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)
Net nutrient balances in the Intensive Land-use Zone [L Indicators 5.1 and 5.1A]
A mid-1990s assessment of the nutrient status of agricultural lands in Australia indicated that inputs (via fertilisers and nitrogen-fixing legumes) are not always balancing exports (via meat, grain and hay removed) (SCARM 1998). This gives cause for concern; not only where excess nutrients leach into waterways and groundwater, but also where nutrient deficiencies lead to poor plant growth and low water utilisation. There is probably as much water leaking into groundwater from lack of efficient fertiliser use as there is from replacement of one form of vegetation by another in some medium and low rainfall environments, such as the inner margins of the cropping belt (Hamblin and Kyneur 1993, Latta 1997).
The cost-price squeeze, intensification, and market signals for higher quality products have all stimulated a significant rise in fertiliser use over the past decade (Figure 64).
Figure 64: Increases in the consumption of nitrogen (N), phosphorus (P) and potassium (K) fertilisers between 1980 and 1999.
Source: International Fertiliser Association (2000)
Recent studies, undertaken for the National Land and Water Resources Audit have extended the SCARM 1998 study, and placed nutrient balance estimates on a continental footing. The continental flux estimates of carbon, water, nitrogen and phosphorus have been developed through a modelling framework using daily climate records for the past twenty years on a 25 km 2 grid across the continent (Raupach et al. 2001). The work of Reuter (2001) has provided detailed nutrient balances at the scale of Statistical Local Area (SLA), that has been used to provide measures of agricultural input and off-take across those parts of the continent where these impacts are greatest. The study does not provide an estimate of what the off-take may be in the rangelands, where fertilisers and legume fixation are not normal inputs.
The nutrient balance project undertaken for the National Land and Water Resources Audit provides an assessment of nutrient balances covering the period 1992-1996 for agricultural land uses in the ILZ. Additional information extends back to 1989 in WA for the major nutrients N, P and K, and supplemental information for sulfur (S), calcium (Ca), and magnesium (Mg) is available (Reuter et al. 2001).
The National Collaborative Project on Indicators for Sustainable Agriculture (NCPISA) provides additional information for the whole country for the period 1987-1995 (SCARM 1998). Most of the data used in these studies come from the farm soil samples referred to in Carbon and its relationship to other nutrients and are therefore limited to the condition of the surface 100 mm of soil. However, general points drawn from this continent-wide study assist the interpretation of the later, more detailed study undertaken for the National Audit.
- 91% of P and 76% of K fertilisers are applied in southern Australia.
- There has been a sharp increase in total fertiliser applied from 1990 onward.
- 46% of P is applied to crops, 37% on pastures, and 17% to horticultural crops.
- 60% of K was applied to temperate pastures in this period.
- There was a similar, but fluctuating increase in use of N fertilisers.
Removal and balance:
- Nearly 70% of the P removed was exported in food and fibre, but nearly twice as much P was applied as was removed.
- Twice as much K was removed as was put back in fertiliser, and 76% of that was exported in products.
- Most of the nutrients exported go in cereals, meat and hay.
Much of northern Australia is considered deficient in phosphorus for the purposes of pasture production. However, very little fertiliser is applied except in the sugarcane and other cropping areas of the east coast.
The phosphorus balance is positive and becoming more so in most of the southern cropping and pasture zone, but the potassium balance is negative and becoming more so, except in Western Australia, where deficiencies at the start of the period have mostly been rectified.
The NLWRA study (Reuter et al. 2001) has focused on the Intensive Land-use Zone, but has had the benefit of ten years of soil testing data from all the major analytical laboratories and the fertiliser records of private fertiliser companies. Nearly half-a-million soil samples were contributed by these companies and government laboratories to the database.
Soil carbon and associated phosphorus values decrease inland from the coast as rainfall decreases. Nearly half the soils had organic carbon values of less than 1%, which indicates low biological fertility. Less than 2% of the total area is considered absolutely deficient in available phosphorus and potassium, but 12% of farmlands sampled are considered deficient in sulfur (less than 5 mg/kg), and over a quarter of Western Australian agricultural soils were low in plant-available potassium.
Records of fertiliser application showed that the nearly all fertiliser is applied to crops (including broadacre grains, horticultural crops, sugarcane and cotton). The exception is for irrigated pastures principally used for dairy cows (and for sheep grazed in rotation with rice). Figure 65 shows this difference very clearly.
Figure 65: Nitrogen, phosphorus and potassium fertiliser application rates for crops and pastures, 1992-1996.
Source: Reuter et al. (2001)
As with soil acidity and the lack of liming of pastures, the lack of phosphorus, and increasingly potassium, appear to be contributing both to low productivity and to undesirable off-site impacts from water leakage.
This finding demonstrates that nutrient management of agricultural lands is not in balance. While rotation of cropland with improved pastures in some districts means that most of these lands receive fertiliser at some stage, upland regions and marginal soils that are too poor (e.g. too acid or too shallow) for cropping are not being maintained at productive capacity. The situation may be exacerbated where legumes are providing more than adequate nitrogen, but deficiencies in other elements reduce its utilisation and lead to nitrate leaching.
At a continental scale, a comparison of carbon, nitrogen and phosphorus fluxes that would occur in the presence and absence of artificial inputs (fertilisers and introduced legumes), using the BIOS model demonstrate the major stimulatory effect that these inputs have had on biomass production in the Intensive Land-use Zone. Figure 66 shows comparisons between 'with and without fertiliser' (plus legumes) on soil carbon and nitrogen.
Figure 66: Continental soil carbon, nitrogen and phosphorus simulations using the BIOS model, with and without the effect of current levels of fertiliser and legume inputs.
Source: Raupach et al. (2001)
The increase in soil carbon has been from one to five times that of unaltered soils. Soil nitrogen has been increased from one to eight times (particularly marked in the infertile sandy soils of Western Australia) and soil phosphorus has been increased from one to ten times. Use of fertilisers is estimated to be increasing the overall continental nutrient store by about 15%.
In normal, undisturbed situations, forests are relatively self-contained nutrient systems, and losses into streams and groundwater are small. Likewise, export of nutrients in timber is low, apart from carbon itself. Nevertheless, disturbance does cause significant sediment loss from unsealed roads, tracks and snig-lines in forests that are harvested, and nutrient loss where soil is exposed to heavy rain for prolonged periods (Croke et al. 1999). Some undisturbed forested areas within large catchments have been monitored in comparative nutrient and sediment erosion studies as described earlier in this section.
Table 32 and Figure 67 show that, where forests occupy the largest proportion of a catchment, they inevitably contribute significantly to nitrogen and phosphorus exports, with rates of loss little different to those of grazed native grasslands or shrublands. These rates (0.05 and 0.1 kg/ha/year) are tens or hundreds of times less than exports from cropping lands, but local sources of disturbance (tracks and earthworks) in many of the more fragmented regions adjacent to populated regions (as in the Hawkesbury-Nepean catchment) have higher rates than this.
The increases in carbon, nitrogen and phosphorus, demonstrated in the synoptic continental modelling study of Raupach et al. (2001), have substantial implications for the off-site impacts from agriculture. Catchment studies show how the effect of land use influences water quality. Cropping regions have much higher levels of nitrogen, phosphorus and even potassium and sulfur than they would in a natural state. Native pastures and rough grazed areas are not fertilised, or only rarely. Forested and wooded areas tend to have higher natural levels of carbon, nitrogen, phosphorus and potassium by virtue of their geographical position. There are thus large disparities in nutrient concentration across the landscape.
In summary, nutrients are not in balance across most of the eastern third of the continent, where agricultural activities and land clearing are contributing both to sediment movement and to imbalance between nitrogen, phosphorus, potassium and other plant nutrient requirements. This is having an effect on the status of water quality in the major catchments of the Intensive Land-use Zone, clearly demonstrating the major impact arising from transported sediments (leading to turbidity) that are rich in nutrients (leading to algal blooms).