Download Agricultural nutrients in the Hawkesbury

Agricultural nutrients
in the Hawkesbury-Nepean catchment
Nutrient export rates
Nutrient exports by sub-catchment
The levels of nutrients entering the HawkesburyNepean River System are specific to land use type
and vary depending on factors such as annual rainfall, soil type, farm management and slope. Figures 1
and 2 show typical nitrogen (N) and phosphorus (P)
export rates for different agricultural land uses in the
Hawkesbury-Nepean catchment.
Map 1 shows the intensity of agricultural phosphorus
exports from each sub-catchment below the main water
supply dams. The Macdonald and Colo River catchments are mostly native forest and, consequently, have
the lowest intensity of agricultural phosphorus exports.
The mid Hawkesbury-Nepean and South Creek catchments have larger areas of horticultural farms than the
other catchments and, consequently, the highest intensities of phosphorus exports.
Figure 1. N export rates
Figure 2. P export rates
150
25
20.3
20
90
kg/ha/yr
15.1
15
62.7
10
5
13.5
Turf
Vegetable
Grazing
0
Colo River
0.8 0.5
0.3
Ag. high intensity
Rural residential
Cropping
Turf
Vegetable
Grazing
0
Ag. low intensity
4.2 3.3
2.4
Macdonald
River
3.2
Ag. high intensity
30
Ag. low intensity
52.5
Rural residential
60
Cropping
kg/ha/yr
21.9
122.
120
Mid HN
Cattai
Blue
Mountains
South
Creek
Agricultural nutrient contributions
Nutrient export rates, together with land area, are used to
calculate the total nutrient exports from a given region. The
graphs below show the contributions of different land uses
to agricultural nutrient exports in the Hawkesbury-Nepean
catchment. Although grazing is by far the dominant agricultural land use by area in the catchment (see Figure 3), it is
not the largest contributor to nutrient exports (see Figures 4
and 5). On the other hand, horticultural farms (turf and
vegetables) make up only a small proportion of agricultural
area but have proportionately far greater nutrient exports.
Figure 3. Land area
2% Vegetable
4% Ag. low
intensity
1%
Cropping
Upper
Nepean
0.0 - 0.5
0.6 - 0.9
1.0 - 1.4
1.5 - 1.8
1.9 - 2.3
>2.3
Figure 4. N from runoff
4% Ag. low intensity
1% Turf
Phosphorus
kg/ha/yr
Figure 5. P from runoff
3% Ag. low
intensity
1% Cropping
8%
25%
8%
1% Ag.
high intensity
67%
Grazing
Cropping
Data are for agricultural areas only.
34%
16%
33%
16%
29%
<1% Cropping
Vegetable
11%
Turf
Rural residential
16%
Other Ag. low intensity
20%
Other Ag. high intensity
Managing agriculural nutrients
The following methods can help to reduce nutrient exports into
river systems, increasing farm efficiency and improving the
health of the river.
Nutrient retention ponds
Nutrient retention ponds capture agricultural runoff, allowing
sediment containing particulate phosphorus to settle, and facilitating greater nitrogen removal.
Where the captured runoff is also reused for irrigation, removal
of nutrients is further enhanced and a valuable supplementary
water source is provided.
Retention pond nutrient removal rates vary depending on a
range of factors including pond size and shape, annual rainfall,
catchment area and retention time.
With sound design and by ensuring that pond water levels are
kept low so the pond has sufficient capacity to capture the
runoff, spills and associated nutrient losses can be kept to a
minimum.
Typically, nutrient removal rates for retention ponds are 74 per
cent for nitrogen and 61 per cent for phosphorus (see Table 1).
Table 1. Summary of the typical nutrient reduction from
agricultural runoff management
Nutrient retention ponds
Riparian buffer zones
Soil aeration
Compost application
Greenhouse leachate recycling
Nitrogen Phosphorus
74%
61%
50%
53%
28%
28%
14%
49%
95%
95%
Greenhouse recycling
Discharging greenhouse leachate (wastewater) offsite can
equate to very large losses of valuable nutrients and water and
can be detrimental to local waterways.
Recycling leachate back through an irrigation system can be
very effective in preventing these losses. Importantly, pathogens and accumulating salt concentrations must be carefully
managed so as to prevent damage to crops.
Generally, leachate recycling systems must be occasionally
purged to release excess salts. However, if these small
volumes are released to a well-managed nutrient retention
pond, constructed wetland and/or open reuse area (e.g. onto
field vegetables), then losses of nutrients can be virtually zero.
Typically, leachate recycling can reduce nutrient exports from
greenhouses by 95 per cent (see Table 1).
This fact sheet summarises agricultural nutrient exports and
typical nutrient reduction rates for nutrient mitigation measures. More detailed information is available in Agricultural
Nutrient Exports and Mitigation in the Hawkesbury-Nepean by
Haine, B., Coade, G and McSorley, A., NSW Office of Environment and Heritage, Sydney (2011).
The Hawkesbury-Nepean River Recovery Program is funded by the
Australian Government through the Water for the Future Program and is
managed by the NSW Government Office of the Hawkesbury-Nepean.
Mechanical aerators
Compacted soils, such as those typically found on turf farms,
tend to have lower infiltration rates and, as a result, generate
more runoff than less compacted soils.
When infiltration rates are increased by mechanically aerating
soils, runoff rates and the associated nutrient exports can be
significantly reduced. Mechanical aeration can also increase
delivery of water to the root zone improving the health of
crops.
Typically, by reducing runoff, aeration can reduce nutrient
exports by 28 per cent (see Table 1).
Compost amendments
Incorporating compost into soils increases soil carbon,
improves soil structure and infiltration, reducing runoff and the
export of nutrients carried with it. Compost can also improve
soil microbial activity and water holding capacity, providing
real benefits to plant health.
Typically, adding compost to soil can reduce nitrogen exports
by 14 per cent and phosphorus exports by 49 per cent (see
Table 1).
Riparian stock-exclusion fencing and
vegetation buffers
Excluding stock from waterways and riparian zones (where the
land and river meet) reduces nutrient exports in three ways.
Firstly, fencing prevents stock from defecating and urinating
directly in and around streams. This can significantly reduce
the levels of nutrients entering the river system. Typical nutrient excretion rates for dairy and beef cattle are indicated in
Table 2. Estimates of nutrient exports through this direct deposition by stock can be calculated by determining the proportion
of time that stock spend in and around streams and applying
this to the rates indicated in Table 2.
The second way that fencing reduces nutrient exports is by
preventing erosion due to trampling of vegetation and banks.
Thirdly, excluding cattle and improving riparian vegetation can
improve the capacity of vegetation to capture sediment and
absorb nutrients before they reach the waterway.
A healthy riparian vegetation buffer combining woody and
grass species will typically capture about 50 per cent of nitrogen and 53 per cent of phosphorus (Table 1).
Table 2. Typical excretion from cattle
Dairy
Beef
kg/head/yr
N
P
125.7
19.3
84.9
13.0
© State of New South Wales through the Office of the Hawkesbury-Nepean, 2011. This material may
be reproduced in whole or in part for educational and non-commercial use, providing the meaning is
unchanged and its source, publisher and authorship are clearly and correctly acknowledged.
Disclaimer: While every reasonable effort has been made to ensure that this document is correct at the
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done in reliance upon the whole or any part of this document.