes or through fertilisers) is being lost to the atmosphere through a combination of disturbance fluxes and volatilisation.
• Contribution to the national nitrogen budget from nitrogen exported in agricultural produce is small, but regionally important in cropping areas.
• Determination of a continental nitrogen balance is difficult and requires major assumptions (described in detail in project reports on the Australian Natural Resources Atlas).

Phosphorus fluxes and total phosphorus balance

The budget applies to plant available phosphorus, the landscape phosphorus store that interacts directly with the carbon cycle. This includes phosphorus in plant, litter, soil organic matter and the labile mineral pool. As noted previously, plant available phosphorus is only part of the total phosphorus store in the landscape, the remainder being only weakly available for plant growth (secondary phosphorus) or effectively unavailable (occluded phosphorus). The fluxes in the model of landscape balance of plant available phosphorus are:

• input of phosphorus from applied fertiliser;
• input of phosphorus from atmospheric deposition (mainly through dry particulates);
• loss of phosphorus from inert soil and rock stores by physico-chemical or biological processes (weathering);
• loss of phosphorus from the plant-available mineral pool by transport in dissolved form in deep drainage (leaching);
• horizontal transport of phosphorus in particulate form by water or wind erosion;
• return of phosphorus to an inert soil store (occluded phosphorus sink) through the secondary phosphorus pool (the opposite process to weathering);
• net removal of phosphorus off-farm in harvested plant or animal product; and
• the major disturbance flux for phosphorus is probably fire, through transport in airborne particulate ash. Phosphorus fluxes due to grazing are likely to involve local recycling on the landscape through plant and soil
  pools and are unlikely to be major contributors to the overall landscape phosphorus balance.

View Drainage Division Map

It is not possible to estimate a steady-state landscape balance for plant-available phosphorus. The very slow exchanges within the inert stores of phosphorus in the landscape render the entire balance non-steady even over time scales of millennia. We can only estimate a few of the major fluxes in the balance (Figure 2.30). These estimates suggest that with present agricultural inputs, phosphorus fertilisation is of the same order of magnitude as losses in the agriculturally managed parts of the country.

Nutrient balances and Australian agriculture

In developing Australia's agriculture, we have increased landscape nutrient stores much more than we have increased landscape production (net primary productivity). This is especially evident in the 400 - 700 mm southern agricultural zone. Predicted increases in plant-available mineral nitrogen and phosphorus have been increased by as much as five times (ratio of current to pre-agricultural level), while in the same areas the landscape yield (measured by net primary productivity) has increased by a factor of about two.

On large scales, nutrients are being applied at higher rates than are needed for optimum plant production levels and are approaching, if not well within, diminishing returns. Higher nutrient levels mean that some landscapes are leaking more nutrients into the atmosphere and into soil water and waterways than they were before introduction of European-style agriculture.

Production benefits and the environmental costs from applying nutrients to agricultural land behave quite differently as functions of the nutrient input (Figure 2.31):

• production benefits from nutrient application approach a plateau or a point of diminishing returns as soil nutrient levels become limiting, because plants can only use a finite amount of nutrient before other resources (e.g. water) become limiting.
• environmental costs tend to increase progressively and more steeply as nutrient inputs increase, because damage (e.g. eutrophication in waterways or estuaries) often has a threshold limit. Exceedance of the threshold causes undesirable and usually expensive changes that adversely affect biota and other users of water bodies which may be impossible to reverse.

In applying nutrient to landscapes, there is an optimum point (A) at which the net benefit (total benefit minus the cost) is maximised . This point occurs at a nutrient input below that required to achieve maximum production. At input rates beyond A, benefits rise progressively more slowly (diminishing returns) while the costs increase at least linearly and probably progressively more steeply.

Production benefits and environmental costs are also usually borne by different groups (benefits accruing on-farm and within farm industries, and costs accruing to users of environmental resources (e.g. drinking water supply, fisheries, environmental amenity).

Overall:

• Australian agriculture is on average beyond the point of most effective nutrient application rates;
• as a priority we need to more closely examine fertiliser and legume regimes to achieve optimum plant productivity—greater precision in use of agricultural nutrients is essential to maintain or reduce costs; and
• attention to on-farm nutrient balance will mean that negative impacts to the Australian environment from agriculture are reduced.

REFERENCES AND FURTHER READING

AWRC 1987, 1985 Review of Australia's Water Resources and Water Use, Australian Water Resources Council/Department of Primary Industries and Energy, Australian Government Publishing Service, Canberra.

Barrett D.J. 2001, NPP multi-biome: VAST calibration data, 1965-1998, available on-line http://www.daac.ornl.gov/NPP/html_docs/vast_des.html from Oak Ridge National Laboratory Distributed Active Archive Center, USA

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