Nitrogen dynamics in modern cropping systems

Take home messages

  • Matching the supply of nitrogen (N) with crop demand is critical to optimising nutrient use and profitability of grain production. Defining the ability of a soil to deliver available N both prior to and within the grain growing season is required to help optimise N fertiliser application rates.
  • The amount of N delivered to crops from soil will be location specific due to variations in the environmental conditions, soil types and their properties and the manner in which agricultural management practices are implemented.
  • N stocks in Australian soils are declining. On average, the production of cereal and oilseed crops is associated with a negative N balance. Performing N balance calculations is important to define the potential impacts of current management practices on long term soil productivity and N supply capacity.
  • A short period of negative N balance is acceptable provided it is followed by a rebuilding phase through implementing practices capable of increasing soil organic N stock.
  • As soil N supply capacity is reduced, a greater reliance on fertiliser N will result. Due to potential losses that N derived from fertilisers is exposed to, the greater reliance on fertiliser N may result in lower yields being associated with the optimisation of profits.

Background

Grain production in Australia occurred on 21Mha in 2015-16 (Australian Bureau of Statistics 2017b). Cereal production accounted for 17Mha with wheat accounting for the largest area (11Mha) and barley the second largest (4Mha). Contributions from oilseed and pulse crops were each approximately 2Mha. Combined, the gross value of Australian grain crops was $12.6 billion across 29,000 businesses, with wheat, barley and canola production providing a total of $9.9 billion (Australian Bureau of Statistics 2017a; Australian Bureau of Statistics 2017c). For individual businesses, setting and maintaining an ability to achieve appropriate yield targets is a critical step to defining the profitability of grain production.

The majority of grain produced in Australia is rainfed and grown under conditions where the availability of water defines potential productivity. Provision of an appropriate supply of nitrogen (N) and other nutrients, matched to the temporal demand of grain crops over the growing season is required to optimise profitability. If N supply is insufficient, water limited grain yields will not be attained. If too much N is present, the potential exists for vigorous early vegetative growth to lead to crops ‘haying-off’ under water limited conditions later in the growing season (van Herwaarden et al 1998).

Synchronising the rate of N supply with the demand of a growing crop is a challenge faced by grain growers. It requires an understanding of temporal crop demand and how best to satisfy that demand through the application of appropriate quantities of fertiliser. A key component associated with deciding how much fertiliser N to apply is estimating the quantity and temporal provision of N from soil.

Given the contribution that purchasing and applying fertilisers makes to the variable cost of grain production in Australia (20-25% of variable costs (IPNI 2013), 12-30% for cereals and oilseeds and 6-16% for pulses depending on crop and rainfall zone (Rural Solutions SA 2017)), developing an ability to accurately predict and maintain the provision of N from soil will lead to more profitable grain production. This paper will consider the importance of soil derived available N to productivity, implications of running soil N supply capacity down and practices with a potential to alter soil N supply capacity.

Nitrogen supply in the context of potential productivity

Variations in the availability of water to grain crops across years and from location to location will mean that different amounts of N are required to optimise productivity and profitability. Defining potential grain yield on the basis of water availability (stored soil water at sowing + growing season rainfall) has been used to guide the definition of yield targets (Figure 1a). In Figure 1b, point B on the solid black line defines potential yield for a particular availability of water.

Combining this potential with a protein target allows grain growers to estimate crop N requirement. However, defining an appropriate fertiliser application rate that matches N supply with crop demand requires a knowledge of the quantity of N that will be delivered from the soil. Without an understanding of soil N delivery, the use of inappropriate application rates of fertiliser N may occur and result in suboptimal yields (point A) and reduced profitability.

Where management practices can be applied that shift the intercept term to lower values (move from point C to D) in response to a reduction in soil evaporation, run-off or deep drainage, yield potential will be enhanced and follow the dotted line. Under such conditions potential yield will move from point B to E, but attaining that yield will again require a knowledge of the amount of N that can be provided by a soil in order to define appropriate fertiliser application strategies.

(a) Water use (mm) (growing season rain + change in soil water), mapped against productivity (grain yield or dry matter - kg/ha), (b) Productivity (grain yield or dry matter - kg/ha) mapped against potential plant available water (mm) (growing season rain + plant available soil water at sowing).

Figure 1. Concept of water limited potential (a) as defined originally by French and Schultz (1984) and (b) as subsequently modified.

The water use and water use efficiency concept can be extended to the efficiency frontier approach described by Keating et al (2013) (Figure 2). To introduce this approach, the change in grain yield or profit as a function of N fertiliser application rate is presented (Figure 2a). The economic optimum (point A on the dashed profit line) is likely to occur at an N fertiliser rate less than that required to maximise yield (point B on the solid yield line).

The contribution to yield or profit due to the ability of a soil to provide N to a crop is defined by point C. As the soil contribution increases, the response curves for yield and profit will shift to the left and the fertiliser N application rate required to optimise profit will be reduced. Conversely, as the N supply capacity of a soil declines, a greater reliance on fertiliser N will result.

In Figure 2b, the efficiency frontier for profit as a function of investment is presented along with the changes that can occur if management actions are taken that can improve the efficiency frontier (e.g. enhanced infiltration of rainfall or water holding capacity). For this example, the investment will be taken to represent the costs associated with fertiliser N application and assuming that other costs are fixed.

The current efficiency frontier is represented by the dotted line and the optimum fertiliser N application where profits are maximised corresponds to point D. If fertiliser N application is too low (point E), profit will be reduced due to the opportunity cost associated with forgone grain yield. If too much fertiliser N is applied (point F), profits will be reduced due to the decreased return on the investment made in buying and applying additional fertiliser. Under the conditions of a new efficiency frontier (solid line), if the fertiliser N addition associated with point D is maintained, profit may increase (point G) but it will not be optimised. To reach the new economic optimum (new profit maximisation), fertiliser N addition would have to be increased to that associated with point H.

Since the availability of N to grain crops results from a the combination of N supplied by the soil and any applied fertiliser N, maximising profits requires a knowledge of the amount of N that can be supplied by the soil (point C) to ensure that optimum fertiliser application rates can be defined. Only in the case where soils are not able to supply any N to grain crops will not knowing the ability of a soil to supply N have no impact on yield and profit outcomes. However, under such conditions, it is likely that the soil will exist in a degraded state and optimisation of production even with the application of additional fertiliser will likely not be possible due to the existence of other factors constraining productivity.

The water use and water use efficiency concept can be extended to the efficiency frontier approach described by Keating et al (2013) (Figure 2). To introduce this approach, the change in grain yield or profit as a function of N fertiliser application rate is presented (Figure 2a).

Figure 2. (a) Application of the efficiency frontier concept as described by Keating et al (2013) and (b) how modifying fertiliser addition rates on the basis of knowing soil nutrient supply will move profitability along existing and improved efficiency frontiers.

The soil nitrogen cycle

The soil nitrogen cycle, showing the various pools and nitrogen transformations and movements, is presented in Figure 3. The majority (>95%) of nitrogen in a soil exists as insoluble organic matter (soil organic matter, soil microorganisms and plant residues). Nitrogen available to crops includes inorganic nitrogen (ammonium and nitrate) and soluble organic nitrogen. The biological processes of decomposition and mineralisation convert insoluble organic N into plant available forms. Although microorganisms mediate the production of available N, they also require N themselves.

Under conditions where the organic matter being decomposed does not contain sufficient N to satisfy the requirements of the microorganisms, they will scavenge available N from the soil. This process is referred to as immobilisation and occurs when crop residues with high C/N ratios decompose in soil. Immobilised N is not lost from the soil system, but is converted into an organic form that can be mineralised back into an available form through decomposition.

Nitrogen can be added to a soil in the form of inorganic (e.g. urea, di-ammonium phosphate (DAP), mono-ammonium phosphate (MAP), etc.) or organic (e.g. manure, compost, etc.) fertilisers or through biological nitrogen fixation associated with free-living N2 fixing organisms or the production of pasture or grain legumes. Several processes exist that can reduce the amount of N present in a soil including: removal in harvested products (grain, hay or meat), leaching of available N (large rainfall events), denitrification of nitrate (prolonged periods of wet soil) and volatilisation of ammonia (high pH soils or poor conditions after surface granular urea application). Increases in the potential for N loss through leaching, nitrification and volatilisation can occur when available N accumulates. Matching the supply of available N to crop N demand will reduce potential accumulations of available N and potential N losses.

The soil nitrogen cycle, showing the various pools and nitrogen transformations and movements

Figure 3. Forms, fluxes and transformations of nitrogen in soil.

Maintaining soil N status — is it important?

Implementing agricultural production on Australian soils has resulted in a decline of soil organic matter stocks to values representative of 30-80% of those in comparable soils under native vegetation (Luo et al 2010). As soil organic matter stocks decline, N bound with carbon (C) in the organic matter is mineralised to an available form. Based on a C:N ratio of 11.7, derived from data presented by Kirkby et al (2011), a loss of 10g C/kg soil within a 0-10 cm soil layer with a bulk density of 1.20Mg/m3 would result in the mineralisation of 1028kg N/ha. Possible fates of the mineralised N include extraction in harvested products or loss from the soil.

Fertiliser N application rates have been guided by results obtained from fertiliser application rate trials. Although such trials are useful, they have tended to be used to define the minimum amount of fertiliser N required to optimise annual (short term) profitability. In essence such trials have taught grain growers how to most effectively mine nutrients from the soil. While in the short term this would appear to be a practice that optimises annual profits, with continuing declines in stocks of soil organic matter and associated N, the ability of soil to continue to supply the N required to meet crop demand will diminish. The result of such a progressive mining of nutrient stocks will be an increasing gap between the amount of N required by grain crops and the quantity that can be supplied by the soil. If not addressed, this will lead to an increasing dependence on fertilisers to achieve desired grain yield outcomes.

Assuming a yield target for wheat of 3t/ha with an 11% protein target, the amount of N required by the crop would be 159kg N/ha as calculated using Equation. Values of 5.7 for the conversion of protein to N, 0.81 for N-harvest index, and 0.45 for N use efficiency were used.

N required (kg/ha) = ((potential crop yield (t/ha) x grain protein content %)/(protein to n conversion x N harvest index x N use efficiency) x 10

Equation [1]

Assuming an N mineralisation rate of 3% of soil N per year, that all mineralised N is derived from the 0-10 cm soil layer with a bulk density of 1.3 Mg/m3, an organic carbon content of 2.0% and a C:N ratio of 11.7, in the first year the soil would be able to supply 67kg N/ha (Equation ) and 92kg fertiliser N/ha would be required to achieve the target wheat yield and protein.

N soil supply kg/ha = soil organic carbon (%) x bulk density (Mg/(m cubed)) x soil layer thickness (cm) x (1/(C:N ratio)) x Percentage of total soil N that mineralises (%) x 100

Equation [2]

If this wheat crop is grown continuously every year and only enough fertiliser N is added to satisfy the N required by the crop above which is supplied by the soil, the changes in soil N supply and required fertiliser N over time are shown in (Figure 4). After 10 years, the soil N supply capacity and the fertiliser N requirement would change to 49 and 110kg N/ha respectively.

After 10 years, the soil N supply capacity and the fertiliser N requirement would change to 49 and 110kg N/ha respectively.

Figure 4. Change in soil N supply capacity (solid line) and fertiliser N requirement (dashed line) over time in the absence of any additions of N addition beyond that required for a 3t/ha wheat grain yield with 11% protein.

Another factor that needs to be considered is the decrease in efficiency of fertiliser N use with increasing application rate (Figure 5a). Each incremental increase in yield will have a higher cost, particularly in progressing towards the biological optimum yield. A main contributor to this relationship resides in the mechanisms by which available N (i.e. fertiliser N) can be lost from the soil/crop system (e.g. volatilisation, denitrification and leaching), and the potential increase in the magnitude of these losses as the concentration of available N in soil increases in response to increasing fertiliser additions.

As a result, where fertiliser N application rates have to increase in response to a decreased ability of the soil to supply N, the cost of achieving additional yield increments will likely increase and the profitability of applying additional fertiliser N will decrease. Under such circumstances, and assuming all other variable costs remain fixed, the economic optimum yield (where marginal benefit = marginal cost, Figure 5a) will decline as the ability of a soil to supply N decreases (point A versus point B in Figure 5b) as will profits (Figure 5c).

Part of the benefit provided by soil N supply, relative to fertiliser N application, resides in the fact that N derived from organic matter decomposition is metered out over the growing season and responds positively to the same environmental variations controlling crop growth and N demand (e.g. soil temperature and availability of water). With an increasing reliance on soil derived N, the supply and crop demand for N are likely to be better synchronised, leading to lower accumulations of available N in the soil. It is important to note that the different responses presented for the soils with a low and high N supply capacity in Figure 5 are conceptual and have been accentuated to demonstrate the points made above. A more complete economic assessment is required to quantify the magnitude of the proposed profitability differences and to fully assess the implications.

Graph (a)  and (b) map proportion of yield potential attained x Fertiliser N application rate, (c) Profit ($/ha) x Fertiliser N application rate.

Figure 5. Changes in (a) fertiliser N use efficiency (b) grain yield and the profit optima and (c) profitability of grain production with increasing fertiliser N application rates for soil with a low (solid line) or high (dashed line) N supply capacity. Note that these diagrams are conceptual and differences between low and high N supply capacity have been accentuated for the purpose of demonstrating potential differences.

A range of performance indicators exist to quantify the effectiveness of fertiliser N management (Table 1). Each of the indicators provide useful, but different information. Partial factor productivity (PFP) provides an overall assessment of yield response per unit of fertiliser N applied, but fails to compensate for the amount of N supplied by the soil. Thus, where similar amounts of fertiliser N are applied to soils with different soil N supply capacities, different values for PFP will be obtained. On soils with a greater N supply capacity, PFP values would be inflated giving the impression of a greater fertiliser N use efficiency. The best use of PFP appears to be as a monitoring tool over a defined land area (e.g. paddock, farm or region). In this situation, increasing PFP likely indicates an improving soil N supply capacity and conversely a falling PFP likely indicates a reduction soil fertility and possible mining of soil N stocks.

Table 1. Common performance indicators used to assess the effectiveness of nutrient management (based on Dobermann, 2005; Norton, 2016).

Performance indicator

Calculation

Units

Interpretation

Partial factor productivity (PFP)

PFP = YF/F

kg grain/kg fertiliser nutrient applied

Defines the return achieved per unit of fertiliser nutrient applied. PFP does not compensate for nutrient provided by the soil. High values could be due to efficiently managed systems or to a high soil nutrient supply capacity.

Values typically range for 40 -70kg grain/kg applied nutrient.

Agronomic efficiency of applied nutrients (AE)

AE = (YF-Y0)/F

kg yield increase/kg fertiliser nutrient applied

AE measures the additional yield achieved by applying fertiliser nutrients. It provides a better assessment of the impact of fertiliser nutrients on yield than PFP through the inclusion of yields measured on unfertilised control plots.

Values range from 10-30kg grain/kg N.

Partial nutrient balance (PNB)

PNB = G/F

kg nutrient in grain/kg fertiliser nutrient applied

PNB is the ratio of nutrient removed to nutrient applied. Values >0.5 suggest background supply is high and/or fertiliser nutrient losses are low. Values >1 imply an extraction and removal of nutrients that is greater than the amount of nutrient applied. Because it is a ratio, PNB does not provide a direct measure of the magnitude of nutrient depletion.

Nutrient balance intensity (NBI)

NBI = G-F

kg nutrient removed in products/ha - kg fertiliser nutrient applied/ha

Defines the difference between nutrient removed in products and fertiliser nutrient added. Positive values indicate nutrient mining and a depletion of soil nutrient supply capacity. The magnitude of NBI provides a direct measure of the extent of nutrient depletion in kg nutrient/ha.

F = the amount of fertiliser nutrient applied (kg ha-1)

YF = crop yield (kg/ha) obtained with the application of fertiliser nutrient.

Y0 = crop yield (kg/ha) in a control treatment with no fertiliser addition.

G = the amount of nutrient present in harvested grain (kg/ha).

In the calculation of agronomic efficiency (AE), the difference in yield between unfertilised and fertilised crops is expressed as a function of the amount of fertiliser applied. This provides a more robust assessment of the magnitude of grain yield increase per unit of fertiliser N applied. It is less confounded by variations in the N supply capacity of the soil than PFP. However, if large variations in soil N supply capacity exist, significant variations in AE could be observed. As noted for PFP, monitoring variations in AE through time over a defined land area would provide a means of defining the direction of changes in soil N stocks.

Partial nutrient balance (PNB) and nutrient balance intensity (NBI) both provide direct measures of the potential direction of change in soil N stocks by quantifying the difference between N supply and removal in harvested products. PNB values >1 imply a mining of the soil N stocks, while values <1 imply an enhancement. Where significant quantities of applied N are lost (e.g. through volatilisation of ammonia in response to an application of urea to a wet soil surface), a value of PNB <1 could be calculated, but would not provide a true indication of the net effect on soil N stocks. Additionally, because PNB is calculated as a ratio, it does not provide an indication of magnitude of potential changes (removals or additions). Calculating the value of NBI will quantify the magnitude of the change in soil N stocks. However, as for PNB, the ability of NBI to reflect actual changes in soil N stocks depends on the ability of the soil/crop system to retain supplied N.

When comparing indicator values across different studies it is important to consider the consistency in how the calculations were performed (what terms were included), whether they were applied to single crops or rotations and the area over which values have been integrated. All calculations in Table 1 are generally performed annually on the basis of fertiliser N being the source of nutrient addition and applied to single paddocks. Excluding N inputs from biological fixation, and manure applications, Norton et al (2015) obtained a PNB value of 1.10 for wheat. This suggests that when cereal crops are considered on their own, outside of crop rotations including legumes, a net removal of N has occurred. It is also likely that this net removal is a conservative estimate of the actual reduction in soil N stocks given that N removals due to leaching, volatilisation, denitrification and erosion were not included.

To provide guidance to Australian grain producing growers, values of PNB and NBI at farm or paddock scale should be performed and integrated over multiple years to account for crop rotations and variations in environmental conditions and yield. Norton (2016) examined data from 514 paddocks from 125 farms across four grain growing regions in south-eastern Australia and calculated PNB and NBI values for N over a period of 4-5 years. For this assessment, inputs from biological N fixation were estimated and included with fertiliser nutrient applications (Table 2). Although the magnitude of regional averages of PNB and NBI varied, the indices indicated an average net removal of N (PNB >1 and NBI<0). These results likely represent a conservative estimate of the real change in soil N stocks because losses due to leaching, ammonia volatilisation, denitrification and erosion were not included. The variability in NBI across paddocks and years was large and as yields increased there was a greater probability of obtaining a negative NBI.

Table 2. Values of PNB and NBI for N derived by Norton (2016) over a 4-5 year time frame for paddocks from the grain growing regions of south-eastern Australia. Values are provided as the mean±standard error of the mean.

Grain growing region

Nitrogen

PNB

NBI

High rainfall zone

1.55 ± 0.10

-13.1 ± 2.0

Mallee

2.09 ± 0.17

-9.5 ± 1.2

Southern NSW

1.20 ± 0.07

-4.1 ± 2.9

Wimmera

1.21 ± 0.11

-2.3 ± 3.1

Each farm, and indeed each paddock, will experience different annual inputs, extractions and losses of N as a function of variations in applied management practices, soil properties and environmental conditions. It is important for grain growers to complete net N balance calculations (Equation) for their own production systems to define how soil N stocks are changing. Although a trend to increasing N stocks is encouraged, it should be acknowledged that temporary periods of mining N stocks are acceptable, provided the extent of N mining is quantified and followed by a rebuilding phase in which N stocks are replenished. It is recommended that annual N balance calculations be performed; however, the values should be integrated and accumulated over time to define the full effect of applied management practices and temporal trends. Such information will allow grain growers to implement appropriate actions to maintain their production base into the future and continue to maximise profitable grain yield outcomes.

N balance = (NF + NOA + Ndfa + Ndep) - (NR + NL + NV + NDen + NE)

Equation [3]

NF = N added to the soil in the form of chemical fertilisers.

NOA = N added to the soil in the form of organic amendments (e.g. manure, composts, etc.).

Ndfa = N derived from atmospheric N2 by symbiotic and non-symbiotic fixation.

Ndep = N deposition from the atmosphere.

NR = N removed in harvested products.

NL = leached from the root zone.

NV = volatilised as ammonia from fertilisers and soils.

NDen = N lost as N2 and N2O by denitrification.

NE = N lost by erosion.

Factors that influence soil N supply capacity

A range of factors (environmental, soil and applied management) all interact to influence the ability of a soil to mineralise organic N and make it available to crops. Although many studies have attempted to quantify the impacts of particular factors, this is difficult given the strong interactions that exist. For example, it is difficult to define the impact of crop residue composition on N mineralisation without considering the impact of temperature and soil water content. However, it is possible to make broad comments in some instances. Across a range of soil textures it has been demonstrated that the production and retention of available N increases, goes through a maximum and then declines with increasing soil water content. It has also been shown that mineralisation and the production of available N will increase with increasing temperature. However it is also known that these two factors will be interactive in their impact in field situations. It is proposed that variables inclusive of both terms (e.g. microbially active degree days) may provide a better integrator of environmental impacts on the delivery of available N from soil.

For soil properties, the content and composition of organic matter, soil texture, soil depth and soil biology have all been identified as factors that can alter the amount of nutrient mineralised. Organic matter is a diverse mixture of different components with varying C:N ratios and biochemical composition. Recent developments in the fractionation of soil organic matter into particulate, humus and resistant forms have found that these materials vary in C:N ratio and their contents of labile carbon. As a result, it is likely that at least a portion of the differences in delivery of available N to crops can be ascribed to compositional differences in soil organic matter (i.e. variations in the allocation of soil carbon to the particulate, humus and resistant fractions).

Soil texture influences mineralisation directly and indirectly. With increasing clay content a greater surface area is available to adsorb and protect organic matter from decomposition resulting in a decline in the proportion of organic N that mineralises over time. The indirect impact of texture manifests through its impact on soil water — soils with higher clay contents retain more water than those with low clay contents and thus alter decomposition rates. Most nutrient mineralisation has been found to occur in the surface soil layers where concentrations of organic matter are greatest. However, nutrient mineralisation can occur at depth particularly where significant quantities of carbon exist (e.g. Vertosols or even in the rhizosphere component of low organic carbon soil). For mineralisation to occur it is also essential that a viable and active decomposer community exists including both microorganisms and soil fauna.

Agricultural management practices influence nutrient mineralisation principally through their impact on the quantity, composition and handling of crop and pasture residues. The quantities of residue returned to or onto the soil will vary substantially across the Australian grain growing region in response to species selection, soil type and environmental conditions. Species selection (e.g. legumes versus cereals) will also have a strong impact on the composition of the residues. Where large inputs of cereal residues with low nutrient content are returned, initial net immobilisations of nutrients are likely. Such removal is not permanent, but rather represents a conversion to a form that will be made available later as decomposition continues. Where legume residues are returned, mineralisation of nitrogen will likely be enhanced; however, as the N harvest index of grain legumes increases the amount of N returned in the residue and potentially made available will decline.

An additional factor that has perhaps received less attention than required is the implication of the degree to which the residue is incorporated into the soil. With the movement towards less tillage, a greater proportion of crop and pasture residues are being retained on the soil surface. The impact of this on the relative amount of residue derived and soil derived organic carbon respired to CO2 and the fate of residual nutrients requires further assessment. All root residue residues are returned to the soil, but significant uncertainty remains in how much organic matter is added below ground in the form of root structures and exudates and what the nutrient content of this material is.

The amount of nutrient mineralisation, particularly for N, that occurs over the non-crop period has typically been assessed through the collection and analysis of soil cores prior to sowing a crop. Significant stocks of available N can be found in soils at this time (>200kg N/ha) with the stocks generally being higher after canola, grain legumes and pastures than after cereals. When collecting samples for this assessment, it is important to ensure that the depth of sampling coincides with the effective rooting depth of the crop being sown.

Conclusion

Soil derived N is a limited resource. It can make significant contributions to the amount of N seen by a crop. As the capacity of a soil to deliver available N to crops declines, a greater reliance on fertiliser N will occur. As fertiliser N rates increase, the potential for N loss increases and typically leads to reduced fertiliser N use efficiency. As a result, with decreasing soil N supply capacity, optimised productivity (where marginal benefit=marginal cost) may move to lower yields.

Completing N balance calculations is essential for grain growers to gain an understanding of how their management practices are altering the stock of N present in their soils. N balance calculations should be completed annually, but integrated over time. Where negative N balances are obtained, the soil N resource is being mined. Under such circumstances, it is important to consider whether future long term (decadal) productivity and potential profit is being eroded to maximise short term (annual) values.

It is acknowledged that short term negative N balances are acceptable if a subsequent rebuilding phase is implemented. A range of management practices including stubble retention, grain and pasture legume production, green manuring, application of appropriate quantities of fertiliser N and application of organic amendments have the potential to rebuild soil organic N stocks.

References

Land Management and Farming in Australia (ABS)

Agricultural Commodities, Australia (ABS)

Value of Agricultural Commodities Produced (ABS)

Fertiliser expenditure in the Australian grains industry.

Keating B, Carberry P, Thomas S, Clark J (2013) Eco-efficient agriculture and climate change: conceptual foundations and frameworks. In 'Eco-Efficiency: From Vision to Reality.' (Eds C Hershey and P Neate) pp. 19-28. (The International Centre for Tropical Agriculture: Cali, Colombia)

Kirkby CA, Kirkegaard JA, Richardson AE, Wade LJ, Blanchard C, Batten G (2011) Stable soil organic matter: A comparison of C:N:P:S ratios in Australian and other world soils. Geoderma 163(3-4), 197-208.

Luo Z, Wang E, Sun OJ (2010) Soil carbon change and its responses to agricultural practices in Australian agro-ecosystems: A review and synthesis. Geoderma 155(3-4), 211-223.

Norton R, Davidson E, Roberts T (2015) Position paper. Nitrogen use efficiency and nutrient performance indicators. Global Partnership on Nutrient Management, 1-14.

Rural Solutions SA (2017) Farm gross margin and enterprise planning guide 2017: A gross margin template for crop and livestock enterprises. ISSN - 2207-2349.

van Herwaarden A, Farquhar G, Angus J, Richards R, Howe G (1998) 'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser. I. Biomass, grain yield, and water use. Australian Journal of Agricultural Research 49(7), 1067-1082.

Acknowledgements

The research reported within this paper was made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC. The authors would like to thank them for their continued support. The financial support provided by the GRDC to projects CSO00207 and CSO00203 is gratefully acknowledged.

Contact details

Jeff Baldock
CSIRO Agriculture and Food,Locked Bag, Glen Osmond, SA 5064
(08) 8303 8537
jeff.baldock@csiro.au

GRDC Project Code: CSP00207,